Inter-annual and Long term Variability of Rainfall in Kerala

Abstract

Long term (141 years) surface meteorological data have been analysed to study inter-annual variability in rainfall for all seasons and months. Summer monsoon rainfall of India and Kerala has considerable inter-annual variability in the date of onset, withdrawal and the activity of the monsoon during the Indian summer monsoon season, even though the summer monsoon is considered to be prevailed during the period 01 June to 30 September. Similar is the case with the northeast monsoon rainfall also. The time series of 141 years rainfall exhibits a large amplitude interannual variability. It is found that a year with large above normal rainfall is generally followed by a year with large below normal rainfall and vice versa, a type of biennial oscillation. This biennial tendency is shown to be associated with the changes in ocean surface temperature of tropical Indian and west Pacific oceans. Results on various analyses show strong and vivid features contributed by climate change for this south western state. The rainfall pattern of the state displays large north-south and east-west gradients. The peculiar geographic orientation of Kerala with Arabian Sea as western boundary makes the weather pattern very unique in this part of Indian subcontinent. Summer monsoon rainfall of Kerala shows a decreasing trend of 10.9cm in 100 years and northeast monsoon rainfall shows an increasing trend of 7.5 cm in 100 years during the period of study. Epochal variability of rainfall with 31 year moving average of seasonal rainfall shows that presently rainfall is in the below normal epoch of its natural variability.