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Marcelo Barreiro, Ping Chang, and R. Saravanan

Abstract

Interannual and decadal variability of the South Atlantic convergence zone (SACZ) during austral summer [season January–February–March (JFM)] is investigated. An attempt is made to separate the forced variability from the internal variability. This is accomplished by applying a signal-to-noise optimization procedure to an ensemble of multidecadal integrations of the latest version of the NCAR Community Climate Model (CCM3) forced with observed SST. The result yields two dominant forced atmospheric responses: a local response to Atlantic SST anomalies with interannual-decadal timescales and a remote response to Pacific SST anomalies at interannual timescales. The former is localized within the South Atlantic Ocean with almost no signal over land, consisting of a dipolelike structure in precipitation close to the coast of South America accompanied by a clockwise anomalous circulation of surface winds. The latter manifests itself mainly in the upper-level circulation, consisting of a northeastward shift of the SACZ with associated rainfall anomalies during warm ENSO events.

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Marcelo Barreiro, Annalisa Cherchi, and Simona Masina

Abstract

Using an atmospheric general circulation model coupled to a slab ocean, the effects of ocean heat transport (OHT) on climate are studied by prescribing OHT from 0 to 2 times the present-day values. In agreement with previous studies, an increase in OHT from zero to present-day conditions warms the climate by decreasing the albedo due to reduced sea ice extent and marine stratus cloud cover and by increasing the greenhouse effect through a moistening of the atmosphere. However, when the OHT is further increased, the solution becomes highly dependent on a positive radiative feedback between tropical low clouds and sea surface temperature. The strength of the low cloud–SST feedback combined with the model design may produce solutions that are globally colder than in the control run, mainly due to an unrealistically strong equatorial cooling. Excluding those cases, results indicate that the climate warms only if the OHT increase does not exceed more than 10% of the present-day value in the case of a strong cloud–SST feedback and more than 25% when this feedback is weak. Larger OHT increases lead to a cold state where low clouds cover most of the deep tropics, increasing the tropical albedo and drying the atmosphere. This suggests that the present-day climate is close to a state where the OHT maximizes its warming effects on climate and raises doubts about the possibility that greater OHT in the past may have induced significantly warmer climates than that of today.

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Verónica Martín-Gómez, Emilio Hernández-Garcia, Marcelo Barreiro, and Cristóbal López

Abstract

Sea surface temperature (SST) anomalies over the tropical oceans are able to generate extratropical atmospheric circulation anomalies that can induce rainfall variability and changes in the sources of moisture. The work reported here evaluates the interdecadal changes in the moisture sources for southeastern South America (SESA) during austral summer, and it is divided into two complementary parts. In the first part the authors construct a climate network to detect synchronization periods among the tropical oceans and the precipitation over SESA. Afterward, taking into account these results, the authors select two periods with different degrees of synchronization to compare the spatial distribution of the SESA moisture sources.

Results show that during the last century there were three synchronization periods among the tropical oceans and the precipitation over SESA (during the 1930s, 1970s, and 1990s) and suggest that the main moisture sources of SESA are the recycling over the region, the central-eastern shore of Brazil together with the surrounding Atlantic Ocean, and the southwestern South Atlantic surrounding the SESA domain. Comparison of SESA moisture sources for the 1980s (a period of nonsignificant synchronization) and the 1990s (a synchronized period) shows that the principal differences are in the intensity of the recycling and in the strength of the central-eastern shore of Brazil. Moreover, the authors find that a region centered at (20°S, 300°E) is a moisture source for SESA only during the 1990s. These differences can be associated with the development of a low-level anticyclonic (cyclonic) anomaly circulation over central-eastern Brazil that favors the transport of moisture from central Brazil (central-eastern shore of Brazil) toward SESA in the 1990s (1980s).

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Alexey Fedorov, Marcelo Barreiro, Giulio Boccaletti, Ronald Pacanowski, and S. George Philander

Abstract

The impacts of a freshening of surface waters in high latitudes on the deep, slow, thermohaline circulation have received enormous attention, especially the possibility of a shutdown in the meridional overturning that involves sinking of surface waters in the northern Atlantic Ocean. A recent study by Fedorov et al. has drawn attention to the effects of a freshening on the other main component of the oceanic circulation—the swift, shallow, wind-driven circulation that varies on decadal time scales and is closely associated with the ventilated thermocline. That circulation too involves meridional overturning, but its variations and critical transitions affect mainly the Tropics. A surface freshening in mid- to high latitudes can deepen the equatorial thermocline to such a degree that temperatures along the equator become as warm in the eastern part of the basin as they are in the west, the tropical zonal sea surface temperature gradient virtually disappears, and permanently warm conditions prevail in the Tropics. In a model that has both the wind-driven and thermohaline components of the circulation, which factors determine the relative effects of a freshening on the two components and its impact on climate? Studies with an idealized ocean general circulation model find that vertical diffusivity is one of the critical parameters that affect the relative strength of the two circulation components and hence their response to a freshening. The spatial structure of the freshening and imposed meridional temperature gradients are other important factors.

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Anand Gnanadesikan, Keith W. Dixon, Stephen M. Griffies, V. Balaji, Marcelo Barreiro, J. Anthony Beesley, William F. Cooke, Thomas L. Delworth, Rudiger Gerdes, Matthew J. Harrison, Isaac M. Held, William J. Hurlin, Hyun-Chul Lee, Zhi Liang, Giang Nong, Ronald C. Pacanowski, Anthony Rosati, Joellen Russell, Bonita L. Samuels, Qian Song, Michael J. Spelman, Ronald J. Stouffer, Colm O. Sweeney, Gabriel Vecchi, Michael Winton, Andrew T. Wittenberg, Fanrong Zeng, Rong Zhang, and John P. Dunne

Abstract

The current generation of coupled climate models run at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Climate Change Science Program contains ocean components that differ in almost every respect from those contained in previous generations of GFDL climate models. This paper summarizes the new physical features of the models and examines the simulations that they produce. Of the two new coupled climate model versions 2.1 (CM2.1) and 2.0 (CM2.0), the CM2.1 model represents a major improvement over CM2.0 in most of the major oceanic features examined, with strikingly lower drifts in hydrographic fields such as temperature and salinity, more realistic ventilation of the deep ocean, and currents that are closer to their observed values. Regional analysis of the differences between the models highlights the importance of wind stress in determining the circulation, particularly in the Southern Ocean. At present, major errors in both models are associated with Northern Hemisphere Mode Waters and outflows from overflows, particularly the Mediterranean Sea and Red Sea.

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