Energetics and mixing of stratified, rotating flow over abyssal hills

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Authors

Varvara E Zemskova , Nicolas Grisouard 

Abstract

One of the proposed mechanisms for energy loss in the ocean is through dissipation of internal waves, in particular above rough topography where internal lee waves are generated. Rates of dissipation and diapycnal mixing are often estimated using linear theory and a constant value for mixing efficiency. However, previous oceanographic measurements found that non-linear dynamics may be important close to topography. In order to investigate the role of non-linear interactions, we conduct idealized 3D numerical simulations of steady flow over 1D topography and vary the topographic height, which correlates to the degree of flow non-linearity. We analyze spatial distribution of energy transfer rates between internal waves and the non-geostrophic portion of time-mean flow, and of dissipation and diapycnal mixing rates. In our simulations with taller, more non-linear topographies, energy transfer rates are similar to previously unexplained oceanographic observations near topography: internal waves gain energy from time-mean flow through horizontal straining and lose energy through vertical shearing. In the tall topography simulations, buoyancy fluxes also play a significant role, consistent with observations but contrary to linear wave theory, suggesting that quasigeostrophy-based approximations and linear theory may not hold in some regions above rough topography. Both dissipation and mixing rates increase with topographic height, but their vertical distributions differ between topographic regimes. As such, vertical profile of mixing efficiency is different for linear and non-linear topographic regimes, which may need to be incorporated into parameterizations of small-scale processes in models and estimates of ocean energy loss.

DOI

https://doi.org/10.31223/X5XC8M

Subjects

Earth Sciences, Oceanography, Oceanography and Atmospheric Sciences and Meteorology, Physical Sciences and Mathematics

Keywords

stratified flow, energy dissipation, mixing efficiency, bottom-driven flow, topographic lee waves, direct numerical simulations, abyssal circulation, ocean energy budget

Dates

Published: 2021-07-12 06:13

License

CC BY Attribution 4.0 International

Additional Metadata

Conflict of interest statement:
None

Data Availability (Reason not available):
The data generated for these simulations exceeds $5$TB and cannot be easily distributed. However, we provided all Nek5000 codes necessary to run the simulations presented in this paper and all Python codes used for post-processing at https://github.com/bzemskova/bottom_topography_flow

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