Interdecadal variations of characteristics of monsoon disturbances and their epochal relationships with rainfall and other tropical features

Interdecadal variations of some characteristics (number and duration) of different monsoon disturbances (low pressure areas (lows), depressions and cyclonic storms (CS)) over the Indian region are studied for 110 years (1889-1998) to find whether an epochal variation exists in such main synoptic components of the Indian summer monsoon at the decadal time scale, similar to other long-term interdecadal variations of tropical features, e.g. Indian summer monsoon rainfall (ISMR), El Nino-Southern Oscillation Index (SOI), sub-tropical ridge (STR) over India, Northern Hemispheric surface temperature (NHST), etc. As ISMR has alternate epochs of below and above normal phases at the broad time scale of 30 years, variations of these features are also examined according to the two epochs of ISMR. The stability of correlation of monsoon disturbance days with ISMR is also examined for the same 30-year time scale for 1889-1998. Stability of relationships of other tropical features and monsoon disturbance days with ISMR is also examined, based on their 11-year moving correlation coefficients (CC). Results do not show any regular trend in decadal frequencies of the number or duration of different monsoon disturbances. However, decadal frequencies of number of depressions, CS and depressions are decreasing from 1969-1978 until the most recent decade. During the recent decade, less than half of the total number of depressions and CS are formed, compared with 1969-1978. On the other hand, the total number of lows increases significantly from 1959-1968 to the recent decade, when more than double the number of lows are formed. By examining the 30-year periodicity of the number of disturbances together or separately, it is found that depressions and CS are higher during the above normal epochs, and significantly less during the below normal epochs of ISMR. The reverse is true for the number of lows and the total number of disturbances and their total duration. The Ii-year running means of monsoon disturbance days with ISMR shows the same behaviour but opposite epochs during 1929-1958 and 1970-1990. Their correlation coefficients are also found to be lower during 1929-1958 compared with epochs when they are in the same phase. Comparison of epochal relationships with other tropical features, e.g. El Nino, SOI, STR, NHST etc., from 11-year running means and their Ii-year moving CC with ISMR, have also shown epochal fluctuations. Copyright (C) 2001 Royal Meteorological Society.