Seasonal and Interannual Variations of Cloudiness and Precipitation over Atlantic Ocean and its Adjacent Land Areas

Phillip A. Arkin^1)* and Pingping Xie²⁾

1) ESSIC, University of Maryland 2) NOAA / NWS / Climate Prediction Center

Seasonal and interannual variations of cloudiness and precipitation over Atlantic ocean and its adjacent land areas have been examined using satellite observed cloud amount data of ISCCP, CMAP / GPCP merged analyses of large-scale precipitation and the precipitation data observed by the TRMM Precipitation Radar (PR). First, 3-dimensional structure of the mean annual cycle of monthly precipitation is defined on a 5^olatitude/longitude grid over the tropics (37^oS-37^oN) using the TRMM PR data for a 7-year period from 1998 to 2004. Over the Atlantic sector, a band of heavy rainfall associated with the ITCZ is observed over the tropics from 10^oS to 15^oN with a maximum of surface rainfall located at 5^oN. The contour of mean rain rate stronger than 0.5mm/day reached a height of above 6 Km. Precipitation over the south Atlantic is associated with the Southwest Atlantic Convergence Zone (SACZ) and extends vertically to 5-6 km above the surface. The rainband associated with the storm tracks, meanwhile, appears over the north Atlantic, but the top of the precipitating clouds are less high (~4 Km) compared to those of the ITCZ and SACZ.

Interannual variability of the cloudiness and precipitation associated with the ENSO, NAO, AO and PNA is then investigated using the ISCCP cloud amount data and the CMAP / GPCP data sets for a 25-year period from 1979 to 2003. While enhanced cloud/precipitation is observed over the extra-tropics during the warm ENSO events, cloud/precipitation over the tropical Atlantic is depressed with a delay of several months to the NINO3.4 index. Anomalous precipitation patterns associated with the NAO, meanwhile, are characterized by enhanced cloud/precipitation over the northeastern Canada, Greenland, extending across the Atlantic to Northern Europe during the high index phase, and over the Middle East, the Mediterranean, and into the Atlantic Ocean between ~25^oN-45^oN during the low index periods. This seesaw pattern of zonally oriented anomaly with alternating signs extends further south but with decreased correlation with the NAO index.

The seasonal and interannual variations of cloudiness and precipitation observed by satellites are then compared to those in the cloud and precipitation fields generated by the NCEP/NCAR reanalysis and several climate models including the NCEP / GFS model. Results of these comparisons will be reported at the conference.

* Corresponding Author: Dr. Phillip A. Arkin, ESSIC, University of Maryland, 2207 Computer and Space Science Building, University of Maryland, College Park, MD 207422465. E-mail: parkin@essic.umd.edu

Predicting the NAO: Role of the Stratosphere

Mark P. Baldwin*

Northwest Research Associates Bellevue, WA USA

Collaborators: Timothy J. Dunkerton (NWRA), David B. Stephenson (University of Reading), David W.J. Thompson (Colorado State University), Andrew Charlton (Columbia University), Alan O’Neill (University of Reading)

I will focus mainly on the predictability of the NAO (or almost equivalently, the Arctic Oscillation) up to ~two months in advance. In this study we used observational (NCEP) data for the past 44 years to demonstrate that the state of the stratosphere is a better predictor of the (N)AO than the troposphere. The reason that the (N)AO is predictable in this time frame involves a connection between the (N)AO and the circulation of the lowermost stratosphere. The stratospheric circulation changes very slowly, but the changes in the stratosphere can be large. Long-lasting changes to the winds just above the tropopause appear to affect both planetary-scale and synoptic waves.

In this study we used an empirical statistical model to demonstrate skill in 10–40-day forecasts of the (N)AO. Our methodology works during the extended winter season in the Northern Hemisphere. Forecast skill derives from both wintertime persistence of the surface (N)AO pattern and from long-lived anomalies in the lowermost stratosphere. We find evidence that enhanced persistence and predictability of the (N)AO depend on the long timescale of wind patterns in the lowermost stratosphere.

I will also discuss the implications for stratosphere-troposphere coupling on decadal and longer time scales. We cannot say with confidence how the stratosphere will change in the coming decades, but we can expect those changes to be reflected in changes at Earth’s surface. If the stratospheric polar vortex becomes colder and stronger, we expect a positive trend in the wintertime (N)AO.

Our main conclusions are:

The circulation of the lower stratosphere acts as a harbinger of long-lived (~two months) shifts in the (N)AO.
Most numerical weather prediction models do not adequately represent the stratosphere, so they are unable to include this effect. Models with a good representation of the stratosphere would include this effect.
The stratosphere represents a source of forecasting that is not currently being exploited.
The lowermost stratosphere appears to be most important for predicting the (N)AO.
The stratospheric effect only occurs during the extended winter season, November to April.
Stratosphere-troposphere coupling is important on multi-decadal time scales.

* Northwest Research Associates, 14508 NE 20^th Street, Bellevue, WA 98007. mark@nwra.com. 425-644-9660 x323.

Relationship of the tropical Atlantic to West African Rainfall in the NCEP coupled model

**James A. Carton^{* 1}, Ching-Yee Chang ¹, Semyon Grodsky ¹, Sumant Nigam ¹, and Jiande Wang ²**

¹Department of Meteorology, University of Maryland, ²National Centers for Environmental Prediction/NOAA, Suitland, MD

We present results from an examination of a four member set of 33 year-long integrations using the NCEP atmosphere/ocean coupled general circulation model. Our diagnostic study focuses on the seasonal and interannual fluctations in winds, heat flux, SST, and ocean heat storage and their relationship to rainfall over west Africa. We begin by presenting the seasonal climatology. We then remove the seasonal cycle from all variables and present an examination of subseasonal variability. The most prominent feature of the subseasonal SST variability in the tropics is associated with year-to-year changes in the cold tongue. Warm years are associated with enhanced rainfall in the coastal Gulf of Guinea during boreal summer, a feature that appears in the observational record as well. The second-most prominent feature is associated with meridional gradients. Thus, the model should be a useful tool for understanding the coupled dynamics of west African rainfall and its interaction with the Atlantic cold tongue.

*301-405-5365; carton@atmos.umd.edu Department of Meteorology, University of Maryland, College Park, MD 20742)

Predictability of monthly precipitation in the Mediterranean Basin

C. Norrant and A. Douguédroit*

The spatial distribution of monthly precipitation during the rainy season (September 1950 - August 2000) at 62 stations of the Mediterranean Basin and the Atlantic coast at the same latitude has been regionalized by RPCAs. Six regions are generally obtained, each corresponding with an EOF: Atlantic region, Mediterranean Spain, the Gulf of Genoa, Greece, the Middle East and the Maghreb. The regions presenting significant trends of their scores (according to the Mann-Kendall test at the 0,05 level) have been retained. They consist in one or txo regions each month; all have decreasing significant trends during the half-century studied. Other RPCAs have been done on monthly 500hPa levels north 20°N during the same period. Scores of the monthly precipitation of the regions previously retained and of monthly 500hPa patterns have been regressed to select the patterns significantly influencing the regional precipitation. Then the potential predictability of the low frequency patterns has been checked by using the 500hPa level with a leading time of one and two months (Tab.1).

Month	Region	1	2	3	4	5	6
Oct	Med. Spain	23	4	18	3	18	4
Dec	Gulf of Genoa	38	4	24	2	15	2
Jan	Greece	32	5	25	4	10	2
March	Atl. Region	36	4	16	1	33	3
April	Gulf of Genoa	26	4	20	3	46	4

Tab.1: Results. 1: explained variance; 2: number of patterns; 3 and 4: explained variance and number of patterns with 1 month lead; 5 and 6: explained variance and number of patterns with 2 month lead.

Explained variances of the monthly regional precipitation are not very high (less than 40%) and lesser than when a North Atlantic-European window is used in the RPCAs on the 500hPa level (not shown here). Regressions between monthly regional precipitation and EOF patterns at the 500hPa with a lead time of two months separate two types of seasons. In autumn and winter, correlation are significant for the Mediterranean Spain in October, the Gulf of Genoa in December and Greece in January but decrease to 10-20% with a two month leading time of the low frequency circulation patterns. During Spring, in March and April, a lead time of two months for the circulation patterns reveals correlation equal or even higher than without lead time. March precipitation in the Atlantic region are significantly linked (up to 33% of explained variance) with three circulation patterns of January: PNA, NW Atl. and South Siberia. The links with PNA remind the known correlation between Spain precipitation in March and the two month lead ENSO index of January. April precipitation in the Gulf of Genoa are rather highly correlated (46% of explained variance) with four circulation patterns of February: NE Pac., Sahara, Egypt-Arabia and W Pac. In the Mediterranean Basin and the Atlantic coast at the same latitude the most rainy months (in winter or autumn according to the region) present a low potential predictability with two month lead; it is very likely due to the importance of the regional factors for precipitation in the area. But spring months have a higher predictability which can present some interest when it is mentioned that they influence the decrease of the soil humidity in summer.

* Institute of Geography, 29 av. R. Schuman, 13621 Aix-en-Provence FRANCE. Tel: 33442953871. Fax: 33442640158. Email: annickd@up.univ-aix.fr

THE PHYSICAL BASIS FOR PREDICTING ATLANTIC SECTOR SEASONAL-TO-INTERANNUAL CLIMATE VARIAIBILITY

Yochanan Kushnir*, Lamont-Doherty Earth Observatory, Palisades, NY, USA Walter Robinson, University of Illinois at Urbana-Champaign, IL, USA Ping Chang, Texas A&M University, College Station, TX, USA Andrew Robertson, International Research Institute for Climate Prediction, Palisades, NY, USA

ABSTRACT

This paper reviews the observational and theoretical basis for predicting seasonal-to-interannual (S/I) climate variability in the Atlantic Sector. The emphasis is on the large-scale picture rather than on regional details. The paper is divided into two main parts: a discussion of the predictability of the North Atlantic Oscillation (NAO) – the dominant pattern of variability in the North Atlantic and a review of the tropical Atlantic prediction problem. The remote effects of El Niño are also mentioned as an important factor in Atlantic climate variability. Only a brief discussion is provided on the subject of South Atlantic climate predictability.

It is argued that, because of its chaotic dynamical nature, the NAO and its related rainfall and temperature variability, while highly significant over Europe and North America, are largely unpredictable. This also affects the predictive skill over the tropical Atlantic. That said, there appears to be an insufficiently understood, and possibly predictable marginal signal in the NAO behavior that may be useful to certain end users. It is manifested in the deviation of the NAO temporal behavior from first-order autoregressive behavior.

Tropical Atlantic variability results from the sensitivity of the marine ITCZ to remote forcing from the equatorial Pacific and from the local interaction with the underlying ocean. Both mechanisms are potentially predictable – that is, given the underlying SST and the strength of El Niño, one could determine with a high degree of skill the anomalies in ITCZ position and intensity. Due to the strong coupling between ocean and atmosphere, however, and perhaps also the lack of sufficient understanding of local air-sea interaction, it is not easy to achieve the level of skill indicated by hindcast experiments. Overcoming this obstacle is a major challenge to improved S/I prediction in the Atlantic Sector.

* Corresponding author address: Yochanan Kushnir, Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964

Tel: 845-365-8669, Fax: 845-365-8736.

E-Mail: Kushnir@ldeo.columbia.edu

Propagation of salinity anomalies from the North Atlantic

subtropics to the high latitudes

Audine Laurian¹, Alban Lazar, Gilles Reverdin Laboratoire d'Océanographie Dynamique et de Climatologie, CNRS-IRD-UPMC, Paris, France

Recent observations in the North Atlantic Ocean [Curry et al., 2003] show an increase of salinity and temperature in the upper layers of tropical and subtropical regions between 1950s and 1990s. In the North Atlantic Ocean, subduction occurs mainly in the salinity maximum water (SMW). Water masses subducting in this region ventilate and salinize the upper layers of the North Atlantic Ocean up to the high latitudes, potentially influencing the thermohaline circulation (THC) variability [Latif et al., 2000]. Subduction regions need to be considered to understand what the observations of Curry et al. can tell us about the THC variability.

In this work, we study the evolution of salinity anomalies that enter the thermocline in the SMW region. We use an OGCM forced by bulk formulae based on NCEP reanalysis fields from 1948 to 2000. Since water masses conserve their density and Bernoulli function (far away from turbulent mixing regions), we describe most of their propagation using isopycnal projection. Formation of the sea surface salinity anomalies is studied. Vertical sections and Hoevmoeller diagrams along the mean pathways are used to describe the propagation of the anomalies. They also permit to evaluate the dissipation rate of the signals. Finally, correlation maps allow us to quantify the life time of the anomalies and to describe their mean pathway.

