Impacts of Tropical North Atlantic and Equatorial Atlantic SST Anomalies on ENSO

The sea surface temperature anomaly (SSTA) in the tropical Atlantic during boreal spring and summer shows two dominant modes: a basin-warming mode and a meridional dipole mode, respectively. Observational and coupled model simulations indicate that the former induces a Pacific La Nina in the succeeding winter whereas the latter cannot. The basin-warming forcing induces a La Nina through a Kelvin wave response and the associated wind-evaporation-SST-convection (WESC) feedback over the northern Indian Ocean (NIO) and Maritime Continent (MC). An anomalous Kelvin wave easterly interacts with the monsoonal westerly, leading to a warm SSTA and a northwest-southeast-oriented heating anomaly in NIO/MC, which further induces easterly and cold SSTAs over the equatorial Pacific. In contrast, the dipole forcing has little impact on the Indian and Pacific Oceans due to the offsetting of the Kelvin wave to the asymmetric Atlantic heating. Further observational and modeling studies of the tropical North Atlantic (TNA) and equatorial Atlantic (EA) SSTA modes indicate that the TNA (EA) forcing induces a CP-type (EP-type) ENSO. In both cases, the Kelvin wave response and the WESC feedback over the NIO/MC are important in conveying the Atlantic's impact. The difference lies in distinctive Rossby wave responses: a marked westerly anomaly appears in the equatorial eastern Pacific (EEP) for the TNA forcing (due to its westward location) while no significant wind response is observed in the EEP for the EA forcing. The westerly anomaly prevents a cooling tendency in the EEP through anomalous zonal and vertical advection according to a mixed layer heat budget analysis.