Robust widening of the Hadley cell from LGM to future (RCP8.5) climate
Abstract:
Recent studies have shown that the width of the Hadley cell (HC) has increased in the latter half of the 20th century and will likely continue to increase in the 21stcentury. This finding is evaluated in the present study by examining the HC in the Last Glacial Maximum (LGM), the latest cold period of the Ice Ages, and in the future climate. The six climate models, which participated in both the Paleoclimate Modelling Intercomparison Projection phase 3 (PMIP3) and the Coupled Model Intercomparison Project phase 5 (CMIP5), are considered for the LGM, pre-industrial (PI) and RCP8.5 conditions. For the equilibrium state of these simulations, the poleward edge of the HC in each hemisphere is identified by tracking the location of subtropical ridge in the zonal-mean sea-level-pressure field and the zero-crossing latitude in the 500-hPa mass stream function.The HC width in the two definitions shows a systematic increase from LGM to PI and to RCP8.5 conditions. The multi-model mean shows that the HC width in the PI condition is larger than in the LGM condition by about 1.5-degree latitude, but smaller than in the RCP8.5 condition by about 1.2-degree latitude. These differences are proportional to those in global-mean surface air temperature, indicating that the width of the HC may be to a first order controlled by the mean temperature. Analysis of individual models further revealed that the poleward shift of the HC edge from LGM to PI conditions is very robust in the two hemispheres. However, some models exhibit no change or even equatorward shift of the Northern-Hemisphere HC edge from PI to RCP8.5. The possible causes of less robust HC change in the future climate are briefly discussed.