Future challenges in shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 352

At trophic levels above the primary production, there is some understanding of
how the patchiness in shelf sea ecosystems, from zooplankton up to commercially
important fish stocks, is linked to the physical environment. While phytoplankton
growth is obviously required to provide the basis of food for higher trophic levels,
there are many instances where the physics plays a more direct role, for instance in
modifying prey distributions, having an effect on the availability of prey or by
providing vital transport pathways between spawning grounds and feeding areas.
While progress has been made in understanding the temperate shelf seas and their
ecosystems, much remains to be done in investigating the detail of process inter-
actions and in refining models of the different shelf sea regimes. We shall now
consider some examples of outstanding research questions relating to the temperate
shelf seas which provide challenges for future studies.

11.1.1 Mixing in the pycnocline

In Chapter 7 , we explained the importance of the rather small amount of mixing
which occurs in the seasonal thermocline and drives nutrients upwards to fuel
primary production in the subsurface maximum. Three candidate mechanisms for
supplying the stirring power for this mixing have been identified, namely: (1) internal
wave motions, (2) wind-driven inertial oscillations and (3) peaks in boundary-forced
mixing in the springs-neaps cycle. The environment characteristics which determine
the relative contributions in different parts of the shelf seas are not fully understood.
In principle, it should be possible to simulate the effects of inertial oscillations with
existing models provided good-quality, high-resolution wind data are available.
Recent studies have pointed to the importance of the shear induced across the
thermocline by the rotating shear vector. Peaks in the magnitude of the bulk shear
(velocity difference between top and bottom layers) occur when the shear vector
aligns with the wind stress and/or the bottom stress vector (Burchard and Rippeth,
2009 ). These shear 'spikes' induce a temporary lowering of the Richardson number
which may be sufficient to induce mixing. Accurate modelling of such intermittent
bursts of mixing is likely to be a severe challenge, and therefore a good test, for
turbulence closure schemes.
Internal tides and waves present an even more difficult challenge. As we noted in
Chapter 10 , most of the large energy input to the internal tide at the shelf break is
dissipated close to the source (typically within 75 km of the slope) and does not

influence the inner regions of broad shelf seas where locally generated internal waves
are the alternative candidate power source. In principle, it should be possible to
simulate the generation and propagation of these waves but, as discussed in Chapter
7 , present-day large-scale models of shelf seas have insufficient horizontal resolution
to represent components of the internal waves, which can have wavelengths of 1 km
or less. An alternative approach would be to use the theory of wave generation in
stratified flow over variable topography to estimate the local power source which,
together with assumptions about horizontal scale of propagation before dissipation,
could be used as a basis for parameterisation of the mixing effect.

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