Interdecadal Change in Development of the Baiu Front in a Coupled GCM

In early boreal summer, June, the Baiu front, which is also called the Mei-Yu front in China, is located along the boundary between the subtropics and the extratropics in the western North Pacific near Japan. In the Baiu frontal activity, an interdecadal change was detected around 1990, which was characterized by larger amount of precipitation in 1990's than in 1980's and by the anomalous upward surface heat fluxes with positive sea surface temperature (SST) anomalies in the Baiu/Kuroshio (B/K) region in 1990's (Tomita et al. 2007). This interdecadal change was locally linked to the Pacific Decadal Oscillation that was submerged under the seasonal thermocline in boreal summer in other regions. To generalize the proposed physical processes through a case study, this work extended the analysis using a 150-year control integration of a coupled GCM. On interdecadal time scales, a significant positive correlation is confirmed between simulated SST and precipitation in the B/K region. In decades when the positive precipitation anomalies appeared in this region, the upward surface heat fluxes were anomalously large with positive SST anomalies. The close examination has further revealed that there are slight spatial phase differences in anomalies among SST, precipitation, and surface heat fluxes. The anomalies in SST and surface heat fluxes are almost coherent in the northern part of precipitation anomalies. This spatial phase difference suggests that the surface air-sea interaction induces an interdecadal modulation of the Baiu precipitation in the seasonal northward moving of the front.