Geoscience Reference
In-Depth Information
effect of clouds (see section 3.2.1). This water is transported over the
continents
by winds; after precipitation in watersheds it returns to the
oceans by flowing in rivers or underground;
- in the ocean, evaporation is also the direct cause of salinity
changes, and therefore of the ocean's surface density. It thereby acts
on the local vertical circulation, or on a large scale due to the
thermohaline circulation. Regions with excess precipitation can be
strongly stratified by this supply of fresh, less dense water. A less
dense surface layer can isolate the ocean beneath from the mixing
caused by exchanges at the surface. As they receive light, the layers
below can be warmed by the penetrating solar radiation, so that they
become as warm as, or warmer than, the surface. In this case, they are
called 'barrier layers'. The impact of such layers on the dynamic of
El
Niño Southern Oscillation
(ENSO) events in the equatorial Pacific
Ocean has often been mentioned (these layers are often present in the
west of this basin) where they contribute to the maintenance high
temperatures, even after wind bursts. The presence of such layers can
also lead to amplification of tropical cyclones (for example, in the Bay
of Bengal and the tropical Atlantic to the east of the Caribbean Sea);
Water is still involved in the ocean-atmosphere exchanges in its
solid form, through the formation of sea ice. Sea ice develops during
the polar winter, because of the sea surface cooling (through
evaporation and radiative cooling). During the formation of ice, the
sea salt is progressively pushed down toward the ice bottom in the
form of brine, which increases the seawater's density underneath. This
very dense, cold water can sink to the ocean bottom. Partially
submerged, the ice layer is in contact with the atmosphere and ocean,
and exchanges energy with both.
Sea ice formation processes are complex. The first crystals appear
at the surface at around -2°C in the form of needles, accompanied by
brine drops. Progressively, the ice crystals agglomerate, forming a
frozen “soup”, a viscous layer moved by
the sea, of depths from 1 or 2 cm to 1 m: the frazil. At the same time,
brine droplets agglomerate and sink. The floe forms progressively
(several weeks are needed for a 60 cm thickness to be reached), little-
by-little, pushing out some the bubbles of air and the pockets of brine.
