The Role of Eddies and Topography in the Export of Shelf Waters From the West Antarctic Peninsula Shelf

Abstract

Oceanic heat strongly influences the glaciers and ice shelves along West Antarctica. Prior studies show that the subsurface onshore heat flux from the Southern Ocean on the shelf occurs through deep, glacially carved channels. The mechanisms enabling the export of colder shelf waters to the open ocean, however, have not been determined. Here, we use ocean glider measurements collected near the mouth of Marguerite Trough (MT), west Antarctic Peninsula, to reveal shelf-modified cold waters on the slope over a deep (2,700 m) offshore topographic bank. The shelf hydrographic sections show subsurface cold features (θ <=1.5°C), and associated potential vorticity fields suggest a significant instability-driven eddy field. Output from a high-resolution numerical model reveals offshore export modulated by small (6 km), cold-cored, cyclonic eddies preferentially generated along the slope and at the mouth of MT. While baroclinic and barotropic instabilities appear active in the surrounding open ocean, the former is suppressed along the steep shelf slopes, while the latter appears enhanced. Altimetry and model output reveal the mean slope flow splitting to form an offshore branch over the bank, which eventually forms a large (116 km wide) persistent lee eddy, and an onshore branch in MT. The offshore flow forms a pathway for the small cold-cored eddies to move offshore, where they contribute significantly to cooling over the bank, including the large lee eddy. These results suggest eddy fluxes, and topographically modulated flows are key mechanisms for shelf water export along this shelf, just as they are for the shoreward warm water transport.Plain Language Summary The glaciers and ice shelves of the West Antarctic Peninsula have been rapidly retreating in the past 50 years, believed to be driven by increasing fluxes of heat from the ocean. While knowledge of the mechanisms that drive this heat from the open ocean onto the continental shelves has improved, our understanding of the counterbalancing flows of shelf waters remains limited. In this study, temperature, salinity, and velocity observations from an underwater autonomous vehicle, alongside numerical model output, are used to elucidate the pathway and mechanisms by which shelf waters are moved into the ocean interior. Small spinning vortices-known as eddies-containing shelf water are periodically generated from dynamical instabilities that derive from the horizontal shear of the flow. These small eddies then move offshore to cool a region of elevated topography near the mouth of Marguerite Trough, a deep glacially carved canyon. Here, cold water accumulates in a larger persistent eddy; satellite observations suggest that this water may ultimately be discharged into the Southern Ocean interior. Given that the topographic conditions of the field site are not unique, eddies and topographic flows are likely key mechanisms that balance the onshore flux of ocean heat in the West Antarctic.

Eddy‐Driven Transport of Particulate Organic Carbon‐Rich Coastal Water Off the West Antarctic Peninsula

“…Our high-resolution model results suggest that about half of the volume transport of near surface coastal water due to trapping by cyclones may occur for small eddies that are missed by altimetry observations. This is consistent with Brearley et al (2019), who have shown that small scale eddies contribute to the export of cold shelf water to the open ocean off the Peninsula. Future satellite missions that will provide sea level anomalies in higher resolution (e.g., NASA's Surface Water and Ocean Topography mission-SWOT; Fu et al, 2012) will allow for investigating transport of coastal water rich in POC by small eddies based on observations.…”

“…The configuration used for the WAP has 1.5 km horizontal resolution and 24 variable‐thickness vertical layers that are concentrated toward the top and bottom surfaces. The 1.5 km resolution model has been shown to simulate well small scale (∼5–10 km) eddy heat transport onto the WAP continental shelf (Graham et al., 2016) and larger scale eddy volume transport off the WAP continental shelf/slope (Brearley et al., 2019). It includes a dynamic sea ice model (Budgell, 2005) and simulates mechanical and thermodynamic interactions between the floating ice shelves in the model domain and the water cavity underneath (Dinniman et al., 2011; Holland & Jenkins, 1999).…”

“…Along the Antarctic continental shelf, small (radii of 5–10 km) eddies are important in transporting offshore heat onto and across the shelf in many locations (Stewart et al., 2018), especially along the West Antarctic Peninsula (WAP) (Couto et al., 2017; Martinson & McKee, 2012; McKee et al., 2019; Moffat et al., 2009). To balance the onshore volume transport, offshore transport may be driven by several mechanisms, including possibly by eddies, with small eddies (∼6 km) shed from the shelf and larger eddies (radii of 50–60 km) detaching from topographic features along the slope (Brearley et al., 2019).…”

Global Drivers on Southern Ocean Ecosystems: Changing Physical Environments and Anthropogenic Pressures in an Earth System

“…Deep, steep canyons and other topographic features play a key role in the across-shelf exchange of properties along this and other Antarctic shelves (Moffat et al, 2009;Martinson and McKee, 2012). This exchange is strongly modulated by small ocean eddies that play a role on both the supply of heat and nutrients to the shelf (Moffat et al, 2009) as well as the export of shelf-modified waters to the open ocean (Brearley et al, 2019). Eddy-driven exchange is a key modulating factor of ocean properties around the continent, and this process is modulated by wind forcing (Thompson et al, 2014;Stewart and Thompson, 2015).…”

The Shelf Circulation of the Bellingshausen Sea

“…These authors did not discuss the circulation in Latady Trough, and this region's contribution to heat transport toward the BellS ice shelves remains unconstrained. The generation of cyclonic circulations supported by glacially carved troughs is a feature that is common throughout the WAP (Brearley et al, 2019;Savidge & Amft, 2009). Coupled ice-ocean simulations carried out by Assmann et al (2005) found the region to be dominated by a large-scale cyclonic circulation, although the model did not resolve circulation features in individual troughs.…”