Impacts of ENSO on Duration of South Asia High Staying Over Southern Asia During Summer-To-Winter Transition

Abstract

This study investigates the impacts of the El Niño–Southern Oscillation (ENSO) on the duration of the South Asia high (SAH) that persists over southern Asia (10°N–27.5°N, 40°E–115°E) during the summer-to-winter transition. The findings indicate that the duration of the SAH over southern Asia displays distinct interannual variations. Specifically, the SAH is observed to manifest as more intense and rapidly revert to the Ocean in a short duration year (SD_SAH) in contrast to a long duration year (LD_SAH). Furthermore, the SD_SAH tends to appear during a period of a La Niña event developing, while the LD_SAH is observed to coincide with the developing phase of an El Niño event. During the developing phase of a La Niña event, distinct variations in sea surface temperature (SST) are observed, with cooler SST in the tropical central-eastern Pacific and warmer SST in the tropical northwestern Pacific. These variations in SST are conducive to the generation of an atmosphere circulation cell, featured by upward flow over the tropical and subtropical western Pacific, and downward flow over the western Indian Ocean and eastern Africa. Above-normal precipitation and apparent heating across the tropical northwestern Pacific could lead to positive geopotential height (HGT) over its northwestern side at upper troposphere. The thermotaxis feature of the SAH results in a strengthening and rapid eastward movement of the system during the summer-to-winter transition. Conversely, during El Niño development, the SST pattern exhibits an opposite phase, characterised by a downward flow over the tropical northwestern Pacific. The deficit precipitation and latent cooling present an obstacle to the SAH shifting eastward to the Ocean. Consequently, the SAH would require a longer duration to persist over southern Asia, exhibiting a distinct east–west oscillational trajectory.