Relationship between surface and subsurface anomalies is discussed [Bindoff et al., 1993]. About seven years are necessary for the anomalies to subduct, enter the Gulf of Mexico, flow through the Gulf Stream and outcrop near the Cape Hatteras. The strongest signals (about 0.2 psu) travel along a preferred isopycnic surface (sigma 26). Their velocity is that of advective mean currents, as was shown in previous papers ([Lazar et al., 2002], [Schneider et al., 1999]). Strong mixing in the Gulf of Mexico and in the Gulf Stream reduces significatively the amplitude of the anomalies.

¹ A. Laurian, Laboratoire d'Océanographie Dynamique et de Climatologie, Université Pierre et Marie Curie, Case 100, 4 place Jussieu, 75256 Paris, Cedex 05, France. (Audine.Laurian@lodyc.jussieu.fr)

Moisture fluxes over the Intra-Americas Sea

Alberto M. Mestas-Nuñez*¹, Chidong Zhang², David B. Enfield³

¹Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida, USA

²Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA

³NOAA Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida, USA

Abstract (poster)

The region of the Atlantic basin comprised of the Gulf of Mexico and the Caribbean Sea is known as the Intra-America Sea. In the Intra-America Sea is found the second largest warm pool in the world that develops in the late spring and covers the entire Intra-America Sea from August through October. The interannual changes of the warm pool size are quite significant and thus may affect the amount of moisture into the US. The ultimate goal of our research is to explore the connection among the warm pool of the Intra-America Sea, its moisture budget, moisture transport from the Intra-America Sea into North America, and warm-season precipitation over North America. Understanding these issues is essential for improving climate predictability over the land regions surrounding the Intra-America Sea.

We use atmospheric observations and model analyses in and around the Intra-America Sea to evaluate uncertainties in calculating moisture flux divergences in that region. The main dataset used is an archive of regional analysis fields from the Eta regional analyses for April 2002 - March 2004. The Eta analyses are 4-times daily and have a resolution of about 32 km. The water vapor fluxes from the Eta analyses compare well with sounding estimates. We check the internal consistency of our Eta flux divergence estimates by applying the Gauss theorem and comparing estimates from line and area integrals. We estimate the uncertainties in the moisture flux divergence calculations due to the coarser space and time sampling resolution of the global NCEP/NCAR reanalysis. The estimates of the moisture flux divergence do not change much when the Eta analyses are decimated to the coarser global reanalysis grid. The flux divergences show large differences from month to month and are generally divergent (evaporation exceeds precipitation) during our two-year period. The seasonal cycle and the natural range of interannual variability of the moisture flux divergence are estimated using the global NCEP/NCAR reanalysis for 1960-2003. The seasonal cycle of moisture flux divergence estimates are compared with direct estimates of evaporation minus precipitation from climatologies.

*Corresponding author: Alberto M. Mestas-Nuñez, NOAA/AOML, 4301 Rickenbacker Causeway, Miami, FL 33149, USA, email: Alberto.Mestas@noaa.gov

Using Tropical Atlantic Circulation to predict Sahel rainfall.

*Ousmane NDIAYE, Dr Neil WARD

*Department of Earth and Environment Science, Columbia University International Research Institute for Climate Prediction, Earth Institute, Columbia University,

Understanding and predicting the year to year and intra-seasonal rainfall variability over Sahel (transition region located between equatorial forest regions and Sahara desert in West Africa) is still a major challenge in climate forecasting. Such prediction would have a tremendous impact on the economy (agriculture, health) of these countries related to rainfall. Many studies, both statistical and dynamical (eg Hastenrath and Lamb 1977, Lamb 1982, Nicholson and Palao 1993, Folland et al. 1991 and Ward 1998), relate Sahelian rainfall variability to the Sea Surface Temperature (SST) at a near-global scale as well as over the Atlantic basin. However, a robust and comprehensive mechanism relating SST variations to Sahelian rainfall has yet to be fully established.

General Circulation Model (GCM) simulations forced with prescribed SST variability provide a powerful tool to study these relationships and to assess predictability. In this study we investigate the linkage between Sahelian rainfall and SST variations in the ECHAM4.5 GCM, using both observed and predicted SSTs.

An EOF analysis is applied to lower-level GCM zonal wind over the tropical Atlantic Basin. The interannual time series of the leading EOF is then correlated with an index of observed seasona l-mean Sahelian rainfall during July-September (JAS). When the GCM is forced with observed SST during boreal summer (JAS) the relationship is very strong (r=0.6) over the period 1968-2002. When the GCM is forced with the June SST anomaly added to the JAS SST climatology the skill is very similar (r=0.57), but when we persist SST anomaly from May the skill drops dramatically (r=0.23). Details of this study will be discussed.

*Ousmane NDIAYE, 133, Monell Building, LDEO, Palisades, New York 10964.

Overlooked November-December Cooling ofthe Equatorial Atlantic and its Effects on Interannual Variability and Predictability

Yuko Okumura^*and Shang-Ping Xie ^{^†}

Department of Meteorology, University of Hawaii, Honolulu, Hawaii

Seasonal cycle of sea surface temperature (SST) in the equatorial Atlantic is characterized by a rapid cooling from April to July and a slow warming that follows. With the onset of summer monsoon over West Africa, enhanced cross-equatorial southeasterly winds cool the equatorial ocean through upwelling, horizontal temperature advection, and evaporation. In response to the enhanced easterly winds, thermocline shoals in the east, significantly contributing to the eastern cooling.

Whereas the annual harmonic dominates the seasonal cycle of SST over much of the equatorial Atlantic, the easterly wind and thermocline depth display significant semiannual signals in the east. Besides the June-July acceleration, the easterlies on the equator intensify also in October-November, causing the thermocline to shoal. Using high-resolution satellite data, we show, for the first time, that the central Atlantic SST decreases by ~0.5ºC from November to early December in response to the accelerated easterlies and shoaling thermocline. This secondary cooling has not been captured well in some widely used climatological data because of their low resolution in space and time. The six-year PIRATA buoy observations support the existence of this secondary seasonal cooling.

The November-December cooling displays considerable year-to-year variability in its strength and extent. As a result, SST variance in the eastern equatorial Atlantic shows a secondary maximum in November-December, suggesting that the shoaling thermocline increases the ocean-atmosphere coupling and hence their variability. Indeed, the associated ocean-atmospheric anomalies bear a strong resemblance to the equatorial mode of the tropical Atlantic variability, the so-called Atlantic Nino which is known to be pronounced in boreal summer (June-August). The November-December Nino is not a simple extension of the better-known June-July-August Atlantic Nino, and the two are not significantly correlated.

Our study of this overlooked cousin of Atlantic Nino has a number of important implications for the predictability of climates of the tropical Atlantic Ocean and surrounding continents. For example, equatorial SST anomalies in November-December correlate well with rainfall anomalies along the Congo-Angola coast in their early rainy season. There is evidence that this November-December Nino affects the tropical Atlantic variability in the subsequent season.

^†Also at International Pacific Research Center, University of Hawaii, Honolulu, Hawaii.

The long-term variability of African dust transport across the Atlantic and the link to climate

Joseph M. Prospero¹, Isabelle Chiapello², and Cyril Moulin^³1Rosenstiel School of Marine and Atmospheric Science, University of Miami, FL, USA. ²Laboratoire d’Optique Atmosphérique, CNRS-USTL, Villeneuve d’Ascq, France. ³Laboratoire des Sciences du Climat et de l’Environnement, CEA-CNRS, Gif-sur-Yvette, France.

Aerosol measurements made on Barbados, West Indies (13.17°N, 59.43°W), since 1965 show that huge quantities of dust are carried each year across the Atlantic from sources in North Africa. We know that dust generation and transport is a strong function of weather and climate. However dust also plays a role in atmospheric radiation processes and in cloud microphysics and, thus, could conceivably play a role in climate feedback processes over the Atlantic. In this report we examine the Barbados record and its link to African climate. Then, using satellite data, we assess the degree to which Barbados dust trends are representative of dust trends over the Atlantic and, finally, we look for links to other climate indicators. A companion presentation by Chidong Zhang will consider the possible impacts of dust on the climate of the tropical- equatorial Atlantic.

Barbados dust concentrations vary greatly on time scales ranging from days to decades. On an annual basis dust variability is closely linked to rainfall in North Africa, most closely to that in the Soudano-Sahel region [Prospero and Lamb, 2003]. Barbados dust concentrations were highly correlated with rainfall deficits in the previous year, less strongly correlated with deficits in the current year, and uncorrelated with those in the coming year. By comparing the Barbados record with TOMS and METEOSAT dust optical thickness (DOT) records we find that Barbados dust trends mirror those taking place over a large area of West Africa and the tropical-equatorial Atlantic. Despite differences in spatial coverage, the Barbados and the satellite dust records are in good agreement at both monthly and annual time scales over the 22 years of concurrent operation. We then use satellite DOT to assess dust trends with respect the North Atlantic Oscillation (NAO) and Sahel rainfall. The analysis shows a large regional impact of Sahel drought on dust emissions and transport both in winter and in summer, whereas the influence of the NAO dominates the winter export and is more geographically limited to the eastern Atlantic north of 15°N, and possibly some localized source-regions (southern Mauritania and the Bodele depression). The combination of the 35 years of Barbados measurements of African dust with 22 years of satellite observations highlights the great impact of African drought over this region, especially in the early-mid 1980s when drought was most severe, and into the 1990s.

The response of the Arctic and Atlantic oceans to the Northern Annular Mode (poster)

Gerd Krahmann - Lamont-Doherty Earth Observatory, Palisades/USA

*Martin Visbeck - Leibniz-Institut fŸr Meereswissenschaften, Kiel/Germany

The sea ice response of the Arctic Ocean to the Northern Annular Mode (NAM) has been studied both in observations and in a numerical ocean general circulation model. The analysis of the observed sea ice concentrations shows the well known seesaw in response between the Labrador Sea and the Greenland and Barents Seas. After band pass filtering the data in order to distinguish decadal and shorter or longer periods, it reveals a variation in response in the Greenland Sea between interannual and multidecadal NAM periodicities. In the numerical model experiments idealized NAM-like wind and windstress forcing anomalies of varying periodicities have been applied to the model. This setup allows us to investigate variations in the response to the NAM in a controlled environment. The analysis of the numerical experiments reveals a similar change in response in the Greenland Sea as we found in the observational data. The changes in the response appear to be caused by a relatively slow response of the heat transport from the North Atlantic into the Nordic Seas, which on interannual timescales has not enough strength to modify the quicker windstress driven response of the sea ice.

Corresponding author: Prof. Martin Visbeck Leibniz-Institut fŸr Meereswissenschaften DŸsternbrooker Weg 20 24105 Kiel Germany

Variability of vertical shear within the tropical North Atlantic

Anantha Aiyyer¹and Chris Thorncroft Dept. of Earth and Atmospheric Sc., University at Albany, SUNY

Past studies have shown that vertical wind shear is a key environmental factor that inﬂuences Atlantic tropical cyclone activity. Thus, there is the need to better understand the factors that contribute to the variability of the vertical shear in order to the improve the prediction of tropical cyclone activity in this region.

In the present study, we consider the inter-annual to decadal variability of vertical shear over the tropical North Atlantic. We examine an ensemble of 6 simulations performed using the HadAM3 GCM that is forced by reconstructed historical SST for the period 1871-1999. The spatial and temporal variability of vertical shear simulated by the GCM is evaluated against the ECMWF and NCEP/NCAR reanalysis data. The relationship between Atlantic shear and forcings such as SST and Sahelian precipitation are investigated using the GCM simulations as well as the reanalysis data.

The results to be presented at the conference will emphasize the role of local processes and teleconnections that inﬂuence the vertical wind shear over the tropical North Atlantic.

1. Corresponding author: Anantha Aiyyer, Es 321 Dept. of Earth and Atmos. Sc., University at Albany, State University of New York, Albany, NY 12222. aiyyer@atmos.albany.edu

CLIMATIC IMPACTS OF INTERANNUAL SURFACE CURRENT VARIATIONS IN THE EQUATORIAL ATLANTIC OCEAN

This presentation is focused on major climatic impacts of the surface current variations in the equatorial Atlantic Ocean, through analyses using space-based observations. Considerable interannual climate disturbances occur in the tropics and have a large societal impact on the surrounding lands and subtropical regions, namely El Niño-Southern Oscillation in the Pacific, the Indian Ocean Dipole, and the Tropical Atlantic Variability (TAV). Past studies have underlined the important role played by the Pacific equatorial surface ocean currents during El Niño and La Niña. But so far, there has been no observational analysis of the tropical Atlantic interannual surface currents and their impact during TAV. We here explore particularly the role of horizontal currents in the year-to-year sea surface temperature changes by quantitatively analyzing the terms of a surface layer partial heat budget including the heat storage rate, horizontal heat advection and net air-sea heat flux. Another presentation by Robert Helber will tackle the connection between equatorial divergence fluctuations and SST changes. The vantage point of satellite remote sensing with high-resolution and complete data sampling is particularly appropriate in these studies. We use surface currents derived from satellite altimetry, scatterometer and sea surface temperature data (SST) from the Ocean Surface Current Analysis Realtime (OSCAR, http://www.oscar.noaa.gov/, for the Pacific), together with space-based SST observations and air-sea reanalysis data products. Results indicate that horizontal advection explains some of the SST variations in the central equatorial area. We will discuss the major climate events of the equatorial Atlantic Ocean over the past twelve years, and especially over the Summer and early Autumn periods of 2004, where intense large-scale currents occurred along the equator (Fig.1).

