Enhanced Bimodal Seasonal Distribution of Tropical Cyclone Genesis Potential Over the North Indian Ocean Toward Glacial Inception

Abstract The annual cycle of contemporary tropical cyclones (TCs) in the north Indian Ocean (NIO) shows a unique bimodal structure, but whether the bimodal seasonal character remains robust across different climates is unknown. Here, we investigate changes in environmental factors important to storm formation in the past warm‐cold transition during 130–115 ka, in which climatic characteristics were comparable to future projections, though with an opposite sign. Based on a non‐accelerated transient global climate simulation at ∼1° resolution, we illustrate that the annual cycle of environmental conditions supporting TC genesis over the NIO bifurcates into two discrete periods during the warm‐cold transition, manifested by a shift from a unimodal to bimodal seasonal distribution. The shift is largely linked to the increased favorability for TC genesis in pre‐monsoon season (April–May–June) toward the last glacial inception (∼115 ka), which is in turn tied to the orbitally‐induced weakening of Indian monsoon‐like circulations.