Seasonal Modulation of Submesoscale Kinetic Energy in the Upper Ocean of the Northeastern South China Sea

Although submesoscale processes are revealed to have a strong seasonality in many regions of global ocean, their seasonal modulation in the northeastern South China Sea (NESCS) remains unexplored by observations. In this study, approaches to diagnose submesoscale kinetic energy (KE) and its conversion terms using data from a single mooring are proposed and validated. Based on these approaches, seasonal modulation of submesoscale KE in the NESCS is for the first time investigated using 8.7-year-long moored velocity and temperature data between August 2010 and April 2019. It is found that the monthly climatology of submesoscale KE shows an asymmetric annual cycle with a steep peak between December and February but a flat trough between April and October. The maximum and minimum of this annual cycle occur in January and May, respectively. Although the submesoscale KE rapidly decreases with depth, its seasonality is still evident at 300 m, which is far beneath the mixed layer. Statistical analysis suggests that the overall seasonality of submesoscale KE is shaped by mixed-layer depth while its steep peak in winter is modulated by mesoscale strain rate. It therefore suggests that the combination of mixed-layer instability and strain-induced frontogenesis determines the detailed annual cycle of submesoscale KE. Energetics analysis demonstrates that submesoscales obtain their KE from the release of available potential energy but lose a small portion of its KE through inverse cascade. This submesoscale inverse KE cascade may play an important role in modulating the temporal variation of mesoscale KE in winter.

Automated summary

This study investigates for the first time the seasonal modulation of submesoscale kinetic energy in the northeastern South China Sea using data from a single mooring. The submesoscale kinetic energy shows an asymmetric annual cycle modulated by mixed-layer depth and mesoscale strain rate, and the inverse KE cascade may play a crucial role in modulating mesoscale KE temporal variation in winter.