Geoscience Reference
In-Depth Information
that u s may be neglected. It should be noted that this relation has the same form as
the bulk parameter representations of evaporation and sensible heat transfer rates
with the relative momentum in the form of (W-u s ) replacing the air-sea temperature
difference, for example, in Equation (2.5) .
Like the other coefficients, C d has been determined by measurements of turbu-
lent transfer in the marine boundary layer made from spar buoys. These measure-
ments reveal that C d is not strictly constant but varies with the degree of
stratification in the atmosphere and with wind speed. Typically C d is reduced by
60% for stable conditions relative to the value for a neutral atmosphere. One of
the simpler formulations linking C d to wind speed is a linear trend (Smith and
Banke, 1975 ):
10 3
C d ¼ð
0
:
63
þ
0
:
066 W
Þ
:
ð
2
:
16
Þ
This result can be interpreted as indicating an increase in the roughness of the
sea surface attributed to the increasing wave height. However, it is fair to say
that the physics of momentum transfer from atmosphere to ocean is not
yet well understood. In addition to the direct effect of the tangential wind
stress on the sea surface, momentum is also transferred by surface waves
which are driven by normal pressure forces. Large waves in a fully developed
sea have considerable net forward momentum in the near surface layers, some
of which is transferred to the mean flow when the waves break (Melville and
Rapp, 1985 ).
As well as injecting momentum and energy into the ocean surface via surface
stresses, the atmosphere also forces the ocean through the action of atmospheric
pressure on the sea surface. The gradients of atmospheric pressure, which drive the
winds, are also applied to the ocean and can induce significant flows as the ocean
adjusts to the varying pressure field. Generally, however, the direct effect of pressure
gradient is smaller by at least an order of magnitude than that of the corresponding
wind stress.
2.5
Tidal forcing
...................................................................................
In many shelf sea areas, the largest and most consistent mechanical forcing comes
from the tides. The tide generation due to the Moon arises from an imbalance
between the forces acting on a particle due to the gravitational attraction of the
Moon and the centrifugal force due to the Earth's rotation about the centre of
gravity of the Earth-Moon system. The balance between these forces is exact only
at the centre of the Earth. At all points on the Earth surface, the small imbalance
in these forces results in a tide generating force, which is
10 7 g. The Sun exerts a
similar force with a magnitude
that of the Moon. Together, these forces act
on the waters of the ocean to drive the tides. Box 2.1 provides a more complete
explanation of the tide generating forces.
0.46
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