Response of the Hadley Circulation to Regional Sea Surface Temperature Changes
The Hadley cell (HC)—the most prevalent feature of global atmospheric circulation—is a key component of the global energy and hydrological cycle, but its relationship to global sea surface temperature (SST) has yet to be examined systematically. A team comprised of researchers from DOE labs (including the Pacific Northwest National Laboratory) and universities used the Community Atmospheric Model 5.3 to investigate the power SST has on the intensification or widening of the HC in both the Northern and Southern Hemispheres. By perturbing a local SST one at a time, researchers were able to construct a global map showing the sensitivity of the HC to SST. This effort helps build a holistic understanding for the first time of the role of oceanic forcing for the HC.
Determining the sensitivity of the HC to SST is crucial to understanding ocean-forced climate change response and climate variability. Applying Green’s function experiments—by perturbing a local SST one at a time—researchers were able to create a holistic sensitivity map of the HC to SST for the first time. They discovered that subtropical warming is the most efficient in expanding the HC, while the tropical Indian Ocean and Pacific Ocean have a distinct impact on the Northern Hemisphere HC edge through stationary wave response. Their sensitivity matrix captured the trend of the HC expansion during the period 1980–2014 and attributed it mostly to the pattern of the SST trend.
Researchers conducted a large suite of experiments with the Community Atmospheric Model 5.3 to investigate the sensitivity of the width and strength of the HC to the location of SST forcing. They used a Green’s function approach by perturbing local SST one at a time to develop a sensitivity map of the HC edge and intensity. They discovered the HC edge shifts poleward in response to SST increases over 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 at the western Pacific Ocean and Indian Ocean. The HC 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 HC in the subtropics. Tropical SST increases off the ITCZ tend to weaken the HC in the corresponding hemisphere and strengthen it in the opposite hemisphere. These results could be used to explain the simulated HC changes induced by recent SST variations, and it is estimated that more than half of the SST-induced HC widening in 1980–2014 is caused by changes in the spatial pattern of SST.