The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 313

10.3.1 Wind stress in the surface boundary layer

In the surface layers, the action of large frictional stresses imposed by the wind can
invalidate the geostrophic balance and induce substantial cross-slope transfer. We
found in Section 3.5 that a steady wind in the x direction exerting an along-slope
stress of t w , in deep water and away from lateral boundaries, induces a vertically
integrated flow at right angles to the wind stress given by the Ekman transport
relation ( Equation 3.43 ) as:

¼ w V

0 f :

ð 10 : 14 Þ

If the wind is blowing parallel to the shelf edge in the northern hemisphere with
the shelf to the left of the wind direction, the transport will be offshore and
normal to the bathymetry. For t w ¼

7.5 m s 1 ), this 0.1Pa(windspeedW ∼

1m 3 s 1 per metre of shelf edge. As we saw in
Section 3.5.3 , continuity requires that such an offshore Ekman transport in the
surface layer must be balanced by onshore flows in the lower part of the water
column with an upwelling flow at the ocean boundary. When the upwelling-
favourable wind switches on, it takes some time for the system to adjust and to
establish the nearbed onshore flow of water. As an example, consider the time
series of alongshore winds and the response of the lower water column collected
by one of us during work at the shelf edge of northeast New Zealand, shown in
Fig. 10.6 . Pulses of upwelling-favourable wind stress ( Fig. 10.6a )oftencorrelate
with onshore near-bed mean flow ( Fig. 10.6b ) and reductions in near bed tem-
perature ( Fig. 10.6c ) indicating the on-shelf transfer of cooler, deeper water. The
most obvious upwelling events in Fig. 10.6 occur at the end of May, end of June,
and following the July 19. A more detailed correlation analysis of the time series
indicates that about 44% of the near bed cross-slope current variability and 65%
of the near bed temperature variability can be explained by changes in the along-
slope wind stress, with the currents lagging the wind by 37 hours and the tem-
perature lagging the wind by 65 hours. The New Zealand shelf is relatively narrow
(the mooring that provided the data in Fig. 10.6 was about 35 km from the coast),
but localised transfers of deeper water across a shelf edge also occur on wider
shelves (Johnson and Rock, 1986 ).
At this stage it is worth noting that the front between the cold, upwelled water
and the warmer shelf surface water is not generally seen to be a stable feature.
The alongshore geostrophic current set up on the warm side of the front gener-
ates mesoscale frontal instabilities, and potentially eddies, analogous to those
seen at the shelf sea tidal mixing fronts ( Section 8.4 ). The structure of the
instabilities in upwelling systems has been observed to take the form of a number
of pronounced filaments of surface water streaming offshore to beyond the shelf
edge at intervals along the coast set by prominent features in the coastal topog-
raphy (Haynes et al., 1993 ).
Upwelling of deep water from seaward of the shelf break has important conse-
quences for the supply of nutrients to the shelf seas, while the wind-driven offshore

cross-shore transport will be V ∼