The Critical Combined Effects of Intraseasonal Oscillations and Tropical Equatorial Rossby Waves on the Tropical Cyclone Genesis Over Bay of Bengal During May

Abstract

Tropical cyclone genesis (TCG) over the Bay of Bengal (BoB) in May can lead to severe meteorological hazards, underscoring the need to identify physical mechanisms with early‐warning potentials. This study quantifies over 90% of TCG events are influenced by northward‐propagating intraseasonal oscillations (ISOs), which transports enhanced low‐level vorticity, reduced vertical wind shear, mid‐level moisture and warm sea surface temperature from the equator to BoB. Most TCG events occur within 20 days after the equatorial intraseasonal convections, highlighting the latter as one early‐warning indicator. Diagnostics reveal the biweekly westward‐propagating equatorial Rossby (ER) waves help channel energy from the intraseasonal envelope into developing tropical disturbances for 86% cyclogenesis. These westward Rossby waves appear over Indochina‐South China Sea 5–10 days before TCG, also serving as a potential TCG indicator. By identifying the synergistic impact of ISOs and ER waves, this study offers scientific guidance for improving subseasonal prediction in this high‐risk region.