Cold Sea Surface Temperature near Cheju Island Responding to Strong Cyclonic Wind and Positive Geopotential Tendency behind a Typhoon Center along its Track

Abstract

The response of sea surface temperature (SST) before and after the passage of Typhoon-18 Songda in the vicinity of Chjeu Island in the southern sea of Korea was investigated using satellite-derived GOES-MCSST (SST), GOES infrared cloud image and a three dimensional WRF-3.0 model with FNL initial meteorological data on September 5 to 8, 2004. On September 4, before the typhoon passed by Cheju Island, the SST extending southwards and eastwards of the island was 27°C. However, after the typhoon passed by the island at 2100 LST, September 7, the SST decreased by 2°C to 25°C. Asymmetrical-cyclonic surface wind generated by the combination of the movement of the typhoon and the cyclonic winds of the typhoon itself, caused divergence of wind driven currents, which induced upwelling of deep sea colder water to the sea surface and outward spreading of cold waters near the island. This resulted in the decrease of SST and a negative maximum geopotential tendency of 500 hPa for 24 hours (∂Φ/∂t; m/day) was detected in the center of the typhoon eye, where the thinner atmospheric layer existed. This induced a decrease in the sea surface height and in the downwelling of surface waters. A positive maximum geopotential tendency was detected behind the typhoon center along its track, where maximum expansion of the atmospheric depth caused the level of the sea surface to rise and upwelling of deep sea cold waters to the surface. Thus, the slope height ranging from 422 to 639 m/day between two maxima of geopotential tendencies could strongly influence the sea surface elevation, thereby inducing a very strong upwelling and outward spreading of deep sea colder waters toward the inclined sea surface, resulting in a decrease of SST in the wake of the typhoon track.