Western North Pacific monsoon depressions: formation, structure, and transition to tropical cyclones
Beattie, Jodi C.
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Even though monsoon depressions are the synoptic precursors for a substantial fraction of the western North Pacific tropical cyclones, little is known about how they form. High resolution reanalyses, satellite imagery and precipitation estimates are used to examine the physical mechanisms for monsoon depression formation in all 44 cases during 2009. An interaction of at least one cross-equatorial airstream from the Southern Hemisphere with the monsoon trough or confluent zone preceded all 44 monsoon depression formations. Four types of cross-equatorial airstreams provide wave-activity flux that contributes to the formations. At the formation time, the elliptical monsoon depressions on average are 1098 km east-west and 812 km north-south, and have vorticity maxima on either end. Wave-activity flux convergence over a vorticity maximum within the monsoon depression was analyzed in all transitions to tropical cyclones during 2009. In about half of these transitions, the inward-directed wave-activity flux was associated with the same primary airstream as during the monsoon depression formation. In the other half, another airstream or the trade easterlies was the source of the wave-activity flux. Persistent wave-activity flux convergence was not analyzed in any of the 25 monsoon depressions that failed to transition to a tropical cyclone.
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