Joint Effects of Winds and Tides on Near-Inertial Internal Waves in the Northern South China Sea: A Three-Dimensional Numerical Study

Abstract

Abstract Joint effects of winds and tides on near-inertial internal waves (NIWs) are numerically investigated via a series of three-dimensional quasi-realistic simulations in the northern South China Sea (NSCS). Model results demonstrate that in the presence of wind-induced NIWs, more tidal energy is transferred to NIWs, while in the presence of tide-induced NIWs, the extreme wind (cyclone) would inject less near-inertial kinetic energy (NIKE). The interaction between wind-induced and tide-induced NIWs produces total NIKE more (or less) than a linear superposition of that generated by wind and tide forcing alone at different sites in the NSCS. Specifically, near the Luzon Strait, both tides and winds make positive contributions to the local near-inertial energy input, resulting in more than 30% enhancement of total NIKE (>0.5 kJ m−2). However, in some deep-water regions along the cyclone paths, energy is transferred from cyclones to NIWs and also from NIWs to internal tides. Due to this “energy pipeline” effect, tide- and wind-induced NIWs contribute to weakening of total NIKE (∼0.3 kJ m−2 or 30%). Additionally, sensitivity experiments with varying initial tidal phases indicate that the interaction between wind-induced NIKE and tide-induced NIKE is robust in most of the model domain (over 80%) under different phase alignments between wind- and tide-induced NIWs. From the perspective of cyclones, tide-induced NIKE is comparable to wind-induced NIKE in the Luzon Strait before the arrival of cyclones, while tide-induced NIKE is two orders of magnitude smaller than wind-induced NIKE in most of the NSCS after the arrival of cyclones. Overall, our results highlight the joint effects of wind and tide forcing on the local NIW dynamics in the NSCS. Significance Statement Near-inertial internal waves (NIWs) are ubiquitous phenomena in stratified water, which can significantly influence the ocean mixing, especially across the thermocline. NIWs are generated mainly by wind forcing near the sea surface, but also by internal tide breaking in the ocean interior. Hence, a question that arises is whether winds or tides play dominant roles in generating NIWs. In fact, due to the interaction between wind- and tide-induced NIWs, the total near-inertial kinetic energy (NIKE) is not merely a linear superposition of that generated by winds and tides forcing alone. By carrying out numerical experiments, we find that extreme winds can help to transfer more energy from tides to NIWs, while tides would suppress energy transferring from winds to NIWs. As a result, this fact is crucial in accurately reproducing NIW dynamics in a targeted region, thereby determining redistribution of local ocean mixing intensity.