Geoscience Reference
In-Depth Information
(a)
30°N
296
300
296
20°N
288
296
300
10°N
300
296
300
Equator
292
288
10°S
300
300
296
20°S
300
300
30°S
304
300
120°E 140°E
160°E
180°
160°W 140°W 120°W 100°W 80°W
60°W
(b)
30°N
2
0
20°N
0
2
10°N
0
2
4
Equator
0
0
2
10°S
0
0
20°S
30°S
120°E
140°E
160°E
180°
160°W 140°W 120°W 100°W 80°W
60°W
Figure 3.4 Sea surface (a) temperatures and (b) temperature anomalies in the
tropical Pacific during the strong El Niño of 1997/98. The rectangle in (b) marks
the Niño3.4 region, which is often used to monitor the progression of El Niño and
La Niña events.
weaken, and may even be replaced by westerly flow. When the easterly winds
slacken in the eastern Pacific, upwelling off the South American coast dimin-
ishes and fewer nutrients are brought to the surface to feed aquatic life. Be-
cause the associated failure in fishing typically occurs near Christmas, people
connected the events with the birth of Jesus and this is the origin of the name
El Niño.
These changes in atmospheric circulation systems are recorded in sea level
pressures across the Pacific. An El Niño event is characterized by a decrease
in the climatological high pressure (high geopotential heights) in the eastern
Pacific and an increase in geopotential heights in the western Pacific. In the
early 1900s Gilbert Walker, a British scientist on assignment as the head of the
Indian Meteorological Service, recorded the inverse correlation between sur-
face pressure in the western Pacific, represented by Darwin, Australia, and the
central Pacific, represented by Tahiti, which he called the
Southern Oscillation
.
Smoothed sea level pressure at these two locations is plotted in
Figure 3.6b,
and it is clear that when surface pressure is anomalously high in the western
Pacific it is anomalously low in the central Pacific, and vice versa. During par-
ticularly strong ENSO warm events (e.g., 1982/83 and 1997/98), pressures at
Darwin and Tahiti may be equal.
