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Diapycnal mixing plays an important role in modulating upper-ocean heat content of the western equatorial Pacific (WEP) that profoundly impacts the global climate. Given the sparsity of long-term in situ observations, the mechanisms driving thermocline mixing in the WEP remain poorly understood. Here, based on yearlong mooring measurements in the WEP, we first report that the occurrence of shear instability was significantly enhanced by an anticyclonic subthermocline eddy (STE). As a result of strong subinertial velocity shear and weakened stratification associated with the STE, the Richardson number was decreased below 1/4 and the estimated diapycnal diffusivity was increased by 400%. Moreover, contrary to surface-intensified anticyclonic eddies, the STE appeared to act as a dynamic barrier for downward penetration of wind-generated near-inertial energy, which may also fertilize mixing near the upper thermocline. Given the frequent occurrence of STEs, they may play a pivotal role in driving thermocline mixing in the WEP.

Plain Language Summary Turbulent mixing across density surfaces can bring cold water from the deep to the near-surface layer of the ocean; it therefore modulates the upper-ocean heat content of the western equatorial Pacific, which is critically important for the global climate system. Here, based on long-term velocity and temperature measurements from a bottom-anchored mooring, we for the first time found that the strong currents associated with a subsurface ocean eddy strongly elevated the upper-ocean mixing in the western equatorial Pacific through increasing the occurrence of shear instability, which occurs in stratified shear flows when the destabilizing effects of the velocity shear exceeds the stabilizing effects of the density stratification. Given their frequent occurrence in the western equatorial Pacific, the subsurface eddies may provide an important route to driving turbulent mixing therein.