The 10 to 20-day westward propagating mode and “Breaks in the Monsoons”

Abstract

This study utilizes daily surface pressure data records for a 40-year period (1933 to 1972). The dominant transient modes of the wave number frequency spectra of surface pressure along the latitudes of the monsoon trough (i.e., 20° to 30° N) are determined from a longitude-time composite of roughly 3 months of surface pressure data for ten separate episodes of Breaks in the monsoons. This data base is composited relative to a reference (0, 0) which denotes the day of commencement of the Break in rainfall over central India and the longitude of central India (i.e., 75° E). These dominant modes, as determined from this composited data, exhibit interesting westward as well as eastward propagating modes. Furthermore, some of the salient modes exhibit steady variations of phase from one day to the next. The period of Breaks in the monsoon rainfall is shown to coincide with a pressure rise associated with the arrival of a ridge of the dominant modes over the reference origin (0, 0). The remaining 30 years of data are next subjected to a test of a hypothesis that the steady propagation of phase of a dominant westward propagating mode can be used to extrapolate, and thus to predict, the arrival of this ridge. The tests show that a 10-day linear extrapolation of the phase to Day 0 exhibits considerable skill in locating the ridge of the “Monsoon Breaks” central India. In over 70% of the cases examined we note that the arrival of the ridge coincides with a period of the observed Breaks in the monsoon. Suggestions for casting this problem in a truly predictive frame are made, the results of which will be reported in a separate study.