Rates and Mechanisms of Turbulent Mixing in a Coastal Embayment of the West Antarctic Peninsula

Quantifying and understanding the processes driving turbulent mixing around Antarctica are key to closing the Southern Ocean's heat budget, an essential component of the global climate system. In 2016, a glider deployed in Ryder Bay, West Antarctic Peninsula, collected hydrographic and microstructure data, obtaining some of the first direct measurements of turbulent kinetic energy dissipation off West Antarctica. Elevated dissipation O(10(-8)) W kg(-1) is found above a topographic ridge separating the 520-m-deep bay, where values are O(10(-10)) W kg(-1), from a deep fjord of the continental shelf, suggesting the ridge is important in driving upward mixing of warm Circumpolar Deep Water. The 12 glider transects reveal significant temporal variability in hydrographic and dissipation conditions. Mooring-based current and nearby meteorological data are used to attribute thermocline shoaling (deepening) to Ekman upwelling (downwelling) at Ryder Bay's southern boundary, driven by similar to 3-day-long south-westward (north-westward) wind events. Anticyclonic winds generated near-inertial shear in the bay's upper layers, causing elevated bay-wide shear and dissipation similar to 1.7 days later. High dissipation over the ridge appears to be controlled hydraulically, being co-located (and moving) with steeply sloping isopycnals. These are observed in similar to 60% of the transects, with a corresponding mean upward heat flux of similar to 2.4 W m(-2). The ridge, therefore, provides sustained heat to the base of the thermocline, which can be released into overlying waters during the bay-wide, thermocline-focused dissipation events (mean heat flux of similar to 1.3 W m(-2)). This highlights the role of ridges, which are widespread across the West Antarctic Peninsula, in the regional heat budget. Plain Language Summary Glacial ice flowing off the Antarctic continent either terminates directly into the ocean or forms floating tongues called ice shelves. Marine-terminating glaciers and ice shelves in West Antarctica are melting rapidly, contributing significantly to global sea-level rise. The melting has largely been attributed to the presence of warm deep waters, which travel at depth from the shelf break toward the coast, before mixing upwards and coming into contact with ice. However, we lack an understanding of the vertical mixing processes. In 2016, an autonomous underwater robot was used to measure the vertical mixing in Ryder Bay, off the West Antarctic Peninsula. Increased vertical mixing was consistently observed above a ridge on the seafloor at the bay's entrance, suggesting that the ridge is a hotspot for bringing sustained deep-ocean heat to shallower depths. The heat rises until it reaches a strong temperature gradient in the ocean, with colder, less-dense water above. This gradient typically prevents rapid mixing; however, strong winds are found to cause bay-wide intense mixing events that release heat across the gradient into overlying waters. Similar ridges are widespread off the West Antarctic Peninsula, suggesting that they are important regionally for bringing heat toward glaciers and ice shelves.

Automated summary

The study measures vertical mixing in Ryder Bay off the West Antarctic Peninsula using a glider, finding increased vertical mixing consistently above a ridge at the bay's entrance, indicating this ridge is a hotspot for transferring deep ocean heat to shallower depths.