Global Teleconnections in Response to Freshening over the Antarctic Ocean

Abstract

In this paper, coupled ocean?atmosphere responses to freshening over the Antarctic Ocean are investigated in a fully coupled model with a series of sensitivity experiments. In the model, 1.0 Sv (1 Sv ≡ 106 m3 s?1) of freshwater flux is uniformly imposed over the Antarctic Ocean for 400 yr, while the ocean and atmosphere remain fully coupled both locally and elsewhere. The model explicitly demonstrates that a freshening of the Antarctic Ocean can induce a significant local cooling coupled with an intensification of the westerly winds and expansion of sea ice. Furthermore, the cooling can extend to the entire southern extratropical and tropical oceans coupled with an intensification of southeasterly trades and the equatorial trade winds. Some modest warm anomalies also occur in the northern extratropical oceans, forming a sharp interhemispheric SST contrast. A series of sensitivity experiments are conducted to understand the mechanisms responsible for transmitting the southern high latitude cooling to the tropics and the Northern Hemisphere. Experimental results demonstrate the important role of the surface coupled wind?evaporation?SST feedback and in turn changes of the subtropical?tropical meridional overturning circulation in conveying the southern high-latitude temperature anomalies to the tropics. The interhemispheric seesaw originates from the tropical?northern extratropical atmospheric teleconnection and is sustained by the subductive process of Antarctic subsurface warming. The Atlantic meridional overturning circulation is intensified in the first few decades of the freshwater forcing over the Antarctic Ocean because of a shutdown of the Antarctic deep convection, but it subsequently decreases because of the spreading of the fresh anomalies from the Southern Ocean to the Northern Ocean.