Structure of the Western North Pacific Monsoon Depression

The monsoon depression is one synoptic-scale feature that is favorable to tropical cyclone formation over the western North Pacific during the summer monsoon. In conjunction with a study of the processes involved in the formation of the monsoon depression, this study has documented the horizontal and vertical structures of the monsoon depressions at formation time. A key objective has been to document the average and variability in the horizontal structure of these monsoon depressions to give a more quantitative description than the original ~1000 km diameter definition developed by Mark Lander for the Joint Typhoon Warning Center (1994; Lander 2004). A second objective has been to differentiate the structure of the monsoon depressions from the structure of the western North Pacific monsoon gyre (diameter of ~2500 km), also as defined by Mark Lander (1994).

This observational study examined the 43 monsoon depressions that occurred from April to December 2009 using the 0.25 degree latitude/longitude resolution analyses from the European Center for Medium-range Weather Forecasts (ECMWF) as archived on the Year of Coordinated Observing Modeling and Forecasting Tropical Convection (YOTC) website. The timing of the monsoon depression formations was established from our previous study that developed a conceptual model of western North Pacific monsoon depressions. Based on the ECMWF analyses, the 850 hPa wind and vorticity diameters were estimated. Separate estimates of the diameters were calculated from the infrared cloud images, the TRMM precipitation product, and the QuikSCAT surface winds. Most of the monsoon depressions (86%) were found to be predominately elliptical in shape, with average longitudinal diameters of 1200 km and latitudinal diameters of 900 km, with standard deviations of about 200 km. A minority of the depressions were more circular (14%), and tended to be smaller in horizontal scale. The elliptical shape is attributed to the mechanisms that create dual vorticity centers, and specifically the special role played by cross-equatorial flows from the Southern Hemisphere in creating the eastern vorticity center as in the Beattie and Elsberry (2010) conceptual model of monsoon depression formation in the western North Pacific. As expected for a warm-core system, the monsoon depressions at formation time have low-level westerlies (easterlies) on the equatorward (poleward) side that change to easterlies (westerlies) with height. In general, the larger, elliptical monsoon depressions also tended to extend to higher levels than the smaller, circular depressions. No monsoon gyres as defined by Lander (1994) were found to exist during the 2009 season. Since 16 of the monsoon depressions evolved into typhoons, this study is in contrast to some previous studies in the literature that attributed a major fraction of western North Pacific typhoon formations to monsoon gyres.