A Comparison of the Response of the Hadley Circulation to Different Tropical SST Meridional Structures During the Equinox Seasons

The response of the Hadley circulation (HC) to different tropical sea surface temperature (SST) patterns during the equinox seasons (i.e., boreal spring and autumn) is analyzed. Although the HC shows quasi-equatorially symmetric structure in both equinox seasons, both the climatology and the interannual variations of the HC display considerable differences, that is, being more equatorially asymmetric (symmetric) during autumn (spring). After decomposing the variations of the HC and zonal SST into the equatorially asymmetric (i.e., HC's equatorially asymmetric variation (HEA) for HC and SST's equatorially asymmetric variation (SEA) for SST) and symmetric (i.e., HC's equatorially symmetric variation (HES) for HC and SST's equatorially symmetric variation (SES) for SST) components, the differences in the HC between spring and autumn are shown to derive mainly from the HEA. Meanwhile, the contrast in the response of the HEA to SEA against HES to SES is greater in spring than in autumn, suggesting the HC is more sensitive to the SST conditions in spring. The variation of the HEA is related to different signals in spring and autumn that is linked to the central Pacific El Nino during spring but to the Atlantic Multidecadal Oscillation during autumn. The different contributors are associated with the differences in both the spatial extent and temporal variations in the HEA. In contrast, both spring and autumn HES are closely linked to the canonical El Nino, favoring similar variations in spring and autumn HES. The results here provide a possible explanation of the different variations in the HC during spring and autumn and highlight the seasonal dependence of the response of the HC to SST.