Across-slope bottom boundary layer (BBL) fluxes on the shelf-edge connect
this region to deeper waters. Two proposed ways in which across-slope BBL
fluxes can occur, in regions that have a slope current aligned to the bathymetry, are:
the frictional veering of bottom currents termed the ‘Ekman drain’; and through local
wind-forced downwelling (wind-driven surface Ekman flow with an associated bottom
flow). We investigate the variability, magnitude and spatial scale of BBL fluxes
on the Shetland shelf, which has a prominent slope current, using a high-resolution
(∼ 2 km) configuration of the MITgcm model. Fluxes are analysed in the BBL at
the shelf break near the 200 m isobath and are found to have a seasonal variability
with high/low volume transport in winter/summer respectively. By using a multivariate
regression approach, we find that the locally wind-driven Ekman transport plays
no explicit role in explaining daily bottom fluxes. We can better explain the variability
of the across-slope BBL flux as a linear function of the speed and across-slope
component of the interior flow, corresponding to an Ekman plus mean-flow flux. We
estimate that the mean-flow is a greater contributor than the Ekman flux to the BBL
flux. The spatial heterogeneity of the BBL fluxes can be attributed to the mean-flow,
which has a much shorter decorrelation length compared to the Ekman flux. We conclude
that both the speed and direction of the interior current determines the daily
BBL flux. The wind does not explicitly contribute through local downwelling, but
may influence the interior current and therefore implicitly the BBL fluxes on longer
timescales.