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The impact of Agulhas leakage variability on the strength of the Atlantic meridional overturning circulation (AMOC) in the Community Climate System Model, version 4 (CCSM4) is investigated. In this model an advective connection exists that transports salinity anomalies from the Agulhas region into the North Atlantic on decadal (30-40 yr) time scales. However, there is no identifiable impact of Agulhas leakage on the strength of the AMOC, suggesting that the salinity variations are too weak to significantly modify the stratification in the North Atlantic. It is argued that this study is inconclusive with respect to an impact of Agulhas leakage on the AMOC. Salinity biases leave the South Atlantic and Indian Oceans too homogeneous, in particular erasing the observed salinity front in the Agulhas retroflection region. Consequently, salinity variability in the southeastern South Atlantic is found to be much weaker than observed. [ABSTRACT FROM AUTHOR]

This study analyzes important aspects of the tropical Atlantic Ocean from simulations of the fourth version of the Community Climate System Model (CCSM4): the mean sea surface temperature (SST) and wind stress, the Atlantic warm pools, the principal modes of SST variability, and the heat budget in the Benguela region. The main goal was to assess the similarities and differences between the CCSM4 simulations and observations. The results indicate that the tropical Atlantic overall is realistic in CCSM4. However, there are still significant biases in the CCSM4 Atlantic SSTs, with a colder tropical North Atlantic and a hotter tropical South Atlantic, that are related to biases in the wind stress. These are also reflected in the Atlantic warm pools in April and September, with its volume greater than in observations in April and smaller than in observations in September. The variability of SSTs in the tropical Atlantic is well represented in CCSM4. However, in the equatorial and tropical South Atlantic regions, CCSM4 has two distinct modes of variability, in contrast to observed behavior. A model heat budget analysis of the Benguela region indicates that the variability of the upper-ocean temperature is dominated by vertical advection, followed by meridional advection. [ABSTRACT FROM AUTHOR]

The role played by interocean fluxes of buoyancy in stabilizing the present-day overturning circulation of the Atlantic Ocean is examined. A 2D model of the Atlantic overturning circulation is used, in which the interocean fluxes of heat and salt (via the Bering Strait, Drake Passage, and Agulhas Leakage) are represented by sources and sinks. The profiles and amplitudes of these sources are based mainly on the heat and salt fluxes in a high-resolution ocean model (OCCAM). When applying realistic sources and sinks, a circulation is favored that is characterized by major downwelling in the Northern Hemisphere (northern sinking pole to pole circulation, NPP), and resembles the present-day Atlantic overturning circulation. The Southern Ocean sources appear to stabilize this circulation, whereas Bering Strait freshwater input tends to destabilize it. Already a small buoyancy input at southerly latitudes is enough to prohibit the existence of a southern sinking circulation (SPP), leaving the NPP circulation as a unique and stable solution. A large, factor 3 increase in Bering Strait freshwater import would be necessary to bring the SPP circulation back into existence. Especially the Indian–Atlantic transfer of heat and salt, brought about by Agulhas Leakage, contributes considerably to the strength and, in particular, the stability of the northern sinking circulation. According to this model, shutting off the Agulhas Leakage, and consequently the so-called warm water route for North Atlantic Deep Water (NADW) compensation, leads to a reduction of the overturning strength by 10% at most. These results imply that the way in which the NADW renewal takes place has implications for both the strength and stability of the Atlantic overturning circulation, giving the discussion about the warm versus cold water route for NADW compensation dynamical significance. Moreover, when the stabilizing effect of the Agulhas Leakage on the overturning disappears, the destabi... [ABSTRACT FROM AUTHOR]

Abstract: The Atlantic Meridional Overturning Circulation (AMOC) is an integral part of the circulation in the Atlantic Ocean. The AMOC is typically characterized by the meridional streamfunction, which is also an indicator of the thermohaline circulation. This paper explores the mean state and long-term variability of the Atlantic meridional streamfunction and Atlantic meridional heat transport from six multidecadal ocean reanalysis. Statistics based on the meridional overturning streamfunction and the meridional heat transport are computed in the same manner across the products, and analyzed. The maximum streamfunction values in the Atlantic are located between 35°N and 50°N for the various ocean reanalyses. The Atlantic heat transport is greater between the Greater Antilles (at about 20°N) and 25°N, similar to the observations. The streamfunction and heat transport have stronger seasonality in the deep tropics. In the North Atlantic the annual harmonic of the streamfunction has greater amplitude in July between 40°N and 50°N. Even though there are differences in the mean strength of the streamfunction and the heat transport, there are robust similarities in their seasonal cycle across ocean reanalyses. Furthermore, although the year-to-year variability of the meridional heat transport is not consistent across products, there are other aspects that are consistent. The relationship between streamfunction and meridional heat transport at 30°N (i.e., in the northern hemisphere) shows greater consistency than that between streamfunction and meridional heat transport at 30°S (i.e., in the southern hemisphere). Also, even though the North Atlantic sea surface temperature (SST) anomalies are greatly consistent, their relationship with the meridional streamfunction is disparate. However, the relationship between North Atlantic SST anomalies and meridional heat transport shows some consistency with respect to the meridional profile. The greater numbers of temperature data assimilated may help in constraining the meridional heat transport more than the streamfunction, thereby indicating the meridional heat transport to be an important field to analyze with respect to observed changes in the AMOC. [Copyright &y& Elsevier]