The relationship between tropical warm pool sea surface temperature and Hadley circulation

Abstract:

The Hadley Circulation has been expanding at an average rate of 50km per decade in each hemisphere during the recent few decades. The symmetry of this expansion –meridionally and zonally–remains unclear and observational data shortcomings prevent a more precise determination. Climate models largely underestimate the magnitude of expansion partly because they are unable to reproduce the main large scale natural variability (such as the El Niño-Southern Oscillation, the annular modes, or the Pacific Decadal Oscillation) in addition to the persistent cold tongue bias present in all models. Various anthropogenic factors, such as increasing greenhouse gases, stratospheric ozone depletion and aerosols, were identified to partially drive the expansion. Physical mechanisms for the expansion include increasing mean temperature and increasing meridional temperature gradients, interaction with extratropical baroclinic eddies and gross static stability. However, these are often seasonally and regionally dependent. Here we explore the regional aspects of the Hadley cells in both hemispheres and based on the three main centers of convergence located over Equatorial Africa, Equatorial America and the Maritime Continent. Preliminary results suggest that most of the expansion occurs in the Maritime Continent sector in association with the local tropical SST warming. Our preliminary assessment indicates that the decadal variability of the SST in the tropical warm pool may be driving the observed changes in the Hadley Circulation in both hemispheres.