Geography Reference
In-Depth Information
200-h Pa
pressure
L
L
Surface
pressure
H
H
L
Indian
Pacific
Atlantic
0
90 E
180
90 W
0
200-h Pa
pressure
H
L
Surface
pressure
H
L
Indian
Pacific
Atlantic
0
90 E
180
90 W
0
Fig. 11.10
Schematic diagrams of the Walker circulations along the equator for normal conditions
(top) and El Nino conditions (bottom). (After Webster, 1983 and Webster and Chang,
1988.)
and by horizontal vapor transport provides a moisture source for convection in the
western Pacific, in addition to that provided by the high evaporation rates caused
by the high sea surface temperatures in that region.
The wind stress due to the time mean equatorial easterly surface winds over the
Pacific has a strong influence on the heat balance of the ocean surface layer. It
advects warm surface waters into the western Pacific and produces poleward drifts
in the oceanic Ekman layer, which by continuity drive an equatorial upwelling.
This upwelling accounts for the cold tongue of water along the equator, which in
turn is a major reason for the equatorial dry zone exhibited in Fig. 11.2.
11.1.6
El Ni no and the Southern Oscillation
The east-west pressure gradient associated with the Walker circulation undergoes
an irregular interannual variation. This global scale “see-saw” in pressure, and
its associated changes in patterns of wind, temperature, and precipitation, was
named the
southern oscillation
by Walker. This oscillation can be clearly seen
by comparing time series of surface pressure anomalies (i.e., departures from the
long-term mean) for locations on the western and eastern sides of the equatorial
Pacific. As shown in the upper portion of Fig. 11.11, surface pressure anomalies at
Darwin Australia and Tahiti are negatively correlated and have strong variations in
