Patterns of K1 and M2 internal tides and their seasonal variations in the northern South China Sea

Seasonal variations of baroclinic tides for K-1 and M-2 constituents were separately studied using two-dimensional numerical simulations along the 21A degrees N section of the northern South China Sea (SCS). Results show that the continental slope of the northern SCS and the west ridge of the Luzon Strait are supercritical to K-1 internal tides, which may be trapped in the deep basin of the SCS and form standing or partial standing waves. Meanwhile, these areas are sub-critical to M-2 internal tides, which can transmit onto the shelf and are seldom reflected back into the basin. The trapped K-1 internal tides are dominated by mode-2 and mode-3 in summer and by mode-1 and mode-3 in winter. Moreover, high mode K-1 internal tides account for nearly 20-40 % of the total energy density in winter and 15-20 % in summer. The pattern of K-1 internal tides in the basin is mainly determined by the percentage of reflected energy from the continental slope. The phase difference between the incoming mode-1 and mode-2 K-1 internal tides near the continental slope are nearly out of phase in winter, which means that the percentage of reflection of the K-1 internal tide is larger than that in summer. Both the convergence and high mode K-1 internal tides can enhance the vertical shear. The above results indicate that, in the deep basin of the SCS, water mixing potentially induced by internal tides in winter is stronger than in summer.