Convection Enhances Mixing in the Southern Ocean

Abstract

Mixing efficiency is a measure of the energy lost to mixing compared to that lost to viscous dissipation. In a turbulent stratified fluid the mixing efficiency is often assumed constant at = 0.2, whereas with convection it takes values closer to 1. The value of mixing efficiency when both stratified shear flow and buoyancy-driven convection are active remains uncertain. We use a series of numerical simulations to determine the mixing efficiency in an idealized Southern Ocean model. The model is energetically closed and fully resolves convection and turbulence such that mixing efficiency can be diagnosed. Mixing efficiency decreases with increasing wind stress but is enhanced by turbulent convection and by large thermal gradients in regions with a strongly stratified thermocline. Using scaling theory and the model results, we predict an overall mixing efficiency for the Southern Ocean that is significantly greater than 0.2 while emphasizing that mixing efficiency is not constant.Plain Language Summary Convection in the ocean occurs when dense (cold and/or saline) water overlies lighter (warmer or fresher) water. Convective events occur primarily when water is cooled (or salinified) at the ocean surface; during such events cold and salty water sinks to the bottom of the ocean, especially along the Antarctic continent. Convection is not well represented in ocean models, and its impact on circulation, transport, and turbulent mixing in the Southern Ocean is unknown. In this paper, we investigate the role of convection and surface winds in the Southern Ocean using a first-of-its-kind high-resolution model. We find that both convection and surface winds enhance turbulent mixing. Mixing rates are largest in the convective plumes in cold regions and near the warm surface of the ocean and the efficiency with which turbulence mixes the ocean is greatest in convecting regions. We predict that the mixing rate in the Southern Ocean is much higher than previously thought, challenging some well-established assumptions held by the broader oceanographic community. These results further help to estimate the convective mixing in many different regions of the global ocean and are crucial for improving ocean models.

The Impact of Turbulence and Convection on Transport in the Southern Ocean

“…We also thank two anonymous reviewers for their constructive feedback and advice. Data supporting the results may be found in Sohail et al () and in the supporting information.…”

“…In response to the enhancement of the density gradients in the system, the flow becomes baroclinically unstable, shedding coherent eddies. These baroclinic eddies migrate from the generation site, playing a major role in flattening the meridional density gradient over depth and impacting meridional and zonal transport (Karsten et al, ; Sohail et al, ). These processes are extremely important in driving Southern Ocean circulation and are visible in the idealized model studied here.…”

“…As shown in Figures c and d, the upper cell is dominated by the mean flow features, namely, by time‐invariant Ekman transport. The lower cell in the mean overturning is dominated by the direct, buoyancy‐driven flow, which is associated with the presence of a persistent plume at the southern boundary (Sohail et al, ). The mean upper cell extends through the surface boundary layer, penetrating up to .…”

“…The DNS solves the incompressible, nonhydrostatic Navier‐Stokes momentum equations with a Boussinesq approximation, linear equation of state, beta‐plane assumption, and heat and volume conservation in a rotating reference frame (see Sohail et al, ). In this study, a hat denotes a nondimensionalized quantity, such that , , , , and .…”

“… Note. Case SO represents the dimensionless parameters for the Southern Ocean, calculated in Sohail et al ().…”