Recent studies have begun to provide new insights into the bottom boundary layer over shelves. Measurements made from bottom tripods, primarily by scientists studying sediment transport [ Cacchione and Drake, 1990], have provided estimates of the bottom stress (the drag the bottom exerts on the flow) and shown that over continental shelves the bottom stress depends not only on the near-bottom currents and the bottom characteristics, but also on surface gravity waves. Observations in a variety of different shelf environments have provided confirmation for models of the nonlinear interaction of waves and currents in the bottom boundary layer [ Gross et al., 1992; Madsen et al., 1993]. The observations have also motivated refinements to the models. For example, Drake and Cacchione [1992] provided observational evidence that the bottom stress is sensitive to the orientation of wave-generated sand ripples relative to the mean current.
The studies cited above have focused primarily on the bottom boundary
layer dynamics near the bed (less than 1 m above the bottom) and processes
influencing the bottom stress. However, Keen and Glenn
[1994] showed that inclusion of wave-current interactions in a numerical
model resulted in as much as a 25% reduction in alongshelf current
magnitudes in shallow water (less than 30 m depth). They found that in
the deeper stratified portion of the water column the impact of
wave-current interactions was complicated even for simple bathymetry and
forcing. There has also been recent progress in our understanding of the
outer portion of the bottom boundary layer which can be 10s of meters
thick over the shelf [ Lentz and Trowbridge, 1991].
Several recent studies have shown that the presence of both stratification
and a bottom slope can profoundly influence the response of the bottom
boundary layer [see Garrett et al., 1993]. If the
cross-shelf transport in the bottom boundary layer is upslope, it will
tend to carry denser water upslope, increasing the stratification near the
bottom and inhibiting growth of the bottom mixed layer (Figure
4).
In contrast, if the cross-shelf transport
in the bottom boundary layer is downslope, it will carry lighter water
under heavier water, tending to enhance growth of the bottom boundary
layer [ Trowbridge and Lentz, 1991]. This implies that
bottom mixed-layers should be thicker during downwelling flows than during
upwelling flows. Lentz and Trowbridge [1991] examined
observations from the northern California shelf and found that this was
indeed the case. Houghton [1995] looked for this
asymmetry using observations made near the shelfbreak off the east coast
of the United States. He found a less clear picture, possibly due to the
presence of the shelfbreak front (see Section 2.2). Another, possibly
more important, consequence of this process is a reduction in the bottom
stress felt by the interior flow because the resulting cross-shelf
buoyancy force within the bottom boundary layer tends to balance, at least
in part, the interior pressure gradient. This is an area where more
research is needed to assess the impact and relevance of these ideas to
continental shelf dynamics.