Tidal currents in the coastal waters east of Hainan Island in winter

Abstract

Using the 25-h continuing hydrographic observations at three successive stations in February 2012 and the mooring time series of current observations from January to March 2015, the tidal currents and tidal energy fluxes in the coastal waters east of Hainan Island in the northwestern South China Sea were analyzed. The diurnal and semidiurnal (using K₁ and M₂ as proxies, respectively) tidal currents and associated isopycnal undulations were derived with harmonic analysis. Results show that the velocities of the diurnal and semidiurnal tides derived from the 25-h observations are comparable to those from the mooring series. The semi-major axes of the tidal ellipses were O(4–7 cm/s) for barotropic tides and O(2–4 cm/s) for baroclinic tides. The directions were in NE-SW at deeper stations to N-S at the shallowest station for the diurnal tide and from NW-SE to NE-SW for the semidiurnal tide. Both the diurnal and semidiurnal isopycnal fluctuations reached O(5 m), O(8 m), and O(10 m) at the cross-shelf stations (H03, H04, and H05) from 35 m, 45 m, to 55 m, respectively, showing insignificant vertical variation, and the barotropic signals were predominate. The baroclinic diurnal tide showed first-mode structures at H03-05, as does the semidiurnal tide at H03. The semidiurnal tide at H04 and H05 exhibited higher-mode structures. The time series of both the alongshore and cross-shore components reveal the vertically propagation features of the baroclinic tidal phase and energy. The calculated horizontal energy fluxes of the diurnal and semidiurnal internal tides decreased from O(0.1 W/m) at H05 to O(0.01 W/m) at H03, implying a propagation and dissipation of energy from offshore to inshore.