Decadal variability of shallow cells and equatorial SST in a numerical model of the Atlantic

Jurgen Kroger¹, Antonio J. Busalacchi¹*, Joaquim Ballabrera-Poy¹, and Paola Malanotte-Rizzoli²

¹Earth System Science Interdisciplinary Center (ESSIC), University of Maryland

²Massachussets Institute of Technology (MIT)

The relative role of extra-equatorial mechanisms modulating decadal sea surface temperature anomalies (SSTA) in the equatorial Atlantic is investigated using a suite of sensitivity experiments based on an ocean general circulation model (OGCM). The model is forced by observed wind stress and/or computed heat flux from an associated advective atmospheric mixed layer model. In addition, the surface forcing is optionally applied on the equator or in off-equatorial regions. Although the long term response of equatorial SST is dominated by local forcing, a weak but significant part of it is caused by remotely induced variability. Subtropical Cells (STCs) provide the oceanic bridging of the climate signals. The dynamical forcing leads to a spin-up and -down of the shallow cells, which, in the case of local forcing included, coincides with cold and warm SSTA. The local heat flux forcing reveals an overall damping tendency on the dynamical SST response. When excluding the local forcing, the isolation of the effect of the northern remote forcing from the one in the south appears to be essential in understanding the respective mechanisms at work. In the northern hemisphere the spin-up and -down of the STC is highly correlated with the (lagging) SSTA, the effect of off-equatorial heat flux forcing on SSTA is negligible. In the southern hemisphere, on the other hand, both long term heat and momentum fluxes that were induced in the subtropics lead to a significant SST response on the equator.

Antonio J. Busalacchi, Earth System Science Interdisciplinary Center (ESSIC), 2207 CSS Bldg., University of Maryland, College Park, 20742, tonyb@essic.umd.edu

Subthermocline Tropical Cells and Equatorial Subsurface Countercurrents

Chunzai Wang

NOAA Atlantic Oceanographic and Meteorological Laboratory

Over most of the equatorial Atlantic and Pacific Oceans, easterly trade winds drive a westward surface flow that produces an eastward pressure gradient force in association with the eastward equatorial undercurrent. Meridionally, the surface Ekman flow is poleward while the eastward pressure gradient force induces equatorward geostrophic flow within the thermocline. Equatorial upwelling occurs near the equator and downwelling is located in convergence zones about 3°-4° from the equator. These meridional circulation cells on either side of the equator, recognized a long time ago, have recently been named the tropical cells (TCs).

In this paper, we present observational evidence for the existence of a new meridional circulation of subthermocline tropical cells (STTCs). The STTCs are below and weaker than the TCs, being characterized by an equatorward flow in the thermocline, an equatorial downwelling, a poleward flow in the subthermocline, and an upwelling about 3°-4° from the equator. Observations show that the STTCs correspond to the eastward subsurface countercurrents (SSCCs) that are observed in both the Atlantic and Pacific Oceans on either side of the equator and are associated with a poleward shoaling of subthermocline isotherms at the poleward flanks of the equatorial 13°C thermostad. We then provide a simple dynamical explanation for the SSCCs in relation to the poleward flow of the lower branch of the STTCs, based on the conservation of absolute vorticity. As a parcel within the subthermocline moves poleward, its gain of planetary vorticity is compensated by a loss of relative vorticity, resulting in the eastward SSCCs. By applying the conservation of potential vorticity, the paper also shows that the poleward shoaling of subthermocline isotherms at the poleward flanks of the thermostad can contribute to the eastward SSCCs.

Corresponding author address: Dr. Chunzai Wang, NOAA Atlantic Oceanographic and

Meteorological Laboratory, Physical Oceanography Division, 4301 Rickenbacker Causeway,

Miami, FL 33149. Tel: (305) 361-4325; email: Chunzai.Wang@noaa.gov.

Dynamical Elements of Predicting Boreal Spring Tropical Atlantic Sea-Surface Temperatures

Ping Chang Texas A&M University College Station, TX 77843

Abstract

The dynamical processes that contribute to the seasonal prediction of the tropical Atlanticsea-surface temperature (SST) anomalies from boreal winter into spring are exploredwith an atmospheric general circulation model coupled to a slab ocean. Taking theadvantage of the reduced-physics model that effectively isolates thermodynamicfeedbacks from dynamic feedbacks, we examine the joint effect of local thermodynamicfeedback and the remote influence of El Nino-Southern Oscillation (ENSO) onthe prediction of SST anomalies by conducting large ensembles of prediction runs. Theseprediction experiments yield the following findings: 1) in the northwestern part of thetropical Atlantic, the positive feedback between the surface heat flux and SST can play animportant role in enhancing the predictability of the SST; 2) the remote influence fromPacific ENSO can enhance the SST predictability through a constructive interferencewith the local thermodynamic feedback, but can also cause the SST prediction moredifficult when the interference is destructive; 3) ocean dynamics plays a fundamental rolefor prediction of SST anomalies in the equatorial and south tropical Atlantic. To shedfurther light on the importance of the ocean dynamics, a statistical procedure ofparameterizing the important ocean dynamics is developed within a linear dynamicalframework. Prediction experiments with the parameterized ocean dynamics included inthe simple coupled model result in an improved forecast skill in predicting the cross-equatorial SST gradient, which subsequently lead to a high skill of the model inpredicting seasonal rainfall anomaly associated with variations in the IntertropicalConvergence Zone during boreal spring. A diagnostic study suggests that the verticaladvection of heat due to anomalous Ekman pumping/suction is a dominant contributingfactor for causing equatorial SST anomalies, thereby a major element of predictabledynamics in this region.

Does the tropical Pacific possess climate variability analogous to the

tropical Atlantic 'meridional mode'?

* John C. H. Chiang, University of California, Berkeley, CADaniel J. Vimont, University of Wisconsin, Madison, WI

We identify from observational analysis a Pacific mode of variability in the Intertropical Convergence Zone (ITCZ)/cold tongue region that possesses characteristics and interpretation similar to the dominant 'meridional' mode of interannual-decadal variability in the tropical Atlantic. The Pacific and Atlantic meridional modes are characterized by an anomalous sea surface temperature (SST) gradient across the mean latitude of the ITCZ coupled to an anomalous displacement of the ITCZ towards the warmer hemisphere. Both are forced by trade wind variations in their respective northern subtropical oceans. The Pacific meridional mode exists independently of ENSO, although ENSO nonlinearity projects strongly on it during the peak anomaly season of boreal spring. We suggest that the Pacific and Atlantic modes are analogous, governed by physics intrinsic to the ITCZ/cold tongue complex.

* Corresponding Author:John Chiang547 McCone Hall University of California Berkeley, CA 94720-4740 510 642 3900 (tel)

510 642 3370 (fax) jchiang@atmos.berkeley.edu

Contributions of African Easterly Waves to Tropical

Atlantic Variability

Kerry H. Cook* and Christina M. Patricola, Cornell University

A climatology of easterly wave activity in the tropical North Atlantic is assembled using the NCEP reanalysis and used to evaluate the contribution of the waves to variability over the tropical Atlantic on intraseasonal to interannual time scales. Waves with 3-5 day periods that travel eastward near 8°N with phase speeds of 12 m s^-1 and 4000 km wavelengths tend to die out in the central Atlantic. Near 17-20ºN, waves with periods around 4.5 days, wavelengths about 3200 km, and periods of approximately 9 m s^-1 are strong at 850 hPa over West Africa, and propagate to 700 hPa over the Atlantic to the west. They propagate across the tropical Atlantic, staying within a few degrees of 20ºN, to influence the Caribbean and North America. Both of these wave types are most active in July, August and September. Two longer period waves are active later in the season. 6-day waves dominate in July-September, while 8-day waves are generated in September-November. Both of these wave types amplify as they move off the African coast into the Atlantic Ocean.

On synoptic time scales, easterly wave activity contributes more than half of the meridional wind perturbation in the 3-9 day range. Interannual variability is also significant, and various modes of interannual variability are identified. Most of the interannual variability in the 3-5 day range is associated with a few strong wave years. In contrast, 5-9 day waves tend to have maximum power in the same latitude range every year (20-27ºN), but the degree to which they amplify over the Atlantic varies greatly from one year to the next.

* Department of Earth and Atmospheric Sciences, 3114 Snee Hall, Cornell University, Ithaca NY 14853. 607-255-9716. khc6@cornell.edu

South America and Central-South Africa Precipitation Variability Associated to the South Atlantic Ocean

Anita Rodrigues de Moraes Drumond –- University of São Paulo – Brazil ^*Tércio Ambrizzi –- University of São Paulo - Brazil

Rotated Empirical Orthogonal Function (REOF) analysis was applied over normalized Reynolds Sea Surface Temperature (SST) seasonal anomalies in order to find the main modes of variability over the Southern Atlantic ocean during the Austral Summer (December to February) associated to the precipitation variability over the adjacent continents. The first twelve modes obtained via EOF were rotated using Varimax. Each rotated mode was examined via composites of extremes events selected by applying a threshold of one standard deviation over its normalized Principal Component time series (CP). The first rotated mode (REOF1), explaining 23.1% of the total variance, consists of negative loading over the Tropical South Atlantic. It is related to the precipitation over Northeastern Brazil through the meridional displacement of the Inter Tropical Convergence Zone (ITCZ). Most of selected cold Atlantic events occurred during neutral El Niño-Southern Oscillation (ENSO) and La Niña (LN) events and they were associated to drought over Northeastern Brazil and to wet conditions over Southern Africa. However, most of the selected warm Atlantic events occurred in El Niño (EN) years and they were associated to wet conditions over Northeastern Brazil and drought in Southern Africa. These results confirm that the Tropical Atlantic acts as a modulator of the precipitation in Northeastern Brazil even in ENSO events. REOF2 (17.8% of total variance) consists of positive loading over the Subtropical South Atlantic with the highest values near to 0ºW; 25ºS. Most of the selected warm SST events occurred in EN years and they were associated to drought over the South Atlantic Convergence Zone (SACZ) region and over southern Africa. An inverse pattern occurs during the cold events when there is the predominance of LN years and wet conditions are observed over Southern Africa. REOF3 (13.4% of total variance) consists of positive loading over the southwestern Atlantic with the highest values located close to the southern Brazilian coast. It is interesting that most of the extremes events observed in the both phases occurred in LN years. Warm waters over southwestern Atlantic were associated to drought over the SACZ region and wet conditions over southern Brazil and southeastern Africa. On the other hand, the opposite pattern over South America occurred in cold Atlantic events. In these episodes, the convection over southern Africa was not well configured. Comparing the LN events observed in both phases, the episodes associated to the warm Atlantic events presented stronger anomalous convection over Indonesia and lower negative SST anomalies over the Central Equatorial Pacific. REOF 5 (7.8% of total variance) consists of positive loading over the Subtropical South Atlantic with the highest values near to 5ºW; 35ºS. Most of the selected warm events occurred during LN years and they were associated to wet conditions over the northern South America and drought over southeastern Brazil. In addition, wet conditions were observed over southern Africa. On the other hand, most of the selected cold events occurred in EN years and they were associated with drought over northern South America and wet conditions over southern Brazil. In these events, it was also noticed drought in the southern Africa. In summary, the South Atlantic seems to be a modulator of the precipitation variability in both continents, being also related to the ENSO pattern. Some dynamical aspects of the observational results will be studied through numerical experiments with a GCM.

* Department of Atmospheric Sciences IAG/USP, Rua do Matão, 1226, São Paulo, SP, Brazil, 05508-090 Email: anitadru@model.iag.usp.br ; Phone: 55-11-30914713; Fax: 55-11-30914714

Intraseasonal oscillations in the tropical Atlantic

Gregory R. Foltz^{^�}and Michael J. McPhaden NOAA/PMEL

Evidence is presented for 30–70 day oscillations in the trade winds of the northern and southern tropical Atlantic. The pattern of intraseasonal oscillations in surface pressure and wind speed in the Northern Hemisphere resembles that of the North Atlantic Oscillation (NAO). Like the NAO, these oscillations have a strong seasonal cycle, with a maximum amplitude in boreal winter/spring. Winds in the southern tropical Atlantic are related to the Southern Hemisphere equivalent of the NAO, exhibiting a weaker seasonality with peak amplitudes in austral winter. The Madden-Julian Oscillation (MJO), which originates over the Indian Ocean, also exerts an inﬂuence on tropical Atlantic surface winds, particularly between about 10N and 10S. Where intraseasonal wind speed oscillations in the Atlantic are strong, they force changes in sea surface temperature (SST) through latent heat loss from the ocean, potentially aﬀecting lower frequency climate variations.

