U.S. CLIVAR

Atlantic Meridional Overturning Circulation

   

 

  • What is the Atlantic Meridional Overturning Circulation?

The Atlantic Meridional Overturning Circulation (hereafter AMOC) is generally described as the large-scale (on the order of 1000 km), low-frequency (interannual to multi-decadal), full-depth, meridional flux of mass, heat and freshwater, and other relevant material properties in the Atlantic Ocean. The AMOC transports these from the mid-depth and upper waters at the southern boundary of the South Atlantic into the northern North Atlantic and beyond into the Arctic Ocean. In these norther polar and subpolar regions, surface waters can become sufficiently cold and salty to sink and flow southward. As such, the AMOC is not a directly observable feature (such an ocean current); rather it is an integrated four-dimensional perspective of the circulation and characteristics of the Atlantic Ocean. Thus one must consider the entire water column (several thousands of meters deep), and the entire Atlantic Ocean, from the Southern Oceans to the Arctic regions in the North to gain a complete picture of the AMOC, and its variability.

Estimates of the net volume transport of the AMOC are generally 17 Sverdrups (106 m^3/s) at 24-deg N (e.g., Bryden and Imawaki, 2001; Ganachaud and Wunsch, 2000). The AMOC transports just over 1 PW of heat northward.

Meridional overturning circulation (MOC) in the North Atlantic. Volume fluxes in Sverdrups (106 m^3/s), obtained by integrating zonally across the Atlantic basin in a general circulation model constrained to observations. The northward near-surface flow includes the Gulf Stream and other dominantly wind-controlled elements. Regions of downward motion near 30°N and 60°N are associated with strong heat losses to the atmosphere. The subsequent flows are, however, determined largely by the global wind distribution. [from Wunsch 2002]

 

  • Why is the AMOC important?

The AMOC is thought to play an important role in maintaining the observed meridional ocean temperature structure in the Atlantic and therefore, if perturbed, the consequences to climate, particularly in the North Atlantic and for the continents surrounding the North Atlantic, could be significant. The AMOC influences much of the Northern Hemisphere (e.g. brings relatively mild climate in Europe) including the tropics. Large and rapid temperature changes over Greenland and Iceland are thought to be related to large changes in the AMOC (Broecker et al, 1992).

A direct attribution of changes in societally-relevant physical and biogeochemical conditions to AMOC variability is difficult, primarily because the description and understanding of the AMOC and its variability are in a nascent stage.  However, it is possible, and as some evidence now suggests, that AMOC variability may bring about a range of impacts. For example, global model simulations show that under normal conditions the AMOC exhibits multi-decadal oscillations in the range of 10-20% of its mean strength.  Coupled climate models indicate that an intensification (weakening) of the AMOC on these time scales results in a warming (cooling) of the upper North Atlantic and for the land regions of eastern North America and western Europe. Moreover, such anomalously warming of the North Atlantic SST impacts can lead to increased probability of droughts in North America; creates a northward shift in the position of the ITCZ; and tends to reduce the vertical wind shear in the northern tropical Atlantic troposphere, which can lead to an increase in the number and intensity of tropical Atlantic hurricanes. The AMOC response to, and role in, anthropogenic climate change is of concern because of its perceived link to past rapid climate change:  AMOC has been invoked to explain rapid changes in the climate during the Pleistocene and the termination of the last glaciation, with less dramatic changes during the Holocene (see the National Academies, Ocean Science Board, 2002 publication: “Abrupt Climate Change: Inevitable Surprises”).  Modeling studies of the more modern era suggest that the AMOC may be sensitive to changes in surface conditions brought about through anthropogenic climate changes (e.g. Gregory et al. 2005). A weakening (or reduction in heat transport) of the AMOC is anticipated as a result of these conditions. Moreover, under such a scenario of increased freshwater into the AMOC, models suggest this could lead to a cooling in Europe (Stouffer et. al., 2006).

A reduction in AMOC strength is likely to bring about less heat transported northward into the Arctic regions, and hence one would expect overall cooling and an expansion of sea ice coverage; however, modeling and observations are inconclusive on this matter mainly because of lack/sparseness of observations particularly for wintertime. The most pronounced impacts of AMOC variations are likely to occur in the seasonally varying ice covered seas (e.g. Baffin Bay/Labrador Sea, GIN Seas, Barents-KaraSeas).

Changes of AMOC may also influence global mean sea level in two ways: (i) by influencing the rate of melting (or accretion) of land-based ice, and (ii) by affecting the uptake of heat and consequent thermal expansion by the ocean in the presence of a warming climate. Regional sea level changes may also be expected due to changes in ocean circulation and atmospheric effects.

The impact of changes in the AMOC are not necessarily limited to the physical climate system.  The anthropogenic Carbon accumulating in the North Atlantic Ocean during the past 20 years is due to localized ocean uptake from the atmosphere (50%) with the rest being due to the northward advective transport of Carbon by the AMOC (Quay et al, 2007). Not only will changes in the AMOC circulation impact the Carbon budget, but ocean processes connected to AMOC such as mixing, and water mass formation may likely influence ocean Carbon as well.

Marine ecosystems are also likely to be impacted through changes of ocean circulation, stratification, and changing local ocean conditions (e.g. temperatures, salinity, nutrient availability).

Figure 1 (Figure 8 from Vikebo et al, 2007) Simulated distribution of pelagic juvenile cod late June, 204 months old, using a) control run and b) a run with a THC reduction of 35%. The color scale indicates wet weight in milligrams.

  • Is the AMOC changing and can future changes be predicted?

