Southern Hemisphere Circulation Change from Paleoclimate (LGM) to Future (RCP4.5) Climate

The recent studies have shown that atmospheric general circulation in the Southern Hemisphere (SH) has systematically changed in response to increasing greenhouse gas concentrations. For example, it is shown that the westerly jet has shifted poleward and the Hadley cell has widened during last few decades. These trends, which are evident in both observations and climate model simulations, are further projected to continue in future climate. The present study addresses whether such trends are also present in the paleoclimate by analyzing coupled model simulations. Specifically, six models, which have participated in both the Coupled Model Intercomparison Project phase 5 (CMIP5) and the Paleoclimate Modeling Intercomparison Projection phase 3 (PMIP3), are examined in the context of westerly jet and Hadley cell changes in the Southern Hemisphere. By comparing Last Glacial maximum (LGM), Pre-Industrial (PI), and future (RCP4.5) climate simulations, it is found that Hadley cell width systematically changes from LGM to future climate. However this is not the case in westerly jet. Although jet intensity change from LGM to future climate is almost linear, jet position change is highly nonlinear. It turns out that while the former can be explained by meridional temperature gradient changes, the latter can not be simply explained by linear baroclinicity change.