Physical forcing of the shelf seas: what drives the motion of ocean? - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 39

Figure 2.11 Depth-averaged
salinity distribution in winter
in Spencer Gulf, South
Australia. From (Nunes and
Lennon, 1987 ), courtesy
of the American Geophysical
Union.

Port
Augusta

33°S

Whyalla

Port Pirie

Cowell

Wallaroo

34°S

10 km

137°E 138°E

surface salinity significantly, with a consequent reduction in density and the
enhancement of water column stability.

2.4 Forcing by wind stress and pressure gradients

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In addition to acting as a source and sink for heat and freshwater, the atmosphere also
exerts a stress on the sea surface which involves the transfer of momentum from the
wind to surface currents in the ocean. In principle this exchange of momentum can go
either way but, since winds in the atmosphere tend to involve much higher velocities
than surface ocean currents, we are mostly concerned with the downward transfer of
momentum and energy to the ocean from the atmospheric boundary layer. The result-
ant stress t w on the sea surface is frequently represented in terms of a quadratic drag law

C d a W 2

t w ¼ C d a W ð W u s Þ ð 2 : 15 Þ

where r a is the density of air, C d is a drag coefficient, W is the wind speed and u s is the
component of surface current in the direction of the wind. In most cases u s <<

W so