The role of the Bjerknes and low-cloud feedbacks in the formation of the eastern equatorial Pacific warming pattern: contrasting two climate models

The majority of climate models predict the development of an enhanced eastern equatorial Pacific (EEP) warming pattern (“El Niño-like”) by century-end, characterized by greater mean warming over the Pacific cold tongue compared to the western Pacific warm pool and the corresponding weakening of the Walker circulation. A number of plausible mechanisms have been proposed to explain this pattern; however, it remains unclear which proposed mechanism is dominant in this response. Moreover, the magnitude of the EEP pattern varies greatly across climate models. To understand these differences, we conduct partially coupled experiments with an abrupt 4xCO2 increase, wherein surface wind stress and shortwave fluxes are overridden to values prescribed from the preindustrial control simulations, using two climate models – CESM1 and CESM2. Although both models were developed at NCAR, their behaviors are very different. In the former model, changes in the east-west SST gradient along the equator are relatively small. In contrast, the latter model, known to have a high climate sensitivity, develops a very strong EEP pattern. We find that the key factors that explains these differences are the different strengths of the  Bjerknes (wind stress-SST) and shortwave (low clouds-SST) feedbacks critical in reducing the Pacific zonal SST gradient, whereas differential evaporative cooling in the equatorial region appears to be similar between the two models. We discuss the implications of these results to the ongoing and future changes in the tropical Pacific.