Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 187

Figure 6.7 The main forcings of the
model and the discrete resolution of the
vertical temperature profile.

Heating Q s (1- A )

Cooling Q u

Wind W

z

T s T

Wind stirring

T(z)

Tidal stirring

û = û M 2 sin ω M 2 t

T b

the temperature profile which is stored as discrete values for sub-layers of thickness
D

z (typically 1 metre), shown in Fig. 6.7 . The operating cycle starts with the input of
solar heat. The heat loss from the sea surface Q u is next extracted from the surface
sublayer of the model vertical grid. The upper sublayers are then mixed by the
available power P w to produce a new surface mixed layer with uniform temperature
which extends downwards as far as the available power allows; i.e. the increase in the
potential energy of the water column matches the energy supplied by the mixing. The
algorithm for the new layer depth includes any energy available from convective
instability introduced by surface cooling. After surface mixing, a similar procedure is
applied at the bottom to create a new bottom mixed layer, which utilises all the
available tidal energy P T arising from the oscillatory tidal currents, thus completing
the heating-stirring cycle. At the next time step, the heat flux is modified according to
the new surface temperature and the cycle is repeated.
Starting from a vertically uniform column at the spring equinox when Q i becomes
positive, the model steps forward in time to simulate the seasonal cycle of water
column temperatures. An example for a mid-latitude site with rather weak tidal
stirring (SH

3m 2 s 3 ) is shown in Fig. 6.8a . Notice how the surface and bottom
temperatures diverge soon after the onset of heating in the spring, which is when the
condition for stratification in Equation (6.26) is met. The temperature difference and
stratification continue to increase during the summer until they reach a maximum in
September. However, the increasing surface temperature leads to large heat losses
( Sections 2.2.2 and 2.2.3), and the net heat flux Q i is significantly curtailed. After the
temperature maximum, there is a rather rapid decrease as surface cooling commences
and starts to induce unstable conditions which drive convective overturning in the
water column. The mixing process is augmented by enhanced wind stirring but, in
contrast to a widespread misconception, it is convection driven by cooling and not
wind stirring which dominates the autumnal overturn and return to a vertically
homogeneous state.

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