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Fig. 2—Convergence of meridional moisture transfer (solid lines) as
computed from the transfer depicted in Fig. 1 expressed in net precipitation (evaporation in case of negative values),
The dashed lines
give the corresponding values deduced from ocean salinity. [G. Wist,
Arch, Meterol. Geophys. u Bioklimatol., Ser. A, 7: 305-328 (1954).]
net evaporation or precipitation is shown. It has been computed as the
divergence or convergence of the meridional water-vapor flux. We
therefore know the net transfer between the atmosphere and the sea
rather well, but our knowledge of the gross exchange is considerably
less accurate.
The water-vapor content of the air decreases quite rapidly with
elevation owing to the decreasing temperature and thus decreasing
saturation vapor pressure. Most of the water in the atmosphere and
therefore aiso the transfer of water vapor in the atmosphere occurs
in layers below about 5 km. The lower branch of the meridional circu-
lation cells (see Fig. 3) in the tropics brings moist air from sub-
tropical latitudes to equatorial regions in the lower part of the atmosphere, but their return branches in the upper troposphere above
10 km are very dry. This obviously means a net transport of water
vapor towards the equator, and Fig. 1 shows that actually the major
part of the horizontal transfer in the tropics is due to these meridional
circulation cells. The upward air motions in the intertropical convergence zone related to the mean meridional circulation cells carry
moist air to higher levels whereby it is cooled and condensed with the
water being brought to the surface of the earth as precipitation. Water