Investigation of monsoon lows by the method of spectrum analysis

Abstract

The method of spectrum analysis is applied to the time-series data over India during the summer monsoon season 1962. The periodicity around 5 days appears as a spatial peak in the lower troposphere over the monsoon trough region, corresponding to the passage of so-called monsoon lows. It is confirmed by the horizontal cross spectrum analysis that these monsoon lows propagate westward and their longitudinal wavelength is about 300.

The mean vertical structure of monsoon lows is investigated and compared by using the data of two stations located on the coast 9f the Bay of Bengal and on the Inland of northern India. The results of both stations show that the cyclonic circulation of monsoon lows prevails in the lower troposphere and their axis tilts slightly westward with height. They also bear warm-cored structure in common in the upper levels.

The most remarkable contrast appears in the field of specific humidity. When monsoon lows are in the vicinity of the Bay of Bengal, moistening occurs in coincidence with southerly winds, i.e., in the east of the trough. On the contrary, moist and cold anomalies appear in the west of the trough with northerly wind over the inland area. These contrasts suggest an .intense geographic effect of Indian subcontinent on the life cycle of monsoon lows.