U.S.
CLIVAR Atlantic Science Conference U.S.
CLIVAR Atlantic Science Conference
Conference Abstracts
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 5o latitude/longitude grid over the tropics (37oS-37oN) 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 10oS to 15oN with a maximum of surface rainfall located at 5oN. 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 ~25oN-45oN 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
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:
* Northwest Research Associates, 14508 NE 20th Street, Bellevue, WA 98007. mark@nwra.com. 425-644-9660 x323.
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)
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
Semyon A. Grodsky1*, James A. Carton1, Christine Provost2, Jacques Servain3,4, Joao A. Lorenzzetti5, and Michael J. McPhaden6
1Department of Meteorology University of Maryland College Park, MD 20742, USA
2Laboratoire d'Océanographie Dynamique et de Climatologie (Paris),
3Institut de Recherche pour le Développement (IRD) - UR 065, France
4Fundação Cearense de Meteorologia e Recursos Hidricos (FUNCEME)
5National Space Research Institute (INPE), Brazil
6NOAA/Pacific Marine Environmental Laboratory
Temperature, salinity, velocity, and wind from a mooring at 00 N, 230 W are used along with satellite sea surface temperature and sea level to examine the contribution of Tropical Instability Waves (TIW) to the energy and heat balance of the equatorial Atlantic mixed layer. The TIW appear as periodic 20-30 day fluctuations of currents, temperature, and salinity, which intensify beginning in June and peak in late boreal summer. The intensification occurs in phase with strengthening of the southeasterly trade winds and the seasonal appearance of the tongue of cold mixed layer temperatures. In 2002 these waves, which warm the mixed layer by 0.350 C during summer months, are maintained by both barotropic and baroclinic conversions that are of comparable size. Salinity fluctuations, previously neglected, increase the magnitude of baroclinic energy conversion.
Kathryn A. Kelly* Applied Physics Laboratory, University of Washington Shenfu Dong Scripps Institution of Oceanography
Much of the heat transported poleward by the oceans is carried in the midlatitude western boundary currents. As these currents separate from the coast and extend eastward into the ocean interior, they flux some of their heat to the atmosphere and store some of their heat in the recirculation gyres south of the current core. An increase in the heat storage is associated with a decrease in the volume of the subtropical mode water. An analysis of upper ocean (400 m) heat content observations (1955–2001) and altimeter data shows substantial interannual variations. About 26% of the heat content variations in the North Atlantic and North Pacific (corresponding to the first principal component and with maxima in the western boundary current extension regions) are in phase and slightly lag the atmospheric Northern hemisphere Annular Mode (NAM or Arctic Oscillation). The simplest explanation, that stronger westerlies cause larger heat loss by the ocean, can be ruled out by by the sign of the correlation: strong westerlies (strong AO) are correlated with positive heat content anomalies. This conclusion is supported by previous analyses of the upper ocean heat budget, which show that the heat content anomalies are primarily caused by variations in ocean advection. The heat content anomalies, rather than being caused by changes in air-sea fluxes, instead appear to force interannual variations in those fluxes.
The (positive) Northern Annular Ocean Mode begins with a zonally coherent increase in winds, which strengthens the ocean heat advection. Increased advection results in greater heat storage in the western boundary current recirculation regions, and the resulting high heat content forces more heat loss from the ocean to the atmosphere. Ocean heat content anomalies have been shown to have predictive skill for air-sea flux anomalies up to one year in advance. To be a truly coupled mode, these heat flux anomalies would need to affect the wind field, but this has not been established. The coherence between the two oceans suggests a stronger effect on the atmosphere than would be expected from North Atlantic variations alone. This simple model of midlatitude western boundary currents affecting interannual to decadal variations in climate variability through local air-sea interaction could be missed by coupled climate models that ignore or underestimate advection by the energetic boundary currents.
*Box 355640, University of Washington, Seattle, WA 98195-5640; 206-543-9810;kkelly@apl.washington.edu
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
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)
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
*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.
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.
Joseph M. Prospero1, Isabelle Chiapello2, and Cyril Moulin3 1Rosenstiel 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.
Yan Xue*(poster)Climate Prediction Center, NCEP/NOAA, Washington D.C.
David Behringer Environmental Modeling Center, NCEP/NOAA, Washington D.C.
