Mesoscale eddies and near-inertial internal waves modulate seasonal variations of vertical shear variance in the northern South China Sea

Diapycnal mixing in the South China Sea (SCS) is commonly attributed to the vertical shear variance S 2 of horizontal ocean current velocity, but the seasonal modulation of S 2 is still poorly understood due to the scarcity of long-term velocity observations. Here, this issue is explored in detail based on nearly 10-yr-long acoustic Doppler current profiler (ADCP) velocity data from a mooring in the northern SCS. We find that S 2 in the northern SCS exhibits significant seasonal variations at both the near-surface (90–180 m) and subsurface (180–400 m) layers, but their seasonal cycles and modulation mechanisms are quite different. For the near-surface layer, S 2 is stronger in late summer, autumn, and winter but weaker in spring and early summer, while in the subsurface layer, it is much stronger in winter than in other seasons. Further analysis suggests that in the near-surface layer, the stronger S 2 in autumn and winter is primarily caused by typhoon-induced near-inertial internal waves (NIWs) and the large subinertial (SI) velocity shear of the baroclinic mesoscale eddies, respectively. With respect to the subsurface layer, the enhanced wintertime S 2 is primarily associated with the “inertial chimney” effect of anticyclonic eddies, trapping wind-forced downward-propagating NIWs and significantly increasing the near-inertial shear at the critical layer. The findings in this study highlight the potentially important roles of mesoscale eddies and NIWs in modulating the seasonality of upper-ocean mixing in the northern SCS. This modulation is attributed not only to the strong shear of these features but also to their interactions. Significance Statement Vertical shear variance of velocity S 2 significantly modulates turbulent mixing in the thermocline, but its climatologically seasonal variations and the associated mechanisms are still obscure due to the scarcity of long-term in situ velocity data. By analyzing nearly a decade of velocity data, we reveal significant seasonal variations in S 2 at different ocean layers in the northern SCS and uncover different seasonal cycles and modulation mechanisms. The study sheds light on the pivotal roles of mesoscale eddies and near-inertial internal waves in modulating seasonality of S 2 in the upper ocean. These findings have important implications for improving mixing parameterizations in numerical models.