Geoscience Reference
In-Depth Information
Domain of monsoon
Domain of Southern Oscillation
Figure 23.1. Region influenced by the summer monsoon and the two key regions
of the Southern Oscillation. During El Ni no, or ENSO, events (negative Southern
Oscillation Index), surface atmospheric pressure is above normal in the stippled
area and below normal in the hatched area. The opposite pattern prevails during
La Ni na, or anti-ENSO, events (positive SOI). For example, when surface atmo-
spheric pressure is below-average off the coast of Peru, it is above-average in the
area encompassing India, southern China, East Africa, Indonesia and Australia, and
drought is common in these localities. The mapped areas (showing statistically signi-
ficant correlations greater than
0.4, at the 95% confidence level) reflect the
difference between annual surface atmospheric pressure at Jakarta, Indonesia, and
global mean sea level pressure. (Modified from Whetton et al.,
1990
, and Williams
et al.,
2006b
.)
+
0.4 or
−
December in certain years, the normally cold and nutrient-rich waters offshore are
replaced by a tongue of warmer water, and the anchovy fisheries fail. They term these
years 'El Ni no years', and we now have a reasonably accurate record of such events
spanning the last 500 years (Whetton et al.,
1990
; Diaz and Markgraf,
1992
; Whetton
et al.,
1992
; Whetton and Rutherfurd,
1994
; Allan et al.,
1996
; Whetton et al.,
1996
;
Diaz andMarkgraf,
2000
; Ortlieb,
2004
). (El Ni no is Spanish for 'little boy' and refers
to the Christ child, because December is the usual month in which an El Ni no event
starts). Such years are accompanied by severe floods in normally arid coastal Peru
and are years of negative SOI, with lower-than-average surface atmospheric pressure
offshore. Years when the waters off Peru are colder than average are termed 'anti-El
Ni no years' or, more simply, 'La Ni na' years. The phrase El Ni no-Southern Oscilla-
tion (or ENSO) event is a concise way of indicating a year marked by an El Ni no event
and a negative SOI. The two phenomena are closely related but are not synonymous,
given that one refers to surface atmospheric pressure (Southern Oscillation) and the
other to sea surface temperature anomalies (El Ni no events).