The new global ocean data assimilation system (GODAS) at NCEP was developed using the Geophysical Fluid Dynamics Laboratory’s Modular Ocean Model version 3 (MOM.v3) and a three-dimensional variational data assimilation scheme. Compared with the operational ODAS developed for the Pacific Ocean (referred as RA6 hereafter), the major changes include 1) an extension from the Pacific basin to the quasi-global domain for 75OS-65ON, 2) a model change from MOM.v1 to MOM.v3 that contains more vertical levels, an explicit free surface, the Gent-McWilliams mixing scheme and an improved vertical mixing scheme (KPP), 3) a forcing change from momentum flux forcing only to momentum flux, heat flux and fresh water flux forcings of the NCEP Reanalysis 2, and most importantly, 4) a data input change from temperature only to temperature and synthetic salinity that is constructed from temperature and a local T-S climatology. The temperature data includes those from XBTs, profiling floats and TAO moorings. The quality of GODAS is evaluated with independent data sets such as the sea level observations from the TOPEX/Poseidon and Jason satellite altimeters, the current data from surface drifter program and subsurface temperature data from the PIRATA array.
Diagnostic studies on Tropical Atlantic Variability (TAV) largely relate atmospheric circulation patterns and precipitation anomalies with sea surface temperature (SST) anomalies in the Atlantic sector. Both statistical and numerical models have been used to understand the underlying physics for TAV. Prediction of the Tropical Atlantic SST has proved to be very difficult, partially due to weak signal and complexity in physical processes. The multivariate EOF analysis by Ruiz-Barradas et al. (2000) suggests that the Tropical Atlantic SST is not only associated to precipitation and surface wind stress but also to subsurface ocean temperature. We will use the global ocean analysis of GODAS in 1979-2004 to search for coupled modes of atmosphere-ocean interaction in the tropical Atlantic sector and to study impacts from other regions. The optimal goal is to use the oceanic information from GODAS to improve prediction skill of SST and precipitation using statistical models.
Corresponding author address: Dr. Yan Xue, Room 605, Climate Prediction Center, NCEP/NOAA, 5200 Auth Rd, Camp Springs, MD 20746. E-mail: yan.xue@noaa.gov
African Aerosol: External Forcing or an Integral Component of the Tropical Atlantic Climate System?
Chidong Zhang RSMAS, University of Miami
Abstract
Africa is the largest source of aerosol, which include biomass burning aerosol and mineral dust. Tremendous research efforts have been made to understand the impact of aerosol on the global climate through absorption and scattering of radiation and modulation of cloud and precipitation as cloud condensation nuclei and ice nuclei. In the tropical Atlantic, which is immediately downstream of Africa, the aerosol concentration undergoes short-term (seasonal, interannual and decadal) climate variability (see presentation at this workshop by Prospero et al). There is, however, a gaping hole in our knowledge regarding the possible role of the short-term climate variability of African aerosol in the tropical Atlantic variability (TAV), which has been viewed as driven mainly by air-sea coupling. This role of aerosol is plausible because of their potential effect on cloud and precipitation in the Atlantic marine ITCZ (AMI) and the connection between the AMI and Atlantic SST. If the short-term climate variability of African aerosol is independent of the Atlantic coupled climate system, then their effect can be viewed as external forcing. However, recent studies have pointed out that African aerosol is related to African rainfall, which in turn is related to Atlantic SST. African aerosol, therefore, might be an integral component, instead of external forcing, of the tropical Atlantic climate system. In this presentation, key issues regarding these two possibilities are discussed and a proposal is introduced on how the potential role of African aerosol in the tropical Atlantic climate variability should be quantified.
