Abstract
Abstract The overflow of dense water from the Nordic Seas through the Faroese Channel system was investigated through combined laboratory experiments and numerical simulations using the Massachusetts Institute of Technology General Circulation Model. In the experimental study, a scaled, topographic representation of the Faroe-Shetland Channel, Wyville-Thomson Basin and Ridge and Faroe Bank Channel seabed bathymetry was constructed and mounted in a rotating tank. A series of parametric experiments was conducted using dye-tracing and drogue-tracking techniques to investigate deep-water overflow pathways and circulation patterns within the modelled region. In addition, the structure of the outflowing dense bottom water was investigated through density profiling along three cross-channel transects located in the Wyville-Thomson Basin and the converging, up-sloping approach to the Faroe Bank Channel. Results from the dye-tracing studies demonstrate a range of parametric conditions under which dense water overflow across the Wyville-Thomson Ridge is shown to occur, as defined by the Burger number, a non-dimensional length ratio and a dimensionless dense water volume flux parameter specified at the Faroe-Shetland Channel inlet boundary. Drogue-tracking measurements reveal the complex nature of flow paths and circulations generated in the modelled topography, particularly the development of a large anti-cyclonic gyre in the Wyville-Thompson Basin and up-sloping approach to the Faroe Bank Channel, which diverts the dense water outflow from the Faroese shelf towards the Wyville-Thomson Ridge, potentially promoting dense water spillage across the ridge itself. The presence of this circulation is also indicated by associated undulations in density isopycnals across the Wyville-Thomson Basin. Numerical simulations of parametric test cases for the main outflow pathways and density structure in a similarly-scaled Faroese Channels model domain indicate excellent qualitative agreement with the experimental observations and measurements. In addition, the comparisons show that strong temporal variability in the predicted outflow pathways and circulations have a strong influence in regulating the Faroe Bank Channel and Wyville-Thomson Ridge overflows, as well as in determining the overall response in the Faroese Channels to changes in the Faroe-Shetland Channel inlet boundary conditions.
Highlights
The south-westwards outflow of relatively cold and fresh water from the Nordic Seas to the North Atlantic Ocean is known to occur partly in the surface waters along the Greenland coast and partly at depth via several overflow locations along the Greenland-Scotland Ridge (GSR; see forOcean Dynamics (2014) 64:273–292 example Hansen and Østerhus 2000; Olsen et al 2008)
The main region of interest for the present study considers the dense water inflow from the Norwegian Sea (Norwegian Sea Deep Water, NSDW) at the lower end of the Faroe Shetland Channel (FSC), its expansion and topographic routing within the Wyville Thomson Basin (WTB) and the resulting outflow pathways either (1) wholly through the up-sloping, converging approach to the Faroe Bank Channel (FBC) threshold sill or (2) partly across the Wyville Thomson Ridge (WTR) and through the Ellett Gully (EG) in addition to the FBC
It is apparent that for a certain parametric range of Bu1: Q1∗ conditions, no spillage is observed across the WTR (e.g. Fig. 4a), whilst other parametric conditions result in strong dense water overflows across the WTR (Fig. 4c, d)
Summary
Open questions remain on the importance of exceedance of the transport capacity (Wåhlin 2002) of the FBC, coupled with topographically induced departures from geostrophy in the FBC deep water overflow (Davies et al 2006), in regulating and limiting the discharge over the sill and promoting spillage over the WTR To investigate such processes, a combined laboratory and numerical modelling study has been undertaken of the deepwater circulation and outflow characteristics in the Faroese Channels, focused on defining the parametric conditions that result in WTR spillage. A combined laboratory and numerical modelling study has been undertaken of the deepwater circulation and outflow characteristics in the Faroese Channels, focused on defining the parametric conditions that result in WTR spillage In this regard, previous field studies of the FBC deep-water outflow (e.g. Lake et al 2005; Johnson and Sanford 1992) have indicated the presence of comparatively homogeneous (i.e. roughly constant temperature) deepand surface water masses, separated by a permanent and welldefined pycnocline. Distorted model approach is in general accordance with previous experimental studies of outflows in the Faroe Bank Channel region (e.g. Davies et al 2006; Cuthbertson et al 2011)
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