Interannual Variation of South Asian High Intensity in September Plays an Important Role in Modulating Indian Rainfall

Abstract

September is widely considered to be the final month of the monsoon season on the Indian subcontinent. Precipitation levels during this month exert a pivotal influence on the duration of monsoon rainfall, with the potential to substantially impact subsequent dry spells in the region. Consequently, further investigation into the variations in September rainfall is imperative for ensuring social and agricultural security. This study, therefore, examined the possible role of the South Asian high (SAH) modulating Indian rainfall in September. The study found that the SAH was generally stable around South Asia in September, prior to its retreat over the ocean. The SAH was found to be weaker and shifted in a southeastward direction in September compared to its summer mean. A strong SAH in September was often concomitant with a delayed withdrawal of Indian summer monsoon (ISM) rainfall and vice versa, with positive rainfall anomalies primarily manifesting over central-northeastern, west-central, and peninsular India. The enhanced SAH was accompanied by stronger westerly and easterly jet streams, respectively, over the southern Caspian Sea and northwest India in the upper troposphere. A notable upper-tropospheric anticyclonic circulation has been observed over the western Tibetan Plateau. Additionally, a significant lower-tropospheric cyclonic circulation has been observed over India, accompanied by an enhanced Somali cross-equatorial flow. The associated anomalous westerly anomaly over southern India and southeasterly anomaly over northern India can transport abundant moisture over most of India. Consequently, there is a tendency for substantial rainfall tends to occur in conjunction with an enhanced SAH.