Geoscience Reference
In-Depth Information
ISMR has large inter-annual variability as may be seen from Fig.
9
. Generally, a
deficient monsoon year is followed by a normal or excess monsoon year, a sort of biennial
oscillation. This phenomenon has been called the tropospheric biennial oscillation (TBO)
in which the summer monsoons of India and Australia and the SST of the tropical Indian
and west Pacific oceans take part. Meehl (
1997
) has studied the observational aspects of
TBO, and Chang and Li (
2000
) have given a modelling study. Both these studies have
analysed the SST variations on the inter-annual time scale in relation to the heat balance on
this scale. The SST anomalies averaged for the five September to November periods
following 5 very severe Indian monsoon drought seasons of 1965, 1972, 1979, 1982 and
1987 are shown in Fig.
10
. Soon after a season of monsoon drought, the SST of the ocean
around India (west Pacific Ocean) warms (cools) and this persists till the following
monsoon which has normal or excess ISMR. This is very good for India and makes it
different from Africa where drought years have plagued that continent during successive
(
a
)
(
b
)
Fig. 11 a 200 hPa vector wind (JJAS) as the mean of the WET epoch 1951-1960 showing the subtropical
jetstream. b Mean wind (JJAS) at 200 hPa of the DRY epoch 1965-1987 of frequent drought monsoons
minus the mean wind (JJAS) of the WET epoch 1951-1960 (wind anomaly). Magnitude of the wind is
shown by the shaded contours. The southwards shift of westerlies over south Asia as a wave number-3
trough in the DRY epoch may be seen
