Geoscience Reference
In-Depth Information
thermocline, which corresponds to a redistribution of heat along the
Equator and between the Equator and the convergence zones. The
Equatorial Undercurrent thus diminishes, as the North Equatorial
Counter Current, and a reversal can even be found in surface layers
with an eastward surface current along the Equator at the culmination
of the event. The Equator becomes a region of convergence rather
than divergence of the surface waters. One of the direct consequences
of El Niño is the disappearance of
upwellings
, which results in a
spectacular impoverishment of the coastal and equatorial waters and
ecological catastrophes, as much for marine life as for sea birds. The
temperature anomaly in the equatorial Pacific (Niño region 3.4,
between 5°N and 5°S, and 120°W to 170°W) is the El Niño index for
oceanic anomaly (Figure 2.22(b)).
Figure 2.22.
Standardized Southern Oscillation Index, which characterizes the
atmospheric signature of El Niño, on the left and on the right, the temperature
anomaly index for the Niño region 3.4 of the Pacific, which characterizes the oceanic
signature of the event (data available on www.ncdc.noaa.gov/teleconnections/
enso/indicators/soi.php)
El Niño is a frequent phenomenon, which repeats itself every two
to ten years in the equatorial Pacific, with a favored frequency of three
to five years. Its nature and origin remain subjects of debate: is it
triggered by a local disturbance in the central Pacific or is its nature
oscillatory? An initial disturbance of heat in the central Pacific
translates rapidly into a growing coupled anomaly. The heat
accumulated in the warmpool in the west Pacific favors the
