“The Ekman Drain”: a conduit to the deep ocean for shelf material

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  • J.H. Simpson
  • R.R. McCandliss
A long (167 days) acoustic Doppler current profiler time series from the European continental slope west of Scotland has been analysed to investigate the influence of bathymetric steering on the slope current and the extent of down-slope transport in the bottom boundary layer. Within an interior region between the surface and bottom boundary layers, the direction of the flow is found to be remarkably consistent as required by the Taylor-Proudman theorem for geostrophic flow. The mean value of this interior flow direction is taken to be the effective direction of the bathymetry in controlling the geostrophic flow and so defines the rotation of coordinates required to determine along and cross-flow transports. Within a bottom boundary layer (BBL) of thickness ~100 m, the direction of the flow was deflected increasingly to the left with the mean veering angle ~12.5° at 12 mab and a down-slope speed of 2.6 cm s−1. The corresponding integrated transport (the “Ekman drain”) had an average value of ~1.6 m2 s−1 over the full observation period. This down-slope flow was significantly correlated (at 0.1 % level), with the stress applied by the along-slope flow although with considerable scatter (r.m.s. ~1 m2 s−1) which suggests the influence of other forcing mechanisms. Combining the BBL volume transport with an estimate of the mean concentration of suspended particulate material indicates an annual down-slope flux of 3.0 ± 0.6 tonnes m−1 year−1, of which ~0.36 ± 0.1 tonnes m−1 year−1 is carbon. Biogeochemical measurements indicate that the carbon flux in the Ekman drain predominates over settlement of organic material through the water column over the slope and provides for relatively rapid delivery of material to deep water.
Original languageEnglish
Pages (from-to)1063-1072
JournalOcean Dynamics
Volume63
Issue number9-10
DOIs
Publication statusPublished - 1 Oct 2013

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