^{^�}NOAA/PMEL 7600 Sand Point Way NE Seattle, WA 98115

p: 206-526-6256

f: 206-526-6744 gregory.foltz@noaa.gov

Local Forcing of the Surface Heat Budget of the Equatorial East Pacific

C. W. Fairall*NOAA Environmental Technology Laboratory Boulder, CO

Meghan F. Cronin NOAA Pacific Marine Environmental Laboratory Seattle, WA

ETL and PMEL have been cooperating since 1999 on a joint program of climate-quality air-sea flux observations in the Equatorial East Pacific which represents an interesting contrast/analog to the Equatorial Atlantic. Each fall and spring, the ETL seagoing flux package is installed on the ship servicing the TAO buoys at 95 W and 110 W. The standard meteorological sensors on the buoys on the 95 W line have been enhanced with solar and radiative flux instruments, allowing measurement of all terms of the surface energy budget. The ship-based observations provide both quality assurance and broader interpretation of the buoy measurements. Using the comprehensive surface, boundary layer, and cloud information from the twice-yearly ETL observations, parameterizations of clear sky radiative fluxes have been developed that allow assessment of cloud surface forcing of radiative fluxes throughout the annual cycle. In this paper we will present results on the annual cycle of each term of the surface energy budget between 8 S and 12 N at 95 W. The net heat flux and cloud contribution will also be shown. Intercomparisons with buoy and satellite observations indicate some problems with operational flux products (NCEP and ECMWF). Comparison of the net surface heat budget and the time derivative of the mixed-layer heat content indicate the balance of local forcing versus ocean processes at this location.

Corresponding author: C. W. Fairall, NOAA ETL6, 325 Broadway, Bouder, CO 80305 chris.fairall@noaa.gov

Inﬂuence of Surface Processes over Africa on the Atlantic Marine ITCZ

Samson M. Hagos and Kerry H. Cook

Department of Earth and Atmospheric Sciences, Cornell University, Ithaca NY14853

Previous study shows that the climatological precipitation over equatorial Atlantic and northeastern South America is inﬂuenced by the presence of the African continent. Here the impact of various aspects of African surface features on the Atlantic Marine ITCZ (AMI) is explored. The relative roles of equatorially symmetric and asymmetric circulation perturbations associated with such surface features are also investigated. Crossequatorial ﬂow over the Atlantic Ocean introduced by northsouth asymmetry in surface conditions over Africa shifts the AMI in the direction of the ﬂow. African topography, for example, introduces an anomalous high over the southern Atlantic Ocean and a low to the north. This results in a northward migration of the AMI and dry conditions over the western equatorial Atlantic and Nordeste region.

Similarly, northerly ﬂow induced by equatorially asymmetric perturbation in soil moisture over northern tropical Africa shifts the AMI southward, increasing the climatological precipitation over western equatorial Atlantic and northeastern South America. Flow associated with an equatorially symmetric perturbation in soil moisture, however, has a very weak cross equatorial component and very weak inﬂuence on the AMI.

Hence the primary inﬂuence of those African land surface features on the AMI is modulation of its meridional position through equatorially asymmetric circulations.

Corresponding Author : Samson Hagos, Cornell University, Ithaca NY14853 (sh282@cornell.edu)

Satellite Derived Surface Current Divergence in relation to Tropical Atlantic SST and Wind

Robert W. Helber* and Robert H. Weisberg, University of South Florida, Saint Petersburg, Florida Fabrice Bonjean and G. S. E. Lagerloef, Earth and Space Research, Seattle, Washington

The relationships between tropical Atlantic surface currents, Sea Surface Temperature (SST), and winds on monthly-to-interannual time-scales are described. Currents averaged over the upper 30 meters are estimated from satellite sea surface height, surface vector wind, and SST data with a quasi-linear, steady state model. The results show good agreement with early ship drift studies.

Surface current divergence (upwelling) patterns are consistent with the annual north/south movement of the InterTropical Convergence Zone and equatorial cold tongue evolution. Goestrophic and Ekman dynamical contributions are considered. Along the equator, the total surface divergence exhibits seasons of upwelling in the central/eastern equatorial Atlantic that peak in May and November within the cold tongue (see Figure 1). While surface currents have a key role in SST variability, surface heat fluxes and subsurface temperature structure are also important. This approach establishes a new method for monitoring tropical currents and related dynamics while providing an observation based product for testing numerical models.

*Corresponding Author: Robert W. Helber College of Marine Science University of South Florida Saint Petersburg, Florida 33701 phone: (727) 553-1627 fax: (727) 553-1189 e-mail: helber@marine.usf.edu

VARIABILITY OF WEST AFRICAN STORM TRACKS AND THEIR RELATIONSHIP WITH ATLANTIC TROPI CAL CYCLONE ACTIVITY

Susanna Hopsch¹, Chris Thorncroft¹,Kevin Hodges,²SUNY at Albany, Albany, NY 12222¹Environmental Systems Science Centre, Reading, UK²

This studyis concerned with improving our knowledge and understanding of the variability of West African storm tracks and their association with Atlantic tropical cyclone variability. Our analy sis uses the automatic tracking technique of Thorncroft and Hodges (2001) to identify coherent vorticity structures over West Africa and the tropical Atlantic. Their study indicated the existence of two dominant source regions, one south of about 15˚N in the rainy zone, with a track density maximum just offshore the West African coast; and a second located north of 15˚N on the fringes of the Sahara.

The present study extends this analysis by considering the relationship between the storm tracks over West Africa and the tropical Atlantic in more detail and includes an investigation of their variabilityon both intraseasonal-to-decadal timescales. In this study the 40-year ECMWF reanalysis (ERA40) dataset was analysed. Results show that the southern storm track provides more than 70% of the systems that reach the main development region where most Atlantic tropical cyclones develop, whereas the northern storm tracks play a much less important role in the tropical Atlantic. Evidence also indicates that the intensity of the storms leaving the West African coast can inﬂuence the likelihood of downstream intensiﬁcation and longevity.

The southern storm track is characterized bymarked intraseasonal-to-decadal variability negating the often-quoted belief that the variability of African weather systems leaving the West African coast is small. Of particular interest is the pronounced low-frequency variability, which is well correlated with both Sahelian rainfall and Atlantic tropical cy clone variability suggesting a possible important role of West African storm track variability on the tropical Atlantic.

1. Susanna B. Hopsch Dept.of Earth and Atmospheric Sciences, University at Albany, SUNY 1400 Washington Avenue, Albany, NY 12222 E-mail: hopsch@atmos.albany.edu

The Influences of the Ocean-Atmosphere Mean Climatology on the Tropical Atlantic Variability

∗

Bohua Huang and Paul S. Schopf

Climate Dynamics Program George Mason University Fairfax, Virginia Center for Ocean-Land-Atmosphere Studies Calverton, Maryland

A series of long-term simulations focused on the tropical Atlantic have been conducted using a coupled ocean-atmosphere general circulation model. In previous studies, a model bias in the ITCZ led to warm SST errors in the southeastern equatorial Atlantic. The focus of this study is to examine whether an improved model mean state can benefit the simulation of the annual cycle and the interannual variability and, later on, the dynamical climate prediction. Toward this end a simple flux correction is applied to the heat flux that varies spatially but is temporally constant. Its magnitude is determined from the mean SST error of a previous long-term simulation of the same coupled model without flux correction. The flux corrected model has been run in global and Atlantic regional coupled modes. Additional experiments are also conducted with a surface momentum flux correction term derived in a similar way.

Our results show that the heat flux correction reduces the model annual mean SST errors in the tropical Atlantic efficiently. Moreover, associated with the improved mean SST gradient, surface wind simulation is improved significantly over the tropical ocean. With a better mean SST and surface winds, the model’s annual cycle and the interannual variability are also significantly improved, especially in the regions along the equator and near the Angolan coast. In particular, the anomalous events in the Gulf of Guinea and the Angolan coast are more realistic in the flux corrected model, which enhances significantly the Southern Tropical Atlantic pattern, which is the leading SST mode in the observations but was poorly simulated by the original model. Our analysis shows that the improvement of the model anomalous events is mostly associated with a better simulation of the low frequency propagation of the equatorial oceanic waves in response to surface wind forcing.

An ensemble of eight hindcasts has been conducted using the flux corrected Atlantic regional coupled model with prescribed observational SST forcing for 1950-1998 over the uncoupled world ocean. A comparison with a set of similar runs using the model without flux correction shows that the flux corrected model produces more realistic pattern of the warm SST signals in the northern tropical Atlantic Ocean during the boreal spring after a maturing El Niño event in the tropical Pacific. In general, an El Niño induces a global warm state nearly symmetric to the equator in the upper tropical atmosphere. The colder mean Atlantic SST to the south of the equator in the flux corrected model, however, seems to suppress local atmospheric convection and prevent the ENSO signals in the upper troposphere from propagating downward. As a result, the observed meridional asymmetry of the tropical Atlantic response to ENSO is better reproduced.

∗

Corresponding address: Bohua Huang, COLA/IGES, 4041 Powder Mill Road, #302, Calverton, MD 20705. Email: huangb@cola.iges.org.

Local and equatorial forcing of Seasonal variability of the North Equatorial Countercurrent in the Atlantic Ocean

Jiayan Yang and Terrence M. Joyce Dept. of Physical Oceanography Woods Hole Oceanographic Institution Woods Hole, MA 02543 E-mail: jyang@whoi.edu

The seasonal variation of the North Equatorial Countercurrent (NECC) in the tropical Atlantic Ocean is investigated by using a simple one-layer reduced gravity ocean model and by analyzing sea level height (SSH) data from TOPEX/POSEIDON (T/P) altimeters. The T/P data indicate that the seasonal variability of the NECC geostrophic transport, between 3°N and 10°N, is dominated by SSH changes in the southern flank of the current. Since the southern boundary of the NECC is located partially within the equatorial waveguide, the SSH variation there can be influenced strongly by the equatorial dynamics. We hypothesize therefore that the wind stress forcing along the equator is the leading driver for the seasonal cycle of the NECC transport. The wind stress curl is an important but smaller contributor. This hypothesis is tested by several sensitivity modeling experiments that are designed to separate the two forcing mechanisms. In the first sensitivity run, a wind stress field that has a zero curl is used to force the ocean model. The result shows that the NECC geostrophic transport retains most of its seasonal variability. The same happens to another experiment in which the seasonal wind stress is applied only within a narrow band along the equator outside the NECC range. To further demonstrate the role of equatorial waves, we ran another experiment in which the wind stress in the southern hemisphere is altered so that the model excludes hemispherically symmetrical waves (Kelvin waves and odd-numbered meridional modes of equatorial Rossby waves), and instead, excites only the anti-symmetrical equatorial Rossby modes. The circulation in the northern tropical ocean, including the NECC, is affected considerably even though the local wind stress there remains unchanged. All these appear to support our hypothesis.

Equatorial Atlantic interannual variability and its relation to ENSO

Noel Keenlyside* and Mojib Latif Leibniz Institut fuer Meereswissenschaften, Kiel, Germany

Abstract

The impact of the El Niño/Southern Oscillation (ENSO) on northern tropical Atlantic and southern subtropical Atlantic variability are now reasonably well established. The impact of ENSO on the equatorial Atlantic is less clear. Several studies have argued that ENSO related perturbations to the zonal Walker circulation over the equatorial Atlantic induce a delayed dynamical response there. However, these studies have been more model based, and evidence in the observations for such an interaction has not been demonstrated clearly.

In this study, the link between ENSO and equatorial Atlantic interannual variability is investigated using observations of sea surface temperature (SST), sea level pressure, surface winds, and ocean heat content. Results show that surface zonal winds over the western equatorial Atlantic are modulated by ENSO activity. During periods of strong ENSO activity, such as in the last 30 years, the modulation of zonal winds results in a slow eastward propagating heat content signal that reaches the east 6 to 18 months later. In the east, these heat content anomalies are able to influence SST only during the boreal summer. This is because only during these months the thermocline is sufficiently shallow to be able to influence SST. Observations indicate that for the 3 large El Niño events of the last 30 years, this mechanism is able to consistently generate positive eastern Atlantic SST anomalies 18 months later. For weaker ENSO variability and for other lead times no such relationship exists. While this mechanism explains only a small fraction of the total variance, the apparent robustness for strong El Niño events is potentially of real practical use.

