Southern Ocean isopycnal mixing and ventilation changes driven by winds

Abstract

Observed and predicted changes in the strength of the westerly winds blowing over the Southern Ocean have motivated a number of studies on the response of the Antarctic Circumpolar Current and Southern Ocean meridional overturning circulation (MOC) to wind perturbations and led to the hypothesis of the “eddy compensation” regime, wherein the MOC becomes insensitive to wind changes. In addition to the MOC, tracer transport also depends on mixing processes. Here we show, in a high‐resolution process model, that isopycnal mixing by mesoscale eddies is strongly dependent on the wind strength. This dependence can be explained by mixing length theory and is driven by increases in eddy kinetic energy; the mixing length does not change strongly in our simulation. Simulation of a passive ventilation tracer (analogous to CFCs or anthropogenic CO2) demonstrates that variations in tracer uptake across experiments are dominated by changes in isopycnal mixing, rather than changes in the MOC. We argue that to properly understand tracer uptake under different wind‐forcing scenarios, the sensitivity of isopycnal mixing to winds must be accounted for.

Implementation of a Geometrically Informed and Energetically Constrained Mesoscale Eddy Parameterization in an Ocean Circulation Model

“…While isopycnal slumping and eddy induced stirring (from McWilliams 1990 andRedi 1982 respectively) are often implemented together (e.g., Griffies 1998;Griffies et al 1998), in this work κ redi was fixed to be a constant in space and time, while κ gm follows the GEOMETRIC prescription. Changing κ redi is expected to affect tracer transport and is thus of great importance in the study of the ocean's role in heat transport and carbon storage, to name a few (e.g., Pradal and Gnanadesikan 2014;Abernathey and Ferreria 2015). This is beyond the scope of this work.…”

Variability and trends in Southern Ocean eddy activity in 1/12° ocean model simulations

“…Since the strengthening of westerly winds is projected to continue under rising greenhouse gas emissions [Gillett and Fyfe, 2013], a concomitant increase in mesoscale eddy activity could have a global climatic impact. It is hypothesized that mesoscale eddies might mitigate the projected reduction of the Southern Ocean carbon sink in response to increasing westerly winds [Le Quére et al, 2007;Dufour et al, 2013;Abernathey and Ferreira, 2015]. Mesoscale eddies have also been shown to affect the properties of mode waters forming on the equatorward side of the ACC [Herraiz-Borreguero and Rintoul, 2010] as well as the transport of heat, salt, and biogeochemical tracers poleward across the ACC [Ansorge et al, 2014;Dufour et al, 2015].…”

Modulation of Lateral Transport by Submesoscale Flows and Inertia‐Gravity Waves

“…Lateral mixing in the upper ocean plays an important role in Earth's climate system. For instance, near-surface lateral mixing processes help determine the ocean's rate of uptake of tracers such as heat and anthropogenic CO 2 (Abernathey & Ferreira, 2015;Balwada et al, 2018;Gnanadesikan et al, 2017; The prevailing assumption is that mesoscale ocean flows, with horizontal scales of ∼100 km and evolutionary times of months, are the dominant contributor to lateral stirring on scales relevant for the large-scale circulation and transport. Phenomenologically, the mesoscale is characterized by distinctive coherent structures such as vortices (i.e., eddies), fronts, and filaments.…”