Etudes et évaluation de processus océaniques par des hiérarchies ...

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18 CHAPTER 4. SURFACE FLUXES, THE FORCING OF THE OCEAN
due to the variability of the solar radiation. The solar irradiance has a marked cycle of 11
years leading to a variabilty of only 0.1%. The effects of (small) shifts in the frequency of the
solar irradiance on the earths heat budget are unexplored. Due to geometry of the earth, its
inclination of its rotation axis to the sun, and its rotation, the short wave heat flux at the top
of the atmosphere during one day is a function of the day in the year and the latitude.
Part of the incoming short wave radiation is reflected back to space by the atmosphere, the
amount strongly depends on the cloudiness.
The quantity of short wave heat energy absorbed by the earth depends also on the extent
to which it reflects radiation. This is measured by the albedo which is the ratio of reflected
energy to incident energy. The albedo is a dimensionless number between zero for a black body
which absorbs all radiation, and one, for an object reflecting all light as, for example, a perfect
mirror. Various values of albedo are given in table. 4.1.1.
tel-00545911, version 1 - 13 Dec 2010
Surface type albedo
fresh snow 0.7 – 0.9
ice 0.3 – 0.4
ocean surface 0.05 – 0.15
Table 4.1: Albedo
The albedo of the ocean depends also on the roughness of the ocean (waves) and the angle
of incidence of the radiation. The errors in the determination of the albedo of the sea surface
present a major source of error in the estimation of the heat uptake of the ocean.
Exercise 4: Search the Internet for typical values of the albedo of the ocean.
Exercise 5: Search the Internet for a global map of the short-wave and long-wave radiative
heat fluxes.
4.1.2 Sensible Heat Flux
The sensible heat flux, that is the heat fluxes between the ocean and the atmosphere due to
molecular exchange of heat (not matter) is mostly negative, as the ocean surface, which is
heated by the radiation of the sun, is on average warmer than the atmosphere just above. The
sensible heat flux depends on the density of air ρ air = 1.3kg m −3 its heat capacity C p = 1.3×10 3 J
(K kg) −1 , the wind speed usually measured at the reference level of 10 meters above the ocean
surface |u 10 | and the local temperature difference between the sea surface temperature (SST)
and the atmosphere 10 meters above the ocean.
The bulk formula to calculate the sensible heat flux is
Q sensible = ρ air C p C s |u 10 |(T atmos − SST), (4.2)
where the sensible heat transfer coefficient C s = 900 is empirically determined.
Exercise 6: Search the Internet for a map of the sensible heat flux of the world ocean.
4.1.3 Latent Heat Flux
Evaporation is the major loss of heat by the ocean. The most energetic water molecules have
enough energy to escape the water reducing this way the average temperature of the remaining