Geophysical Research Letters

Horizontal structure of monsoon depressions in the western North Pacific at formation time


Corresponding author: J. C. Beattie, Department of Meteorology, Naval Postgraduate School, 589 Dyer Rd., Monterey, CA 93943, USA. (


[1] Tropical cyclones that form from monsoon depressions in the western North Pacific have a larger outer wind structure, and those that form from monsoon gyres tend to be smaller than average. Quantitative values of the horizontal wind structure of monsoon depressions are analyzed from high-resolution European Centre for Medium-Range Weather Forecasts analyses and satellite imagery during 2009. Rather than a circular shape with a diameter of about 1000 km, 80% of the 44 monsoon depressions were elliptical in shape with average diameters of about 812 km north-south and 1098 km east-west. The latitudinal-to-longitudinal ellipticity factor of 0.69 ± 0.14 emphasizes the noncircular structure of these cases, which is important for the weather distribution and likely also for potential tropical cyclone formations. Seventeen tropical cyclones during 2009 formed from monsoon depressions and none from monsoon gyres, which contradicts earlier studies that about 70% form from monsoon gyres.