Geoscience Reference
In-Depth Information
disregarding that these formulae are only valid at small scales in time
and space (in general, less than an hour and less than 50 km) which
correspond to the scale of turbulent eddies in the atmosphere.
3.2.2.2.
Turbulent fluxes and the ocean
For the upper layers of the ocean, the turbulent fluxes are the
defining factors:
- the heat fluxes (marked here positively for an upward transfer of
heat, in accordance with meteorological convention) cool the surface
by temperature transfer and evaporation. These effects are added to
the radiative balance to determine the proportion of heat transferred to
the ocean (see Figure 3.2);
- the momentum flux, which conveys the friction exercised by the
atmosphere on the sea's surface, has several effects:
- transfer of momentum, causing a displacement of surface water;
- turbulent mixing in the upper layers;
- generation of waves that then spread deeper;
- formation of swell (sea swell from the wind).
The shortest waves contain fairly little energy and rapidly
retransmit their quantity of movement to the ocean when they
break, which can cause turbulence in the ocean's upper layers in
combination with the movement of orbital waves (or Langmuir cells).
The longest waves are organized in the form of swells, whose energy
propagates fairly rapidly from the regions where they were formed to
the coastal zones, where they can sometimes trigger (especially in
combination with a local wind) strong, breaking waves and the
phenomenon of storm surges.
The quantity of momentum transported by the turbulence in the
oceanic boundary layer (often bounded at its base by a stably stratified
layer, the thermocline) will allow the creation of currents on a larger
scale. Apart from the Equator, these currents are constrained by the
Coriolis force linked to the Earth's rotation. Because of their
horizontal structure (the same as that of the winds) they can be
associated with horizontal convergences or divergences, and so cause