Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 190

much greater heat storage than strongly stratified regions. The contrast by a factor of
∼

2 in the amplitude of heat storage between mixed and stratified regimes predicted
by the two-layer model is close to that observed (Simpson and Bowers, 1984 ;
Simpson and Bowers, 1990 ). On the down side, the ability of the two-layer model
to represent the evolution of vertical structure is ultimately limited by the strong
assumptions on which it depends. The model does not give us a detailed picture of the
vertical structure, nor does it indicate where and when mixing is occurring in the
water column. It also neglects the fact that the efficiencies of mixing are not constant
but depend on the degree of stratification, as we saw with the laboratory results of
Turner (Turner, 1973 )( Fig. 6.3 ). This latter point will be revisited in Chapter 8 when
we discuss the adjustment of the tidal mixing fronts as the tides change over the
spring-neap cycle.

Physics Summary Box

The net heat input to the sea surface is the difference between the incident solar
radiation and the loss of heat back to the atmosphere (by infra-red radiation,
conduction and evaporation).

As with light, heat flux declines exponentially with depth through the water
column. In a typical shelf sea, 90% of the net heat crossing into the sea surface will
be absorbed within the upper 5 metres of the water column. In the absence of any
mixing, the temperature profile of the water would also be exponential; i.e. a stratified
water column with surface water becoming much warmer than deeper water.

Whether or not a shelf sea water column will become thermally stratified is
dependent on the balance between the rate at which heat input can generate
surface water warmer than the deep water, and the rate at which mixing processes
can re-distribute the heat throughout the water column.

In areas where mixing is dominated by turbulence generated by friction between
tidal currents and the seabed, the balance between heating and mixing results in
h/u 3 being a useful parameter to map the horizontal distribution of stratified and
mixed waters (h is the water depth and u is some measure of the strength of the
tidal currents). High h/u 3 (deep water and/or weak tides) is associated with regions
that stratify during summer, low values of h/u 3 (shallow water and/or strong tides)
are found in regions that remain vertically mixed all year.

The competition between heating and stirring divides the shelf seas into mixed
and stratified regimes. The heating and stirring effects balance at a critical value
of h/u 3 , leading to the formation of the shelf sea fronts (see Chapter 8 ).

In parts of the world where tides are generally weak, the effects of wind-driven
mixing should be included in assessing the likelihood of seasonal stratification in
shelf seas.