Shelf-Ocean Exchange in the Polar Regions

Abstract

The polar oceans are of paramount importance to Earth’s climate system. The unprecedented changes that these regions are now experiencing have the potential to impact everyone on our planet, via sea level rise, extreme weather events and by threatening food security. The complex exchange processes that occur between the deep ocean and the polar continental shelves move climatically-important quantities such as heat, salt, and nutrients, and are thus essential to the functioning of the polar oceans within the climate system. This thesis uses oceanographic observations from both polar regions to further our understanding of two components of this shelf-ocean exchange: (i) vertical mixing on the West Antarctica Peninsula (WAP) and (ii) eddies in the Arctic Ocean. In 2016, an ocean glider deployed in Ryder Bay, WAP, collected hydrographic and microstructure data, obtaining some of the first direct measurements of turbulent kinetic energy dissipation off West Antarctica. These data reveal significant spatio-temporal variability in hydrographic and dissipation conditions, with elevated dissipation and heat fluxes observed above a topographic ridge at the bay’s entrance, suggesting that the ridge is important in driving upward mixing of warm Circumpolar Deep Water. 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 ∼ 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 ∼ 1.7 days later. High dissipation and heat fluxes over the ridge appear to be controlled hydraulically, being co-located (and moving) with steeply sloping isopycnals. The ridge thus provides sustained heat to the base of the thermocline, which can be released into overlying waters during the bay-wide, thermocline-focused dissipation events. This highlights the role of underwater ridges, which are widespread across the WAP, in the regional ocean heat budget. A rapid, high-resolution hydrographic and current shipboard survey provides a unique three-dimensional view of an anti-cyclonic, cold-core eddy in the Arctic Ocean. The eddy was situated 50-km seaward of the Chukchi Sea shelfbreak, embedded in the offshore side of the Chukchi Slope Current. The eddy core (at 150-m depth) consisted of saline, newly ventilated Pacific winter water, which is important for ventilating the cold Arctic halcoline. Subtracting out the slope current signal, the eddy’s velocity field was symmetrical and approximately in geostrophic balance, with a peak azimuthal velocity of ∼ 10 cm s−1 . The eddy’s age is estimated to be on the order of months, and different scenarios are discussed regarding how the eddy became embedded in the slope current, the eddy’s life-span, and the ramifications for halocline ventilation.

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ORCID for Alberto Naveira Garabato: orcid.org/0000-0001-6071-605X

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