Strengthening of the Southern Side of the Jet Stream and Delayed Withdrawal of Baiu Season in Future Climate

The withdrawal of Baiu, the end of early summer rainy period over Japan, is marked by a stepwise northward migration of the westerly jet in the upper troposphere. The Coupled Model Intercomparison Project Phase3 (CMIP3) dataset was utilized to investigate possible changes in the seasonal development under a warmer climate. To obtain realistic model simulations, we evaluated model performance by comparing simulated and observed westerly jets for the present-day climate, selecting the top five models. Future climate projections using these models show that the westerly jet will be strengthened to the south of the jet axis, and the amount of precipitation in late July will be increased over the main land of Japan. These findings indicate that, by the end of the 21st century, withdrawal of the Baiu season will be delayed. The CMIP3 models also allowed changes in larger-scale atmospheric circulation to be predicted. Most of the CMIP3 models project that the Asian jet will be intensified on its equatorward side. Decreased upper tropospheric divergence in the western North Pacific and simultaneous shrinking of the Tibetan high suggest that the simulated change in the westerly jet is closely connected to the weakening of monsoonal circulation in Asia.