Geoscience Reference
In-Depth Information
The two satellite images of sea surface chlorophyll and sea surface temperature in
Fig. 10.16a
show a band of locally increased chlorophyll coincident with the region of
cool water, while the section in
Fig. 10.16b
illustrates the sub-surface chlorophyll
maximum either side of the shelf edge beingmixed upward at the shelf edge and reaching
high surface concentrations. This link between the physics and the chlorophyll, and the
broader ecological consequences, will be explored further in
Section 10.9.1
.
Physics summary box
The very different regimes of the deep ocean and shelf seas meet and adjust to each
other over the continental slope which is generally a narrow region of steep
topography.
Steady, geostrophic flow over a steep slope on a rotating earth is constrained to be
parallel to the isobaths at all depths. This 'bathymetric steering' tends to trap
currents (e.g. the Gulf Stream) over the slope and prevents them from moving onto
the shelf.
The steep slope topography and the latitudinal density gradient combine through
the JEBAR mechanism to drive slope currents.
Slope currents, forced by JEBAR, have a large barotropic (depth-independent)
component so that their influence extends to the seabed over the upper part of the
slope.
Bathymetric steering inhibits cross-slope exchange, but some cross-slope flow can
occur when the geostrophic constraint is broken, notably in the top and bottom
boundary layers where friction is an important term in the dynamics.
Friction at the seabed can undermine the geostrophy of an along-slope flow.
Persistent downslope flow in the bottom boundary layer of a steady slope current
at the eastern boundary of the ocean provides an important mechanism for the
transfer of material from the shelf to the deep ocean (the Ekman drain). Upslope
flow (upwelling) in the bottom boundary layer can occur under western boundary
currents as they impinge against the shelf slope.
Episodic downslope flow may occur infrequently due to 'cascading' after winter
cooling helps to increase the density of shelf water so that it exceeds that of deeper
water on the slope.
In the surface boundary layer, wind forcing may drive cross-slope transports with
consequent upwelling and downwelling at the shelf edge as well as on the shelf.
The barotropic tide forces stratified water up and down the slope, which acts as a
wave maker and generates an internal tide which propagates as a series of internal
waves on to, and away from, the shelf.
Most of the energy transported by these waves is dissipated close to their source
and results in a region of increased mixing in mid-water with a consequent
reduction in sea surface temperature which has been detected in I-R imagery.
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