The 30–50 Day Variation of Convective Activity over the Western Pacific Ocean with Emphasis on the Northwestern Region

Abstract

Coherent 30?50 day oscillations between regions north and south of 20°N were observed using the latitude?time (y?t) diagram of a convective index (Ic) along 140°E over the entire 1979 summer (May?September). The oscillation in the northern region exhibits a relatively regular north?south movement of deep convective clouds, while the oscillation in the southern region shows enhancement or suppression fluctuations of broad and intensive deep convective clouds with a period of 30?50 days. The southward (northward) movement of the midlatitude oscillation coincides with the suppression (enhancement) of the tropical deep convective clouds. The midlatitude deep convective clouds are usually maintained by the divergent circulation and are associated with fronts. The empirical orthogonal function (EOF) analysis of velocity potential at 200 mb shows that the north?south, 30?50 day oscillation of deep convective clouds over the northwestern Pacific is a response of the regional weather system to the eastward propagation of this low-frequency mode. The surface charts in the vicinity of Japan show that 1ocations of fronts passing through 140°E north of 20°N undergo a similar oscillation as indicated by the y?t diagram of Ic. With both the EOF analysis of velocity potential, and the comparison between the time series of the 850-mb height field averaged over the center of Pacific anticyclone and those over the equatorial trough of the western Pacific, it is shown that the coherent 30?50 day oscillations in the tropics and midlatitudes revealed in this study are induced by the eastward propagation of the 30?50 day mode.