Relationship between the Hadley Circulation and Different Tropical Meridional SST Structures during Boreal Summer

The relationship of the Hadley circulation (HC) to different tropical sea surface temperature (SST) meridional structures during boreal summer is investigated over the period of 1979-2016. After decomposing the variations of the HC into the equatorially asymmetric HC (HEA), zonal-mean equatorially asymmetric SST (SEA), equatorially symmetric HC (HES), and equatorially symmetric SST (SES) components, the ratio of the HEA associated with SEA with respect to the HES associated with SES is around 2 across multiple reanalyses, which is a smaller ratio than in the annual and seasonal cycle. The reduced ratio of the HC to SST is due to the regional SST variation in the Asian summer monsoon (ASM) domain. The first leading mode (EOF1) of the regional SST variability in the ASM domain is dominated by a homogeneous warming pattern. This pattern is associated with an equatorially asymmetric HC, but it has an opposite direction to the climatological HEA and so weakens the HEA. The second dominant mode has an El Nino-like pattern, which resembles the distribution of the principal mode of the SST in the non-ASM region. Both modes are responsible for the variation of HES. However, the SST EOF1 in the ASM domain displays a significant upward trend, favoring a suppressed HEA, and leading to the smaller ratio of the HC to SST during boreal summer. Moreover, the variation of the SST EOF1 is closely linked with the intensity of the ASM, highlighting the potential modulation by the ASM of the relation between the HC and SST during boreal summer.