Hadley Circulation Changes Depend on Surface Temperature Anomaly Patterns
Scientists at Lawrence Livermore National Laboratory in collaboration with colleagues from Nanjing University, Pacific Northwest National Laboratory, and Peking University have found that changes in the width of the Hadley circulation in response to global warming are strongly affected by the spatial pattern of that warming. Having quantified these dependencies using a novel Green’s function approach, they show that the Hadley circulation expands poleward in response to subtropical sea surface temperature increases that occur on or near its equatorward flank. Conversely, it contracts in response to warming over the Tropics, unless the warming is confined to the western Pacific and Indian Oceans. Using the Green’s functions, the team estimated that about 60% of the Hadley circulation widening between 1980 and 2014 is caused by changes in the spatial pattern of surface temperature anomalies rather than by increases in tropical mean warming. The observed pattern of warming is not reproduced in coupled model simulations of the recent past, likely contributing to their large underestimate of observed trends in Hadley cell expansion.
The responses of the atmospheric circulation to future warming have important impacts on regional precipitation changes that affect many people. This is especially true for those living within the sharp climatological gradients that characterize the edge of the tropical Hadley circulation. Despite its importance, the response of the Hadley circulation edge remains a highly uncertain component of future climate projections. In this study, the role of surface warming patterns on the Hadley cell edge is systematically quantified using a novel Green’s function approach. This aids in understanding how, for given globally-averaged warming, different Hadley cell responses can arise depending on the spatial pattern of warming.
Idealized experiments performed with the Community Atmospheric Model 5.3 indicate that the width and strength of the Hadley circulation are sensitive to the location of sea surface temperature (SST) increases. The Hadley circulation edge shifts poleward in response to SST increases over the subtropical regions near and on the equatorward flank of the HC edge, and shifts equatorward in response to warming over the tropical area except for the western Pacific Ocean and the Indian Ocean. The Hadley circulation is strengthened in response to SST increases over the intertropical convergence zone (ITCZ) and is weakened in response to SST increases over the subsidence branch of the Hadley circulation in the subtropics. Tropical SST increases off the ITCZ tend to weaken the Hadley circulation in the corresponding hemisphere and strengthen it in the opposite hemisphere. These results could be used to explain the simulated Hadley circulation changes induced by recent SST variations, and it is estimated that more than half of the SST-induced Hadley circulation widening in 1980–2014 is caused by changes in the spatial pattern of SST.