64
1
0 r
-1
-1
0 .,
501 (invened)
Ja.
1
45
50
55
60
65
70
75
80
85
90
95
Figure 22: Inverted time series of the Southern Oscillation Index (SOl) , defined as the
normalized difference between the normalized sea-level pressure anomalies at Tahiti and
Darwin (12 S, 131 E) for the recent period of record. The corresponding segment of the
time series of the cold tongue index (Fig. 8 of chapter 1) is repeated here for comparison.
Both time series have been smoothed with a 5-month running mean filter.
so-called "Southern Oscillation Index" (SOl), tends to be negative during
the warm phase of the ENSO cycle, which is usually accompanied by EI
Nino events along the South American coast. An extended time series of
the SOl, comparable in length to the SST time series in Fig. 8 of chapter
1 has been compiled by the NOAA Climate Analysis Center. The segment
corresponding to the recent period of record is shown in Fig. 22 , together
with the index of equatorial Pacific cold tongue SST, repeated from Fig
8 of chapter 1. The negative correlation between the SOl and equatorial
Pacific SST is clearly evident.
Figure 23 shows the shows the anomalous rainfall pattern depicted in
the bottom panel of Fig. 21 superimposed upon contours of vertically averaged (surface to 300-mb) tropospheric temperature anomalies, as revealed
by the same MSU instrument. The dipole pattern in tropospheric temperature, with positive anomalies straddling the region of enhanced equatorial
precipitation during the warm phase of the ENSO cycle, is a robust feature
which has been noted in previous observational studies of EI Nino (e.g. ,
Horel and Wallace 1981), and it has been reproduced in numerous GCM
simulations of the atmospheric response to tropical SST anomalies. Since
geopotential surfaces in the lower troposphere tend to be rather flat in
comparison to those in the upper troposphere, the centers of action of the
dipole pattern in Fig. 23 are indicative of upper level anticyclones, with
anomalous easterly flow over the region of enhanced equatorial rainfall and
a strengthening of the climatological mean westerlies in the subtropics of
1
0 r
-1
-1
0 .,
501 (invened)
Ja.
1
45
50
55
60
65
70
75
80
85
90
95
Figure 22: Inverted time series of the Southern Oscillation Index (SOl) , defined as the
normalized difference between the normalized sea-level pressure anomalies at Tahiti and
Darwin (12 S, 131 E) for the recent period of record. The corresponding segment of the
time series of the cold tongue index (Fig. 8 of chapter 1) is repeated here for comparison.
Both time series have been smoothed with a 5-month running mean filter.
so-called "Southern Oscillation Index" (SOl), tends to be negative during
the warm phase of the ENSO cycle, which is usually accompanied by EI
Nino events along the South American coast. An extended time series of
the SOl, comparable in length to the SST time series in Fig. 8 of chapter
1 has been compiled by the NOAA Climate Analysis Center. The segment
corresponding to the recent period of record is shown in Fig. 22 , together
with the index of equatorial Pacific cold tongue SST, repeated from Fig
8 of chapter 1. The negative correlation between the SOl and equatorial
Pacific SST is clearly evident.
Figure 23 shows the shows the anomalous rainfall pattern depicted in
the bottom panel of Fig. 21 superimposed upon contours of vertically averaged (surface to 300-mb) tropospheric temperature anomalies, as revealed
by the same MSU instrument. The dipole pattern in tropospheric temperature, with positive anomalies straddling the region of enhanced equatorial
precipitation during the warm phase of the ENSO cycle, is a robust feature
which has been noted in previous observational studies of EI Nino (e.g. ,
Horel and Wallace 1981), and it has been reproduced in numerous GCM
simulations of the atmospheric response to tropical SST anomalies. Since
geopotential surfaces in the lower troposphere tend to be rather flat in
comparison to those in the upper troposphere, the centers of action of the
dipole pattern in Fig. 23 are indicative of upper level anticyclones, with
anomalous easterly flow over the region of enhanced equatorial rainfall and
a strengthening of the climatological mean westerlies in the subtropics of