Sources of Intermodel Diversity in the Regional Hadley Circulation over the Western Pacific during Boreal Winter

Abstract The Hadley circulation (HC) is the strongest meridional overturning circulation on Earth and plays a crucial role in linking the tropics and extratropics. Compared to the zonal-mean HC, the regional HC is much less studied. This study investigates the capability of state-of-the-art climate models in phase 6 of the Coupled Model Intercomparison Project (CMIP6) to reproduce the western Pacific HC (WPHC) during the boreal winter. While most CMIP6 models successfully reproduce the spatial structure of the winter WPHC, there is considerable intermodel variability in the simulation of the intensity, centerline, and poleward edge of the WPHC. The intermodel empirical orthogonal function (EOF) is then used to identify the dominant modes of diversity. The first three EOF modes explain above 80% of the total variation of the meridional mass streamfunction. The first EOF mode represents intermodel spread in the simulation of WPHC intensity in both hemispheres. This spread is due to model differences in simulating meridional sea surface temperature (SST) gradient in the western Pacific and zonal SST gradient in the tropical Pacific. The second EOF mode highlights model discrepancies in the position of the WPHC in the Northern Hemisphere, which is related to model differences in simulating meridional position of the intertropical convergence zone. The third EOF mode characterizes model diversity in simulating the Southern Hemisphere WPHC position, which is related to the differences in the tropical cold tongue. Our study highlights the critical role of the regional SST structure in the representation of the regional HC and provides insights for the further improvement of coupled climate models. Significance Statement The Hadley circulation (HC) is the strongest meridional atmospheric circulation in Earth’s climate system, transporting energy and momentum flux from the tropics to the midlatitudes. The descending branch of the HC generally corresponds to the subtropical arid zone. Changes in the intensity and position of the HC could have significant impacts on global weather and climate, such as precipitation distribution and tropical cyclone activity. It is therefore crucial to assess the ability of state-of-the-art climate models to simulate the HC. Although many previous studies have examined the characteristics of zonally averaged HC, it remains unclear how well climate models can reproduce regional HC characteristics. Our study examines the ability of phase 6 of the Coupled Model Intercomparison Project models to simulate the western Pacific HC (WPHC) during the boreal winter. The results show a large model spread in capturing the WPHC, highlighting the link between the ability to reproduce the meridional and zonal regional SST structures and the regional HC characteristics.