Large-scale environmental parameters associated with tropical cyclone formations in the western North Pacific

The local environmental conditions associated with 405 tropical cyclone (TC) formations in the western North Pacific during 1990-2001 are examined in this study. Six large-scale parameters are obtained and computed from the NCEP reanalyses with 2.5degrees latitude resolution. These include the sea surface temperature, 200-850-hPa vertical and meridional zonal shear, 500-700-hPa average relative humidity, convective available potential energy, and 200-hPa divergence. It is found that these parameters show quite consistent values in almost all the cases examined, and near-Gaussian distributions are found for them. The mean values of these distributions are 28.9degreesC, -3.2 m s(-1), -2.3 m s(-1), 61.8%, 1184.1 J kg(-1), and 5.2 x 10(-6) s(-1) for the above six domain-averaged parameters, respectively. Based on these distributions of values, a formation potential area is defined as a location that possesses these environmental parameters, all within the thresholds (one standard deviation from the mean value found above) of favorable formations. The correlations of this formation potential area with the observed submonthly, seasonal, and interannual variations in tropical cyclone activity are discussed.