Geoscience Reference
In-Depth Information
(a) Climatology
20°N
10°N
Equator
10°S
20°S
120°E
140°E
160°E
180°
160°W
140°W
120°W
100°W
80°W
60°W
(b) ENSO warm event
20°N
10°N
Equator
10°S
20°S
120°E
140°E
160°E
180°
160°W
140°W
120°W
100°W
80°W
60°W
(c) ENSO cool event
20°N
10°N
Equator
10°S
20°S
120°E
140°E
160°E
180°
160°W
140°W
120°W
100°W
80°W
60°W
-6
-5
-4
-3
-2
-1
012345
Figure 7.6 Vertical
p
-velocity at 500 hPa from (a) a 50-year climatology, (b) an average
for DFJ of the 1982/1983 and 1997/1998 ENSO warm events, and (c) an average for
DFJ of the 1975/1976 and 1988/1989 ENSO cool events. Units are 0.01 Pa/s and the
0 line is dashed.
of maximum negative values (upward motion) is located in the central Pacific
instead of in the west, and vertical velocities in the western Pacific are weak.
During ENSO cool events (La Niña), sinking motion over the eastern Pacific
is enhanced and extends to the west along the equator (
Fig. 7.6c)
. Upward
motion in the far western Pacific (120°E-160°E) and over South America is
stronger than in the climatology.
The Walker circulation across the equatorial Pacific reinforces the easterly
low near the surface (
Fig. 2.13)
. These low-level easterly winds push warm
surface water to the western Pacific, raising sea level there about 20 cm above
that in the eastern Pacific. This easterly flow helps maintain the longitudinal
sea surface temperature gradient across the Pacific
(Fig. 2.15)
which, in turn,
maintains the Walker circulation. In this way, the atmosphere and ocean are
“coupled.”
