The sea surface height oscillation with a quasi-four-month period (SSHO4) along continental slope in the northern South China Sea (NSCS) is detected using satellite altimeter data and an ocean model simulation. The SSHO4 is at southwest of Dongsha Island, and is characterized by a wavelength of ~600 km and a southwestward phase speed of ~0.1 m/s. Crossing the climatological background SST front, geostrophic currents corresponding to the SSHO4 generally induce sea surface temperature (SST) “tongues” during January-March. The cold and warm SST tongues appear southwest of cyclonic and anticyclonic eddies, respectively. The distance between the warm and cold SST tongues is about half the wavelength of the SSHO4. The geostrophic currents play an important role in lateral mixing, as manifested by the SST tongue phenomena in the NSCS. The sea surface height oscillation with a quasi-four-month period (SSHO4) along the continental slope in the northern South China Sea (NSCS) is detected using satellite altimeter data and an ocean model simulation. The SSHO4 is at southwest of Dongsha Island, and is characterized by a wavelength of ~ 600 km and a southwestward phase velocity of ~ 0.1 m / s. Crossing the climatological background SST front, geostrophic currents corresponding to the SSHO4 generally induced sea surface temperature (SST) March. The cold and warm SST tongues appear southwest of cyclonic and anticyclonic eddies, respectively. The distance between the warm and cold SST tongues is about half the wavelength of the SSHO4. The geostrophic currents play an important role in lateral mixing, as manifested by the SST tongue phenomena in the NSCS.