Trends and periodicities in annual rainfall in monsoon areas over the northern hemisphere

Abstract

Trends and periodicities in annual rainfalls in monsoon areas over the northern hemisphere have been examined. Stations selected for Southeast Asia are Colaba (Bombay), Trivandrum and Colombo; for West Africa they are Accra, Warri, Tower Hill (Freetown) and Dakar; and for Central America, Quito and Colon*. The data were mostly collected from World Weather Records. For finding out long term trends a 25-point ultra low pass sine terminated filter was used to cut off all periods below 12 years. In Southeast Asia, Colaba showed a rise between 1910 and 1955 and then a fall. Trivandrum does not show any significant variation whereas the trend at Colombo is oscillatory. In West Africa amongst the places to the south of Sahara, Accra and Dakar do not show any trend, a general decrease is noticed at Tower Hill and Warri shows an increase. In Central America Colon shows an oscillatory trends whereas Quito does not show any significant variation. There does not appear to be current decrease in the rainfall over African monsoon countries and over India as has been reported by Winstanley (1973). As regards periodicities, analysis were confied to only 11-year and 22-year cycles. Besides being of physical significance these two cycles have long enough period to cause imbalance in drought prone areas. These periods seem to be present everywhere but they do not show and systematic correlations with sunspot numbers. The amplitudes of the oscillation do not exhibit any systematic dependence either on latitude or on longitude. Large differences were noticed even at neighbouring places. Rainfall in these areas because of its strong dependence on local geography, is modulated strongly by other factors to show a poor correlation with sunspot number.