Dissipation of the energy imparted by mid-latitude storms in the Southern Ocean

Julien Jouanno,Gurvan Madec,Guillaume Roullet,Xavier Capet,Patrice Klein

doi:10.5194/os-12-743-2016

Abstract

Abstract. The aim of this study is to clarify the role of the Southern Ocean storms on interior mixing and meridional overturning circulation. A periodic and idealized numerical model has been designed to represent the key physical processes of a zonal portion of the Southern Ocean located between 70 and 40° S. It incorporates physical ingredients deemed essential for Southern Ocean functioning: rough topography, seasonally varying air–sea fluxes, and high-latitude storms with analytical form. The forcing strategy ensures that the time mean wind stress is the same between the different simulations, so the effect of the storms on the mean wind stress and resulting impacts on the Southern Ocean dynamics are not considered in this study. Level and distribution of mixing attributable to high-frequency winds are quantified and compared to those generated by eddy–topography interactions and dissipation of the balanced flow. Results suggest that (1) the synoptic atmospheric variability alone can generate the levels of mid-depth dissipation frequently observed in the Southern Ocean (10−10–10−9 W kg−1) and (2) the storms strengthen the overturning, primarily through enhanced mixing in the upper 300 m, whereas deeper mixing has a minor effect. The sensitivity of the results to horizontal resolution (20, 5, 2 and 1 km), vertical resolution and numerical choices is evaluated. Challenging issues concerning how numerical models are able to represent interior mixing forced by high-frequency winds are exposed and discussed, particularly in the context of the overturning circulation. Overall, submesoscale-permitting ocean modeling exhibits important delicacies owing to a lack of convergence of key components of its energetics even when reaching Δx = 1 km.

Highlights

Knowledge gaps pertaining to energy dissipation and mixing distribution in the ocean greatly limit our ability to apprehend its dynamical and biogeochemical functioning and its role in the climate system evolution (Naveira-Garabato, 2012)
The aim of this study is to (i) further clarify the mechanisms implicated in near-inertial wave (NIW) penetration into the ocean interior, (ii) more precisely quantify the resulting near-inertial energy (NIE) dissipation intensity including its vertical distribution and (iii) better understand the current OGCM limitations in representing NIE dissipation. (Findings on ii will be specific to the Southern Ocean while we expect those on i and iii to be more generic.) For that purpose, we perform semi-idealized Southern Ocean simulations for a wide range of model parameters and different numerical schemes covering eddy present to submesoscale-rich regimes
Zhai et al (2009) analyzing a global 1/12◦ model found that nearly 70 % of the wind-induced near-inertial energy at the sea surface is lost to turbulent mixing within the top 200 m

Summary

Introduction

Knowledge gaps pertaining to energy dissipation and mixing distribution in the ocean greatly limit our ability to apprehend its dynamical and biogeochemical functioning (globally or at smaller scale, e.g., regional) and its role in the climate system evolution (Naveira-Garabato, 2012). A great deal of effort is currently deployed to address the issue but the difficulties are immense: dissipation occurs intermittently, heterogeneously and in relation to a myriad of processes, whose importance varies depending on the region, depth range, season, proximity to bathymetric features, etc. In this context, establishing an observational truth based on local estimates involves probing the ocean at centimeter scale (vertically) with horizontal- and temporal-resolution requirements that will need a long time to be met (e.g., MacKinnon et al, 2009 or DIMES program, Gille et al, 2012).

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