Geoscience Reference
In-Depth Information
First, let us describe the state of equilibrium of the atmospheric and
oceanic circulations in the Pacific Ocean. The atmosphere is
characterized by a constrated distribution of pressures in the tropics.
Along a meridional section, the circulation is ensured by Hadley cells
that, in the lower troposphere, draw masses of dry, hot air from
tropical regions of high pressure toward the low pressure convergence
zone, situated next to the Equator; these air masses, which gain
humidity during their displacement, are then drawn at altitude by
ascendance in the convergence zone and return at altitude towards the
mid-latitudes, where they subside to low layers in the high tropical
pressure regions situated either side of the Equator. Along a zonal
section (Figure 2.20(a)), the circulation known as Walker circulation
consists of a large loop of zonal circulation over the Pacific, marked
by winds from the east in the lower troposphere, an ascendance of
masses of hot, humid air over the West Pacific accompanied by heavy
precipitation, then a return of air masses eastward, in the upper
troposphere where the air dries and cools progressively, to return to
the surface in the eastern Pacific. The winds from the east are
supported by the difference in pressure between the eastern Pacific
and the western Pacific. The ocean's response to these two cells is
marked by an accumulation of hot surface waters in the west where
the surface is higher than in the east by 60 cm, an important deepening
of the equatorial thermocline (more than 200 m in the west), whereas
the thermocline is close to the surface, around 50 m, in the east. The
zonal circulation consists of a westward surface current (the South
Equatorial Current) and an eastward current in subsurface
(the Equatorial Undercurrent). Along a meridional section, the surface
waters diverge from the Equator at the surface (divergence of
the Ekman transport resulting from the trade winds) and converge in
the subsurface. One peculiarity of the ocean is the complex
organization of its circulation between two regions with singular
properties: the geographic Equator, which is a region of geometric
singularity for Ekman pumping, and the convergence zone of the
winds, also called the meteorological Equator. The existence of the
meteorological Equator is associated with a dipole of Ekman pumping
( upwelling in the convergence region, downwelling between the
convergence zone and the Equator) which gives rise to a very dynamic
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