Long-Term variability of the indian summer monsoon and related parameters |
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| Authors: | G B Pant K Rupa Kumar B Parthasarathy H P Borgaonkar |
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| Institution: | Indian Institute of Tropical Meteorology, Pune-411005, India,Indian Institute of Tropical Meteorology, Pune-411005, India,Indian Institute of Tropical Meteorology, Pune-411005, India,Indian Institute of Tropical Meteorology, Pune-411005, India |
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| Abstract: | The long-term variability of the Indian summer monsoon rain-fall and related regional and global parameters are studied. The
cubic spline is used as a digital filter to smooth the high frequency signals in the time series of the various parameters.
The length of the data series varies from 95 to 115 years during the period 1871-1985. The parameters studied within the monsoon
system are: (a) monsoon rainfall of the country as a whole; (b) number of break-monsoon days during July and August; (c) number
of storms/ depressions in Bay of Bengal and Arabian Sea during summer monsoon season; and (d) dates of onset of summer monsoon
over South Kerala Coast. The parameters studied outside the monsoon system are: (a) the Wright’s Southern Oscillation Index
(June-July-August); (b) the January mean Northern Hemispheric surface air temperature anomaly; and (c) the East-equatorial
Pacific sea surface temperature anomaly.
In order to examine the variability under various degrees of the smoothing, the series are filtered with splines of 50% variance
reduction frequency of one cycle per 10, 20 and 30 years. It is observed that the smoothed time series of the parameters within
the monsoon system comprise a common slowly varying component in an episodic manner distinctly showing the excess and deficient
rainfall epochs. The change of intercorrelations between the time series with increasing degree of smoothing throws some light
on the time scales of the dominant interactions. The relation between Southern Oscillation and East equatorial Pacific sea
surface temperature and the Indian summer monsoon seems to be dominant on the interannual scale. The low frequency variations
are found to have significantly contributed to the instability of the correlations of monsoon rainfall with parameters outside
the monsoon system. |
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