Oscillations of a Monsoon System. Part I. Observational Aspects.

In this paper the elements of a monsoon system are defined, and its oscillations are determined from spectral analysis of long observational records. The elements of the monsoon system include pressure of the monsoon trough, pressure of the Mascarene high, cross-equatorial low-level jet, Tibetan high, tropical easterly jet, monsoon cloud cover, monsoon rainfall, dry static stability of the lower troposphere, and moist static stability of the lower troposphere. The summer monsoon months over India during normal monsoon rainfall years are considered as guidelines in the selection of data for the period of this study. The salient result of this study is that there seems to exist a quasi-biweekly oscillation in almost all of the elements of the monsoon system. For some of these elements, such as the surface pressure field, monsoon rainfall, low-level cross-equatorial jet and monsoon cloudiness, the amplitude of this oscillation in quasi-biweekly range is very pronounced. For the spectral representation of the time series, the product of the spectral density times frequency is used as the ordinate and the log of the frequency as the abscissa. Dominant modes are also found in the shorter time scales (<6 days). A sequential ordering of elements of the monsoon systems for the quasi-biweekly oscillation is carried out in terms of their respective phase angle. The principal result here is that soon after the maximum dry and moist static instabilities are realized in the stabilizing phase, there occur in sequence an intensification of the monsoon trough, satellite brightness, Mascarene high, Tibetan high and the tropical easterly jet. Soon after that the rainfall maximum over central India, arising primarily from monsoon depressions, is found to be a maximum.In the second part of this paper we offer some plausible mechanisms for these quasi-biweekly oscillations.