The interannual relationships between the southern subtropical cell (SSTC) in the Indian Ocean (IO) and the El Ni & ntilde;o-Southern Oscillation (ENSO) are examined. The results demonstrate that the impacts of ENSO on the boreal winter IO SSTC display a notable negative skewness, with the amplification of the SSTC during La Ni & ntilde;a being far stronger than its reduction during El Ni & ntilde;o. This difference is mainly attributed to an asymmetry in anomalous meridional geostrophic transport, characterized by more robust southward (weaker northward) geostrophic transport anomalies during La Ni & ntilde;a (El Ni & ntilde;o) events. Further analyses reveal that the differing longitudinal positions of the ENSO-related sea surface temperature anomaly (SSTA) are the primary drivers of the asymmetry in anomalous meridional geostrophic transport via the oceanic and atmospheric teleconnections. Compared to the warm SSTA in El Ni & ntilde;o events, the La Ni & ntilde;a-related cool SSTA in the central-eastern equatorial Pacific displays a greater westward extension, which induces stronger surface easterlies over the western equatorial Pacific, and then initiates a more substantial sea surface height anomaly (SSHA) in the southeast Indian Ocean (SEIO) through oceanic waveguides. This leads to a heightened zonal SSHA gradient over the southern IO, resulting in a larger anomalous meridional geostrophic transport. On the other hand, the westward extension of negative SSTA during La Ni & ntilde;a winters triggers anomalous cyclonic northerlies along the western coast of Australia by shifting the Walker circulation westward. These coastal northerlies amplify the positive SSHA signals in the SEIO and enhance the anomalous meridional geostrophic transport, ultimately contributing to a more robust IO SSTC response.
The southern subtropical cell (SSTC), driven by wind over the Indian Ocean (IO), is characterized by southward surface flow, subduction in the subtropics, and upwelling in the tropics. This study revealed that the boreal winter SSTC weakens during El Ni & ntilde;o and enhances during La Ni & ntilde;a, with the latter exhibiting a notably stronger response, contrary to the expected El Ni & ntilde;o/La Ni & ntilde;a amplitudes. These asymmetric ENSO impacts predominantly result from disparities in anomalous meridional geostrophic transport. In-depth analysis further uncovered that the distinct longitudinal positions of ENSO-related sea surface temperature anomalies (SSTA) primarily drive the asymmetry in anomalous meridional geostrophic transport. During La Ni & ntilde;a events, the cool SSTA in the central-eastern equatorial Pacific extends significantly westward compared to the El Ni & ntilde;o-related warm SSTA, which is evident during both the preceding season and the contemporaneous winter. During the preceding season, this westwardly extended SSTA leads to a more substantial anomalous meridional geostrophic transport in the subsequent winters through the propagation of oceanic waveguides. In La Ni & ntilde;a winters, the westward extension of negative SSTA further amplifies the anomalous meridional geostrophic transport by shifting the Walker circulation westward. These intensified meridional geostrophic transport anomalies ultimately result in a more pronounced SSTC response during La Ni & ntilde;a events.
The boreal winter southern subtropical cell (SSTC) in the Indian Ocean displays a negative skewness in response to El Ni & ntilde;o-Southern Oscillation (ENSO) Such asymmetric SSTC responses are primarily attributed to an asymmetry in anomalous meridional geostrophic transport Differing longitudes of ENSO-related sea surface temperature anomaly cause the asymmetric meridional geostrophic transport via oceanic and atmospheric teleconnections