What controls the warming of the Antarctic bottom water aupply to the Atlantic Ocean from the Weddell Sea?

Abstract

Antarctic Bottom Water (AABW) is the densest oceanic water mass and plays a pivotal role in regulating the overturning within the Southern Ocean and beyond. Over the past three decades Weddell Sea sourced AABW has experienced a marked contraction, warming and freshening with large implications for the ocean's ability to draw down both atmospheric heat and carbon. Accompanying this decadal trend, AABW displays significant variability across a range of timescales. Much of this variability has been linked to the role of wind forcing within the region, although the timescales and mechanisms linking surface forcing to the abyss remains an open question. This thesis shows on annual to interannual timescales, from moored observations, strengthening winds reduce AABW transport through a key export site due to the growth of a bottom boundary layer. This boundary layer has direct consequences for mixing on shorter timescales leading to enhanced dissipation and altering the water column structure. On longer timescales, wind drives changes in connectivity of the gyre with wider regions such as the ACC, linking transport with changes in formation due to varying eddy energy and gyre strength.

Using a moored array within the passage, AABW export in 2015 is shown to reduce substantially, concurrent with increased wind stress. Strengthening winds accelerate the boundary current transporting AABW from the northern Weddell Sea. This drives a thickening of the bottom boundary layer. Within the boundary layer transport is reduced alongside reduced stratification. The boundary layer and the sloping terrain create the conditions for instabilities. Reduced stratification and velocity shear within the layer are observed through the moorings, causing sub-mesoscale instabilities to develop. These instabilities promote exchange between the boundary layer and the interior, thinning the boundary layer. These changes are episodic, with rapid transitions between quiescent and turbulent conditions.

These changes are imposed onto interannual variability, partially driven by changes in wind. An idealised model of the Weddell region explores the longer timescales. The changes to transport through an idealised passage and the gyre dynamics, are diagnosed in response to changing wind forcing. Strengthening zonal and meridional winds, across the region decreases export of dense waters. For meridional winds, enhanced off shelf export reduces the cooling of dense shelf waters, whilst zonal winds increase isopycnal overturn, increasing available potential energy dissipated through eddy activity, increasing poleward heat transport and warming the gyre. The eddies also act to enhance exchanges with the shelf, leading to further warming of export.

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Identifiers

ORCID for Christopher Auckland: orcid.org/0000-0001-7613-6789

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

ORCID for Eleanor Frajka-williams: orcid.org/0000-0001-8773-7838

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