On the response of the Antarctic Circumpolar Current transport to climate change in coupled climate models
Source: LASG Viewed: time(s) Time: 2010-7-22
Speaker
Dr. Zhaomin Wang
Affiliation
British Antarctic Survey, UK
Time
22 July, 2010
Location
Introduction
Abstract
The IPCC AR4 models show a consistent intensification and poleward shift of the westerlies over the Southern Ocean (SO) during the 21st century. However, the responses of the Antarctic Circumpolar Currents show great diversity. We investigate in detail the processes that control the ACC responses in these models.
Coincident with the intensification of the westerly winds are significant changes in buoyancy forcing. Such fluxes are correlated with changes in wind forcing in such a way that Ekman pumping act to increase the isopycnal slope and ACC transport whilst simultaneously the associated changes in heat and freshwater flux act to retard the ACC.
Also coincident with the intensification of the westerlies are increases in cyclonic wind forcing at higher latitudes, and increases and poleward displacements of anticyclonic wind forcing at lower latitudes. This latter process leads to the previously-recognised poleward shift of the subtropical gyres, whilst the increased cyclonic wind forcing at higher latitudes acts to intensify the subpolar gyres. For models with reduced ACC transport, the subpolar gyres expand equatorward significantly; the combined effect of this and the subtropical changes is to narrow the ACC considerably, and limit its increase in transport. This narrowing of the ACC, combined with the effects of increased Ekman pumping, acts to increase isopycnal tilt and enhance the (parameterized) eddy response.
The great diversity of the simulated ACC changes is due to different representations of the above processes in each model. In addition, different density stratifications are also responsible for diverse ACC changes.
These findings give insight into the reliability of IPCC-class climate model predictions of the SO circulation, and have implications for aspects of regional and global climate that depend on it. They also give insight into the observed relative insensitivity of ACC transport to the increases in westerlies over the past several decades.
