Intraseasonal Variations in the Indian Ocean Cross-Equatorial Cell During the Subsequent Boreal Summer of ENSO

Abstract

The interannual relationships between the cross-equatorial cell (CEC) in the Indian Ocean (IO) and the El Niño-Southern Oscillation (ENSO) are examined through observational data and numerical simulations. The findings indicate a notable intraseasonal variation in the boreal summer IO CEC responses to preceding ENSO events, showing a weakening in early summer (May–June) and a strengthening in late summer (August–September) subsequent to an El Niño events. These contrasting responses are primarily driven by opposite meridional Ekman transport anomalies, characterized by anomalous northward Ekman transport in early summer and southward transport in late summer. Further analysis reveals that ENSO-induced surface zonal wind anomalies predominantly influence these anomalous meridional Ekman transport in the upper layer, accounting for over 80% of the variation. In early summer, an antisymmetric wind pattern over the IO, induced by El Niño in the decaying spring, along with the westward extension of the anomalous western North Pacific anticyclone (WNPAC), generates anomalous easterlies over the North IO (NIO), leading to weakened meridional Ekman transport and a diminished CEC. Simultaneously, those anomalous easterlies and the associated weakened CEC in early summer trigger a wind-CEC-SST (WCS) negative feedback mechanism. The resulting anomalous northward transport and reduced upwelling lead to sea surface temperature (SST) warming in the northern NIO, creating a northward surface ocean temperature gradient in the July–August period. This gradient, along with the eastward retreat of the WNPAC, causes anomalous NIO westerlies in late summer, enhancing meridional Ekman transport and ultimately strengthening the CEC.