Eddy transport as a key component of the Antarctic overturning circulation

The exchange ofwatermasses across theAntarctic continental shelf break regulates the export of dense shelf waters to depth as well as the transport of warm, mid-depth waters towards ice shelves and glacial grounding lines1. The penetration of the warmer mid-depth waters past the shelf break has been implicated in the pronounced loss of ice shelf mass over much of west Antarctica2–4. In high-resolution, regional circulation models, the Antarctic shelf break hosts an energetic mesoscale eddy field5,6, but observations that capture this mesoscale variability have been limited. Here we show, using hydrographic data collected from ocean gliders, that eddy-induced transport is a primary contributor to mass and property fluxes across the slope. Measurements along ten cross-shelf hydrographic sections show a complex velocity structure and a stratification consistent with an onshore eddy mass flux. We show that the eddy transport and the surface wind-driven transport make comparable contributions to the totaloverturningcirculation.Eddy-induced transport is concentrated in the warm, intermediate layers away from frictional boundaries. We conclude that understanding mesoscale dynamics will be critical for constraining circumpolar heat fluxes and future rates of retreat of Antarctic ice shelves. Residual-mean theories, which propose a leading-order balance between windand eddy-induced overturning cells, have illuminated the dynamics of material exchange across the Southern Ocean’s Antarctic Circumpolar Current7 (ACC). The Antarctic margins share similarities with the ACC: the circulation is composed of strong, narrow frontal currents; the flow is unblocked by continental boundaries; and surface forcing is largely due to zonal, down-front winds—easterlies around Antarctica, westerlies over the ACC. Evidence of persistent eddy variability in both idealized and realistic numerical models5,6,8–10, as well as mooring data11, has led to the proposal that wind-driven and eddy overturning cells may be active at the Antarctic margins9,10,12. Observational confirmation requires resolving variability in alongstream velocity and cross-stream buoyancy fields. We present new hydrographic data that achieves this coverage and supports the presence of an eddy overturning at the shelf break. Three ocean gliders were deployed in the northwestern Weddell Sea between 20 January and 13 March 2012 (Fig. 1a,b). This region is a key gateway for the delivery of Antarctic Bottom Water (AABW) to the Scotia Sea, and eventually to the global circulation13. Furthermore, the export of iron-rich shelf waters gives rise to elevated chlorophyll levels in the Scotia Sea14,15, which distinguishes this region from most of the Southern Ocean, where nutrients are replete but chlorophyll levels are low. Therefore, cross-slope exchange processes influence both the global overturning circulation and Southern Ocean biogeochemical and ecosystem dynamics. Properties of AABW are modified near the Antarctic coast owing to entrainment or mixing with modified Circumpolar Deep Water16 (MCDW). These interactions are enhanced at the shelf break, where MCDW penetrates onshore, establishing strong lateral water-mass gradients (Fig. 1c; glider section A). MCDW intrudes on the shelf, separating cold Winter Water above and relatively fresh Weddell Sea Deep Water below; other glider sections are similar. The injection of warm MCDW onto the shelf occurs through intermittent pulses, consistent with an energetic mesoscale field. Temperature/salinity diagrams (Fig. 1d,e) indicate that the space enclosed by the three water-mass endmembers is populated by many observations, indicative of strong mixing both along and potentially across density surfaces. Lateral stirring throughout the water column will expose a broad range of density classes to mixing, which will in turn influence the closure of the meridional overturning circulation’s lower cell at the southern boundary17,18. The high-resolution glider sections provide a striking view of the cross-slope structure of the Antarctic Slope Front (ASF) system19. Rather than a single ASF shelf-break current1, the glider sections reveal a more complex picture (Supplementary Fig. 1). A narrow, bottom-intensified current is nearly always found at, or slightly o shore of the shelf break. Yet, as many as three distinct velocity cores may span a distance as short as 50 km. These smallscale fronts have a baroclinic structure and are not simply a signature of a variable barotropic transport. The front separation remains larger than the first baroclinic Rossby deformation radius , which we estimate as = NH/f ⇡ 5–10 km, where N is the buoyancy frequency, H is the water column depth and f is the Coriolis frequency (see Methods for values). In a turbulent flow, potential vorticity (PV) gradients, due to a change in water column depth for instance, can give rise to banded flows, or jets. A standard scaling for the separation of these jets depends on an eddy velocity Ue and the background PV gradient, in this case a topographic beta T, yielding the Rhines scale `R ⇠pUe/ T. Using the root mean square value of velocities over the continental slope, Ue⇡ 0.1m s 1, and the scaling T⇠ fH 1@H/@y , we arrive at `R ⇡ 20 km. This is consistent with the observed spacing of the velocity cores, and further evidence of an active mesoscale eddy field.