1
Improved Estimates of Net Air-Sea Fluxes over The Atlantic Ocean
A. Bentamy1*, K. B. Katsaros2 , R. T. Pinker3, A. M. Mestas-Nu–ez4, L. H. Ayina1
1. Department of Oceanography from Space, IFREMER, France 2. Adjunct Professor, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, U.S.A. 3. Department of Meteorology, University of Maryland, College Park, Maryland, U.S.A. 4. Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida, U.S.A
Information on the turbulent and radiative fluxes over the Atlantic Ocean is essential for improving model simulations of climate variations and in climate process studies. A long time series of surface winds, latent and sensible heat fluxes, sho rt and long wave radiative fluxes are estimated from merging satellite data. The methodology for obtaining the fluxes uses physical properties of radar and radiometer measurements, empirical and inverse models relating satellite observations and surface parameters, and objective analysis merging various satellite estimates. A high-resolution dataset is prepared for the Atlantic Ocean, with a spatial resolution between 0.5¡ and 1¡, and temporal resolution between one day and one week. The satellite data come from the European Remote Sensing satellite scatterometer (ERS-2), NASA scatterometer Seawinds onboard QuikScat, and several defense Meteorological Satellite Program (DMSP) radiometers (Special Sensor Microwave/Imager [SSM/I] F10 – F14), Meteosat, and GOES. The reliability of the derived surface fluxes is examined and validated through comprehensive comparisons with available in-situ data. Using the resulting remotely sensed fluxes, spatial and temporal characteristics of both fluxes are investigated in the Atlantic Ocean. The results are compared to NCEP/NCAR re-analysis and to ECMWF analysis and re-analysis flux patterns. For instance, the wind large-scale characteristics from satellite and models compare well. However, significant discrepancies are found in the tropical areas, and especially between satellite and ECMWF analysis. At higher latitudes, differences in the spatial structure are found in the wind stress magnitude as well as in the Northern Hemisphere sub-polar gyres. The wind-driven response of the ocean is investigated through a simulation with the oceanic general circulation model (ORCA). The annual mean ORCA responses to the satellite winds and to surface winds from the ECMWF analyses and re-analysis are investigated in terms of mean and variability of sea surface temperature.
*
Corresponding author: Abderrahim Bentamy, Department of Oceanography from Space, IFREMER, Plouzane 29280, France, email: Abderrahim.Bentamy@ifremer.fr
Changes in temperature, salinity, oxygen and carbon across24¡N
Stuart
A. Cunningham*, Ute Schuster+, Harry L. Bryden*, Andrew J. Watson+
*Southampton Oceanography Centre, Empress Dock, Southampton,
SO14 3ZH, UK, +School of
Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K
During April-May
2004 on board RRS
Discovery we
completed a transatlantic hydrographic section across 24¡N. 125 CTD/LADCP
stations were occupied with continuous top-to-bottom profiles of temperature,
salinity, oxygen and east and north velocity components and with 24 water
samples collected through the water column and analysed for salinity, oxygen,
nitrate, phosphate, silicate, chlorofluorocarbon, total inorganic carbon,
alkalinity and organic nutrient concentrations. This section effectively
repeats transoceanic hydrographic sections across this latitude in 1957,
1981, 1992 and 1998, with carbon observations in 1992 and 1998. We present
estimates of the temporal changes in water mass properties (temperature,
salinity, oxygen and carbon) for this time series of sections.
Previously reported warming and salinification of the main thermocline from 1957 to 1992 (9-22¡C) and intermediate waters (4-9¡C) to 1992 has reversed, and these waters have cooled and freshened, so that water properties are approaching 1981 values. Mediterranean Overflow Water is responsible for the maximum variability of Intermediate waters. Upper North Atlantic Deep Waters of Labrador Sea Water origin cooled from 1957 to 1981, warmed to 1981 and have cooled abruptly through to 2004, so that they are –0.080¡C cooler and –0.015 fresher than in 1957. Lower North Atlantic Deep Water continues to cool and freshen steadily, at rates of –0.015¡C/decade and –0.004 /decade, and the 2¡C isotherm has shallowed by 333m since 1957.
Previous analysis of the 1992, and 1998 carbon data sets show a changing carbon inventory consistent with the increasing concentrations of atmospheric carbon dioxide. We will present results of our most recent section, and calculate the flux and divergence of carbon and anthropogenic carbon for the Atlantic north of 24¡N.
Corresponding author: Stuart A. Cunningham, Phone: 44-23-80596436, e-mail:
s.cunningham@soc.soton.ac.uk
Symposium: What are the trends and long time scale modes of Atlantic basin climate?
