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.
While total rainfall is positively correlated with SST, the 4-year average pattern of rainfall deviates substantially from this commonly assumed equilibrium. The heaviest rainfall is associated with propagating events of long duration; as long as 4 days and up to 2000km zonal distance traveled. These events occur under conditions of moderate easterly winds and shear above the marine boundary layer, within which there is westerly flow. Rainfall amounts monotonically decline with decreased westerly flow, the weakest events occurring during calm surface wind conditions.