Eddy saturation of the Southern Ocean: a baroclinic versus barotropic perspective

Abstract

Key Points:• An isopycnal layered model, with a varying number of fluid layers, is used to assess relative importance of barotropic and baroclinic processes in the Southern Ocean.• Both baroclinic and barotropic flows exhibit regimes in which mean zonal transport is insensitive to wind stress.• Eddies actively shape the time-mean flow, irrespective of the instabilities from which they originate.Abstract "Eddy saturation" is the regime in which the total time-mean volume transport of an oceanic current is relatively insensitive to the wind stress forcing and is often invoked as a dynamical description of Southern Ocean circulation. We revisit the problem of eddy saturation using a primitive-equations model in an idealized channel setup with bathymetry. We apply only mechanical wind stress forcing; there is no diapycnal mixing or surface buoyancy forcing. Our main aim is to assess the relative importance of two mechanisms for producing eddy saturated states: (i) the commonly invoked baroclinic mechanism that involves the competition of sloping isopycnals and restratification by production of baroclinic eddies, and (ii) the barotropic mechanism, that involves production of eddies through lateral shear instabilities or through the interaction of the barotropic current with bathymetric features. Our results suggest that the barotropic flow-component plays a crucial role in determining the total volume transport. Plain language summaryWind stress at the surface of the ocean is an important driver of ocean currents. However, what sets up the strength of the currents remains puzzling. The strongest ocean current flows around Antarctica: the Antarctic Circumpolar Current (ACC). It is believed that the ACC is close to a so-called "eddy saturated" state, a regime in which changes in the strength of the wind stress forcing do not alter the strength of the mean current. Instead, the swirling oceanic eddy motions that accompany the current are enhanced. Here, we investigate the physics and assess the relative importance of the two mechanisms proposed in the literature to explain this phenomenon: the most commonly invoked baroclinic mechanism and the recently proposed barotropic mechanism that crucially involves the interaction of the oceanic flow with bathymetric features. Our results suggest that the oftentimes ignored depth-averaged (barotropic) component of the ocean flow and its interaction with bathymetry play a dominant role in setting up the strength of the current in certain regimes.

Connecting flow–topography interactions, vorticity balance, baroclinic instability and transport in the Southern Ocean: the case of an idealized storm track

“…Our work contributes to its interpretation and strives to unravel the underlying causal chain of processes. Our results complement those of Barthel et al (2017), Constantinou et al (2019) and Patmore et al (2019) in drawing attention to the barotropic flow component. Although baroclinic instability is, in our simulations, what ultimately sets the ACC density structure and transport, the barotropic flow plays a key role in modulating the propensity of the eddies to relax baroclinicity.…”

“…In turn, this affects the efficiency of baroclinic instability processes at releasing APE and limit the circumpolar transport. The role of the ACC barotropic component is a subject of active research and our work complements the recent studies of Patmore et al (2019) and Constantinou et al (2019) in this regard.…”

“…The standing meanders that form through the interaction of the barotropic flow with the topography contribute to the bottom form stress and may also participate in the saturation process (Thompson and Naveira Garabato, 2014;Katsumata, 2017). Constantinou and Hogg (2019) recently highlighted the role played by the eddy production through lateral shear instabilities of the barotropic circulation or interaction of the barotropic current with the topography, in establishing the eddy saturated state of the Southern Ocean. Overall, our findings confirm the robustness of the saturation process with respect to major changes in model configuration, which translate into a wide range of baroclinic instability regimes and efficiency (as previously noted in Nadeau et al, 2013), mean flow with widely distinct barotropic characteristics and ACC transports.…”

Kinetic Energy of Eddy‐Like Features From Sea Surface Altimetry

“…This hypothesis has been verified several times in numerical models, for example, by Hallberg and Gnanadesikan (), Meredith and Hogg (), Nadeau and Straub (), Marshall et al. (), and Constantinou and Hogg (), but only indications of it have been seen in observations (Böning et al., ; Chidichimo et al., ; Firing et al., ).…”

Regional Trends in Southern Ocean Eddy Kinetic Energy

“…Our understanding about changes in eddy kinetic energy (EKE) has been greatly improved over the past decades with the advent of satellite altimetry and increasingly high‐resolution ocean models that resolve or permit eddies. Numerical simulations suggested that enhanced wind stress will cause an increase in the transient eddy activity over time that counteracts increases in the ACC transport, resulting in increases in EKE but no significant change to volume transport (Constantinou & Hogg, 2019; Hallberg & Gnanadesikan, 2006; Hogg et al., 2015; Marshall et al., 2017; Meredith & Hogg, 2006; Straub, 1993). This has become to be known as the “eddy‐saturation” hypothesis.…”