Influences of Indian and Pacific Ocean coupling on the propagation of tropical intraseasonal oscillation

This study examines the influences of Pacific and Indian Ocean coupling on the propagations of tropical intraseasonal oscillation (ISO) in boreal winter (November-April) and summer (May-October). A series of three basin-coupling experiments are performed with the UCLA CGCM, in which the air-sea coupling is limited to the Indian Ocean, the Pacific Ocean, or both the Indian and Pacific oceans. Using extended EOF and composite analyses, the leading ISO modes are identified and compared among the observations and the experiments. Results show that the Indian Ocean coupling is an important part of ISO generation mechanism in both the winter and summer seasons. Without the coupling, the intensity of simulated ISO is reduced. As for the propagation characteristics, this study finds the Pacific Ocean coupling important to the zonal propagation of wintertime ISO and the Indian Ocean coupling crucial to the meridional propagation of summertime ISO. Without the Pacific Ocean coupling, the propagation of the wintertime ISO is mostly limited within the Indian Ocean sector. Lacking of the Indian Ocean coupling, the simulated ISO in boreal summer can not propagate northward into the Asian-Pacific monsoon regions. This result suggests that the Indian Ocean coupling is an indispensable ingredient to the summertime ISO dynamics. This study also indicates that basic-state SST and low-level westerly biases in coupled experiments give rise to erroneous standing components of ISO in the central Indian and Pacific oceans.