Mechanism of Interannual Cross-Equatorial Overturning Anomalies in the Pacific Ocean

The meridional overturning circulation (MOC) transports heat and mass between the tropics and the extratropics. Recent research has shown that the variability of the Indo-Pacific MOC dominates the variability of the global MOC on interannual timescales, and this variability is characterized by a prominent cross-equatorial cell (CEC) spanning the tropics. This CEC is a potentially important influence on interannual climate variability, but the mechanism responsible for the CEC is not understood. This study seeks to elucidate the mechanism of the CEC using two observational estimates of the ocean. Our analysis shows that the CEC can be explained by the following mechanistic chain: (a) Anomalies in the atmospheric circulation and hydrological cycle produce equatorially antisymmetric density anomalies in the upper Pacific Ocean (above approximately 500 m); (b) these density anomalies generate equatorially antisymmetric anomalies of sea surface height (SSH); (c) these SSH anomalies generate a cross-equatorial flow above approximately 1,000 m; and (d) this anomalous cross-equatorial flow drives compensating flow below approximately 1,000 m. This mechanism contrasts with that responsible for anomalous cross-equatorial overturning on seasonal timescales, which is primarily the Ekman response to equatorially antisymmetric anomalies of zonal wind stress. On interannual timescales, the zonal wind stress anomalies associated with the CEC are equatorially symmetric, and steric SSH variations are the dominant driver of the CEC. These insights may lead to improved understanding and prediction of interannual climate variability.