The role of the dry static stability for the recent change in the Pacific Walker circulation

Abstract

There is an uncertainty in how the Pacific Walker circulation (PWC) will change in response to increased greenhouse gas (GHG) warming. On average, climate models predict that the PWC will weaken. Observational evidence is mixed, with some evidence supporting the models while others do not. In this study, insight into the PWC trend is provided by examining the tropical dry static stability, a quantity that is inversely proportional to the strength of the PWC. For the 1979-2012 period, the static stability increased markedly in all phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, far more so than in the satellite and global reanalysis data, which show a strengthening of the PWC. The stabilization is greater for a subset of models that simulate a significant weakening of the PWC.With the observed sea surface temperature as the lower boundary condition, over the western tropical Pacific, atmospheric models that belong to the weakening-PWC-CMIP5 group produce greater stabilization than those that belong to the strengthening-PWC-CMIP5 group. Compared with the latter group, the former group of atmospheric models simulates weaker trade winds over the western and central tropical Pacific and, consistent with the Bjerknes mechanism, the corresponding CMIP5 models produce a weaker west-east gradient in tropical SST. Given that the models' convective parameterizations overstabilize the atmosphere compared with an explicit convection, the findings here suggest that the models' representations of tropical convection and stability contribute to the models' tendency to simulate a weakening of the PWC and an El Nino-like SST.