Spatial and seasonal variations of near-inertial kinetic energy in the upper South China Sea: Role of synoptic atmospheric systems

The spatial distribution and seasonal variation of near-inertial kinetic energy (NIKE) in the upper ocean of the South China Sea (SCS) are examined using the Global 1/12° Analysis Hybrid Coordinate Ocean Model and the Navy Coupled Ocean Data Assimilation reanalysis product. The annual mean NIKE in the upper SCS is characterized by a rapid decay with depth and a southwestward decrease from the west of Luzon Island to the southern SCS, reflecting the pattern of near-inertial energy input by the atmospheric wind field. Owing to the changes of near-inertial wind forcing as well as the mixed layer depth, NIKE in the upper SCS exhibits a pronounced seasonal cycle. The magnitude of mixed layer NIKE averaged in the SCS in November-January is found to be approximately twice of that in April-June. Further analysis shows that the variation of near-inertial wind forcing in the SCS can, to a large extent, be explained by northerly cold surges during the winter SCS monsoon and strong tropical cyclones from the Tropical Pacific. In addition, the wind reversal during the monsoon transitional period and the low-level convergence formed by the elevated terrain of islands also contribute to oceanic near-inertial energy in the SCS.