Exceptional Equatorial Extension of Extreme Indian Ocean Dipole Cooling: Shaping Effect From Strong El Niño

Abstract

The extreme positive Indian Ocean Dipole (EXpIOD) peaking in 2023 boreal fall exhibited the strongest variations in sea level and subsurface temperature in the eastern Indian Ocean (IO), and the most equatorially westward extension of cooling observed during the satellite era to date. Its pattern resembled the 1997 EXpIOD, contrasting with other coastally-concentrated EXpIODs. Here we show that the distinctive air-sea responses of equatorial EXpIODs stem from the energetic central IO zonal winds fueled by concurrent strong El Niños, which were absent in other cases. The exceptionally strong easterly anomaly generates prominent nonlinear vertical advection cooling the central IO, with nonlinear zonal advection and the Ekman feedback playing supportive roles. Current coupled models can capture this Pacific shaping effect, despite with divergent strengths. Under strong El Niño's inter-basin forcing, the westward extension of cooling has profound implications for regional hydrology and coral ecology, and warrants closer attention in future predictions.