Description of a Monsoon Gyre and Its Effects on the Tropical Cyclones in the Western North Pacific during August 1991

Abstract

This paper describes the character and evolution of the low-level wind, sea level pressure, and satellite-observed cloudiness over the western North Pacific (WNP) during August 1991 when the low-level monsoon circulation there became organized as a monsoon gyre. The specific configuration of the monsoon circulation, which herein is called a monsoon gyre, is an episodic event?occurring roughly once per year, for two or three weeks during July, August, or September. As a monsoon gyre, the low-level circulation of the WNP becomes organized as a large cyclonic vortex associated with a nearly circular 2500-km-wide depression in the contours of the sea level pressure. A cyclonically curved band of deep convective clouds rims the southern through eastern periphery of this large vortex. Once this pattern is established, it becomes a prolific generator of mesoscale vortices that emerge from the downstream end of the major peripheral cloud band. These mesoscale vortices form the seed disturbances for midget or small-sized tropical cyclones. The large area encompassed by the outermost closed isobar of the monsoon gyre of August 1991 (the centroid of which moved slowly westward along 20°N) was the site of genesis for two tropical depressions, two tropical storms, and two typhoons during its 20-day westward journey. Initially, small tropical cyclones formed in the peripheral circulation of the gyre and later, the gyre itself evolved into a very large tropical cyclone; this is suggestive of two distinct modes of tropical cyclogenesis: one mode operates to produce small tropical cyclones in the eastern periphery of the gyre, and the other mode operates to accelerate the winds of the monsoon gyre until it becomes a giant tropical cyclone.