Effect of the Ocean-Atmosphere Interaction on the Northwestward Propagation of the Intraseasonal Oscillation in the Western North Pacific

The study finds that the convection of the intra-seasonal time scale in the western North Pacific during the boreal summer tends to propagate northwestward in the Philippine Sea to near 20N and then continues propagating westward. The formation of enhanced convection in the western North Pacific is a result of the merging of a convective system moving eastward along the equator and a westward-propagating low-level convergence anomaly, located to the east of a vortex, in the subtropics. A positive feedback between the anomalous circulation and convection leads to a rapid enhancement of both. The strengthened southwesterly associated with the vortex enhances evaporation over the oceans (e.g., the Indian Ocean, the Bay of Bengal, and the South China Sea) and transports moisture northeastward. The moisture converges at the northwestern corner of the convection and results in a potentially unstable atmosphere. The result is the northwestward propagation of the coupled circulation-convection system in the western North Pacific. A similar mechanism also affects the following westward-propagating feature in the subtropics, which exhibits characteristics of a Rossby wave. It was found that the ocean-atmosphere interaction plays an important role in supplying energy to sustain the circulation and convection during the course of propagation. The circulation-convection interaction is the key factor in maintaining the system's strength until it reaches the Asian landmass when the supply of moisture is cut off. It is suggested that the combination of ocean-atmospheric interaction and circulation-convection interaction lead to the propagation of this system.