Evolution of the meridional shift of the subtropical and subpolar westerly jet over the Southern Hemisphere during the past 21,000 years

Deciphering the variations in the subtropical westerly jet (STJ) and the subpolar westerly jet (SPJ) over the Southern Hemisphere in Earth's history will help us to better understand how the westerlies may respond to various external forcings in the future. Using Simulation of Transient Climate Evolution over the past 21,000 years simulations, we demonstrate that the STJ migrated equatorward in the Last Glacial Maximum relative to the present, which followed by abrupt poleward, equatorward, and poleward shifts during the Heinrich Stadial I, Antarctic Cold Reversal, and Younger Dryas, respectively. These large -amplitude fluctuations are controlled by changes in meltwater fluxes and polar ice-sheet topography. During the Holocene, the STJ shows no obvious long-term trend, due to the offsetting effect between increased orbital insolation in austral summer and decreased Antarctic ice-sheet topography. The meridional shift of the SPJ shows an in-phase change with the STJ during the past 21 kyr. In terms of the dynamic mechanisms, the meridional shift of the STJ and SPJ is induced by changes in the meridional temperature gradient and the maximum eddy growth rate, respectively; and is also linked to changes in the mean meridional circulations (that is, the Hadley and Ferrel cells). The modeled meridional shift of the southern westerlies is supported by the results from the Paleoclimate Modelling Intercomparison Project Phase III and is consistent with several reconstructions, but it is difficult to constrain robustly by geological evidence as the reconstructed westerlies vary across sites and proxies. Nevertheless, our study provides a possible scenario for the evolution of the southern westerlies during the past 21 kyr and identifies the dominant forcing and the associated mechanisms, which may advance our knowledge on the future behavior of the southern westerlies. (C) 2020 Elsevier Ltd. All rights reserved.