Topographic Modulation of the Wind Stress Impact on Eddy Activity in the Southern Ocean

Abstract

Previous studies proposed that the increase in the eddy kinetic energy (EKE) in the Southern Ocean in recent decades is primarily caused by the strengthening of circumpolar surface westerlies. However, the spatial pattern of EKE change does not match the pattern of wind change. Here, we revisit the relationship between EKE and wind stress through an observational analysis and model experiments and show that the change in EKE is primarily determined by the mean flow. The increasing wind stress intensifies the circumpolar mean flow contributing to increasing EKE; yet strong EKE variations are generally confined downstream of major topographic features. This arises from the releasing of available potential energy as the mean flow passes through the topography. Our results indicate that the change in Southern Ocean eddy activity has a distinct localization characteristic due to the strong dynamical influence of topography.

Contrasting trends in short-lived and long-lived mesoscale eddies in the Southern Ocean since the 1990s

“…4, the ACC jet is largely zonal upstream of the topography where the eddy energy is relatively low. When the jet encounters the major topography, the water columns are squashed/stretched and move equatorward/poleward, leading to a meander curvature and an unstable flow, which shapes the features of the elevated eddy field downstream of the topography (Barthel et al, 2017;Rintoul, 2018;Cai et al, 2022). Because the SO experienced pronounced warming in recent decades, more (less) warming north (south) of the ACC caused greater isopycnal tilting and robust zonal acceleration (Shi et al, 2021).…”

“…The energy conversion from the MPE to the EPE (BC) due to baroclinic instabilities is also centralized around the eddy-rich regions, and its value is much larger than that of the BT, reaching ~5 − 10 × 10 𝑊𝑚 , which indicates more energy is being released from the baroclinic instabilities. Moreover, the locations of the elevated eddy energy and the BC coincide with the bottom topography but not with those of strengthened winds, which implies the primary role of topography in shaping eddy activity patterns along the ACC (e.g., Graham et al, 2012;Thompson & Sallée, 2012;Barthel et al, 2017;Cai et al, 2022). between the accelerated mean flow and topography.…”

“…Besides external wind-forced changes in the EKE, high-resolution modeling has suggested that the eddy field also exhibited a chaotic internal nature, which may mask wind-driven changes (Meredith, 2016;Patara et al, 2016;Hogg et al, 2022). Another important feature is the spatial pattern of the EKE field, which is collocated with major topography and is primarily determined by the instability of the mean flow (Graham et al, 2012;Barthel et al, 2017;Chapman, 2017;Youngs et al, 2017;Cai et al, 2022). Model experiments showed that the EKE depends on the shape and height of the topography as well as manuscript submitted to Geophysical Research Letters on the baroclinicity of the jet, but it is not very sensitive to increased wind stress (Barthel et al, 2017;Cai et al, 2022).…”

“…Another important feature is the spatial pattern of the EKE field, which is collocated with major topography and is primarily determined by the instability of the mean flow (Graham et al, 2012;Barthel et al, 2017;Chapman, 2017;Youngs et al, 2017;Cai et al, 2022). Model experiments showed that the EKE depends on the shape and height of the topography as well as manuscript submitted to Geophysical Research Letters on the baroclinicity of the jet, but it is not very sensitive to increased wind stress (Barthel et al, 2017;Cai et al, 2022). The nonlinear evolution of the instability leads to an inverse cascade of energy and likely determines the eddy properties (Pedlosky, 1987;Venaille et al, 2011;Scott & Wang, 2005); however, the long-term changes in barotropic and baroclinic instabilities and their connections with eddy variations in the SO remain unexplored.…”

Characteristics and Dynamics of the Interannual Eddy Kinetic Energy Variation in the Central Pacific Sector of the Southern Ocean

“…The spatial inhomogeneity of EKE and its associated generation mechanism in the SO have been well studied. Several previous studies suggested that bottom topography plays a key role in generating standing meanders of the ACC, which are associated with localized regions of high EKE (Barthel et al., 2017, 2022; Cai et al., 2022; Thompson & Sallée, 2012). Both baroclinic instability and barotropic instability are found to be responsible for the EKE generation in these highly localized regions (Wilson et al., 2015; Youngs et al., 2017).…”