Western North Pacific Monsoon Depression Formation

Relatively few studies have been carried out as to the conditions leading to the formation of monsoon depressions in the western North Pacific. Two monsoon depression formations during July 2007 were analyzed using ECMWF analyses and satellite observations. Wave-activity flux calculations indicated that cross-equatorial flow from the Southern Hemisphere played an important role in the formation of these monsoon depressions. A new conceptual model of monsoon depression formation in the western North Pacific is proposed that includes three southerly airstreams in the Southern Hemisphere that lead to cross-equatorial flows into the Northern Hemisphere. Examination of 44 monsoon depressions from April to December 2009 confirms the critical role of these cross-equatorial flows in monsoon depression formation. All of the monsoon depressions in the 2009 sample for which formation conditions could be established had at least one of three possible airstreams that interacted with a confluent region and, thus, may be a necessary condition for monsoon depression formation. This conceptual model of monsoon depression formation was further confirmed by means of wave-activity flux calculations and backward trajectory ensembles for the 2009 cases.