Jet–Topography Interactions Affect Energy Pathways to the Deep Southern Ocean

Abstract

In the Southern Ocean, strong eastward ocean jets interact with large topographic features, generating eddies that feed back onto the mean flow. Deep-reaching eddies interact with topography, where turbulent dissipation and generation of internal lee waves play an important role in the ocean’s energy budget. However, eddy effects in the deep ocean are difficult to observe and poorly characterized. This study investigates the energy contained in eddies at depth, when an ocean jet encounters topography. This study uses a two-layer ocean model in which an imposed unstable jet encounters a topographic obstacle (either a seamount or a meridional ridge) in a configuration relevant to an Antarctic Circumpolar Current frontal jet. The authors find that the presence of topography increases the eddy kinetic energy (EKE) at depth but that the dominant processes generating this deep EKE depend on the shape and height of the obstacle as well as on the baroclinicity of the jet before it encounters topography. In cases with high topography, horizontal shear instability is the dominant source of deep EKE, while a flat bottom or a strongly sheared inflow leads to deep EKE being generated primarily through baroclinic instability. These results suggest that the deep EKE is set by an interplay between the inflowing jet properties and topography and imply that the response of deep EKE to changes in the Southern Ocean circulation is likely to vary across locations depending on the topography characteristics.

A mechanism for jet drift over topography

“…This is also in agreement with the linear predictions which say that any zonal variation in bottom topography enhances the baroclinic instability [Chen and Kamenkovich, 2013;Hart, 1975]. In a study of the North Atlantic jets, Kamenkovich et al [2009] also found both kinds of eddy feedbacks [also see Barthel et al, 2017;Melnichenko et al, 2010;Youngs et al, 2017]. It is important to note that if eddy field is computed by high-pass time filtering in a stationary frame of reference, then the resulting eddy field would also include the drifting jets.…”

“…For example, eddies play a major role in the dynamics of the Antarctic Circumpolar Current (ACC) [Abernathey and Cessi, 2014;Thompson and Naveira Garabato, 2014]. Recent studies have shown that the behaviour of the Reynolds stress term and form stress term can change abruptly around the topographic features in the Southern Ocean, where in different parts of the ocean eddies are seen to be forcing the large-scale flow as well as gaining energy via a barotropic energy source [Barthel et al, 2017;Youngs et al, 2017].…”

A Geometric Interpretation of Southern Ocean Eddy Form Stress

“…This zonal heterogeneity has also been reported in the upwelling associated with the residual overturning circulation (Tamsitt et al 2017). The meanders form when largescale flow navigates submarine topographic obstacles and are associated with elevated deep-reaching eddy kinetic energy (Hallberg and Gnanadesikan 2001;Bischoff and Thompson 2014;Barthel et al 2017). In addition both eddy-resolving models and observations show that the transport of the Antarctic Circumpolar Current is insensitive to a strengthening of the zonal wind stress (Hallberg and Gnanadesikan 2006;Meredith and Hogg 2006;Böning et al 2008;Munday et al 2013;Morrison and Hogg 2013;Marshall et al 2017;Poulsen et al 2018).…”

Eddy Saturation of the Southern Ocean: A Baroclinic Versus Barotropic Perspective

“…In addition to the evidence for strong bottom flows, some recent work has emphasized the importance of the bathymetry in setting up standing meanders in the course of the ACC and in producing strong interaction between barotropic and baroclinic fluid motions (Barthel et al, 2017;Youngs et al, 2017). Furthermore, these meanders play a crucial role in balancing the momentum through topographic form stress and thus determining the ACC transport (Katsumata, 2017;Thompson & Naveira Garabato, 2014).…”