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

J Alexander Brearley,Hugh J Venables,Michael S Dinniman,Carlos Moffat,Michael P Meredith

doi:10.1029/2018jc014679

Abstract

AbstractOceanic 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.

Highlights

The West Antarctic Peninsula (WAP) in the Bellingshausen Sea is one of the most climatically variable environments on Earth
This illustrates that a water mass source for the deeper water over the bank is distinct from that found in the open Southern Ocean
Such detachment events may be implicated in the final movement of modified CDW (mCDW) into the interior of the Southern Ocean, though the altimetry lacks sufficient horizontal resolution to determine the total magnitude of this volume/buoyancy flux associated with this process or whether eddies or filaments smaller than can be observed using satellite altimetry are responsible for fluxing mCDW into the Southern Ocean interior

Summary

Introduction

The West Antarctic Peninsula (WAP) in the Bellingshausen Sea is one of the most climatically variable environments on Earth. Since 2000, there has been an absence of statistically significant atmospheric warming (though the overall positive trend from 1950 to 2014 is still statistically significant; Turner et al, 2005, 2016). The extent and duration of sea ice cover in the Bellingshausen Sea decreased rapidly since the start of the satellite era (Stammerjohn et al, 2008), though since 2000 the Journal of Geophysical Research: Oceans northern part of the WAP has seen sea ice increases (Turner et al, 2016). The overall trend is divergent from other parts of Antarctica such as the Ross Sea, where long‐term increases in sea ice extent have been observed (Comiso et al, 2016). Summertime temperatures in the upper water column increased by more than 1 °C between the 1950s and 1990s (Meredith & King, 2005) with deep warming on the shelf being observed by Schmidtko et al (2014). Significant freshening is apparent close to the coast, where glacial meltwater is a significant input, but further offshore upper layer salinification has been observed (Bers et al, 2013; Schloss et al, 2012)

Results

Discussion

Conclusion