Classification and 3-D distribution of upper layer water masses in the northern South China Sea

Using the fuzzy cluster analysis and the temperature-salinity(T-S) similarity number analysis of cruise conductivity-temperature-depth(CTD) data in the upper layer(0–300 m) of the northern South China Sea(NSCS), we classify the upper layer water of the NSCS into six water masses: diluted water(D), surface water(SS),the SCS subsurface water mass(U_S), the Pacific Ocean subsurface water mass(U_P), surface-subsurface mixed water(SU) and subsurface-intermediate mixed water(UI). A new stacked stereogram is used to illustrate the water mass distribution, and to examine the source and the distribution of U_P, combining with the sea surface height data and geostrophic current field. The results show that water mass U_P exists in all four seasons with the maximum range in spring and the minimum range in summer. In spring and winter, the U_P intrudes into the Luzon Strait and the southwest of Taiwan Island via the northern Luzon Strait in the form of nonlinear Rossby eddies, and forms a high temperature and high salinity zone east of the Dongsha Islands. In summer, the U_P is sporadically distributed in the study area. In autumn, the U_P is located in the upper 200 m layer east of Hainan Island.

Tide- and wind-driven variability of water level in Sansha Bay, Fujian, China

This study analyzes water-level variability in Sansha Bay and its adjacent waters near Fujian, China, using water-level data observed from seven stations along the coast and wind data observed from a moored buoy near Mazu Island. At super-to near-inertial frequencies, tides dominated the water-level variations, mainly characterized by semi-diurnal （prmafily M2, S2, and N2） and diurnal tides （primarily Kb O1）. The correlation coefficients between residual （non-tidal） water-level time series and the observed wind-stress time series exceeded 0.78 at all stations, hinting that the wind acting on the study region was another factor modulating the water-level variability. A cross-wavelet and wavelet-coherence analysis further indicated that （i） the residual water level at each station was more coherent and out-of-phase with the alongshore winds mostly at sub-inertial time scales associated with synoptic weather changes; and （ii） the residual water-level difference between the outer and inner bay was more coherent with the cross-shore winds at discrete narrow frequency bands, with the wind leading by a certain phase. The analysis also implied that the monsoon relaxation period was more favorable for the formation of the land-sea breeze, modulating the residual water-level difference.