Excitation of rainfall over the Tropical Western Pacific

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+°C range. The signal is disproportionately large at SSTs that are 0.25°C above the mean, near 29.5°C; disproportionately weak for SST ≤ 28.8°C; and proportionately neutral for SST ≥ 30.3°C. The calculations suggest that a characteristic strength of this lower-boundary forcing (~3 × 10−5 s−1) is approximately one order of magnitude stronger than the mean regional background forcing (~3 × 10−6 s−1).