14 August 2017

Understanding Current and Eddy Contributions to Ocean Mixing

Mixing is dominated by mean and eddy flow interactions; new mixing parameterizations are needed.

Science

Ocean gyres and currents in the ocean interact to transport carbon and heat into the deep ocean.  A simplified and idealized Antarctic Circumpolar Current flow is analyzed to understand what causes mixing by identifying mixing contributions by gyres, currents, and their interactions.

Impact

Combined mixing by ocean gyres and currents dominate ocean mixing, suggesting that high resolution is necessary to compute the correct transport of carbon and heat into the deep ocean.  Modifications are needed to existing mixing models using mixing suppression and critical layer theory; our diagnosed vertical variability and gyre-current interactions are key for improvement.

Summary

Transport of heat and carbon into the ocean from the atmosphere and melting of ice sheets by ocean flows is largely mediated by ocean mixing, quantified with a diffusivity. We diagnose mixing in an Idealized Circumpolar Current that approximates the Antarctic Circumpolar Current and Antarctic shelf break.  We measure a reduced diffusivity over the shelf break, which is a mechanism that helps inhibit on-shelf mixing of ocean water with ice sheet cavities to help constrain the rate of land ice melting.  Mixing is produced by the combined action of ocean gyes and currents, e.g., large eddies and the mean flow.  We decompose the full diffusivity into its key mixing contributions by the eddy, mean, and residual (combined eddy-mean) flows.  Results indicate that mixing is largely produced by interactions of the mean and eddy flows.  Eddy and mean contributions are small as compared with the nonlinear residual. The diffusivity decomposition provides a path for improved understanding of ocean mixing. The importance of residual contributions of eddy-mean interactions indicates high resolution is key to resolving mixing.  We find that existing diffusivity models require new and improved parameterization strategies.

Contact
Phillip J. Wolfram
Los Alamos National Laboratory
Publications
" Quantifying Residual, Eddy, and Mean Flow Effects on Mixing in an Idealized Circumpolar Current." Journal of Physical Oceanography 47, 1897-1920 (2017). [10.1175/JPO-D-16-0101.1].