Comparison of the seasonal evolution of the South Asian high associated with two types of El Niño event

Abstract

El Niño (EN) episodes can be classified based on their time of onset as spring onset EN (SPEN) events and summer onset EN (SUEN) events. To evaluate the different influences of SPEN and SUEN events on the South Asian high (SAH), this study compared the seasonal evolution of the SAH (SESAH) associated with SPEN and SUEN events through analysis of geopotential height and zonal wind data derived from NCEP–NCAR Reanalysis-1 and sea surface temperature data obtained from the Hadley Center. The main features of the SESAH during an EN event are similar to its climatological characteristics. Climatologically, the SAH forms in May, strengthens, and moves northwestward in June and July. It does not change much in August, but then it returns south and weakens during September and October. However, its lifespan is shorter and its intensity weaker during EN periods. Furthermore, there are significant differences between the SESAH during SPEN and SUEN events. During a SPEN episode, the movement of the SAH to the northwest during May and June is slower than during a SUEN event, i.e. the SPEN SAH has a shorter lifespan. In comparison with the SUEN SAH, the SPEN SAH in July and September tends more towards the Tibetan high mode rather than the Iranian high mode. The SPEN SAH in October moves southeastward faster than the SUEN SAH, which also indicates that the SAH has a shorter lifespan during a SPEN event than during a SUEN episode.