Sea surface height variations in the South China Sea from satellite altimetry

Sea surface elevation in the South China Sea is examined in the Topex/Posei don altimeter data from 1992 to 1995. Sea level anomalies are smoothed alon g satellite tracks and in time with tidal errors reduced by harmonic analys is. The smoothed data are sampled every ten days with an along-track separa tion of about 40 km. The data reveal significant annual variations in sea l evel. In winter, low sea level is over the entire deep basin with two local lows centred off Luzon and the Sunda Shelf. In summer, sea level is high o ff Luzon and off the Sunda Shelf, and a low off Vietnam separates the two h ighs. The boundary between the Vietnam low and Sunda high coincides with th e location of a jet leaving the coast of Vietnam described in earlier studi es. Principal component analysis shows that the sea level variation consist s mainly of two modes, corresponding well to the first two modes of the win d stress curl. Mode 1 represents the oscillation in the southern basin and shows little inter-annual variation. The mode 2 oscillation is weak in the southern basin and is strongest off central Vietnam. During the winters of 1992-1993 and 1994-1995 and the following summers, the wind stress curl is weak, and the mode 2 sea level variation in the northern basin is reduced, resulting in weaker winter and summer gyres. Weakening of the Vietnam low i n summer implies diminishing of the eastward jet leaving the coast of Vietn am. The results are consistent with model simulations. (C) Elsevier, Paris / Ifremer / Cnrs / Ird.