Sensitivity of deep ocean mixing to local internal tide breaking and mixing efficiency

There have been recent advancements in the quantification of parameters describing the proportion of internal tide energy being dissipated locally and the “efficiency” of diapycnal mixing, i.e. the ratio of the diapycnal mixing rate to the kinetic energy dissipation rate. We show that oceanic tidal mixing is non‐trivially sensitive to the co‐variation of these parameters. Varying these parameters one at the time can lead to significant errors in the patterns of diapycnal mixing driven upwelling and downwelling, and to the over and under estimation of mixing in such a way that the net rate of globally‐integrated deep circulation appears reasonable. However, the local rates of upwelling and downwelling in the deep ocean are significantly different when both parameters are allowed to co‐vary and be spatially variable. These findings have important implications for the representation of oceanic heat, carbon, nutrients and other tracer budgets in general circulation models.

Plain Language Summary

Deep ocean basins are filled with dense waters that form at high latitudes and sink to the abyss. The overturning circulation of the ocean, a key regulator of the climate system, is only feasible if such dense waters can resurface. The breaking of internal waves makes such resurfacing possible. In the deep ocean, internal waves are largely generated by the flow of tides over topography. Their breaking mixes dense deep waters with lighter waters above them, bringing them upward. Two key parameters in climate models for modeling such mixing are: (I) the ratio of energy in the wave field that is spent near rough topography due to breaking as opposed to what is radiated away; and (II) the amount of energy from wave breaking that goes to mixing versus what is wasted through dissipation by viscosity of seawater. Both parameters are considered constant in climate models. In this work, we quantify the roles of variations in each of these two parameters in setting the patterns of deep ocean upwelling of dense waters and argue that the two parameters need to be changed realistically and inter‐dependently to avoid significant inaccuracies in the quantification of the mixing‐induced deep branch of ocean circulation.

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Cimoli Laura, Caulfield Colm‐cille P., Johnson Helen L., Marshall David P., Mashayek Ali, Naveira Garabato Alberto C., Vic Clement (2019). Sensitivity of deep ocean mixing to local internal tide breaking and mixing efficiency. Geophysical Research Letters. 46 (24). 14622-14633.,

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