Geoscience Reference
In-Depth Information
Changes in density caused by heat or freshwater may lead to vertical and horizontal
gradients which contribute to water column stability and circulation respectively.
The mechanical forcing of the shelf seas, i.e. the direct input of momentum and
kinetic energy, arises mainly from forces applied to the sea surface by the atmosphere
and from the tidal energy input from the deep ocean. The principal atmospheric
forcing is in the form of tangential stresses exerted by the wind on the sea surface.
This stress is represented by a quadratic drag law with a drag coefficient which
increases somewhat with wind speed.
The most regular and, in many cases, the dominant mechanical forcing of the shelf
seas is the result of momentum and energy inputs from the tides. A tidal record of
speed or sea level can be broken down into a sum of sinusoidal components, known
as the tidal constituents. These constituents all arise from the orbital characteristics
of the Moon about the Earth and the Earth-Moon system about the Sun. There are
about 400 known constituents, though generally only a few dominate any particular
region. The tide generating forces exerted by the Moon and the Sun inject energy
mainly into the deep ocean. This energy takes the form of long waves which
propagate on to the shelf and induce large tidal oscillations in which most of the
incoming energy flux is dissipated by frictional effects. The energy consumed is taken
from the kinetic energy of the Earth's rotation as the frictional stresses on the seabed
act as a brake which is gradually slowing the rate of rotation. Most (
75%) of the
∼
tidal energy is dissipated in the shelf seas.
FURTHER READING
Changing Sea Levels: Effects of Tides, Weather and Climate, by David Pugh, Cambridge
University Press, 2004.
Atmosphere-Ocean Dynamics, by Adrian E. Gill, Cambridge University Press, 1982.
See particularly chapters 1 and 2.
Atmosphere and Ocean: A Physical Introduction, by Neil Wells, Wiley, 1997.
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