Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 183

( b )

(a)

log( h / u s 3 )

log Φ

E

55°N

A

1.0

Ireland

United
Kingdom

B

C

4

50°N

2

D

France

5°W 0°

5°W 0°

= u s , with u s the amplitude of spring tide currents, west of
the UK; (b) A corresponding map of the distribution of the potential energy anomaly as log 10 F
in August. In (b) the dashed lines A-E mark the positions of the tidal mixing fronts. Based on
Simpson and Pingree, 1978 with permission from Springer-Verlag.

Figure 6.6 (a) A map of log 10

h

due to differences in the heat input rate Q i and also in the expansion coefficient a which
varies significantly with temperature. Differences may also be attributable to changes
in the relative importance of the other main source of boundary-driven turbulence,
the winds. We shall address the impact of wind mixing in the following section.

6.1.4 Adding the effect of wind stress

In the development of Section 6.1.3 , we included only mechanical stirring by the tides
and neglected the influence of stirring by wind stress. The fact that the observed
partitioning of the shelf seas and, in particular, the positions of the tidal mixing
fronts, conforms to the simplified theory provides post facto justification for our
assumption. Comparison of the stresses produced by the wind acting at the surface
and tidal flow at the bed supports the idea that the tides will usually predominate in
the stirring in many of the world's shelf seas. In the tidally energetic regions of the
Irish Sea the stress in a tidal flow of 1 m s 1 , for example, is equivalent to that of a
hurricane wind acting on the Earth's surface. However, in shelf areas with weaker
tidal flows we might expect the wind stress to make a significant contribution
especially in shallow areas (h

30 metres or less) where wind stirring alone may be
sufficient to achieve complete mixing of the water column.
To include wind forcing in the argument, we need to insert a second stirring term in
Equation (6.14) . The rate of working by the wind stress t s (Nm 2 ) on the sea surface
with a wind-induced surface current speed of u s is just

∼