Geoscience Reference
In-Depth Information
3.3. Examples of energy exchanges
3.3.1.
Tropical cyclones
Tropical cyclones are among the sea's most intense meteorological
phenomena. They develop over warm waters (temperatures higher
than 26°C), although not at the Equator, since the Coriolis force is null
there: it is this force that puts into rotation the convective systems
from which cyclones originate.
Thus, they form on either side of the Equator, at around 10° latitude,
and when mature are made up of bands of intense convective systems
organized in a spiral around a calm center (the eye) (see Figure 3.4).
Their spatial characteristics can change radically in just one or two
days. Some cyclones move rapidly, whereas others remain fairly
stationary. When they approach a continent, their trajectory can branch
off abruptly toward higher latitudes, transforming them into intense
storms in temperate zones, as they collide with low pressure systems
circulating from West to East (fairly frequent in the North Atlantic).
The passage of a cyclone causes a surface water cooling (through
vertical mixing above all, but also heat loss by evaporation), which
acts, in turn, on the cyclone itself, and possibly contributes to its
evolution. On average, for identical conditions, it appears that a
cyclone that causes less cooling will grow more dramatically and
spread less quickly than a cyclone that causes a strong cooling.
The impact of a cyclone on the ocean surface temperature is very
variable: the regions where the mixed layers are not very deep will be
more rapidly cooled, just as in cases where the thermal stratification
beneath is strong. Depending on the duration of the cyclone's passage
and its intensity, surface cooling variations reaching as far as factor 8
can be induced. The air-sea fluxes of momentum, heat and water
vapor tend to be reinforced on the right side of the cyclone track (in
the northern hemisphere), because of the addition of its rotation speed
with its displacement speed. In the same place, the strongest sea
currents are generated, the swells moving in the direction of the
cyclone being greatly increased with the associated momentum flux.
