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eastward geostrophic current: the North Equatorial Counter Current
(Figure 2.21).
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Figure 2.21. Diagram illustrating the presence of the North Equatorial
Counter Current, an eastward geostrophic current, between the Equator
and the intertropical convergence zone (ITCZ)
When El Niño occurs in the Pacific (Figure 2.20(b)), the ocean-
atmosphere system reaches a state of equilibrium very different from
the preceding state. The warm temperature anomaly at the surface in
the central eastern Pacific diminishes the equatorial pressure gradient
between the east and west of the Pacific, which slows down the
Walker circulation. The Hadley cells are also affected; the
intertropical convergence zones situated to the north and south of
the Equator merge into a single region of convergence next to the
Equator, in the central eastern Pacific, the location of strong rains.
The pressure anomaly at sea level is negative in the high pressure
region of the south-east Pacific and on the contrary is positive in the
region to the north of Australia-Indonesia. The difference between
these two centers of pressure is called the Southern Oscillation Index
(SOI), which is recognized as the atmospheric signature of El Niño
(Figure 2.22(a)).
The ocean is also subject to deep modifications: the occurrence of
warm temperature anomalies in the central-eastern Pacific reduces the
east-west temperature difference as much as the north-south
temperature difference. The sea level evens out, as well as the
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