Widening of the Hadley cell from Last Glacial Maximum (LGM) to future climate

Abstract

The Hadley Cell (HC) change from paleoclimate to future climate is examined by comparing coupled model simulations archived for the Paleoclimate Modelling Intercomparison Project phase 3 (PMIP3) and the Coupled Model Intercomparison Project phase 5 (CMIP5). Specifically, HC width and strength are evaluated using 100-year equilibrium simulations for the Last Glacial Maximum (LGM), Pre-Industrial (PI), and Extended Concentration Pathway 4.5 (ECP4.5) conditions. Where available, ECP8.5 simulations are also examined to increase the sample size. All models show a systematic widening of the HC from the LGM to the PI, and to the ECP4.5/8.5 simulations. Such widening, which is found in both hemispheres with more robust change in the Southern Hemisphere (SH) than in the Northern Hemisphere (NH), is significantly correlated with global-mean surface air temperature change and the associated static stability change in the subtropics. Based on the zero-crossing latitude of 500-hPa mass streamfunction, about 4.5° latitude widening of the HC results from global warming of 10 °C. HC strength also exhibits a systematic weakening in the NH. However, in the SH, HC strength shows a rather minor change from LGM to ECP4.5 conditions, because of the cancellation between HC weakening during the austral summer/fall and its strengthening during the spring. This result, which suggests no systematic relationship between HC width and strength changes, is discussed in the context of quasi-geostrophic zonal-mean dynamics. Overall findings are also compared with recent studies that are based on transient climate model simulations.