Features of Kinetic Energy Exchange Between Balanced Motions and Near‐Inertial Internal Waves in the Northeastern South China Sea

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Abstract The energy exchange between balanced motions and near‐inertial internal waves (NIWs) serves as a key pathway for cross‐scale energy redistribution; however, its characteristics remain poorly understood. Based on a submesoscale‐permitting simulation, the kinetic energy exchange rates (KER) between balanced motions (consisting of large‐ and mesoscale geostrophic motions, and high‐frequency balanced motions) and NIWs are investigated in the northeastern South China Sea (NESCS). The results suggest that in the Luzon Strait, NIWs gain kinetic energy from balanced motions year‐around, with an annual mean 0–1,000 m integrated KER of 1.15 × 10 −6 m 3 /s 3 , accounting for 200.0% and 9.5% of wind work on NIWs and balanced motions, respectively. This indicates that energy exchange is an important source for NIWs and a corresponding sink for balanced motions. In the NESCS interior, the KER are substantially weaker than the wind work and slightly reverse seasonally, being positive in summer and negative in winter. The contributions of high‐frequency balanced motions are quantified by comparing the KER between geostrophic motions and NIWs with those between balanced motions and NIWs. Their annual means are 5.51 × 10 −7 m 3 /s 3 in the Luzon Strait and 2.83 × 10 −8 m 3 /s 3 in the NESCS interior, respectively equivalent to 91.2% and 98.7% of the magnitude of KER between geostrophic motions and NIWs. Further analysis reveals that high‐frequency balanced motions enhance the kinetic energy transfer from balanced motions to NIWs by increasing the strain rates and elevating positive Okubo‐Weiss parameter. These results emphasize the crucial role of high‐frequency balanced motions in facilitating energy transfer from balanced motions to NIWs.

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