Geoscience Reference
In-Depth Information
Thus, the western boundary regions, characterized by strong
contrasts in oceanic temperature (just like the strong fronts along the
circumpolar currents, in particular to the south-east of South Africa),
are also regions of strong atmospheric variability during the winter
season. Therefore, it is often possible to observe intense winds
associated with cold, dry air coming from the continents. What are the
exchange mechanisms that occur in these conditions? It has long been
known, from the analysis of scatterometer winds in particular, that the
surface winds and ABL properties are very different on either side of
the strong fronts that border these currents or the ocean vortices that
they generate.
These mechanisms are caused mainly by the contrasts in surface
temperature, and therefore by the changes in the ABL. A high
instability is observed over warm waters when continental air arrives.
The strong winds and strong vertical gradients in temperature and
humidity, therefore, induce very strong sensible and latent heat fluxes
over the warm sectors, and it is not unusual for these combined heat
fluxes to exceed 1,000 Wm
-2
. As strong momentum fluxes are
associated with sea waves, and sometimes with strong currents beside
these fronts, this can lead to very heavy seas and to the formation of
rogue waves. The intense winds and the resulting coolings reinforce
the exchanges of gas, in particular of CO
2
. The reason is that the
vertical mixing caused by the strong momentum flux pulls waters rich
in inorganic carbon into the surface layer, while the cooling would
tend to diminish the partial pressure of CO
2
in the seawater. In the
atmospheric boundary layer, the marked instabilities trigger a rapid
deepening of this layer and eventually the formation of shallow
convection. Associated clouds are, likewise, also very different on
either side of the fronts.
3.3.4.
Marginal ice zones and associated air-sea fluxes
Sea ice occupies the main part of the icy Arctic Ocean in winter,
while its surface is reduced in summer. For the last 10 years, the
amount of ice which is melting in summer has been significantly
increasing (see Figure 3.7). The mechanism responsible for this
reduction has not yet been established, since the number of
in situ
