Poleward Expansion of the Hadley Circulation in CMIP5 Simulations

Abstract:

Observational analyses have demonstrated that the Hadley circulation has expanded poleward in recent decades. Important issues are what caused the widening of the Hadley circulation and whether the observed widening is related to anthropogenic forcing. Here, we use CMIP5 simulation results to analyze anthropogenic forcings on the width of the Hadley circulation. It is found that greenhouse gas (GHG) forcing generate widening of the Hadley circulation in all seasons for both hemispheres. Ozone depletion causes significant widening of the Hadley circulation only in DJF for the Southern Hemisphere and MAM for the Northern Hemisphere, but no significant widening in other seasons. However, anthropogenic aerosols have no significant forcing on the width of the Hadley circulation. The widening in historical simulations with all forcings is ~0.17° per decade. In contrast to that in CMIP3, AMIP-type simulations of CMIP5 show significant widening. In CMIP5 projection simulations for the 21st century, magnitudes of widening of the Hadley circulation increase with radiative forcing. For RCP8.5, the total annual-mean widening of the Hadley circulation is ~0.27° (10 yr)^-1 in the 21st century. In addition, we will also show different model performances in simulating the climatology and trends of the Hadley circulation.