An Inside Passage for NADW(poster)
R.A. Fine1*, W.M. Smethie, Jr.2, J.D. Happell1, D.A. Le Bel2, Z. Garraffo1, R. Curry3,
T.M. Joyce3, J.M. Toole3, A.M. Macdonald3, S. Cunningham4, T.Tanhau5
1 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 2 Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 3 Woods Hole Oceanographic Institution, Woods Hole, MA 4 Southampton Oceanography Centre, Southampton, UK 5 University of Kiel, Kiel, Germany
Chlorofluorocarbon (CFC) data are used to document an inside passage that provides an alternate pathway from the Deep Western Boundary Current for transport of climate anomalies and newly ventilated deep waters from the subpolar regions into the subtropical interior. This work builds on interior recirculations that have been described in earlier studies. As part of the CLIVAR and Carbon global repeat hydrography, tracer, CO2 program, sections along 66¡W (A22) and 52¡W (A20) were occupied in October-November 2003 and 24.5¡N in April-May 2004. CFCs (CFC-11, CFC-12, CFC-113) and CCl4 were measured. Observations show increasingly higher tracer concentrations in time. In the western North Atlantic Ocean, there are cores of high CFCs coincident with Upper North Atlantic Deep Water (UNADW: ULSW and LSW, 1000-2000 m) and LNADW (Overflow waters, 3000-4500 m). The general pattern is that CFCs decrease in the equatorward direction in the DWBC, and they decrease toward the interior. Superimposed on this general pattern are interior CFC maxima. Due to the changing CFC atmospheric source function, maxima were observed in different CFC compounds during different occupations of the sections. Most prominent are the CFC maxima associated with UNADW, ~1500 m. At that level, there is a high potential vorticity ridge extending westward between about 35¡N and 25¡N, it is coincident with the Mediterranean outflow. The ridge prohibits a direct connection between the Gulf Stream Recirculation waters and the subtropical interior, as exemplified by the broad westward extending CFC minimum. Interior subtropical CFC maxima together with property-property plots, and layer thickness and velocity maps from the high resolution MICOM are used to map transport along an inside passage for export of newly ventilated water into the subtropical interior. Furthermore, there may be North Atlantic Oscillation dependent variations in the inside passage transport.
*Rana A. Fine, University of Miami, Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Causeway, Miami, FL 33149-1098; tel: 305-421-4722; fax: 305-421-4917; e-mail: rfine@rsmas.miami.edu
Atlantic Ocean Model Response to Realistic NAO-Related Atmospheric Forcing at Quasi- to Multi-decadal Periods
George Halliwell*, University of Miami
Atlantic Ocean climate variability is simulated by the HYbrid Coordinate Ocean Model (HYCOM) within a low-resolution domain with realistic topography. The model is driven by monthly surface forcing fields from January 1948 through December 2003 derived from the NCEP/NCAR and NCEP/DOE atmospheric reanalyses. HYCOM contains multiple vertical mixing submodels, and the Goddard Institute for Space Studies level 2 turbulence closure is chosen based on statistical comparisons between observed and simulated SST. The goal is to characterize the model ocean response to the NAO pattern of atmospheric forcing, emphasizing how properties of this response change as a function of time scale. All observed and simulated fields are temporally filtered into four frequency bands: interannual (IA, 2-7 yr periods), short quasi-decadal (QD1; 7-12 yr), long quasi-decadal (QD2; 12-22.5 yr) and multi-decadal (MD; >22.5 yr). The dominant atmospheric forcing structure closely resembles the NAO pattern in all four bands. As expected, ocean dynamics becomes more important relative to local thermodynamical processes with increasing time scale. The SST anomaly response in the IA, QD1, and QD2 bands has the classic North Atlantic tripole structure, with local thermodynamical forcing clearly dominating in the IA and QD1 bands. In the MD band, ocean dynamics associated with (1) meridional shifts in the subtropical/subpolar gyre boundary and (2) advection around the two gyres exerts the dominant influence on SST. Local forcing and ocean dynamics are both important in the QD2 band. In all bands, anomalous wind stress curl forcing associated with high NAO index is negative roughly from 30-55 ¡N and positive to the north. In the IA band, simulated dynamic topography responds locally in a manner consistent with northward migration of the subtropical/subpolar gyre boundary and strengthening of the subpolar gyre to the north during high NAO index. Opposite polarity is observed for low NAO index. In the QD1 and QD2 bands, the dynamic topography response pattern extends westward from the strong forcing region and peaks two years after index extrema. Anomaly propagation is slightly evident in these two bands, but is very important in the MD band. The peak MD dynamic topography response occurs 8-10 years after index extrema. While the response magnitude is building, the anomaly features north of 30 ¡N propagate around the subpolar gyre so that anomaly pattern has already reversed sign when the forcing reverses polarity. Also, subtropical mode water anomalies form across the basin, with denser water associated with high index, and then propagate southward and westward across the southern part of the gyre to enter the western boundary current. The anomalies then reinforce the anomaly sign change along the gyre boundary produced by the forcing polarity change. The timing of these simulated propagating signals suggests the possibility that coupled ocean-atmosphere dynamics is important in the MD band.
