A Marginal Stability Paradigm for Shear‐Induced Diapycnal Turbulent Mixing in the Ocean

C P Caulfield,B B Cael,A Mashayek,L E Baker

doi:10.1029/2021gl095715

C P Caulfield, B B Cael + Show 2 more

Open Access

https://doi.org/10.1029/2021gl095715

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Abstract

AbstractTurbulent mixing induced by breaking internal waves is key to the ocean circulation and global tracer budgets. While the classic marginal shear instability of Richardson number ∼1/4 has been considered as potentially relevant to turbulent wave breaking, its relevance to flows that are not steady parallel shear flows has been suspect. We show that shear instability is indeed relevant in the ocean interior and propose a new marginal stability paradigm that relates the stability criterion based on Richardson number to one based on the ratio of Ozmidov and Thorpe turbulence scales. The new paradigm applies to both ocean interior and boundary layer flows. This allows for accurate quantification of the transition from downwelling to upwelling zones in a recently emerged paradigm of ocean circulation. Our results help climate models more accurately calculate the mixing‐driven deep ocean circulation and fluxes of tracers in the ocean interior.

Highlights

Breaking internal waves induce widespread turbulent mixing at all ocean depths and across the globe, thereby playing an important role in (a) closure of the ocean circulation by upwelling of dense waters that form in polar regions and sink to the abyssal ocean basins and (b) regulating the budgets of heat, carbon, nutrients and other tracers important to the climate system (Talley et al, 2016; Wunsch & Ferrari, 2004)
We show that shear instability is relevant in the ocean interior and propose a new marginal stability paradigm that relates the stability criterion based on Richardson number to one based on the ratio of Ozmidov and Thorpe turbulence scales
We have investigated the relevance of a criterion based on the concept of marginally stable shear instabilities and found that in the ocean interior such a criterion is relevant even in dynamically complex regions

Summary

Introduction

Breaking internal waves induce widespread turbulent mixing at all ocean depths and across the globe, thereby playing an important role in (a) closure of the ocean circulation by upwelling of dense waters that form in polar regions and sink to the abyssal ocean basins and (b) regulating the budgets of heat, carbon, nutrients and other tracers important to the climate system (Talley et al, 2016; Wunsch & Ferrari, 2004). One of the two primary goals of this manuscript is to investigate whether there is a connection between the marginal stability paradigm based on Ri and the Goldilocks paradigm of MCA21 based on ROT, that is, is it possible to identify this emergent optimal state (i.e., most energetic and most efficient state) conceptually with some marginally stable state of the flow. It is our second primary goal to test whether in complex, realistic flows, those not characterized by directly forced shear instability We consider mixing in the Drake Passage, in the Brazil Basin tidal region, and in an abyssal canyon in the Brazil basin

Drake Passage

Marginal Stability and Goldilocks Mixing

Direct Application to Ocean Data

Discussion

Findings

Data Availability Statement