The AMOC involves a very large and complex ocean circulation system and multiple environmental variables (e.g. temperatures, salinity, currents). Many processes involving the ocean, atmosphere (changes in winds, precipitation, and heat entering the ocean) as well as adjacent land and ice regions (e.g. melting of ice sheets, runoff from rivers and land masses) are believed to be involved in changes of the AMOC. This complexity makes it very difficult to observe how, where, and even if the AMOC is changing.

In theory the AMOC is observable; however, because the AMOC extends very deep and has numerous circulation pathways (see Lumpkin figure below) and currents (e.g. the Gulf Stream), it is difficult to measure with confidence. The ongoing Rapid Climate Change Program (RAPID) (http://www.noc.soton.ac.uk/rapid/rapid.php) funded largely by the UK, with contributions from the US (and to a lesser extent Norway and the Netherlands), deployed a mooring array across the North Atlantic near 26°N and some monitoring capability in the subpolar North Atlantic (e.g., the WAVE array near the western boundary). These observations, although helpful, are not adequate for monitoring the meridional transport of mass, heat, and freshwater across high-latitudes. Moreover, there is no corresponding measurement system is in place in the South Atlantic to monitor the lower part of the AMOC that connects the Southern Ocean with the upper part of the MOC in the North Atlantic. However, several global observation systems are giving us more information from which we can infer information about the entire AMOC system. The Argo system (link) provides temperature and salinity information down to approximately 2000m, and satellites tell us about the upper ocean conditions (e.g. SST, SSH, vector winds, air-sea fluxes), but such information has not yet been mined for clues on variability of the AMOC. Relatively infrequent research cruises over the past 30 years are providing some information on variability, especially with regards to water properties. Paleoclimate data indicate in the past the AMOC has changed dramatically.

Observational evidence suggests parts of the AMOC are changing. Some studies based on in-situ data indicate that the sub-polar N. Atlantic is freshening over the past four decades (Dickson et al. (2002), Curry et al. (2003)). An analysis of satellite data shows that the subpolar gyre surface circulation for 1992-2003 may be slowing (Hakkinen and Rhines, 2004). An observational study (Bryden et. al., 2005) of five hydrographic cruises over the past 50 years provides an estimate of the envelope of AMOC variability. It also indicated a notable (30%) slowing of the AMOC, however later studies (see Latif et al., 2006; Searl et. al., 2007) now suggest these observed changes may reflect shorter-term variability not resolved by the limited number of hydrographic cruises. Thus in summary, we have evidence of some interesting changes in parts of the AMOC, but we simply don’t have enough information to assemble a complete and coherent picture of its current state, how it has changed, and if these changes are significant.

There are strong decadal to multidecadal variations of climate in the Atlantic region. Modeling studies suggest part of this variability (that related to the Atlantic ocean AMOC circulation) may be potentially predictable (Latif et.al., 2006a), but more work is required to scope the bounds and uncertainties of this predictability.

Does the MOC have something to do with rapid climate change? (Coming soon...)

 

 
 

Broecker W. S., G. Bond, M. Klas, E. Clark, and J. McManus, 1992: Origin of the northern Atlantic’s Heinrich events. Climate Dyn., 6, 265–273.

Bryden H.L., Longworth H.R. & Cunningham S.A., 2005: Slowing of the Atlantic Meridional Overturning Circulation at 25N, Nature, 438, 655-657

Ganachaud, A. and C. Wunsch, 2000: Improved estimates of global ocean circulation, heat transport and mixing from hydrographic data, Nature, 408, 453-457.

Gregory, J. M., and Coauthors, 2005: A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO2 concentration. Geophys. Res. Lett., 32, L12703, doi:10.1029/2005GL023209.

Hakkinen, S. and P. B. Rhines, 2004: Decline of the North Atlantic subpolar circulation in the 1990s. Science, 304, 555-559.

Knight, J. T., R. J. Allan, C. K. Folland, M. Vellinga, and M. E. Mann, 2005: A signature of persistent natural thermohaline circulation cycles in observed climate, Geophys. Res. Lett., 32, L20708, doi:10.1029/2005GL024233.

Latif, M., Boning, C., Willebrand, J., Biastoch, A., Dengg, J., Keenlyside, N., Schweckendiek, U. and Madec, G., 2006: Is the thermohaline circulation changing? J. Clim., 19, 4631-4637.

Latif, M., Collins, M., Pohlmann, H. & Keenlyside, N., 2006a: A review of predictability studies of the Atlantic sector climate on decadal time scales. J. Climate 19, 5971-5987.

Quay, P., R. Sonnerup, J. Stutsman, J. Maurer, A. Körtzinger, X. A. Padin, and C. Robinson, 2007: Anthropogenic CO2 accumulation rates in the North Atlantic Ocean from changes in the 13C/12C of dissolved inorganic carbon, Global Biogeochem. Cycles, 21, GB1009, doi:10.1029/2006GB002761.

Searl, Y., H. T. Banks, S. Stark, and R. A. Wood (2007), Slowing of the Atlantic meridional overturning circulation: A climate model perspective, Geophys. Res. Lett., 34, L03610, doi:10.1029/2006GL028504.

Stouffer, R. J., et al., 2006: Investigating the causes of the response of the thermohaline circulation to past and future climate changes, J. Clim.,19, 1365-1387.

Vikebo FB, Sundby S, Adlandsvik B, Ottera OH, 2007: Impacts of a reduced thermohaline circulation on transport and growth of larvae and pelagic juveniles of Arcto-Norwegian cod (Gadus morhua), Fisheries Oceanography, 16, 216-228.

Wunsch, C., 2002: What Is the Thermohaline Circulation?, Science, 298, 1179-1180.