Excitation of Rainfall Over the Tropical Western Pacific

We have examined 4-year 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 ~75% of ~10,000 rainfall onset events. The signal is disproportionately large for slightly above average SST (circa 29.5°C); disproportionately weak for cool SST (<28.8°C); and proportionately neutral at high SST (>30.4°C). The signal for rainfall onset locations is defined by horizontal convergence, as inferred by the Laplacian of SST on scales from 50-200 km. Calculations infer the characteristic strength of this lower boundary forcing (~10-5s-1) to be approximately one order of magnitude stronger than the mean regional background forcing (~10-6s-1). There is a statistical preference for excitation of rainfall on eastward-directed SST gradients. For example, under prevailing westerly surface flow, this location corresponds to the windward shoulder of a warm SST patch. Large amplitude SST anomalies appear daily in mesoscale patches throughout the region, thereby suggestive of local non-equilibrium and a causal relationship with deep moist convection.