*Leibniz Institut fuer Meereswissenschaften, Dusternbrooker Weg 20, D-24105, Kiel, Germany email:nkeenlyside@ifm-geomar.de

OGCM study of the Western Hemisphere Warm Pool

By S.-K. Lee* (Cooperative Institute for Marine and Atmospheric Science, Univ. of Miami),

D. B. Enfield (NOAA Atlantic Oceanographic and Meteorological Laboratory), and

C. Wang (NOAA Atlantic Oceanographic and Meteorological Laboratory)

The Western Hemisphere Warm Pool (WHWP) is characterized by its large interannual fluctuations in size, the largest of which are associated with El Niño-Southern Oscillation (ENSO) and occur during the summer following the maximum SST anomalies in the Pacific. To better understand the atmosphere-ocean processes involved in the WHWP variability, a series of ocean general circulation model experiments is carried out using the Hybrid Coordinate Ocean Model (HYCOM). First, we fine-tune the model by assessing the model's sensitivity to surface heat flux data and by optimizing the tunable model parameters, such as light attenuation and vertical turbulent mixing scheme. Consistent with Enfield and Lee (2004), we find that the Southampton constrained heat flux climatology provides a realistic simulation of the WHWP’s annual cycle, but that the NCEP reanalysis heat flux data contain a significant bias, which results in a large SST bias in the model. Therefore, we have reconstructed the 50-year surface heat flux data by combining the Southampton constrained heat flux climatology and the anomalous heat flux fields from the NCEP reanalysis data, which is then used to force the HYCOM. The WHWP indices derived from the model output are compared with the corresponding indices from the observations to assess the model's skill in reproducing the WHWP interannual variability. After validating the model results, we diagnose the WHWP heat budget to gain insights into the major forcing mechanisms responsible for the WHWP variability.

Dr. Sang-ki Lee*, CIMAS, University of Miami 4600 Rickenbacker Causeway Miami, FL 33149 Tel) 305-361-4521 Fax) 305-361-4412 E-mail) sang-ki.lee@noaa.gov

THE RELATIONSHIP BETWEEN EQUATORIAL ATLANTIC SST VARIABILITY AND EL NIÑO

Yochanan Kushnir*, Lamont-Doherty Earth Observatory, The Earth Institute at Columbia University, Palisade, New York, USA

During the boreal summer SST variability in the tropical Atlantic is governed by an equatorially pattern of variability, with a maximum on the eastern side of the Basin, reminiscent of El Niño. Modeling studies of this mode of variability suggest that it is governed by similar dynamics but that it is not self-sustained. There is evidence that one of the ways by which it is invoked is linked with the evolution of the Pacific El Niño. The relationship is characterized by a negative correlation between the variability in the eastern equatorial Pacific and Atlantic regions and is observed during the boreal spring and summer months. Here we investigate this link and find that it tends to change with time on multi-decadal times scales: strong in the first and last few decades of the 20 Century and weak in between. We describe the nature of this link as exhibited by the seasonal behavior of SST and speculate regarding the reasons for its non-stationarity.

* Corresponding author address: Yochanan Kushnir, Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964

Tel: 845-365-8669, Fax: 845-365-8736.

E-Mail: Kushnir@ldeo.columbia.edu

Remote Impact on Tropical Atlantic Climate Variability: Statistical Assessment and Dynamic Assessment

Zhengyu Liu*, Qiong Zhang, Lixin Wu Center for Climatic Research, University of Wisconsin-Madison 1225 W. Dayton St., Madison, WI 53706

The remote impact of tropical Pacific and North Atlantic climate forcing on the tropical Atlantic sea surface temperature variability is assessed using both a traditional statistical correlation method and a model-aided dynamic method. Consistently, both assessment methods suggest that the remote impact contributes to nearly half of the variance of the tropical Atlantic sea surface temperature variability at interannual and decadal time scales. In the mean time, the other half of the sea surface temperature variability is generated predominantly in the tropical Atlantic climate system, with local ocean-atmosphere coupling playing a critical role. Furthermore, the leading sea surface temperature variability modes seem also to originate predominantly internally in the tropical Atlantic climate system. The main effect of the remote impact is therefore an enhancement of the variance of these variability modes. Our model study also show some differences between the statistical and dynamic assessment methods, which may have implications on the methodology of the assessment as well as the dynamics of the system.

Name: Zhengyu Liu* Affiliation: Center for Climatic Research, University of Wisconsin-Madison Mailing Address: 1225 W. Dayton St., Madison, WI 53706 Telephone: 608-262-0777 Fax: 608-263-4190 Email: zliu3@wisc.edu

North Atlantic decadal variability: Air-Sea Coupling, Oceanic Memory and Potential Northern Hemisphere Resonance

**Lixin Wu^* and Zhengyu Liu**

Center for Climatic Research University of Wisconsin-Madison

Abstract

In this paper, the causes and mechanisms of North Atlantic decadal variability are explored in a series of coupled ocean-atmosphere simulations. The model captures the major features of the observed North Atlantic decadal variability. The North Atlantic SST anomalies in the model control simulation exhibit a prominent decadal cycle of 12 to 16 yr, and a coherent propagation from the western subtropical Atlantic to the subpolar region. A series of additional modeling experiments are conducted in which the air-sea coupling is systematically modified in order to evaluate the importance of air-sea coupling for the North Atlantic decadal variability being studied. We shall refer to this as “modeling surgery”. The results suggest the critical role of ocean-atmosphere coupling in sustaining the North Atlantic decadal oscillation at selected timescales. The coupling in the North Atlantic is characterized by a robust NAO-like atmospheric response to the SST tripole anomaly, which tends to intensify the SST anomaly and meanwhile also provide a delayed negative feedback. This delayed negative feedback is predominantly associated with the adjustment of the subtropical gyre in response to the anomalous wind stress curl in the subtropical Atlantic. Atmospheric stochastic forcing can drive SST patterns similar to those in the fully coupled ocean-atmosphere system, but fails to generate any preferred decadal timescales. The simulated North Atlantic decadal variability, therefore, can be viewed as a coupled ocean-atmosphere mode under the influence of stochastic forcing.

Our modeling study also suggests some potential resonance between the Pacific and the North Atlantic decadal fluctuations mediated by the atmosphere. The modeling surgery indicates the Pacific climate, although is not a necessary precondition, can impact the North Atlantic climate variability substantially.

*Corresponding author: Lixin Wu, Center for Climatic Research, University of Wisconsin-Madison, 1225 West Dayton Street, Madison, WI 53706. Email: lixinwu@facstaff.wisc.edu

The interannual variability of the North Atlantic Ocean revealed by combined data from TOPEX/Poseidon and Jason altimetric measurements

Lee-Lueng Fu* Jet Propulsion Laboratory California Institute of Technology Pasadena, CA 91109

A decade-long record of sea surface height (SSH) from combined altimeter data taken by the TOPEX/Poseidon and Jason satellites was analyzed for studying the interannual variability of the North Atlantic Ocean. On time scales of 5-6 years, variations of sea surface height have maximum amplitudes in three areas: the subpolar gyre, the Gulf Stream gyre (the Gulf Stream and its recirculation), and the subtropical region. The variation of the subpolar gyre is 180 degrees out of phase with that of the Gulf Stream gyre at its eastern end. The variation of the Gulf Stream gyre at its western end is connected to that of the subtropical region, exhibiting phase propagation from the subtropics all the way to the eastern end of the Gulf Stream gyre. The patterns of phase change suggest possible roles of Rossby waves in the dynamics of the basin-wide variability. The spatial pattern of SSH variability is similar to that of the sea surface temperature tripole of the North Atlantic, but the phase propagation might indicate the ocean’s dynamic and non-local response to atmospheric forcing.

*Tel: 818-354-8167, Fax: 818-393-6720, Email: llf@pacific.jpl.nasa.gov

Transport Variability across 48N in the Atlantic Ocean

Rick Lumpkin and Kevin Speer

Velocity and transport across 48N are estimated from air-sea flux climatologies and repeat WOCE hydrographic lines near this latitude spanning the period 1993-2000. A box inverse model method was used to estimate unknown thermal wind reference velocities, diapycnal fluxes, and adjustments to air-sea fluxes subject to various constraints on the system. The net overturning strength and heat transport show no significant change in the solutions over this period, in contrast to previous studies based on hydrography and winds. Significant variability is found only for export of Labrador Sea Water (LSW) and air-sea flux quantities in the subpolar North Atlantic. These results suggest that local forcing in the subpolar gyre, rather than upper-limb transport variations driven by more remote forcing, dominates the observed deep variability.

Diapycnal fluxes are found to be important in the mean rate of LSW and Lower Deep Water (LDW) formation, but are relatively constant from one repeat section to the next. The year-to-year variations in flux-driven formation dominate the observed variability of LSW export across the 48N sections, which is significantly and positively correlated with the North Atlantic Oscillation. LDW variations are large compared to the time-mean magnitude, but not significant due to considerable uncertainty in the year-to-year export estimates. These LDW variations are interpreted as as adiabatic volumetric changes, implying mass storage in deep layers at interannual time scales.

Tropical Atlantic SST Forcing of Coupled North Atlantic Seasonal Responses

Shiling Peng*, Walter A. Robinson**, Shuanglin Li, and Martin P. Hoerling

NOAA-CIRES Climate Diagnostics Center, University of Colorado – Boulder; **Department of Atmospheric Sciences, University of Illinois - Urbana

Recent observational studies reveal that a fall Pan-Atlantic sea-surface temperature (SST) anomaly, composed of a horseshoe-like dipole in the North Atlantic and a southern center in the equatorial Atlantic, tends to precede the winter North Atlantic Oscillation (NAO) and its related SST tripole. We seek to understand this relationship using large ensembles of atmospheric GCM (AGCM) experiments and experiments with the AGCM coupled to a mixed-layer ocean (AGCM_ML). The models are forced either by the North Atlantic horseshoe (NAH) or by the tropical SST anomalies over the boreal winter months.

The AGCM results show that the NAH anomaly induces a baroclinic response in geopotential heights throughout the winter, with little projection on the NAO. In contrast, in the AGCM_ML, the coupled North Atlantic response forced by the tropical anomaly exhibits a strong seasonal dependence. In early winter, the positive anomaly induces a trough east of Newfoundland with a wavetrain to the northeast, and in late winter the response projects strongly on a negative NAO. Correspondingly, the extratropical SST response features a NAH-like pattern in early winter and a tripole in late winter. These results suggest that the observed relationship between the fall NAH SST and the winter NAO (or the SST tripole) may result from the seasonal march of coupled responses to persistent tropical forcing.

To determine the effects of the Ekman transport on the NAO response to the tropical forcing, the mixed-layer ocean is further extended to include the Ekman advection. Preliminary results from the experiments with the new coupled model (AGCM_EML) suggest that the tropical-forced NAO response is enhanced by about 30% with also intriguing pattern differences, in comparison with that in the AGCM_ML.

*Shiling Peng NOAA-CIRES Climate Diagnostics Center R/CDC1 325 Broadway Boulder, CO 80305-3328

Tel: (303) 497-6644 Fax: (303) 497-7013 Email: shiling.peng@noaa.gov

Does the oceanic meridional overturning cell really have more than one stable state ??
Doron Nof, Dept. of Oceanography, Florida State University*, Stephen Van Gorder, Dept. of Oceanography, Florida State University*, Agatha M. DeBoer, Atmospheric and Oceanic Sciences Program, GFDL, Princeton University*

Numerical climate and ocean models consistently show that the meridional overturning cell (MOC) has two stable states for the same fresh water flux (into the ocean). One of these two states usually corresponds to a high northward transport of surface water (and, hence, to a warm northern hemisphere climate) whereas the other corresponds to low northward transport (and, hence, to a cool northern hemisphere climate). This two-states scenario leads to the broadly quoted concern that our present day warm climate can perhaps spontaneously flip to a much cooler state.
We present new analytical and numerical runs showing that these two states are an art-effect of the high eddy diffusivities most often used in the commonly employed numerical models. For the Atlantic, these diffusivities artificially introduce unrealistic upwelling into the thermocline within the limits of the Atlantic itself. These, in turn, introduce the familiar hysteresis corresponding to the two-states. In the real ocean, the convection in the north Atlantic is too strong and the vertical diffusivities in the thermocline are too small to allow for such upwelling within the limits of the Atlantic itself. As a result, the water which ultimately sinks in the north Atlantic is drawn into the Atlantic from regions far away--the Southern and Indian oceans.
Both the analytical and the numerical results show that the two states converge into one in the limit of small diffusivity. This one-state scenario doesn’t at all mean that the MOC cannot collapse due to a large fresh water flux--it certainly can do so if the fresh water flux is large enough. Rather, it means that there are no two-states for the same fresh water flux, implying that the transition from warm to a cold state cannot happen spontaneously.

