The 30–50 Day Mode at 850 mb During MONEX

Abstract

The analyzed wind field at 850 mb during the summer monsoon experiment (MONEX) is subjected to a time series analysis to confirm the existence of a peak in the time range of 30?50 days. Having established that, this study presents a mapping of the motion field for this time scale. In this short paper we illustrate the steady meridional propagation of a train of troughs and ridges that seem to form near the equator and dissipate near the Himalayas. The meridional scale of this mode is around 300 km, and its meridional speed of propagation is ?0.75° latitude per day. The amplitude of the wind for this mode is around 3?6 m s?1. The salient contributions here are the mapping and the demonstration of a very regular behavior of this mode; its existence is here noted from a period well before the onset of monsoon, i.e., from early May to late July. Three major storms during MONEX were noted to form within the trough line of this system, and the period of major cessation of rains over the Indian subcontinent was noted to occur around the period of arrival near 20°N of a ridge line of this mode. A more detailed analysis of this strongly divergent mode will be published at a later date.