Delay of the Baiu Withdrawal in Japan under Global Warming Condition with Relevance to Warming Patterns of SST

Abstract

Climatologically, an abrupt enhancement of convection occurs in mid-July over the western North Pacific (WNP) around 20°N, 150°E known as a convection jump (CJ), which is associated with a rapid diminishment of the Baiu rainband over central Japan through excitation of stationary Rossby wave. In view of the subtropical-origin teleconnection, we examined a projected future delay of the Baiu withdrawal under global warming conditions with relevance to future changes in seasonal evolution over the WNP. Based on reproducibility evaluation of the Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model outputs with respect to seasonal evolution of summertime precipitation over central Japan, we computed weighted multi-model ensembles, which are used for diagnosis of the projected future CJ changes. The result shows that CJ under global warming will occur more southeastward. In the future projection, increases in sea surface temperature (SST) over the equatorial Pacific and subtropical North Pacific are larger than those in the adjacent regions. Sensitivity experiments with a general circulation model (MJ98 AGCM) utilizing these salient warm SST anomalies also show the southeastward shift of CJ in comparison with a control simulation. These results indicate that the change in the location of subtropical convections associated with the spatial distribution of warm SST anomalies could be an important factor for the late termination of the Baiu season over central Japan under global warming conditions.