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

Abstract Diapycnal mixing in the South China Sea (SCS) is commonly attributed to the vertical shear variance (S2) of horizontal ocean current velocity, but the seasonal modulation of the S2 is still poorly understood due to the scarcity of long-term velocity observations. Here, this issue is explored in detail based on nearly 10-year-long ADCP velocity data from a mooring in the northern SCS. We find that the S2 in the northern SCS exhibits significant seasonal variations at both the near-surface (90–180 m) and sub-surface (180–400 m) layers, but their seasonal cycles and modulation mechanisms are quite different. For the near-surface layer, the S2 is stronger in late summer, autumn, and winter but weaker in spring and early summer, while in the sub-surface layer, it is much stronger in winter than other seasons. Further analysis suggests that in the near-surface layer, the stronger S2 in autumn and winter is primarily caused by typhoons-induced near-inertial internal waves (NIWs) and the large sub-inertial velocity shear of the baroclinic mesoscale eddies, respectively. With respect to the sub-surface layer, the enhanced wintertime S2 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.