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Atlantic. The MJO is superimposed on the monsoon circulations in Asia, Aus-
tralia, Northern America, Africa, and South America and can lead to intrasea-
sonal variations in those systems. It can also influence the location and strength
of both the SACZ and the SPCZ, and the distribution of tropical cyclones.
3.3 INTERANNUAL CLIMATE VARIABILITY
Climate variations also occur on time scales of a few years, called interannual
variability. The most prominent mode of interannual variability in the climate
system is a coupled atmosphere/ocean oscillation known as the El Niño—
Southern Oscillation, or ENSO.
Figure 3.3 is a Hovmöller diagram depicting the time evolution of sea sur-
face temperatures in the equatorial Pacific from January 1995 through January
2003. The western Pacific (~120°E to 180°) is characterized by relatively con-
stant, warm temperatures without evidence of a seasonal cycle. In the eastern
Pacific, east of 140°W, the seasonal signal has an amplitude of about 5 K, with
coolest temperatures in early boreal spring and warmest temperatures in late
fall. This seasonal oscillation was interrupted in 1997, however, as warm wa-
ters stretched across the entire tropical Pacific. This type of episode is known
as an El Niño event , or an ENSO warm event .
A map of sea surface temperature for DJF in 1997/98 is shown in Figure
3.4a. Comparison with the climatological sea surface temperature distribution
(Fig. 2.16a ) shows that an ENSO warm event is characterized by a relaxation
of the longitudinal temperature gradient across the tropical Pacific Ocean. The
sea surface temperature anomaly ( Fig. 3.4b) is centered on the equator, exceed-
ing 4 K in the eastern Pacific. Note that there are no sea surface temperature
anomalies in the western Pacific during this warm event.
Returning to Figure 3.3, we see that the cool waters of the eastern Pacific
extended farther west than usual in late 1998 and 1999. (Track the posi-
tion of the 300 K isotherm.) These occurrences are called La Niña events . A
comparison of the climatological sea surface temperature distribution for DJF
( Fig. 2.16a ) with the distribution during a strong La Niña event in 1998/99
shown in Figure 3.5a indicates that during a cool event the eastern Pacific
cool tongue is exceptionally well developed and extends farther west along
the equator. The La Niña sea surface temperature anomaly, shown in Figure
3.5b , is typically weaker in magnitude than the El Niño anomaly ( Fig. 3.4b)
and located farther west.
Sea surface temperature anomalies in the area denoted by the rectangles in
Figures 3.4b and 3.5b are often used to monitor ENSO events. This is the Niño
3.4 region . (It was formed by a refinement of two other monitoring areas known
as the Niño 3 and Niño 4 regions.) Figure 3.6a shows the record of Niño 3.4
sea surface temperature anomalies (the deviations from the 1971-2000 mean
temperature) from 1900 through 2009. The record has been smoothed using
an 11-month running mean to emphasize variations on interannual time scales.
The telltale warming of an El Niño is evident in many fall and winter seasons,
including strong events in 1941/42, 1972/73, 1982/83, and 1997/98. Strong
cool events occurred in 1955/56, 1975/76, 1988/89, and 2000/2001.
 
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