Mechanisms for Poleward Expansion of the Hadley Circulation: Observations and Simulations

As one of the most prominent atmospheric circulation systems, the Hadley circulation must be affected by global climate changes. In turn, changes of the Hadley circulation will impact on regional and global climates. Recent studies have shown poleward expansion of the Hadley circulation over the past few decades, using independent datasets. The expansion would lead to poleward shift of the subtropical dry zone, which has important influences on ecosystems and societies. In this presentation, we will first show more observational evidence of poleward expansion of the Hadley circulation since the late 1970s. The observational data include: outgoing longwave radiation (OLR), sea-level pressure, precipitation. Second, we show ensemble GCM simulation results forced by observed time-varying sea surface temperatures (SST). Both observations and simulations demonstrate asymmetry of poleward expansion of the Hadley circulation between the two hemispheres, with greater expansion in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Third, we examine the proposed mechanisms that changes in tropical SST and extratropical wave activity may cause the poleward expansion of the Hadley circulation, using reanalysis data and multimodel simulation results. Our results show that for NH weakening of baroclinic instability is the most important reason in causing the widening of the Hadley circulation, while for SH changes of static and baroclinic stabilities play the nearly equal role. Using multimodel ensemble average results from IPCC-AR4 simulations for the 20th century, we find that models which generate El Nino-like warming pattern of tropical SSTs have narrower poleward expansion of the Hadley circulation than that in models which have broad tropical SST warming. These suggest that meridional SST distribution has important impacts on poleward expansion of the Hadley circulation.