The combined influence of west Pacific subtropical high and tropical cyclones over the northwest Pacific on Indian summer monsoon rainfall

Abstract

AbstractWe examined the interrelationship between the Indian summer monsoon rainfall (ISMR), the west Pacific subtropical high (WPSH), and tropical cyclone (TC) activity over the western north Pacific (WNP) Ocean during the peak monsoon season in 1951–2019. When WNP TCs are inactive (active), there is a noticeable 10° westward (eastward) shift observed along with intensification (weakening) of the WPSH. The WPSH and WNP TC activities are divided into four categories—an eastward shift of WPSH with active TCs (EA) and inactive TCs (EI), and a westward shift of WPSH with active TCs (WA) and inactive TCs (WI)—in order to investigate the combined influence of the WPSH and WNP TCs on ISMR during non‐El Niño–Southern Oscillation (non‐ENSO), El Niño, and La Niña phases. In the EA non‐ENSO cases, a surplus (deficit) rainfall was noticed over central (southern peninsular) India as a result of moisture convergence (low convective available potential energy, abnormal moisture divergence and downdraft); however, the rainfall is above normal over the Indian subcontinent. During the EI non‐ENSO case, an excess (deficit) regional rainfall activity was observed over east‐central India and parts of north India (southern peninsular India) through regional changes in atmospheric circulations and midtropospheric vertical velocity. In contrast, in WI (WA) non‐ENSO cases, the majority of India (southern peninsular and northeast India) experiences excessive precipitation while east‐central India (east and west central India) undergoes a deficit rainfall. This pattern is consistent with the spatial distribution of moisture flux convergence, convective available potential energy, and midtropospheric vertical velocity. In the case of non‐ENSO, the westward shift of the WPSH associated with inactive TCs is one of the favourable conditions for ISMR. In general, the effect of an eastward (westward) shift of the WPSH along with more (less) TCs during the non‐ENSO case is favourable to ISMR. On the other hand, rainfall patterns during ENSO are significantly associated with the circulation changes teleconnected to El Niño and La Niña conditions.