Shengan Wang

Diurnal wind and nonlinear interaction between inertial and tidal currents in the South China Sea during the passage of Typhoon Conson

Diurnal wind (DW) and nonlinear interaction between inertial and tidal currents near the Xisha Islands of the South China Sea (SCS) during the passage of Typhoon Conson (2010) are investigated using observational data and a damped slab model. It is found that the DWs, which are dominated by clockwise wind components, are prominent at our observational site. The DWs increase after the passage of the typhoon from 1 to about 4 m/s, which may be due to the decrease of the sea surface temperature caused by the passage of the typhoon. Kinetic energy spectra and bicoherence methods reveal nonlinear interactions between the inertial currents and the 2MK3 tidal constituent at our observational site. The slab damped model reproduces the inertial currents successfully induced by the total observed winds, and it is shown that the inertial currents induced by DWs are positively proportional to the DWs speed. Even though the observed inertial currents are distinct, the proportion of inertial currents induced by DWs to those induced by the total observed winds is just 0.7%/4% before/after the passage of typhoon. This shows that the inertial currents induced by the DWs are unimportant near the Xisha Islands during the typhoon season.

Seasonal upper circulation in the Sulu Sea from satellite altimetry data and a numerical model

Eight years of Absolute Dynamic Topography from satellite altimetry data are used to study the seasonal variability of the circulation in the Sulu Sea (SS) through Empirical Orthogonal Function (EOF) analysis. It is revealed that first seasonal EOF mode shows a basin-scale anticyclonic/cyclonic circulation in summer/winter and second seasonal EOF mode shows a weak basin-scale anticyclonic/cyclonic meander flow from the Mindoro Strait to the Sibutu Passage, and the typical surface circulation structure in the SS is shown as a basin-scale anticyclonic/cyclonic circulation (or meander) centered at about 120.8 degrees E, 8.6 degrees N in August and December. According to the numerical experiments by a connected single-layer and two-layer model, it is shown that the upper circulation in the SS is closely related to the outflow via the Sibutu Passage and seasonal local wind stress. Either an outflow via the Sibutu Passage or the summer monsoon may cause an anticyclonic circulation in the SS, while the winter monsoon may cause a cyclonic circulation. Either an outflow via the Sibutu Passage or the winter monsoon would push the water out of the SS via the Balabac Strait but bring the water into the SS via the Mindoro Strait, while the summer monsoon would bring the water into the SS via the Balabac Strait but push the water out of the SS via the Mindoro Strait. Thus, in summer, the induced anticyclonic circulation with the negative relative vorticity is stronger in the SS but the water transports via the Mindoro and the Balabac straits are less, while in winter, the induced cyclonic circulation with the mainly positive relative vorticity is weaker but the water transports via the Mindoro and the Balabac straits are larger. The inflow via the Mindoro Strait is also significant since the outflow via the Sibutu Passage is mainly compensated by the inflow via the Mindoro Strait. The western strengthening near the Palawan Island and the asymmetry of the circulation in the SS is caused by the beta effect. The transport via the Mindoro Strait is generally much larger than that via the Balabac Strait. An inflow into the SS via the Dipolog Strait contributes little to the circulation in the SS except for the current field near this strait. A stronger lower layer current than the upper layer one near the Mindoro Strait is also discussed.

The correlation of the surface circulation between the Western Pacific and the South China Sea from satellite altimetry data

Fifteen years of satellite altimetry data are used to study the correlation of the surface circulation between the Western Pacific and the South China Sea (SCS) based on the Singular Value Decomposition method. There is a high correlation between the circulation of the SCS and the North Equatorial Current (NEC) current system. In summer/winter, the sea surface height (SSH) of the NEC rises/falls while the two SCS sub-basin-scale eddies are anti-cyclonic/cyclonic. It reveals both annual and eight-year variability in the circulation. The NEC bifurcation latitude is highly correlated with the water exchange via the Luzon Strait (LS) and the Luzon Cyclonic Eddy (LCE). When the NEC bifurcation latitude moves northward/southward, the water exchange via the LS increases/reduces and the SSH of the LCE reduces/increases. The annual mean number of eddies propagating westward into the Western Pacific within (121∼135°E, 15∼25°N) is about 18∼27 with a 3∼4 year fluctuation. No eddy is found to propagate into the SCS via the LS from the Western Pacific, and the eddy density is defined to show the different distributions between the cyclonic eddies (CEs) and anti-cyclonic eddies (ACEs) in the north-eastern SCS and north-western Pacific, and the difference in the concentration and interaction with the Kuroshio between the CEs and the ACEs is also discussed. The monthly statistics of the number, tracks, lifetimes and moving speeds etc. of the eddies in the north-western Pacific are also shown.