*Contact Information – Address: MPO/RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami, FL, 33149-1098; telephone: 305-421-4621; e-mail: ghalliwell@rsmas.miami.edu
An Observing System to Monitor the Atlantic Meridional Overturning Circulation and Heat Flux at 26¡N (poster)
William Johns1*, Stuart Cunningham2, Molly Baringer3, Lisa Beal1, Deb Shoosmith1, Joel Hirschi4, Johanna Baehr4, Jochem Marotzke4, and Harry Bryden2
1 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Fl USA 2 Southampton Oceanography Centre, Southampton, UK 3 NOAA Atlantic Oceanographic and Meteorological Laboratory, Miami, Fl USA 4 Max Planck Institute for Meteorology, Hamburg, Germany
During February-March 2004 an array of moored instruments was deployed to measure temperature, salinity and velocity profiles at 22 locations across the Atlantic Ocean at 26¡N. This is the beginning of a 4-year pilot monitoring effort to measure the strength and variability of the Atlantic meridional overturning circulation and heat flux across 26¼N under the joint UK/US funded RAPID/MOCHA program. Deployment of the array in two ocean circulation models (OCCAM and FLAME) has demonstrated that the planned array measurements can be used to accurately reproduce the variability in overturning circulation and heat transport within the models. Moorings are concentrated on the western side of the 26¡N section to measure the deep western boundary currents, on the eastern side of the section to measure eastern boundary currents, and on either side of the Mid Atlantic Ridge to separate the contributions to the overturning circulation from the eastern and western basins. Top-to-bottom profiles of temperature and salinity at the edges of the basin from 10 tall moorings will be used to measure the temporal variability in the basin-wide geostrophic velocity profile. The overall observing system relies on ongoing measurements of the Gulf Stream transport through Florida Straits by submarine electromagnetic cable and shipboard observations, quarterly trans-basin XBT sections, and continuous estimates of surface Ekman transport derived from operational wind products. The moored array is due to be recovered and redeployed in May 2005, at which time we will begin transmitting data from several key moorings to test a real-time observing system.
* Corresponding Author
William Johns Rosenstiel School of Marine and Atmospheric Science Division of Meteorology and Physical Oceanography 4600 Rickenbacker Causeway Miami, Florida 33149-1098, USA Phone: 305 361 4054 Fax: 305 361 4696 Email: wjohns@rsmas.miami.edu
ÒThe Busy 2004 Atlantic Hurricane Season: Was it an Anomaly, Trend or Cycle?Ó
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.
The Spatial Pattern of North Atlantic Warming over the Past Fifty Years M. Susan Lozier* and Nathan J. Moore, Duke University Susan Leadbetter, Ric Williams and Vassil Roussenov, University of Liverpool
Recent evidence that the worldÕs oceans have warmed over the past fifty years, and that the attendant increase in the oceanÕs heat content is an order of magnitude larger than the increase in the atmospheric and cryospheric heat content, has made it abundantly clear that a determination of how our global climate is changing in response to long-term natural and/or anthropogenic forcing depends on the effectiveness of the ocean as a heat reservoir. To facilitate predictions of future oceanic heat uptake, it is important to assess the spatial pattern of the warming to discriminate between competing mechanisms and to provide validation for climate modeling studies. To establish the spatial pattern of warming in the North Atlantic we chose to study volume changes of fixed temperature classes. Following on a study over thirty years ago, we created a North Atlantic volume census for temperature ÒclassesÓ that stretch from the sea surface to the ocean floor using historical hydrographic station data (pressure, salinity and temperature) from the National Oceanic Data Center World Ocean Database 1998 and the WOCE Hydrographic Program for the years 1950 to 2000. We show that the heat content gain in the North Atlantic is marked by considerable spatial variability: while the subtropics have warmed over the past fifty years, the subpolar ocean has cooled. To investigate the mechanisms responsible for these heat content changes, a modeling study was conducted using the Miami Coordinate Ocean Model (MICOM). A comparison of model runs using NCEP/NCAR fields from 1950-1969 and from 1980-1999, and also from NAO+ and NAO- years, illuminates the crucial role played by the NAO phases in ocean heating throughout the basin. These results raise the question as to the relative role of anthropogenic forcing and decadal variability in warming the North Atlantic.
