Exchange of water masses between the southern Weddell Sea continental shelf and the deep ocean

Abstract

The exchange of water masses between the shallow Weddell Sea continental shelf and the deep ocean is of climatic importance. Firstly, on-shelf transport of warm oceanic water masses, contributes to basal melting of ice shelves, which affects the stability of the Antarctic Ice Sheet. Secondly, export of denseWeddell Sea shelf water, contributes to production of Antarctic Bottom Water (AABW), which feeds into the lower limb of the global thermohaline circulation. This thesis considers processes related to both on-shelf heat transport, and to export of dense shelf water. Based on a combination of idealized numerical model experiments and observations of hydrography and current at various locations in the southern Weddell Sea, the thesis discusses pathways and variability of the oceanic circulation along the continental slope, on the continental shelf and inside the Filchner Trough (FT), a deep trough across the continental shelf. The response to wind forcing is assessed, along with mechanisms contributing to mixing and transformation of water masses. The Antarctic Slope Front (ASF) protects the Weddell Sea continental shelf from the salineWarm DeepWater (WDW) offshore. Most of the on-shore transport occurs in the summer season, when the thermocline is shoaling at the shelf break, and through the FT, where the sill depth is 200 m below the shelf depth. However, on-shelf transport of warm water may also occur when dense shelf water is present on the shelf. An isopycnal connection between the dense shelf water and the WDW offshore creates a pathway for WDWto access the shelf without doing work against the buoyancy forces. The southward transport of heat into the FT is sensitive to the slope current properties, and to the characteristics of the dense shelf water. The slope current is associated with the ASF, and flows westward along the continental slope. The core of the slope current is found to migrate shoreward onto shallower isobaths under strong wind forcing, leading to different inflow regimes in the FT. In the weak wind regime, the slope current core is located over deep isobaths, and does not interact with the FT topography. In this regime, the southward heat transport into the FT is dominated by eddies, and is sensitive to dense shelf water properties. In the strong wind regime, the slope current is topographically steered southward into the Filchner sill region. However, potential vorticity constraints at the sill edge force the slope current to turn and exit the Filchner sill, without reaching the deeper part of the trough. The recirculating slope current efficiently advects eddies out of the Filchner sill area, and limits the southward heat transport. A recirculation of the slope current across the Filchner sill could influence the export of dense Ice Shelf Water (ISW) from the FT. The slope current is mainly wind-driven, and responds to monthly scale variability in the along-slope wind stress. Here, it is shown that the variability of the ISW overflow velocity is also connected to the upstream wind stress, which could result from interaction between the ISW overflow and the recirculating slope current. New insight is gained regarding the ISW pathway from the Filchner Ice Shelf (FIS) front toward the continental slope. The ISW is flowing northward as a mid-depth jet along the eastern side of the FT. An eastward turning of the ISW across the FIS front could be related to potential vorticity constraints associated with the large increase in water column depth at the FIS front. The dense ISW overflows the Filchner sill and continues westward along the continental slope as a bottom intensified plume. The plume thickness fluctuates strongly on time scales of a few days. High shear and mixing is observed during the early stage of a thick plume event, and indicate efficient entrainment of WDW, which is essential for production of AABW. The efficient mixing is, at least partly, related to topographic vorticity waves. In summary this thesis discusses key elements of the current circulation and its variability in the southern Weddell Sea. The thesis demonstrates the inter-connection between the cold water masses on the continental shelf and the warmer off-shelf water masses. Today, the ASF is an important barrier which protects the Weddell Sea continental shelf from the warm water off-shore. However, the stability of the ASF is vulnerable to climate change, and is not well resolved in global climate models. This study highlights the need for improved understanding of the processes related to the slope front system, and the importance of the exchange and interaction between water masses in the Filchner Trough.