Decadal Weakening in the Connection between the Sea Surface Temperature Anomalies over the North Tropical Atlantic and the North Indian Ocean around the Early 2000s

Abstract The sea surface temperature anomalies (SSTAs) over the north tropical Atlantic (NTA) in spring are positively correlated with the subsequent summer SSTAs over the north Indian Ocean (NIO). We find that this NTA–NIO connection experienced a decadal weakening around the early 2000s. Before that, the warming in NTA leads to significant positive NIO SSTAs via tropospheric temperature mechanism and wind–evaporation–SST feedback, being consistent with previous studies. Nevertheless, after the early 2000s, the positive NTA SSTAs in spring could efficiently trigger negative SSTAs over the central and eastern tropical Pacific in the following seasons, in contrast to the weak response in the tropical Pacific to the NTA before the early 2000s. The negative SSTAs in the central and eastern tropical Pacific induce negative SSTAs over the NIO, which offsets the influence of the NTA on the NIO and leads to a weakened NTA–NIO connection. Additionally, the decadal variation in the NTA–tropical Pacific connection, as well as the NTA–NIO connection, tends to be linked to the decadal change in climatological SST over the tropics. The NTA SST warms faster relative to the central and eastern tropical Pacific, which increases the Atlantic–Pacific SST gradient and thus intensifies the response of the tropical Pacific to the NTA and in turn leads to the weakened NTA–NIO connection. Both observations and the Coupled Model Intercomparison Project phase 6 (CMIP6) preindustrial control simulations support the results above and highlight the importance of transbasin interactions among tropical oceans to the variation in NIO originating from the NTA. Significance Statement This study recognizes that the strong connection between the sea surface temperature anomalies (SSTAs) over the north tropical Atlantic (NTA) and the north Indian Ocean (NIO) occurs before the early 2000s but is weakened after that. The weakened NTA–NIO connection is due to the fact that the warming in the NTA efficiently triggers negative SSTAs over the central and eastern tropical Pacific which induces negative SSTAs over the NIO and in turn offsets the influence of the NTA on the NIO. Further analysis proposed that the decadal variation in the NTA–NIO connection, as well as the NTA–tropical Pacific connection, is related to the change in climatological SST over the tropics. The results highlight the role of three-ocean interactions in the variations in NIO originating from the NTA.