*Doron Nof
nof@ocean.fsu.edu
850-644-2736 850-644-2581 fax

*Stephen Van Gorder
vangorder@ocean.fsu.edu
850-644-2447
*Agatha M. DeBoer Agatha.DeBoer@noaa.gov 609-452-6518 609-987-5063 fax

Dynamic coupling between NAO and North Atlantic SSTA tripole

Lin-Lin Pan* and Fei-Fei Jin Department of Meteorology, Florid State University, FL

Masahiro. Watanabe Graduate School of Environmental Earth Science, Hokkaido University, Japan

A five-layer primitive equation model with synoptic eddy and low-frequency flow (SELF) feedback was coupled with an ocean mixed-layer model to study the effect of air-sea coupling on the NAO. It is shown that the NAO-like atmospheric circulation anomalies can produce tripole-like SST anomalies (SSTA) in the North Atlantic region and a NAO-like dipole with an equivalent barotropic structure over the North Atlantic can be excited by the SSTA tripole. The NAO-like response is primarily maintained by a positive SELF feedback. Without SELF feedback, this covarying pattern cannot exist. However, without air-sea coupling, the NAO-like mode can still exist due to atmospheric internal dynamics.

*Corresponding author address: Dr. Lin-Lin Pan, Meteorology Department, 2525 Correa Rd., HIG 341, Honolulu, HI, 96822. E-mail: lpan@hawaii.edu

Local and remote forcing of South Atlantic climate variability

Chris Reason*, Frank Colberg, Mathieu Rouault Dept. of Oceanography, University of Cape Town Rondebosch, 7701, South Africa 27 21 650 5311 (t), 27 21 650 3979 (f) cjr@egs.uct.ac.za

Several patterns of variability are prominent in the South Atlantic Ocean on interannual to multidecadal scales and some of these are known to impact on the climates of the neighbouring landmasses. Evidence of both local and remote forcing of these patterns exists. Of most importance for southern Africa are the so-called Benguela Niños and Niñas, the South Atlantic response to ENSO, and interdecadal scale modulations of the subtropical gyre and South Atlantic anticyclone. Isolating local atmosphere-ocean interaction from remote forcing such as ENSO is crucial in working towards prediction of South Atlantic SST variability and climate prediction over neighbouring southern Africa. Modulations of the tradewinds over the South Atlantic are an important component of the forcing of Benguela Niños, the South Atlantic response to ENSO, and the evolution of several other modes in the basin. This presentation addresses the local versus remote forcing of the winds over the basin, the ocean response and implications for the design of an appropriate observing system.

The influences of South American rainfall on the climate variability of the tropical Atlantic Ocean

Rong Fu, Mingxuan Chen and Hui Wang School of Earth and Atmospheric Sciences, Georgia Institute of Technology

We have examined the influences of South American rainfall on the variabilities of the tropical Atlantic sea surface temperature (SST) and Intertropical Convergence Zone (ITCZ) in boreal spring (March-April-May) using QuikSCAT ocean surface winds, TRMM daily rainrate and ECMWF reanalysis (ERA40). On synoptic scales and intraseasaonal scales, an increase of South American rainfall can cause episodes of wind anomalies, consequently anomalous ocean surface sensible, latent and momentum fluxes. The accumulative effect of these episodes of strong South American rainfall on sea surface temperature anomalies during boreal spring varies significantly on interannual scales. A stronger South American rainfall could cause as much as 0.5°C of anomalous cooling over the northern tropical or the equatorial Atlantic.

We also observed direct influence of South American rainfall on rainfall and surface wind in the Atlantic ITCZ, namely, a strong episode of South American rainfall can excite anomalous strong rainfall and surface wind convergence center propagate eastward within the Atlantic ITCZ. We are exploring the underlying dynamic process of this observed phenomena and will report our results in the presentation.

Meridional heat transport in the South Atlantic.

Silvia L. Garzoli and Molly Baringer NOAA/AOML

The South Atlantic Ocean is a major conduit for the warm upper layer water that flows northwards across the equator, compensating for the southward flowing North Atlantic Deep Water. This large-scale circulation is responsible for the northward heat flux through the South Atlantic. Estimates of the heat transport in the South Atlantic in the 30° to 35°S band varies from negative values to more that 1 PW. This variability may be a consequence of the different methods used to calculate the heat transport, however natural variability, including different circulation pathways, cannot be ruled out. In 2002, a high-density XBT line began in the South Atlantic that transits between Cape Town, South Africa and Buenos Aires, Argentina. The line was repeated twice a year until 2004 when occupations became quarterly. The line effectively closes the upper layer mass budget in the South Atlantic by providing an estimate the variability of the upper limb of the MOC transport. In this presentation, the methodology developed to estimate heat transports from the XBT data is presented. This methodology is tested using the CTD data collected along the A10 WOCE line and the product of a GCM numerical model. The model product is also used to estimate errors. The Ekman component of the flow is estimated from different wind products to evaluate the uncertainty induced by using different wind fields. Results from the first six realizations of AX18 will be presented and compared with model estimates.

*Silvia L. Garzoli, Director PHOD/AOML, tell: 305.361.4338, e-mail: Silvia.Garzoli@noaa.gov

Molly Baringer, Oceanographer PHOD/AOML, tell: 305.361.4345, e-mail: Molly.Bariber@noaa.gov

The impact of horizontal resolution on the tropical heat budget in an Atlantic ocean model

Markus Jochum, ^¹Massachusetts Institute of Technology

Raghu Murtugudde, Earth System Science Interdisciplinary Center

Raffaele Ferrari, Massachusetts Institute of Technology

Paola Malanotte-Rizzoli, Massachusetts Institute of Technology

An ocean general circulation model (OGCM) of the tropical Atlantic is coupled to an advective atmospheric boundary layer model. This conﬁguration is then used to investigate the hypothesis that resolving tropical instability waves (TIWs) in OGCMs will remove the equatorial cold bias that is a feature common to coarse resolution OGCMs. It is shown that constant horizontal diffusivity in coarse resolution models is a reasonable approximation to the equatorward heat transport by tropical instability waves. However, this diffusion leads to a warmpool that is too cool by approximately 1K. It is demonstrated that the reason for this is that diffusion removes heat from the warmpool to heat the equatorial cold tongue, whereas TIWs draw their heat mostly from the atmosphere, not from the warmpool as hitherto assumed, and thus can bring more heat to the equatorial cold tongue without cooling the warmpool. The equatorial warming due to TIWs is slightly larger than the warming due to diffusion but this increased equatorial heat ﬂux in the high resolution experiment is compensated by increased equatorial entrainment there. This is attributed to the Equatorial Undercurrent being stronger, thereby increasing the entrainment rate through shear instability. Thus, higher resolution does not signiﬁcantly increase the total oceanic heat ﬂux convergence in the equatorial mixed layer. The different resolution does, however, lead to changes in the atmospheric heat ﬂux convergence, because the sharper cross-equatorial temperature gradient in the high resolution experiment leads to reduced latent and sensible heat losses over the equator.

¹Corresponding author's address: markus@ocean.mit.edu, 617-2533573 MIT, 77 Massachusetts Avenue, 54-1410, Cambridge, 02138 MA

The Response of the Labrador Sea to a Variable Atmosphere and Ocean

Fiammetta Straneo* Woods Hole Oceanographic Institution

Variability in the Atlantic sector on decadal and multidecadal timescales is thought to be, in part, associated to changes in the convective activity in the Labrador Sea. The latter, in turn, is influenced by a variety of parameters including past convective activity, atmospheric variability and variability in the oceanic circulation. The impact of each of these is investigated using a two-layer, idealized model of the Labrador Sea. The model represents the interior and boundary currents of the Labrador Sea as two distinct regions whose exchange is regulated by parameterized eddy fluxes. The boundary current velocity is assumed to be both wind and buoyancy driven. For simplicity, convection is limited to the interior region. The model's skill is illustrated through a comparison of historical and recent data from the Labrador Sea.

The model is instrumental in clarifying the connection between the volume of newly formed dense water, the associated downwelling (vertical mass flux), the dense water exported, the surface forcing and the wind-driven circulation. As such, it is a useful tool for the interpretation of global circulation models' results. Also, the model is used to assess the magnitude of the response of convection related fields to changes in both the local surface forcing and in the external (remote) oceanic circulation. Conclusions derived from the model help unravel the causes of the variability observed in the Labrador Sea, and make predictions of the response to variable forcing.

*Corresponding Author Address: fstraneo@whoi.edu Fiamma Straneo WHOI MS #21 Woods Hole, MA 02543

The Causes of Tropical Atlantic Precipitation Biases in AGCMs:in search of a comprehensive diagnosis.

Michela Biasutti* (LDEO)
Adam Sobel(ColumbiaUniversity)
Yochanan Kushnir(LDEO)

This study focuses on the biases in monthly mean Atlantic marine ITCZ (AMI) in reanalyses and AMIP like GCM simulations. We establish a set of diagnostics that provides a comprehensive, multi-variable description of the thermodynamical and dynamical characteristics of precipitation and its response to changes in local and remote conditions. Maps, vertical pro€les, and budget analysis of key variables provide a description of the large scale environment; joint probability density functions and correlation analysis document the relationship between precipitation and its local environment.
Comparisons between the Atlantic and the Paci€c ITCZs in each model, among different models, and between models and observations allow us to hypothesize explanations for shortcomings in the simulation of AMI. For example, convection in CCM3 responds more strongly to surface humidity than to the integrated humidity in a deeper boundary layer. In turn, surface humidity is determined to a large extent by SST and evaporation, not mostly by convergence. In both cases, CCM3 is at odds with observations, reanalysis products, and other models. Thus, we hypothesize that the bias in the CCM3 simulation of AMI is due both to the way the convection scheme responds to its environment (i.e., the weak dependance on lower tropospheric humidity), and to the way the environment is simulated (what determines the surface humidity).

MichelaBiasutti,biasutti@ldeo.columbia.eduLamont-DohertyEarthObservatory61Route9W

Palisades,NY10968Tel:845.365.8512FAX:845.365.8736

Three Dimensional Structure of Easterly Wave Disturbances over Africa and the Tropical North Atlantic

Chris Thorncroft^* (SUNY at Albany) George Kiladis (NOAA Aeronomy) Nick Hall (LTHE, Grenoble, France)

African easterly waves (AEWs) are synoptic-scale weather systems that characterize tropical Africa and downstream tropical Atlantic during boreal summer. They are important systems to study because of their association with rainfall over tropical Africa, and because they can serve as precursors for tropical cyclones in the Atlantic. AEWs typically have a complex structure that arises due to a combination of adiabatic and diabatic processes. They are characterized by a synoptic scale evolution related to interactions between large-scale potential vorticity gradients and low-level theta gradients but embedded within them are sub-synoptic scale structures that develop in association with non-linear developments and mesoscale convective systems.

This talk will emphasize the nature and variability of synoptic aspects of AEWs. The statistical structure of easterly waves over Africa and the adjacent tropical Atlantic is isolated using a combination of satellite, radiosonde, and various reanalysis products. Space-time filtered OLR is used to identify individual synoptic-scale waves and to characterize the amount of wave activity on intraseasonal or interannual time scales. The relationship between variability in AEW-activity and changes in the basic state will be presented together with stability analyses of the 3-dimensional basic states that support the AEWs.

* Corresponding Author SUNY at Albany Department of Earth and Atmospheric Sciences Earth Science 226 Albany, NY 12222

North Atlantic During 1950-1999: An Ocean GCM Study
Bin Zhao* Thomas W. N. Haine
Department of Earth and Planetary Sciences, Johns Hopkins University

Interannual variability in the North Atlantic during the last 50 years is studied using a 1 degree
resolution ocean GCM. When forced with monthly-varying NCEP fields, the model reproduces
the observed SST variability moderately well. To understand the origin of this variability,
experiments are performed with different components of the forcing fields. They show that surface
thermal variability is dominated by air-sea heat fluxes. For example, cold air outbreaks are largely
responsible for winter SST variations in the western mid-latitudes through large sensible and latent
heat variations. Ocean circulation changes induced by wind stress anomalies play a secondary role.
In some regions, such as the western subtropics, circulation changes can cause similar variability
to the air-sea fluxes, however. Heat budget analysis shows that SST anomalies from heat transport
changes - mainly anomalous currents - are smaller in lateral scale than those caused by surface
heat flux perturbations, and are concentrated in the Gulf Stream and its extension. Subsurface
thermal variability in the western subtropical gyre has longer periods than at the surface and is
mainly associated with long Rossby wave propagation. These anomalies are then advected by the
Gulf Stream and North Atlantic Current into the subpolar region. The impact of these thermal
anomalies on the subpolar mixed layer as they reemerge will be reported.

