Influence of the South Asian high-intensity variability on the persistent heavy rainfall and heat waves in Asian monsoon regions

This study investigates the modulation of the South Asian high's (SAH) intensity variation on the persistent heavy rainfall (PHR) and heat wave (HW) events over the Asian summer monsoon regions during summer 1979-2013. The first mode of an empirical orthogonal function (EOF) analysis performed on standardized subseasonal anomalies at 200-hPa geopotential height over the SAH's main body (20 degrees-5 degrees N, 35 degrees-110 degrees E), reveals the intensity variation of the SAH with a 10-40-day periodicity. The strengthening (weakening) of the SAH's main body is accompanied by negative (positive) outgoing longwave radiation (OLR) anomalies and more (less) anomalous rainfall over the Indian subcontinent, southern foot of Tibetan Plateau (TP) and subtropical East Asia. Using different thresholds, the proportion for modest PHR events is approximately 30% and can reach up to 40-50% for extreme PHR events during the strong phase of SAH. About 20% of modest PHR events and only 10% of extreme PHR events occur under the weak phase of SAH, suggesting that SAH has a more significant impact on the occurrence of extreme PHR events over the south and East Asian regions. In contrast, HW events associated with strong SAH account for approximately 5-10% whereas an increased occurrence of HW events is observed during the weak phase of SAH with a ratio up to 15-20%. A much more intensified updraft flow, enhanced cross-equatorial moisture flux and western North Pacific moisture flux is accompanied with strong SAH, which provide favourable conditions for occurrence of extreme PHR events, leading to less surface solar radiation anomalies. The mirror image is seen in the weak SAH phase, contributing to the decrease of extreme PHR events and increase of HW events. The above analysis indicates that the SAH provides significant modulation and impact on extreme rainfall and temperature events over the Asian monsoon regions on subseasonal timescale.