Two interannual dominant modes of the South Asian High in May and their linkage to the tropical SST anomalies

Abstract

The variations of the intensity and position of the South Asian High (SAH) in May are critical for the interannual change in the Asian summer monsoon onset process. The present work has defined two dominant modes of the SAH in May via the principal component analysis of SAH’s strength, zonal extension and ridgeline position indices during 1979–2015. The first mode (SAH intensity mode) features a coherent enhancement of the SAH strength and zonal extension, while the second mode (SAH position mode) exhibits a meridional swing of the SAH ridgeline. Both diagnosis analysis and numerical experiments validate that the distinct evolution of ENSO in boreal spring determines its different influences on the two modes. When an El Nino decays quickly during March–May, its resultant warm anomalies of sea surface temperature (SST) in the tropical Indian Ocean can affect the SAH intensity mode by modifying the tropical convection over the southwestern Bay of Bengal, acting as a local SST forcing to the circulation. However, the SAH position mode depends on the persistence of the ENSO event during boreal spring. The remote El Nino-related SST forcing can enhance the large-scale descent over the Indo–Pacific Ocean, which suppresses the tropical convection to modify the mid–upper-level thermal structure over South Asia, inducing the interannual variation of the SAH ridgeline in May.