Geoscience Reference
In-Depth Information
(a)
Normal conditions
Convective loop
Equator
120°E
80°W
(b)
El Niño conditions
Increased
convection
Equator
Figure 9.12
Water
temperatures in the Pacific
Ocean during (a) non-El
Niño and (b) El Niño years.
(From http://www.cotf.edu/
ete/modules/elnino/crwhatis.
html.)
120°E
80°W
From time to time there is less upwelling of cold water in the eastern Pacific and
therefore a relative warming of the SST in this region. This phenomenon is called
El Niño, from the Spanish for 'the little boy, referring to the Christ child, because
the phenomenon is usually noticed around Christmas off the west coast of South
America. Different theories have been advanced for why the upwelling of cold water
in the eastern Pacific is suppressed but as yet no definitive explanation as been
established. However, when this surface warming in the western Pacific occurs,
there is greater atmospheric convection locally and the resulting advection of air to
support this ascent moderates the strength of the trade winds. This causes equatorial
Pacific surface temperature pattern to be less extreme, with less warm water
concentrated in the warm pool and more warm water farther east. The presence of
El Niño tends to be self-supporting for a period of 6 to 18 months because the
anomalously warm water and reduced trade winds leads to moderation of the
equatorial current that would otherwise drive warming water westward (Fig. 9.12b).