* Bin Zhao Department of Earth and Planetary Sciences
Johns Hopkins University
3400N. Charles Street
Baltimore, MD 21218

A Numerical Study of the Bifurcation of the South Equatorial Current in the Atlantic Ocean

Regina R. Rodrigues*, Graduate School of Oceanography, University of Rhode Island

Lewis Rothstein, Graduate School of Oceanography, University of Rhode Island

Mark Wimbush, Graduate School of Oceanography, University of Rhode Island

The role of the wind-driven subtropical circulation in determining the tropical thermocline has been extensively studied in the last decade, with many publications on the pathways between subtropical and tropical oceans (the subtropical cells). Previous studies have shown that most of the water encountered in the equatorial Atlantic thermocline comes from the South Atlantic. The main route of the subducted subtropical waters to the tropics is through the low latitude western boundary current window, which is in this case through the north branch of the bifurcation of the South Equatorial Current (SEC) into the North Brazil Undercurrent/Current system. In this study, a reduced-gravity, primitive-equation OGCM (Gent and Cane, 1989) is used to investigate the seasonal variability of the SEC bifurcation. Meridional velocity averaged within 2°-longitude band off the South America coast shows that the SEC bifurcation occurs at about 14°S near the surface, shifting poleward with increasing depth, reaching 27°S at 1000m (Figure 1 & 2, from observations and model, respectively). The bifurcation latitude reaches the southernmost position in July and the northernmost position in November (Figure 3). Preliminary results show that most of the seasonal variability of the bifurcation latitude is associated with the variability of the amplitude of local wind stress curl (Figure 4) due to the annual north-south excursion of the marine ITCZ complex, with the remote forcing being less important.

* Corresponding author address: Graduate School of Oceanography, University of Rhode Island, Box 200, Bay Campus, Narragansett, RI, 02882. E-mail: rrodrigues@gso.uri.edu

Sensitivity of tropical Atlantic climate to vertical mixing.
Wilco Hazeleger
KNMI, Oceanographic Res. Dept.
PO Box 201, 3730 AE De Bilt
The Netherlands
tel./fax +31-30-2206718/2202570

Simulations of tropical Atlantic climate with coupled models suffer from severe biases. Here, results from a modeling study will be presented in which the sensitivity of tropical Atlantic climate to upper ocean mixing is studied. An ocean-only model and a fully coupled ocean-atmosphere model (SPEEDO) is used. The upper ocean thermal structure and associated atmospheric circulation proves to be strongly related to the strength of upper ocean mixing. The cold bias in the coupled model can be completely removed by enhancing the entrainment efficiency. Slightly deeper mixed layer reduce the upper ocean divergence resulting in realistic cold tongue development and realistic heat balances. Also, the annual cycle of SST, the thermocline structure, and the Atlantic Marine ITCZ improves markedly.

Atlantic Air-sea Heat Fluxes in the 1990s and Their Connection to Basin-scale Atmospheric and Oceanic Variations
Lisan Yu* and Robert A. Weller
Department of Physical Oceanography, Woods Hole Oceanographic Institution
Woods Hole, MA 02543

The heat exchange between the atmosphere and the ocean in the Atlantic basin underwent a major change in 1990s (Fig.1), as suggested by two newly available surface turbulent and radiative heat flux datasets. The turbulent latent and sensible heat fluxes were from our Objective analyzed Air-sea Flux (OAFlux) project and the surface radiation fluxes were from International Satellite Cloud Climatology Project (ISCCP). The change is characterized by a shift from high flux mode (more oceanic heat gain) to low flux mode (less oceanic heat gain) in both turbulent and radiative heat flux fields with the former dominating the net heat flux pattern; and the shift was more pronounced in the southern Atlantic Ocean with the amplitude of the net heat flux altered by ~30 Wm-2 between the two modes.
Similar mode shift was also detected in observed upper ocean heat content and wind stress and stress curl fields. It appears that the changes in the surface heat fluxes were coordinated with the changes in near-surface atmospheric circulation and upper ocean thermal field. The study will show the analysis of the covariations and interactions between the atmosphere and the ocean during the 1990s and the role of heat fluxes. The study will also show the comparison with the NCEP reanalysis fluxes and discuss the impact of the quality of flux data on studying climate variability.

* Corresponding author contact information: Dr. Lisan Yu, Department of Physical Oceanography, MS#21, Woods
Hole Oceanographic Institution, Woods Hole, MA 02543. Email: lyu@whoi.edu. Phone: 508 289 2504.

Decadal Changes in the Tropical Atlantic Pycnocline
1 Dongxiao Zhang 1,2 and Michael J. McPhaden
1. 1. NOAA/Pacific Marine Environmental Laboratory, Seattle, WA
2. 2. JISAO/U. of Washington, Seattle, WA Dongxiao.Zhang@noaa.gov

The shallow meridional overturning circulation connects the equatorial thermocline with regions of subduction in the north and south subtropical Atlantic. Using historical hydrographic data, we identify the trend and decadal anomalies in water mass properties in the tropical Atlantic pycnocline. We also calculate geostrophic equatorward transports in the pycnocline associated with this shallow overturning for different decades. The transport values will be compared to those from linear Sverdrup theory forced by various wind products. Possible linkages between pycnocline water mass anomalies, ocean circulation changes, and anomalies in equatorial mixed layer temperatures will be described. Relationship of these oceanic changes to changes in atmospheric circulation associated with the NAO will also be discussed.

The Vertical Structure of Ocean Heat Transport
Giulio Boccaletti, MIT; Raffaele Ferrari, MIT; Alistair Adcroft, GFDL;
and John Marshall, MIT* Massachusetts Institute of Technology

The partition of ocean heat transport between wind-driven and thermohaline circulation is considered from the perspective of a ‘heatfunction’ which, unlike the traditional mass transport overturning streamfunction, allows one to more clearly identify the contribution of different components of the ocean circulation to heat transport. A new view of the ocean emerges in which a shallow surface intensified circulation dominates the poleward heat transport. Guided by the heat function we:
(i) Argue that the attention devoted to abyssal circulations is disproportionate to their role in heat transport. In illustration, we show that changes in abyssal mixing have no direct impact on the global meridional heat transport by an ocean model.
(ii) Discuss implications of our findings for mechanism of abrupt climate change triggered by Atlantic processes.

*Authors’ Contact Information:
Giulio Boccaletti Massachusetts Institute of Technology
Dept. of Earth, Atmospheric and Planetary Sciences, 54-1410
Cambridge, MA 02139

“The Busy 2004 Atlantic Hurricane Season: Was it an Anomaly, Trend or Cycle?”
Chris Landsea NOAA/AOML/Hurricane Research Division

The Atlantic hurricane season of 2004 will go down as one of the busiest on record. At least five hurricanes struck the United States, with four of these directly impacting Florida for the first time in recorded history. A presentation will be made regarding the climatological significance of this season and whether it represented an anomalous outlier, part of a long-term trend toward active seasons, or a portion of a multidecadal oscillation.

Deep Boundary Current circulation and variability in the western subpolar North Atlantic
* Marcus Dengler, Jürgen Fischer, Friedrich Schott, Peter Brandt and Lothar Stramma
Leibniz Institute of Marine Sciences (IFM-GEOMAR), Kiel, Germany

Observations of interannual to decadal variability in the western subpolar North Atlantic were carried out based on multiyear mooring arrays installed since 1996 in the Labrador Sea and the Newfoundland Basin, by repeat shipboard observations of currents and hydrography at key positions of the boundary current system, and by profiling floats. In the Labrador Sea at 56°N and at 53°N, a well defined Deep Western Boundary Current (DWBC), the Deep Labrador Current (DLC), carries about 26 Sv of deep water comprising Labrador Sea Water, Gibbs Fracture Zone Water and Denmark Strait Overflow Water. Offshore of the DLC at both locations, weak top to bottom recirculations of about 8 Sv transport are present, carrying warmer and more saline water northwestward into the Labrador See. Farther to the south, at Flemish Cap and at location of the Northwest Corner of the North Atlantic Current, the deep western boundary flow is more variable due to energetic eddies. There, the average DWBC transport is less than the transports determined for the northern sections and comparable to the net deep water outflow (boundary current minus recirculation) from several year long moorings at the western boundary, which was 18 Sv at about 53°N. At the transition from the subpolar to the subtropical regime near the tail of the Grand Banks at 44°N, the DWBC transport decreased to 12 Sv, and float trajectories suggest that other pathways, e.g., eastward and possibly along the MAR, have to be taken into account. Water masses changed markedly throughout the observational period, showing not only a warming trend in the deep central Labrador Sea but also considerable warming of the LSW exported by the DLC. However, moored velocity time series at 56°N and 53°N from the core of the DLC do not show a similar trend for the DLC Transport

.
Corresponding author:
Marcus Dengler Leibniz Institute of Marine Sciences at Kiel University (IFM-GEOMAR) Physical Oceanography I Düsternbrooker Weg 20 24105 Kiel Germany Phone: 0049-431-600-4107 Fax: 0049-431-600-4102 e-mail: mdengler@ifm-geomar.de

Impact of anomalous ocean heat transport on the NAO
by F. D'Andrea (LMD), A. Czaja (MIT) 1 and J. Marshall (MIT)

Based on a series of experiments with a simpli ed yet realistic model of the atmosphere coupled to a slab ocean mixed layer with parameterized ocean heat transport, a negative feedback between ocean gyre and the NAO is isolated. For a moderate to strong efficiency of the gyres to carry heat across 50N, this feedback is shown to be responsible for a significant diminution of the model NAO variability. When the eÆciency of the gyres is increased further, the NAO vanishes from the leading modes of low-frequency variability of the model atmosphere. The relevance of the feedback to the observed
climate system is discussed.

1Massachusetts Institute of Technology, Department of Earth, Atmosphere and Plan-
etary Sciences 77 Massachusetts Avenue, Cambridge, MA 02139. tel: (1) 617 253 5458;
fax: (1) 617 253 4464; email: czaja@ocean.mit.edu

The tropical Atlantic extension of El Niño: How is it forced and why does it not always respond?
David B. Enfield*1, Sang-ki Lee2 and Chunzai Wang1
1 Atlantic Oceanographic & Meteorological Laboratory
2 Cooperative Institute for Marine and Atmospheric Science

Research shows that 25% of the tropical Atlantic SSTA variance is El Niño-induced through atropospheric bridge, and that models that neglect the other-ocean extensions of ENSO produce less realistic simulations of the El Niño-related climate impacts. The 4-6 month delay of the Atlantic extended ENSO response means that its continental climate impacts will be felt during the boreal summer following El Niño when the Pacific thermal anomalies have abated and have no direct effects. The Atlantic response to El Niño consists of a Western Hemisphere warm pool that may be twice its normal size for early summer when the North American Monsoon is developing. The historical large warm pools resulted from a tropical heating anomaly in the Pacific that persisted into the second (+1) year and was reflected in a stronger divergent circulation and associated Walker-Hadley outflows over South America and/or the western equatorial Atlantic. The problem for extended forecasts of ENSO climate impacts is that the tropical Atlantic response does not always ensue. Of nine recognized El Niño events after 1950, only five were followed by large warm pools. Thus, the mere knowledge of a culminating El Niño at the end of the onset year cannot be used to issue an empirical forecast of climate impacts during the following summer.

In this paper we examine two hypotheses as to why large warm pools do not invariably follow the mature phase of the Pacific event: (1) that differences in the evolution of the Pacific event may result in a weak or non-existent tropospheric bridge; and (2) that ocean-atmosphere interactions internal to the Atlantic sector and unrelated to ENSO sometimes interfere with the development of the bridge. By contrasting the canonical evolutions of El Niño (+1) years with and without large warm pools, we see that years with large warm pools typically follow longlasting El Niño events that persist into the boreal spring of the second (+1) year, but that the intensity of the Pacific event is not important. For those events the tropospheric bridge and surface forcing in the Atlantic are robust, whilst years without large warm pools lack these
characteristics and the Pacific anomalies end early. Two kinds of forcing internal the to Atlantic were also examined: Forcing of the tropical Atlantic by boreal winter NAO patterns, and tropical Atlantic variability (TAV) involving windevaporation - SST (WES) feedbacks between the tropical North and South Atlantic. In some, but not all cases a positive or negative pattern of atmospheric pressure anomalies similar to the NAO coincided in a consistent way with interference or enhancement of the teleconnection from the Pacific. This effect appears to be of secondary importance, however, and we cannot be certain that the NAO-like pattern is indeed an independent expression of the NAO or simply an extension of the divergent circulation anomaly forced by El Niño. Only one event showed clear evidence of a tropical Atlantic SST evolution consistent with the WES feedback process.

* NOAA Atlantic Oceanographic & Meteorological Laboratory, 4301 Rickenbacker Causeway,
Miami FL 33149. Tel: (305) 361-4351; email: David.Enfield@noaa.gov.

