The Role Of Wind Stress And Freshwater Flux Over The Southern Ocean In Global Ocean-Atmosphere Coupled System

Wind stress and freshwater work as two important forcings over the Southern Ocean (SO). Wind stress directly drives the most powerful circulation among global oceans, i. e., the Antarctic Circumpolar Current (ACC). Freshwater flux over the SO, which once led to the melt-water-pulse events during the last deglaciation, is the important product of Antarctic ice-melting induced by global warming. However, so far, there have been few comprehensive and systematic researches discussing the climatic impact of wind stress and freshwater flux over the SO.In this paper, we focus on the role of SO wind stress and freshwater flux in global climate system as a key scientific issue. A series of sensitivity experiments, combined with the "modeling surgery" strategy based on the "Partial-Coupling" and "Partial-Blocking" schemes are carried out in the fully coupled ocean-atmosphere model, FOAM1.5 (Fast Ocean-Atmosphere Model, version 1.5), to investigate the influence of wind stress and freshwater flux over the SO on global ocean-atmosphere coupled system.In order to explore the climatic impact of SO wind stress, we remove the wind stress south of 40°S in the coupled model. Experimental results demonstrate that the disappearance of wind stress weakens both local vertical diapycnal mixing and the upwelling south of ACC, leading to the SO covered by large cold SST anomalies and equatorward expansion of sea ice. Due to the strengthened oceanic stratification, cooling in the surface layer triggers significant warming in the subsurface layer. Meanwhile, cold SST anomalies induce the quasi-barotropic atmospheric "trough" response in the troposphere, coupled by the acceleration of westerly wind. The enhancement of westerly wind further intensifies local SST cooling, and forms local positive feedback. Our modeling results suggest a strong control of southern high-latitude wind stress on the Atlantic Meridional Overturning Circulation (AMOC). When surface wind stress is turned off, the pumping effect of wind stress on deep water is also eliminated, which leads to the Antarctic Meridional Overturning Circulation (AnMOC) almost disappears. Meanwhile, the transport of AMOC is also reduced by 50%. The weakened AMOC drives less cross-equator heat transport, which helps to generate anomalous warming in the South Atlantic. However, the Wind-Evaporation-SST (WES) feedback in the subtropical South Atlantic works to transmit the southern mid-latitude cold anomalies to tropics, which inhibits warm anomalies in the South Atlantic, and in this way, the bi-polar SST seesaw induced by weakened AMOC can not be formatted.To investigate local and remote impacts of freshwater forcing over the SO, 1.0Sv freshwater flux is uniformly imposed over the Antarctic Ocean (south of 60°S). The modeling results demonstrate that surface freshwater perturbation enhances local stratification, inhibiting deep convection near the Antarctic, vertical diapycnal mixing in the SO and upwelling south of ACC, and leads to baroclinic response in local ocean with anomalous cooling and warming prevails in the upper layer and subsurface layer, respectively. Meanwhile, an intensification of westerly wind is also detected. The high-latitude cooling can be conveyed to the ACC region by the joint effect of upper-ocean advective process and atmospheric process. Subsequently, southern mid-latitude cold anomalies can be further transmitted to the tropics by the upper ocean-lower atmosphere coupled relay teleconnection mechanism. That is, WES feedback triggers southern subtropical cooling, and accelerates Subtropical-Tropical Cell (STC) in the Southern Hemisphere. The combined impact of intensified STC and mean subduction causes the tropical ocean displays anomalous cooling.Freshwater forcing over the SO can also remotely influence the climate of the Northern Hemisphere. In the initial several decades, northern extratropical region shows anomalous cooling due to the atmospheric teleconnection. As the Antarctic subsurface warming propagates northward and comes up to the surface due to vertical diapycnal mixing process, the initial cooling in the Northern Hemisphere is gradually replaced by anomalous warming. The cold and warm anomalies in the Southern and Northern Hemisphere generates a significant bi-polar SST seesaw.Surface freshwater perturbation over the SO strengthens oceanic stratification, which weakens the formation of Antarctic Bottom Water, and the competition between Antarctic Bottom Water and North Atlantic Deep Water leads to the enhancement of AMOC in the initial several decades. As the fresh anomalies from the SO spread to the North Atlantic with upper-ocean advection, the transport of AMOC gradually decreases.The shift of modeling climatology induces important change of climate variability. Under the SO freshwater forcing scenario, the amplitude of ENSO is significantly intensified, and meanwhile, the frequency of ENSO shifts towards the low-frequency range. The enhancement of ENSO can be attributed to the sharper zonal-tilt of tropical thermocline, and the general uplift of entire tropical thermocline also contributes to intensify ENSO. The period of ENSO is enlarged primarily because the upper-ocean meridional temperature gradient between equatorial and off-equatorial region is decreased, which is favorable to extend the period of ENSO. Besides, two types of El Nino, i. e., the eastern Pacific El Nino and center Pacific El Nino display different response to freshening over the Antarctic Ocean. Generally speaking, freshwater perturbation can significantly influence eastern Pacific El Nino but has little effect on center Pacific El Nino.In addition, local freshwater forcing triggers distinct change of the Southern Annular Mode (SAM). The interannual variability of SAM is largely weakened, but the interdecadal variability is amplified. The change of SAM is similar at different vertical layers.