NSCAT tropical wind stress maps: Implications for improving ocean modeling

Using wind vectors from the NASA scatterometer (NSCAT), daily maps of pseud ostress have been constructed for the tropical Pacific Ocean and compared w ith pseudostress maps derived from the European Centre for Medium-Range Wea ther Forecasts (ECMWF) 10-m wind product. The map resolution for the NSCAT pseudostress maps was selected using both a statistical measure of the expe cted mapping errors and tests on realistic wind fields. The selected map re solution is 5 days and 2 degrees, which minimizes residual effects from the NSCAT sampling pattern, while maximizing temporal and spatial resolution. Comparisons with the ECMWF maps showed significant differences in most regi ons, corresponding to mean wind speeds of 2-3 m s(-1), particularly in the Intertropical Convergence Zone (ITCZ) and at 20 degrees S and 20 degrees N. A canonical correlation analysis between NSCAT and ECMWF fields showed a h igh degree of correlation of temporal variations and systematic differences in spatial structure. In the NSCAT fields the ITCZ is narrower, stronger, and is located 1-2 degrees latitude farther south than in the ECMWF fields. The high degree of correlation between the two fields suggests that "hybri d" fields can be generated. The dynamical implications of the differences i n wind forcing are illustrated using estimates of the Sverdrup stream funct ion and the Ekman pumping. A simple reduced-gravity, linear vorticity model , forced by both the ECMWF and hybrid winds to examine predicted difference s in ocean response, showed higher skill for the NSCAT winds.