Abstract:

Turbulent mixing across isopycnals, as a key driver for the vertical exchange of momentum, heat, carbon dioxide and nutrient in the ocean, plays an important role in maintaining the equilibrium of ocean circulation, and in modulating the biogeochemical processes as well as long-term climate changes. Although it is well understood that the turbulent mixing in ocean interior is closely related to finescale velocity shear (S2) through shear instability, the latitude-dependent generation processes of S2 remain poorly known due to the lack of geographically extensive, long-term finescale velocity measurements. Here, using one-year ADCP velocity data from 17 moorings along 143oE, we for the first time show that the upper-ocean S2 and its resultant mixing rate have a W-shaped latitudinal distribution in the tropical-extratropical northwest Pacific with peaks at 0-2oN, 12-14oN, and 20-22oN, respectively. Further analyses reveal that these S2 peaks are primarily caused by vertically-sheared equatorial currents, parametric subharmonic instability of diurnal internal tide, and anticyclonic eddy's inertial chimney effect, respectively. In the equatorial region, it is also found that the turbulent mixing was significantly elevated by an anticyclonic sub-thermocline eddy (STE) through enhanced shear instability. Moreover, contrary to surface-intensified anticyclonic eddies, the STE appeared to act as a dynamic barrier for downward penetration of wind-generated near-inertial waves, which may also fertilize mixing near the upper thermocline. As climate model simulations are sensitive to the mixing parameterizations, our findings highlight the need to incorporate the latitude-dependent generation mechanisms of turbulent mixing to improve climate models' simulation and prediction capabilities.

(Host faculty: Prof. Jianping GAN)

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