Atmospheric influence on warm water intrusions into Filchner Trough in the 21st century

Abstract

Heat transport between the deep ocean and the southern Weddell Sea continental shelf plays a key role in a potential future melting of the western Antarctic ice sheet. Recent studies have highlighted the Filchner Trough as an area for a potential cold-to-warm tipping point in the ocean circulation, governed by the density gradient across the shelf break, influenced by sea ice formation and wind stress. This thesis firstly explores the representation of on-shore heat transport in the Filchner Trough in climate simulations for a suite of carbon dioxide emission scenarios from high-mitigation to high-emission for the 21st century. The model reproduces observed seasonal pulses of warm water transport into the Filchner Trough, mainly driven by seasonally increased Dense Shelf Water (DSW) formation and export, and a wind driven shoaling of the thermocline at the shelf break. The two high-emission scenarios suggest a strongly increased heat transport onto the continental shelf before the year 2100. Over the course of the 21st century, seasonal pulses increase in temperature and duration until a tipping point is reached and the regime shift occurs. The effect of sea ice and DSW formation on the continental shelf, as well as the positionof the thermocline at the shelf break, are regulated by the atmospheric conditions. Especially temperature and wind fields influence the sea ice formation in coastal polynyas – areas of open ocean surrounded by sea ice, adjacent to the coast and with high sea ice productivity – on the continental shelf of the southern Weddell Sea. While a reference ocean simulation forced with atmospheric data from a high-emission climate simulation does not produce a regime shift, dynamic downscaling of the atmospheric data to 15 km decreases the DSW formation on the shelf enough for a regime shift to be initiated. The better resolution of orographic features decreases the wind speed of the westerlies crossing the peninsula, while stronger offshore winds can be found along several sectors of the coastline of the southern Weddell Sea. The combination of reduced wind-driven sea ice export and higher summer air temperatures decreases the density of the shelf waters at an accelerated rate compared to the reference simulation, making a regime shift possible. The Antarctic Slope Front prevents the transport of saline Warm Deep Water onto the continental shelf. In the southern Weddell Sea, the density distribution of the front takes on the form of a distinct V-shape, formed by the interplay of Ekman downwelling and DSW export. With the occurrence of a regime shift, the V-shape is temporarily disturbed and confined to the upper ocean afterwards. This shows that a weakening of the Slope Front in the upper ocean is no indicator for an imminent regime shift in the Filchner Trough. In summary, this thesis examines controls of the heat transport into Filchner Trough for present and future climate and shows that the possibility for a regime shift in the Filchner Trough is high with current climate policies in effect. However, a regime shift might be avoided if global warming is kept below the 2°C limit.