First Assessment Report - IPCC

7 Observed Climate Variation and Change
777
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Figure 7.21: (a) Schematic diagram of areas and times of the year with a consistent ENSO precipitation signal (adapted from
Ropelewski and Halpert, 1987). (b) Monthly tropical sea surface and land air temperature anomalies 1961-1989; land data from P.D.
Jones and sea surface temperature data from the UK Meteorological Office. Tropics extend from 20°N to 20°S. Arrows mark
maximum ENSO warmth in the tropics.
strong SO fluctuations from 1880 to 1920 led to the
discovery and naming of the SO (Walker and Bliss, 1932)
and strong SO events are clearly evident in recent decades.
A much quieter period occurred from the late 1920s to
about 1950, with the exception of a very strong multi-year
ENSO in 1939-42 (Trcnbcrth and Shea, 1987; Cooper et
al., 1989). Quinn et al. (1987) (covering the past 450 years)
and Ropelewski and Jones (1987) have documented
historical ENSO events as seen on the northwest coast of
South America. Therefore, the potential exists for a longer
palaeo-record based on river deposits, ice cores, coral
growth rings and tree rings.
During ENSO events, the heat stored in the warm
tropical western Pacific is transferred directly or indirectly
to many other parts of the tropical oceans. There is a
greater than normal loss of heat by the tropical oceans,
resulting in a short period warming of many, though not all,
parts of the global atmosphere (Pan and Oort, 1983).
Consequently, warm individual years in the record of
global temperatures (Figure 7.10) are often associated with
El Nifios. Maxima in global temperatures tend to occur
about three to six months after the peak warmth of the El
Nino (Pan and Oort, 1983). Figure 7.21b shows monthly
anomalies of combined land surface air temperatures and
SST for the global tropics from 1961-1989. The strong,
coherent, warming influence of the 1972-73, 1982-83 and
1986-88 ENSO events on the record of tropical temperature
is very clear, as is the cold influence of the strong La Nina
episodes of 1974-75 and 1988-89.