Geoscience Reference
In-Depth Information
11.1 El Niño and the Southern Oscillation
El Niño episodes of warm coastal currents with accompanying disastrous consequences for marine
life and birds recur about every four to seven years and consequently were long known along the
West Coast of South America. The related Southern Oscillation (SO) of sea-level pressure between
Tahiti (normally high pressure) and Jakarta (or Darwin) (normally low pressure) was identified by
Sir Gilbert Walker in 1910 and re-investigated in the mid-1950s by I. Schell and H. Berlage, and in
the 1960s by A.J. Troup and J. Bjerknes. A.J. Troup linked the occurrence of El Niño conditions to
an oscillation in the atmosphere over the equatorial Pacific in the 1960s. Their wider implications
for air-sea interaction and global teleconnections were first proposed by Professor Jacob Bjerknes
(of polar front fame) in 1966 who noted the linkages of El Niño or non-El Niño conditions with the
SO. The worldwide significance of ENSO events only became fully appreciated in the 1970s-1980s
with the strong El Niño events of 1972-1973 and 1982-1983. The availability of global analyses
showed clear patterns of seasonal anomalies of temperature and precipitation in widely separated
regions during and after the onset of warming in the eastern and central equatorial Pacific Ocean.
These include droughts in northeast Brazil and in Australasia, and cool, wet winters following El
Niño in the southern and southeastern United States.
The occurrence of ENSO events in the past has been studied from historical documents, inferred
from tree ring data, and from coral, ice core and high-resolution sediment records. The net effect
of major El Niño events on global temperature trends is estimated to be about +0.06
°
C between
1950 and 1998.
Reference
Diaz, H.F. and Markgraf, V. (eds) (1992) El Niño. Historical and Paleoclimatic Aspects of the Southern Oscillation, Cambridge
University Press, Cambridge, 476pp.
Pacific feeds the return airflow in the upper
troposphere (
i.e.,
at 200mb), closing and strength-
ening the Walker circulation. However, this
airflow also strengthens the Hadley circulation,
particularly its meridional component northward
in the northern winter and southward in the
southern winter.
Each year, usually starting in December, a
weak, southward flow of warm water replaces
the northward-flowing Peru Current and its
associated cold upwelling southward to about 6
extensive and the coastal upwelling ceases entirely.
This has catastrophic ecological and economic
consequences for fish and bird life, and for the
fishing and guano industries of Ecuador, Peru and
northern Chile.
Figure 11.51
shows the occurrence
of El Niño events between 1525 and 1987 classified
according to their intensity. These offshore events,
however, are part of a Pacific-wide change in sea
surface temperatures. Moreover, the spatial
pattern of these changes is not the same for all El
Niños. Recently, K. E. Trenberth and colleagues
showed that during 1950-1977, warming during
an El Niño spread westward from Peru, whereas
after a major shift in Pacific basin climate took
place in 1976-1977, the warming spread east-
ward from the western equatorial Pacific. The
S
along the coast of Ecuador. This phenomenon,
known as El Niño (the child, after the Christ
child), strengthens at irregular intervals of two
to ten years (its average interval is four years)
when warm surface water becomes much more
°
