Geoscience Reference
In-Depth Information
(a)
30°N
296
296
300
20°N
288
296
296
292
30
300
10°N
296
300
300
300
Equator
292
296
300
288
10°S
300
296
20°S
300
300
292
284
30°S
304
120°E 140°E
160°E
180°
160°W 140°W 120°W 100°W 80°W
60°W
(b)
30°N
0
20°N
0
0
10°N
0
-2
-2
Equator
0
0
10°S
0
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 3.5 Sea surface (a) temperatures and (b) temperature anomalies in the
tropical Pacific during the ENSO cool event of 1998/99. The rectangle in (b)
marks the Niño3.4 region, which is often used to monitor the progression of El
Niño and La Niña events.
The
Southern Oscillation Index
(SOI) is a metric commonly used to track
and quantify the ENSO oscillation. It is based on the surface pressure anoma-
lies at Tahiti minus those at Darwin, so negative values of the SOI indicate an
ENSO warm event.
Figure 3.6c
shows the time series of the SOI. The periods
1982/83 and 1997/98 stand out as particularly strong warm events, and a
series of weak warm events occurred through the early 1990s. Other indexes
may be used to monitor ENSO, for example, the
Multivariate ENSO Index
,
which takes into account surface wind, surface temperature, and cloudiness
anomalies in addition to surface pressure.
The
quasi- biennial oscillation
(QBO) is an example of interannual variabil-
ity in the stratosphere; it is a result of interactions between the stratosphere and
the troposphere in the tropics. The oscillation occurs in the middle and lower
tropical stratosphere, where the wind direction shifts from easterly (westward)
to westerly (eastward) every 26 months or so.
Figure 2.10
suggests that the climatology of the zonal mean zonal wind in
the lower tropical stratosphere is small and easterly when averaged over many
years. But this is misleading because in averaging over a number of years, strong
positive (or westerly) wind speeds are averaged with strong negative (or easterly)
wind speeds to produce apparently small mean velocities in the climatology.
