Geoscience Reference
In-Depth Information
2.3.5.
An ocean moved by thermohaline fluxes
We have just highlighted the importance of surface wind stress as a
motor of oceanic circulation (and conversely, this wind stress at the
surface of the ocean acts as a brake on atmospheric circulation).
This exchange of mechanical energy is only one of the methods of
exchange at the air-sea interface. Ocean and atmosphere also
exchange heat, mass and numerous chemical components. The
thermohaline fluxes correspond to surface exchanges that modify
temperature and salinity.
Let us consider the surface heat budget. The ocean receives energy
from the sun, of which it reflects a part; it also emits into the
atmosphere in long waves, and it exchanges two sorts of thermal energy
with the atmosphere: the sensible heat flux and the latent heat flux. The
first flux depends on the difference in temperature between air and
water as well as on the surface wind stress, the second flux depends on
the specific humidity difference and on the wind stress. What interests
us for the climate, is to understand the distribution of this surface budget
and to analyze it in the context of the distribution in temperature in the
oceans (Figure 2.14). The budget map shows that the maximum
absorption of thermal energy in the oceans occurs in the central-eastern
part of the equatorial oceans with close to 200 W/m
2
absorbed. The
regions where the ocean loses a great deal of thermal energy to
the atmosphere correspond to the large western boundary currents in the
wake of which enormous quantities of energy are transmitted into the
atmosphere. What we note here, is that these exchanges do not follow a
regular distribution between Pole and equator, but they are strongly
affected by the characteristics of the oceanic dynamics: the absorption
of energy along the equator does not correspond to a maximum
temperature; the presence of cold equatorial waters (the presence of
upwelling
) inhibits the release of latent and sensible heat in these
regions, and favors a broad absorption of solar flux. In the west of the
tropical basins, where the surface temperatures are between 28 and
30°C, the very significant latent heat flux tends to compensate the solar
flux, which renders the gain very slight. In the large western boundary
currents, it is the latent and sensible heat fluxes that very effectively
extract the heat from the ocean to warm up the atmosphere above.
