A CGCM study on the northward propagation of tropical intraseasonal oscillation over the asian summer monsoon regions

摘要

This study performs numerical experiments to (1) examine the influences of Pacific and Indian Ocean couplings on the propagation of tropical intraseasonal oscillation (ISO) in the extended boreal summer (May-through-October) and (2) determine the relative contributions of the ocean coupling and internal atmospheric dynamics to the ISO propagation over the Asian-Pacific monsoon regions. For (1), three basin-coupling experiments are performed with a coupled atmosphere-ocean general circulation model (CGCM), in which the air-sea coupling is limited respectively to the Indian Ocean, the Pacific Ocean, and both the Indian and Pacific oceans. For (2), three forced experiments are performed with the atmospheric GCM (AGCM) component of the CGCM, in which the sea surface temperature (SST) climatologies are prescribed from the CGCM experiments. Using extended Empirical Orthogonal Function and composite analyses, the leading ISO modes are identified and compared between the observation and the model experiments. The CGCM modeling results show that the Indian ocean coupling is more important than the Pacific Ocean coupling to promoting both zonal and meridional propagations of the summertime ISO. In this season, the Indo-Pacific warm pool retracts westward and shifts into the Northern Hemisphere allowing the Indian Ocean coupling to become more important. The Indian Ocean coupling is found to promote the northward propagation mainly through wind-evaporation feedback, whereas in observations the cloud-radiation feedback is found to be equally important. The AGCM modeling results indicate that monsoonal dynamics aid the meridional ISO propagation mainly in the low-level winds. Without the ocean coupling, the northward ISO convection feature is weaker and is limited by the northern boundary of climatic easterly vertical shear. The ocean coupling enables the simulated ISO-related convections to cross the northern boundary of the shear. This modeling study concludes that the Indian Ocean coupling plays a crucial rather than a secondary role for the observed northward propagation of summertime ISO.