Asymmetric Impacts of Extreme Positive and Negative Indian Ocean Dipole Events on Late‐Summer Monsoon Rainfall in Western South Asia

Abstract Observations show a fragile linear relationship between South Asian monsoon rainfall and the Indian Ocean Dipole (IOD), reflecting their complex interactions. Our analysis reveals that both extreme positive and negative IOD (EXpIOD/EXnIOD) events could enhance rainfall over western South Asia during August–September, even when ENSO signals are removed. This asymmetric influence arises from moisture convergence driven by anomalous winds. AGCM simulations indicate that EXpIOD‐related suppressed convection over the eastern pole induces a westward‐extended Gill‐type anticyclone, whose southwesterlies reinforce the background monsoon and promote orographic rainfall near the Western Ghats. Contrastingly, EXnIOD features an eastward‐shifted dipole; positive convection in the east drives a cyclonic circulation that transports Bay of Bengal moisture westward. This flow converges with cross‐equatorial southwesterlies generated by the western pole, enhancing rainfall over western India. These results suggest that the pattern asymmetry between EXpIOD and EXnIOD is a key factor that deserves consideration when assessing IOD‐monsoon interactions.