Impact of Sea Surface Temperature Uncertainty on the Western North Pacific Subtropical High (WNPSH) and rainfall

This paper examines the sensitivity of short-term forecasts of the Western North Pacific Subtropical High (WNPSH) and rainfall to sea surface temperature (SST) uncertainty using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®). A comparison of analyzed SST with satellite observations of SST indicates that SST analysis errors are particularly pronounced on horizontal scales from 100-200 km, similar to the mesoscale eddy scales in the Kuroshio region. Since significant oceanic variations occur on these scales, it is of interest to examine the effect of representing this small scale uncertainty with random, scale dependent perturbations. A SST ensemble perturbation generation technique is used here that enables temporal and spatial correlations to be controlled and produces initial SST fields comparable to satellite observations.

The atmospheric model develops large uncertainty in the Korea and Japan area due to the fluctuation in horizontal pressure gradient caused by the location of the WNPSH. This, in turn, increases the variance of the low-level jet (LLJ) over southeast China, resulting in large differences in the moist transport flux from the tropical ocean and subsequent rainfall. Validation using bin-mean statistics shows that the ensemble forecast with the perturbed SST better distinguishes large forecast error variance from small forecast error variance. The results suggest that using the SST perturbation as proxy for the ocean ensemble in a coupled atmosphere and ocean ensemble system is feasible and can save computational time.