Global mapping of diapycnal mixing rates in the deep ocean

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Global mapping of diapycnal mixing rates in the thermocline is essential to improve the ability of global thermohaline circulation models in predicting future climate changes. For this purpose, we have first estimated diapycnal mixing rates over a large area in the interior of the North and South Pacific, the North Atlantic and the South Indian Ocean by incorporating the fine-scale vertical shear of horizontal velocity measured by expendable current profilers (XCP) into Gregg's formula (Gregg, 1989). The remarkable finding from the XCP survey is that the estimated diapycnal mixing rate in the thermocline is strongly dependent on the latitude.Then, these estimates have been related to the main energy source for diapycnal mixing, the semidiumal internal tide energy numerically calculated at each location, and an empirical formula has been found to predict the global distribution of diapycnal mixing rates in the deep ocean. Incorporating the numerically predicted semidiurnal internal tide energy at each longitude and latitude into the resulting empirical formula, we have found that strong diapycnal mixing (mixing hotspot) is limited to prominent topographic features at latitudes 20^0 -30^0 where the available semidiurnal internal tide energy can be efficiently transferred to dissipation scales by resonant interactions amongst internal waves called parametric subharmonic instability.Embedding the obtained distribution of diapycnal mixing rates into global thermohaline circulation models should yield significant improvement in their ability to predict future climate change.