Rainfall over the southern slopes of the Himalayas and the adjoining plains during ‘breaks’ in the monsoon

Abstract

AbstractDuring ‘breaks’ in the south‐west (or summer) monsoon there is a general cessation of rainfall activity over most of the Indian area, but this activity increases over the southern slopes of the Himalayas and its adjoining plains. In this study, rainfall distribution during the ‘break’ monsoon situations has been studied, over the southern slopes of the Sikkim, Nepal and Uttar Pradesh Himalayas (i.e. from longitude 78°E to 89°E) and their adjoining plains. Rainfall distributions during the major ‘break’ situations which occurred during the principal monsoon months of July and August of the period from 1957 to 1969 were studied. This study has shown that, on a ‘break’ day, positive percentage departures of rainfall as high as 100 to 300 per cent have occurred at stations located in the outer Himalayas and the adjoining plains of north Bengal, east and central Nepal, Bihar and east Uttar Pradesh. However, the width of this heavy rain‐belt is maximum over the hills and plains of south‐eastern Nepal and the adjoining plains of north Bihar. This study has also shown that on a ‘break’ day there is a decrease of rainfall activity over the Himalayas west of longitude 80°E. Decrease of rainfall also takes place along a narrow belt immediately to the south of the great Himalayan range to the east of longitude 80cE outside Sikkim. Over the adjoining Gangetic plains, an increase in rainfall is noticed even to the south of the Ganga river in Bihar and its neighbourhood. Thus, owing to the increase in rainfall activity during ‘break’ monsoon situations in the catchments of Himalayan rivers located in Sikkim and Nepal, a peculiar situation arises which is responsible for causing floods in the downstream plains which experience almost drought conditions.