*Corresponding Author:
M. Susan Lozier Box 90230, Earth and Ocean Sciences Duke University Durham, NC 27708-0230 Ph: 919.681.8199 Fax: 919.684.5833 Email: mslozier@duke.edu
Interannual Variability of the North Atlantic and GIN Seas from
two High-Resolution Ocean Models
*Julie McClean1, Steven Piacsek2, Carmyn Priewe3, and Detelina Ivanova1
1Naval Postgraduate School, Monterey California, USA 2Moss Landing Marine Laboratory, Moss Landing, California, USA 3Naval Research Laboratory, Stennis Space Center, Mississippi, USA
Two realistic ocean simulations are used to study interannual variability in the North Atlantic and that of Atlantic water entering the Greenland-Iceland-Norwegian (GIN) Seas. An eddy-resolving (0.1¡, 40-level) North Atlantic (20¡S-72¡N) basin configuration of the Parallel Ocean Program (POP) is used to study the former variability while a global eddy-permitting (1/3¡, 32level) POP simulation is analyzed to understand the latter. Both models were forced with realistic atmospheric fluxes for at least two decades starting in 1979 following appropriate spin-up periods.
In the North Atlantic basin simulation differences are sought in the modeled ocean state in the years prior to and after the switch in phase of the North Atlantic Oscillation (NAO) index in 1995/1996. First we assess the ability of the model to reproduce the mean surface ocean state during these two phases by comparing model and drifter data statistics; particularly a James test is used to identify regions where the model mean flow is statistically different to the observed. A James test is also used to identify statistically different model flow regimes for the two periods. Differences in the three-dimensional model ocean state are then sought by examining potential vorticity (PV) on isopycnal surfaces during the two phases to understand changes in water mass formation (Labrador Sea and 18¡Water) as represented by the model. Using the global model, changes in the transports and thermohaline structure of the GIN Sea from the 1980s to the 1990s are discussed. Biennial and interannual variability of the North Atlantic layer are examined using extended empirical orthogonal functions and their relationship to the NAO explored.
*Department of Oceanography (OC/Mn). Naval Postgraduate School, Monterey CA 93943. (831) 656-2437 mcclean@nps.edu
The Florida Current: Long term measurements of the transport of the upper limb of the Meridional Overturning Cell
Christopher S. Meinen� - Cooperative Institute for Marine and Atmospheric Studies, University of Miami
Molly O. Baringer - NOAA/Atlantic Oceanographic and Meteorological Laboratory
Deb Shoosmith - Rosenstiel School of Marine and Atmospheric Science, University of Miami
Rigoberto F. Garcia - Cooperative Institute for Marine and Atmospheric Studies, University of Miami
Abstract
The Florida Current carries roughly 30 Sv (1 Sv = 106 ms−1 ) of warm water to the north through the Florida Straits. This ßow carries both the western boundary ßow for the Sverdrup wind-driven subtropical gyre as well as the northward ßowing upper limb of the Meridional Overturning Cell (MOC). Since the early 1980s the transport of the Florida Current near 27�N has been monitored through both repeat section observations and via nearly-continuous observations from a submarine cable spanning the Straits. This long time series of observations, now exceeding twenty years, represents one of the most complete data sets for the study of the MOC in the Atlantic Ocean. Utilizing the full time series of cable and section data, long term trends in the transport of the Florida Current will be discussed in the context of other indices and measures of the MOC.
�Corresponding Author:
Dr. Christopher Meinen NOAA/AOML/PHOD 4301 Rickenbacker Causeway Miami, FL 33149, USA Office phone: 305-361-4355 Office FAX: 305-361-4412 Email: Christopher.Meinen@noaa.gov
Equatorial Atlantic SST Variability and its Relationship to ENSO and Precipitation
Matthias Munnich� ,
J. David Neelin
Inst. for Geophysics and Planetary Physics and Dept. of Atmos. and Oceanic Sciences University of California, Los Angeles
Rank correlation maps of sea-surface temperature anomalies (SSTA) in the equatorial Atlantic with precipitation anomalies indicate a strong relationship of precipitation at the margin of the Atlantic ITCZ to equatorial Atlantic SSTA in all seasons. In May (Figure), the region of signiÞcant rain correlation with equatorial SSTA reaches into equatorial South America, during July and August Upper Guinea.
