Excitation of Rainfall over the Tropical Western Pacific

The authors have examined 4 years of satellite-derived SST and rainfall data in anticipation of a relationship between SST structure and the excitation of convective rainfall. The results exhibit a strong excitation signal consistent with the presence of mesoscale SST gradients in about 75% of approximately 10 000 rainfall onset events. Rainfall onset events occur at locations with enhanced horizontal convergence, as inferred by the Laplacian of SST on scales of order 100 km. The daily SST field exhibits multiscale patchiness, spanning a 2+degrees C range. The signal is disproportionately large at SSTs that are 0.25 degrees C above the mean, near 29.5 degrees C; disproportionately weak for SST <= 28.8 degrees C; and proportionately neutral for SST >= 30.3 degrees C. The calculations suggest that a characteristic strength of this lower-boundary forcing (similar to 3 x 10(-5) s(-1)) is approximately one order of magnitude stronger than the mean regional background forcing (similar to 3 x 10(-6) s(-1)). The periphery of warm oceanic patches exhibits both convergent and divergent Laplacian values of similar frequency and magnitude; however, rainfall onset favors the locally convergent locations by a 3:1 ratio.