Upper ocean shear in the northern South China Sea

The shear in the upper ocean of the northern South China Sea (SCS) is examined based on current observations from six moorings in 2010–2011. Spectral analysis results indicate that the sub-inertial currents, near-inertial waves (NIWs), diurnal and semidiurnal internal tides (ITs) dominate the current and shear in the northern SCS. Through comparing current variance and shear caused by these motions, this study shows the great contribution of NIWs to shear: Although NIWs only account for 2–7% of the total current variance, the shear caused by NIWs is approximate one fifth to one-quarter of the total shear. Moreover, the NIWs are dominated by the component with upgoing phase and downgoing energy, whereas the upgoing and downgoing components are comparable in both diurnal and semidiurnal ITs. Because the incoherent component has a larger contribution to shear than the coherent component, the shear of both diurnal and semidiurnal ITs exhibits significant signals with frequencies larger than the spring-neap cycles of approximate 14 days. The larger contribution to shear and smaller proportion in current variance suggest that the incoherent component of ITs has a larger vertical wavenumber than the coherent component. In addition, a case study shows that the mesoscale eddy pair occurring between 22 October and 2 December 2010 does not significantly enhance the ocean shear at two moorings especially below 150 m depth, although it contributes a lot to the current variance.