Using both models and observations we provide evidence for active involvement of equatorial ocean dynamics in equatorial SSTA variability, with antecedent equatorial wind anomalies in the western Atlantic the likely forcing. A special role for the equatorial region appears to hold despite the strong correlation of equatorial SSTA with SSTA in the southern tropical Atlantic originating from local trade wind variations. In May-June the correlation to ENSO indices is strong enough to raise the question of teleconnection link involving western equatorial Atlantic winds.
Further results on El Nino signals in tropical Atlantic SSTs
Cecile Penland and Ludmila Matrosova NOAA-CIRES/Climate Diagnostics Center, Boulder, CO 80305-3328.
A dynamically-based filter is
used to separate tropical sea surface temperatures (SSTs) into three components:
the evolving El Nino signal, the global tropical trend, and the background.
An intriguing result of separating the signal this way is to identify the
south tropical Atlantic as an important part of the El Nino phenomenon.
In fact, on the seasonal timescale, the combination of El Nino and a large-scale
trend account for more than half of the variance of both the equatorial
and south tropical Atlantic sea surface temperature anomalies. In this
presentation, we show that the Atlantic contribution to the El Nino signal
is highly significant (correlation = - 0.6) and leads the Nino 3.4 signal
by about 9 months.
Climate and Hydrologic Conditions over Northernmost South America on Interannual to Orbital Time-Scales as Recorded in Cariaco Basin Sediments
Larry C. Peterson*, Rosenstiel School of Marine and Atmospheric Science, University of
Miami, Miami, FL 33149 Gerald H. Haug, GeoForschungsZentrum-Potsdam, Potsdam D-14473, Germany David E. Black, Dept. of Geology, University of Akron, Akron, OH 44325 David W. Lea, Geological Sciences, UC Santa Barbara, Santa Barbara, CA 93106 Konrad A. Hughen, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 Richard W. Murray, Earth Sciences, Boston University, Boston, MA 02215 Robert C. Thunell, Geological Sciences, University of South Carolina, Columbia, SC 29208
Beyond the reach of instrumental time-series, paleoclimate records provide the only means to reconstruct and assess trends and long time scale modes of climate variability. In the northern tropical Atlantic, a detailed history of past conditions that can be related to the Atlantic Intertropical Convergence Zone (ITCZ) is recorded in sediments of the Cariaco Basin, a modern anoxic marine basin located on the northern continental shelf of Venezuela. Here, a strong seasonal contrast in the input of materials to the basin, high sedimentation rates (50 to >150 cm/kyrs), and the lack of bioturbational mixing combine to produce an annually-laminated sediment sequence that preserves information on time-scales ranging from interannual to orbital. Today, the Cariaco Basin experiences two distinct seasons that reflect the annual migration of the Atlantic ITCZ. Between January-March, when the ITCZ lies south of the equator, northeasterly trade winds sit directly over Cariaco Basin and strong coastal upwelling and dry conditions dominate. Beginning in June-July, as the ITCZ moves north, local rainfall reaches a maximum and the upwelling diminishes or disappears. This presentation will summarize paleoclimatic data from Cariaco Basin sediments, as well as from other regional locations, which suggest a coherent climatologic response in the tropical Atlantic triggered by a pattern of ITCZ migration that mimics the seasonal cycle. During periods of cooler North Atlantic SSTs, on time-scales ranging from the Little Ice Age to the Younger Dryas to the cold stadials of the last glacial period, decreased delivery of terrigenous sediment to Cariaco Basin from local rivers indicates a southward shift in the mean latitudinal position of the ITCZ. During warm interstadials and periods of Holocene and deglacial warmth, northward shifts in ITCZ position and its belt of convective rainfall are inferred from increased riverine delivery to the basin. Whether these rapid shifts in ITCZ position reflect a response to forcing originating in the high latitude Atlantic or to forcing potentially sourced in the tropics is a key question that requires additional paleoclimate record coverage and insights from modeling.
Contact information: Larry C. Peterson, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149; Phone: 305-361-4692; Fax: 305-361-4632; lpeterson@rsmas.miami.edu
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
Anantha Aiyyer1 and 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
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).
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
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.
By
Ping Chang Texas A&M University College Station, TX 77843
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
* 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
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 pert