Interior mixing and phytoplankton survival in stratified environments - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 215

of 10 metres (Green et al., 2010). These movements of the pycnocline indicate the

presence of internal wave motions which frequently involve large velocity shear across
the pycnocline, with upper and lower layers moving out of phase (see Section 4.2 ).
Such shear can contribute to reducing the Richardson number and hence to produ-
cing internal mixing. Internal waves can have a wide range of frequencies extending
from the inertial frequency f to the buoyancy frequency N. Much of the energy input
to internal waves comes from the barotropic tidal movement of a stratified water
column over uneven bottom topography. The most obvious example of this mechan-
ism in shelf seas is the particularly large internal waves of the semi-diurnal period (the
internal tide) generated over the steep topography of the shelf edge (more on this will
follow in Chapter 10 ). Waves generated in this way travel on to the shelf and become
distorted through non-linear processes to form shorter period internal waves, termed
solibores. These modified waves transmit significant amounts of energy on to the shelf
and can contribute to mixing in the shelf sea pycnocline close to the shelf edge.
Internal waves of tidal period may also be produced away from the shelf edge by flow
over banks and depressions in the seabed (Moum and Nash, 2000 ).

∼

(ii) Inertial motions

As we saw in Section 3.4 , an impulse of wind at the sea surface generates a 'slab'
motion of the surface layer over the thermocline, leading to inertial oscillations in
surface currents. Where the water column is stratified, the pycnocline is a region of
reduced frictional stress (low N z ) so the damping of inertial oscillations in the surface
layer is weak and they may persist for many days. Moreover, in shelf seas where there
are land boundaries influencing the flow, the wind stress can set up an opposing
pressure gradient which induces an out-of-phase oscillation in the layer below the
pycnocline (Rippeth et al., 2002 ) .

(iii) Spring-neap contrasts in tidal mixing

Turbulent dissipation arising from friction between tidal currents and the seabed
can often be detected high within the water column (Sharples et al., 2001b ) ( Fig. 7.5a ),
potentially impacting on the base of the thermocline. The intensity of this dissipation
is modulated by the spring-neap cycle in the tidal currents, resulting in a fortnightly
cycle in the mixing at the base of the thermocline (Sharples, 2008 ). As tidal currents
increase towards spring tides, so does the strength of the current shear and mixing at
the base of the thermocline and the lower few metres of the thermocline are eroded.
As the tidal flow and current shear decreases between spring tides and neap tides, the
base of the thermocline again deepens. The position of the base of the thermocline
oscillates due to a fortnightly shift in the balance between the turbulent mixing
generated by the tides in the bottom layer and by winds in the surface layer.
Processes (i) and (ii) above are mechanisms internal to the water column, while
(iii) is a boundary-driven mechanism. The easiest process to simulate with a numer-
ical model is the spring-neap cycle in the erosion/re-formation of the thermocline
base. Our TC model can simulate the modulation of the tidal shear, and as long as
the vertical resolution of the model

is sufficient, the shifting position of the