Gang Chen

A hierarchy of models of the general circulation of the atmosphere is employed to understand the poleward shift of surface westerlies in the Southern Hemisphere in recent decades.  I will start by describing a basic picture for the generation and maintenance of climatological zonal mean winds, emphasizing the role of extratropical synoptic eddies.  Next, I will examine the movements of surface westerlies in a dry atmospheric dynamical core with simple physics, which captures the dynamics that controls the jet variability in the observation.  As the strength of surface friction is reduced in this simple model, the midlatitude jet moves poleward, and this is used as a test case for our understandings of the mechanisms that determine the latitude of surface westerlies.  An ensemble of switch-on experiments and the space-time eddy momentum flux spectra suggest that the key to the shift is a poleward movement in the subtropical critical latitude associated with the faster eastward phase speeds in the dominant midlatitude eddies.  Next, I will compare the poleward shift of the Southern Hemisphere surface westerlies in recent decades in reanalysis data and in a GFDL climate model.  The space-time eddy spectra revel a similar trend in both the observation and the model that marks an increase in the eastward phase speed of the midlatitude eddies.  I will conclude with a mechanism that can plausibly explain the observed trend of surface westerlies, and discuss some open questions for future research.