Geoscience Reference
In-Depth Information
Figure 7.26
Generalized depiction of the major atmosphere-ocean interaction processes. The sea ice thickness is not to scale.
Source
: Modified from NASA (n.d.). Courtesy of NASA.
of 1000 m (at the equator from about 200 to 800 m;
at 40° latitude from about 400 to about 1100 m).
Poleward of 60° latitude, the colder deep-layer water
approaches the surface. The location of the steepest
temperature gradient is termed the
permanent
thermocline
, which has a dynamically inhibiting
effect in the ocean similar to that of a major inversion
in the atmosphere. However, heat exchanges take
place between the oceans and the atmosphere by
turbulent mixing above the permanent thermocline,
as well as by upwelling and downwelling.
driven by density variations, commonly due to salinity
differences (i.e. having a
thermohaline
mechanism).
In terms also of circulation the ocean may be viewed
as consisting of a large number of layers: the topmost
subject to wind stress, the next layer down to frictional
drag by the layer above, and so on; all layers being acted
on by the Coriolis force. The surface water tends to
be deflected to the right (in the northern hemisphere)
by an angle averaging some 45° from the surface wind
direction and moving at about 3 per cent of its velocity.
This deflection increases with depth as the friction-
driven velocity of the current decreases exponentially
(Figure 7.27). On the equator, where there is no Coriolis
force, the surface water moves in the same direction
as the surface wind. This theoretical Ekman spiral
was developed under assumptions of idealized ocean
depth, wind constancy, uniform water viscosity and
constant water pressure at a given depth. This is seldom
the case in reality, and under most oceanic conditions
During spring and summer in the mid-latitudes,
accentuated surface heating leads to the development
of a
seasonal thermocline
occurring at depths of 50 to
100 m. Surface cooling and wind mixing tend to destroy
this layer in autumn and winter.
Below the thermocline layer is a
deep layer
of cold,
dense water. Within this, water movements are mainly