Simulated Global Response to a Substantial Weakening of the Atlantic Thermohaline Circulation

Rong Zhang¹* and Thomas L. Delworth^{2
1}GFDL / AOS Program, Princeton University ²GFDL, NOAA

In this study we use a newly developed fully coupled global ocean-atmosphere general circulation model to investigate the global scale response of the climate system to a sustained addition of fresh water to the model's North Atlantic, such as may have occurred during glacial periods. In response to this forcing, the model's thermohaline circulation weakens substantially, thereby cooling the North Atlantic and warming the South Atlantic. The associated global response involves a southward shift of the intertropical convergence zone (ITCZ) over both the Atlantic and Pacific sectors, an El Nino like condition and weakened Walker circulation in the southern tropical Pacific, a La Nina like condition and strengthened Walker circulation in the northern tropical Pacific, a dipole subsurface temperature response in the western tropical Pacific, and weakened Indian and east Asian summer monsoons. The substantial weakening of the thermohaline circulation leads to a more symmetric annual mean zonally averaged ITCZ and zonally integrated Hadley circulation about the Equator. These responses are consistent with the global-scale synchronization of millennial-scale abrupt climate change as indicated by paleoclimate records.

*Corresponding author address: Rong Zhang, GFDL/AOS Program, Forrestal Campus, Route 1, Princeton University, Princeton, NJ 08542. Email: Rong.Zhang@noaa.gov

Atlantic SST gradient and the influence of ENSO
Huei-Ping Huang1*, Andrew W. Robertson2, Yochanan Kushnir1
1Lamont-Doherty Earth Observatory of Columbia University
2International Research Institute for Climate Prediction, Columbia University

The tropical Atlantic SST gradient, defined as the difference between North (5°N-25°N) and South (5°S-25°S) Atlantic SSTs, is known to regulate precipitation anomalies over the northern Brazil. In March-May, a positive gradient (North Atlantic warmer than South Atlantic) suppresses Nordeste rainfall, similar to the effect of a positive El Niño SST anomaly. Giannini et al. (2004) found that, for 1950-1994, concordant cases (with same sign of Atlantic SST gradient
and NINO3 index) dominate La Niñas while discordant cases dominate El Niños, resulting in a diminished impact of the latter on Nordeste rainfall. Building on this insight, the ENSO-Atlantic SST gradient relationship is revisited using a long (1870-present) and detrended SST data set. Three major findings are: (i) Unlike 1950-1994, with more samples the asymmetry between warm and cold ENSO events disappears. Both are slightly dominated by concordant cases,
consistent with the fact that the (positive) correlation between the NINO3 index and North Atlantic SST anomaly is stronger than that between NINO3 and South Atlantic SSTA. (ii) The ratio of the numbers of concordant and discordant cases is about 4:3 for all ENSO events, indicating a strong presence of noise due to non-ENSO influences. (iii) In the March-May composite for concordant cases, a well-defined pattern of SST anomaly, resembling the
" Benguela Niños", emerges off the southwest coast of Africa with the sign of SSTA opposite to that of NINO3 index. For the discordant cases, almost the entire South Atlantic has the same sign of SSTA as NINO3, with a notable local maximum/minimum of SSTA located in the far south Atlantic around 25°S. Finding (iii) indicates the need to investigate the relationship between El Niño and SST variability in the southeast and far south Atlantic. Although the asymmetry in Atlantic SST gradient between warm and cold events shown by Giannini et al. is not robust, their insight that the cooperation between the Atlantic SST gradient and El Niño SST forcing regulates South American rainfall remains very useful. We will next investigate the ENSO-Atlantic SST gradient relationship in an initial value problem setting to determine the impact of preconditions in the Atlantic SST on the seasonal forecast for South America.

*Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964
E-mail: huei@ldeo.columbia.edu

CLIMATE VARIABILITY AND PREDICTABILITY OVER THE ATLANTIC SECTOR
Roberta Boscolo [rbos@iim.csic.es] and the CLIVAR Atlantic Panel
International CLIVAR Project Office, Southampton, UK

The Climate Variability and Predictability (CLIVAR) project of the WCRP (World Climate Research Programme) aims at describing, analyzing, modeling and predicting the variability of the global climate from seasonal to centennial timescales.
It is well known that the ocean holds the primary memory of the climate system. The wind driven circulation carries deep heat and freshwater anomalies around the ocean basins, which present a slowly changing SST field to the winter atmosphere. The thermohaline circulation, involving high latitude sinking and distributed upwelling and mixing at lower latitudes, is heavily involved in the longer time scales of climate variability. Tropical SST changes have displayed decadal trends and strong correlations with long-term rain and drought cycles as well as tropical storm frequency.
The major elements of CLIVAR study on decadal climate variability are focusing on regional processes and climate phenomena. The Atlantic region is a CLIVAR special geographical focus: large climatic variations during the ice ages have been linked to changes in the circulation of the Atlantic Ocean. The North Atlantic is the largest ocean contributor of the total northward heat transport but little is known about how it varies with time. Substantial variability has also been observed in the Tropical Atlantic region on decadal timescales. Sea Surface Temperature (SST) anomalies have been observed both north and south of the equator, along with regional changes in winds and precipitation (displacements of the Inter-Tropical Convergence Zone). Atmospheric variability in the extra-tropical Northern Hemisphere winter is dominated by the North Atlantic Oscillation (NAO). Extensive climate impacts have been documented for the NAO phases that affect mostly Europe and North America. Seasonal and longer term predictions of the NAO would have enormous socio-economic benefits however the accuracy of the interannual forecasts are not optimistic.
CLIVAR in the Atlantic sector is addressing the following scientific objectives:
• to describe and understand decadal to centennial climate variability and predictability through the analysis of observations and the modelling of the coupled climate system;
• to extend the record of decadal to centennial variability through paleoclimatic studies, data archaeology, reanalysis of atmospheric and oceanic data;
• to develop and implement appropriate observing, computing and data archiving and dissemination programmes needed to understand the mechanisms of decadal to centennial climate variability and predictability, in co-operation with other relevant climate research and observing programmes.
Further information can be found at: http://www.clivar.org/organization/atlantic/

Internal Variability of the Tropical Atlantic Ocean
Markus Jochum, 1 Massachusetts Institute of Technology
Raghu Murtugudde, Earth System Science Interdisciplinary Center
Paola Malanotte-Rizzoli, Massachusetts Institute of Technology
Antonio J. Busalacchi Earth System Science Interdisciplinary Center

A 100 year integration of an eddy resolving numerical model of the tropical Atlantic is analyzed to quantify the interannual variability caused by internal variability of ocean dynamics. It is found that, except for the spring position of
the SST maximum, the strength of internal variability in the tropical Atlantic is comparable to published mid-latitude values but is dwarfed by the strength of the seasonal cycle. During spring however, the equatorial meridional SST gradient is very weak, and internal oceanic variability causes a variability in the position of the SST maximum that is comparable to its observed variability. It is shown that these variations in the SST are due to tropical instability waves whose strength varies from year to year, even under climatological forcing. The results suggests that in winter, the predictability of the location of the tropical SST maximum is limited to the persistence time of SST anomalies which is approximately 100 days.

1Corresponding author's address:
markus@ocean.mit.edu, 617-2533573
MIT, 77 Massachusetts Avenue, 54-1410,
Cambridge, 02138 MA

The role of tropospheric temperature in the El Niño-driven surface temperature warming over the remote tropics
*John C. H. Chiang, University of California, Berkeley CA
Benjamin R. Lintner, University of California, Berkeley, CA
Adam H. Sobel, Columbia University, New York, NY

We demonstrate through atmospheric general circulation model simulations of the 97-98 El Niño that the observed ‘remote’ (i.e. outside the Pacific) tropical surface warming appearing a few months after the peak of the El Niño event is causally linked to the tropics-wide warming of the troposphere resulting from increased atmospheric heating in the Pacific, with the latter acting asa conduit for the former. Unlike surface temperature, the surface flux behavior in the remote tropics in response to El Niño is complex, with sizable spatial variation and compensation between individual flux components; this dissimilarity suggests a more fundamental control (i.e. tropospheric temperature) for the remote tropical surface warming. For the remote oceans, increasing clear-sky downwelling longwave, a consequence of the increased tropospheric temperature and associated water vapor increase, acts as the important warming influence. However, it is the latent heat flux acting through boundary layer humidity variations is the important regulator linking the surface warming in the model simulations to the tropospheric warming over the remote tropical oceans. The remote ocean surface warming seen in the models used is generally consistent with the “tropospheric temperature mechanism” previously proposed for the tropical ENSO teleconnection, with the zonal propagation of tropospheric temperature anomalies from the eastern Pacific to the remote tropics accomplished by wave dynamics and thetroposphere-to-remote surface connection mediated by moist convective processes requiring the boundary layer moist static energy to vary in sympathy with the free tropospheric moist staticenergy. Over the remote land regions, idealized model simulations suggest that sensible heat flux regulates the warming response to El Niño, though the underlying mechanism is yet unclear.

* Corresponding author 547 McCone Hall, University of California Berkeley, CA 94720-4740 (510) 642-3900 (tel), (510) 642-3370 (fax)jchiang@atmos.berkeley.eduInternal Variability of the Tropical Atlantic Ocean

31 January – 2 February 2005 Rosenstiel School of Marine and Atmospheric Science at the University of Miami

Seasonal and Interannual Variations of Cloudiness and Precipitation over Atlantic Ocean and its Adjacent Land Areas

Phillip A. Arkin1)* and Pingping Xie2)

1) ESSIC, University of Maryland 2) NOAA / NWS / Climate Prediction Center

Predicting the NAO: Role of the Stratosphere

Relationship of the tropical Atlantic to West African Rainfall in the NCEP coupled model

James A. Carton* 1, Ching-Yee Chang 1, Semyon Grodsky 1, Sumant Nigam 1, and Jiande Wang 2

Predictability of monthly precipitation in the Mediterranean Basin

Tropical Instability Waves at 0oN, 23oW in the Atlantic. A case study using the PIRATA mooring data

The Northern Hemisphere Annular Ocean Mode: Coherent Heat Transport Fluctuations Force Air-Sea Heat Exchange

THE PHYSICAL BASIS FOR PREDICTING ATLANTIC SECTOR SEASONAL-TO-INTERANNUAL CLIMATE VARIAIBILITY

subtropics to the high latitudes

Moisture fluxes over the Intra-Americas Sea

Using Tropical Atlantic Circulation to predict Sahel rainfall.

Overlooked November-December Cooling ofthe Equatorial Atlantic and its Effects on Interannual Variability and Predictability

The long-term variability of African dust transport across the Atlantic and the link to climate

A Study of Tropical Atlantic Variability Using the NCEP's Global Ocean Data Assimilation System

Variability of vertical shear within the tropical North Atlantic

CLIMATIC IMPACTS OF INTERANNUAL SURFACE CURRENT VARIATIONS IN THE EQUATORIAL ATLANTIC OCEAN

Decadal variability of shallow cells and equatorial SST in a numerical model of the Atlantic

Subthermocline Tropical Cells and Equatorial Subsurface Countercurrents

Dynamical Elements of Predicting Boreal Spring Tropical Atlantic Sea-Surface Temperatures

Abstract

tropical Atlantic 'meridional mode'?

Atlantic Variability

South America and Central-South Africa Precipitation Variability Associated to the South Atlantic Ocean

Intraseasonal oscillations in the tropical Atlantic

The Influences of the Ocean-Atmosphere Mean Climatology on the Tropical Atlantic Variability

Local and equatorial forcing of Seasonal variability of the North Equatorial Countercurrent in the Atlantic Ocean

Equatorial Atlantic interannual variability and its relation to ENSO

OGCM study of the Western Hemisphere Warm Pool

THE RELATIONSHIP BETWEEN EQUATORIAL ATLANTIC SST VARIABILITY AND EL NIÑO

Remote Impact on Tropical Atlantic Climate Variability: Statistical Assessment and Dynamic Assessment

North Atlantic decadal variability: Air-Sea Coupling, Oceanic Memory and Potential Northern Hemisphere Resonance

Lixin Wu* and Zhengyu Liu

Tropical Atlantic SST Forcing of Coupled North Atlantic Seasonal Responses

Dynamic coupling between NAO and North Atlantic SSTA tripole

Local and remote forcing of South Atlantic climate variability

Meridional heat transport in the South Atlantic.

The impact of horizontal resolution on the tropical heat budget in an Atlantic ocean model

The Response of the Labrador Sea to a Variable Atmosphere and Ocean

Three Dimensional Structure of Easterly Wave Disturbances over Africa and the Tropical North Atlantic

Simulated Global Response to a Substantial Weakening of the Atlantic Thermohaline Circulation

31 January – 2 February 2005
Rosenstiel School of Marine and Atmospheric Science at the
University of Miami

Phillip A. Arkin^1)* and Pingping Xie²⁾

**James A. Carton^{* 1}, Ching-Yee Chang ¹, Semyon Grodsky ¹, Sumant Nigam ¹, and Jiande Wang ²**

Tropical Instability Waves at 0^oN, 23^oW in the Atlantic. A case study using the PIRATA mooring data

**Lixin Wu^* and Zhengyu Liu**