Geoscience Reference
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bottom is called upwelling and we speak of oceanic upwelling due to
Ekman pumping. The reverse instance, or downwelling , is produced
under anticyclonic atmospheric circulation. This mechanism of Ekman
pumping shows that the wind stress only acts on the surface layer but it
will also impact the interior ocean through deformations in the
thermocline that it sets in motion. The formalization of this equilibrium
is calculated by integrating the continuity equation in the surface layer:
He-> s div u dz = ∫ He-> s { div h u h + ∂ w /∂ z } dz = 0
We introduce w E , Ekman's pumping speed, which is measured at
the depth He of the thermocline:
w E = div h T E = div h {
τ
x k /
ρ
0 f } = curl {
τ
/
ρ
0 f }
Two particular cases are interesting to consider. We will first take
the case of the equatorial region where the trade winds blow. It is
possible to schematically assimilate the trade winds with a wind
blowing along the Equator, from east to west. These winds are
associated with an Ekman transport to the right in the northern
hemisphere and conversely to the left in the southern hemisphere. The
surface waters thus diverge toward the Poles and this loss of mass is
compensated by a powerful equatorial upwelling (Figure 2.11). It is in
this way that in the eastern part of the equatorial basins of the Atlantic
and Pacific, we encounter surges of cold, deep water that are rich in
nutritive salts.
The second particular case that we present concerns the example of
the Senegal-Mauritanian Coast. The winds blow toward the south,
along the coast and create a transport toward the west, pulling the
surface waters toward the open sea. The departure of these waters
creates a rise of water from the bottom along the coast, called coastal
upwelling (Figure 2.12). The rise of cold water from the bottom also
brings nutritive salts and favors the emergence of coastal productivity.
The regions of upwelling have been identified for a long time as
regions of intensive fishing and they are found along the subtropical
coasts: the coast of Senegal-Mauritania, the coast of Namibia, the
coast of Peru and the coast of California.
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