Circulation Change on Southern Hemisphere from Paleoclimate to Future Climate

Abstract

Recent studies have shown that the westerly jet and Hadley-cell (HC) edge in the Southern Hemisphere (SH) have systematically shifted poleward during the last few decades. These trends are qualitatively well reproduced by climate model simulations, and projected to continue in the future climate. However, it is unclear whether opposite trends held in the paleoclimate when the global surface air temperature was much colder than the present climate. To better understand zonal-mean circulation change in a wide range of climate states, the present study compares the westerly jet and HC edge in the Last Glacial Maximum (LGM), the latest cold period of the Ice Ages about 21,000 years ago, Pre-Industrial (PI) condition, and Extended Concentration Pathway 4.5 (ECP4.5) by analyzing coupled model simulations archived for the Paleoclimate Modelling Intercomparison Project phase 3 (PMIP3) and the Coupled Model Intercomparison Project phase 5 (CMIP5). Only SH circulations, which can be largely explained by zonal-mean dynamics, are considered in the equilibrium state of each model simulation. In all six models analyzed in this study, the HC edge systematically shifts poleward from LGM to PI and to ECP4.5 conditions. However, the jet latitude presents non-robust changes. Although all models exhibit a poleward shift of the westerly jet in a warm climate as in HC edge change, they show mixed responses in a cold climate. Only three models show equatorward jet shift, whereas the other three show the jet moving poleward in the LGM condition. This large intermodel spread in jet-latitude change is closely related with uncertainty in tropical upper-tropospheric temperature change. By integrating a dynamic core GCM with imposed tropical upper-tropospheric warming or cooling, it is shown that a systematic poleward shift of zonal-mean circulation in a warm climate and nonsystematic change in a cold climate can be largely explained by quasi-geostrophic zonal-mean dynamics. It is also found that the jet shift in the cooling experiment has a larger uncertainty than in the warming experiment. Some cooling experiments in particular reproduce poleward shift of jet with HC contraction. These results from the dynamic core, which are similar to those from LGM simulations, imply that this simple numerical analysis could be the key to understanding the inconsistent jet shifts among models in a colder climate than today.