Assimilating altimetric data into a South China Sea model

Sea surface heights from the TOPEX/Poseidon altimeter are assimilated into a three-dimensional primitive equation model to derive the circulation in t he South China Sea. With data assimilation the model resolves not only the basinwide circulation but also a dipole off Vietnam and a low/high feature near the Luzon Strait. Mesoscale features are missing in the simulation wit hout data assimilation because of poor resolution in the wind field and ina dequate knowledge of the transport through the Luzon Strait. Compared to th e case without data assimilation, data assimilation reduces the root mean s quare error between the simulated and observed sea surface heights by a fac tor of 2-3. Circulation derived from data assimilation under climatological conditions is contrasted with that during El Nino. In the normal winter of 1993-1994, flow at 50 m depth is strong and cyclonic. Flow at 900 m depth is cyclonic as well. The deep cyclone persists into the following summer. D uring the 1994-1995 Fl Nino winter, features in the flow field at 50 m dept h either weaken or disappear, and circulation at 900 m depth is anticycloni c. In the summer of 1995 the dipole and the eastward jet off Vietnam at 50 m depth are missing, and the anticyclonic circulation at 900 m depth persis ts. Temperature at 65 m shows significant warming from fall 1994 to summer 1995. A weakened flow field and warming in the upper ocean are consistent w ith findings from earlier Fl Nino events.