Geography Reference
In-Depth Information
2.5
2.0
1.0
1.5
0.5
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-0.5
-1.5
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-2.0
-2.5
2.5
1.5
0.5
2.0
1.0
0.0
-0.5
-1.5
-1.0
-2.0
1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
Year
Fig. 11.11
Time series showing SST anomalies (˚C) in the eastern Pacific (lower curve) and anomalies
in sea level pressures (hPa) at Darwin (upper solid curve) and Tahiti (upper dashed curve).
Data are smoothed to eliminate fluctuations with periods less than a year. Figure courtesy
of Dr. Todd Mitchell, University of Washington.
the period range of 2-5 years. During periods of large pressure difference between
these stations the Walker circulation is unusually strong and has the same general
structure as its time mean pattern. In periods of weak pressure difference the
Walker circulation weakens and the region of maximum rainfall shifts eastward
(see Fig. 11.10, bottom).
The weakening of trade winds during periods of small Darwin-Tahiti pres-
sure difference reduces wind-driven oceanic upwelling and deepens the oceanic
thermocline in the eastern Pacific. This in turn, as shown in the lower portion of
Fig. 11.11, leads to an increase in the sea surface temperature that is referred to as
El Nino
(Spanish for “the child”). This term was originally applied to a warming
of the coastal waters of Peru and Ecuador that occurs annually near Christmas
time (hence, El Nino refers to the Christ child), but is now used in a more gen-
eral sense for the large-scale oceanic anomalies associated with the weak wind
phase of the Southern Oscillation. In the opposite phase, strengthening of trade
winds associated with large Darwin-Tahiti pressure differences enhances oceanic
upwelling and deepens the oceanic thermocline. This causes a cooling of the sea
surface temperatures in the equatorial Pacific that is now referred to as
La Nina
.
The ocean anomalies associated with weakening in the trade winds may begin
near the coast, but over the course of several months spread westward along the
equator to produce large-scale positive SST anomalies over much of the equatorial
Pacific. These SST anomalies in turn cause a further weakening of the trade winds.
This entire complex of atmospheric and oceanic variations is now referred to as
