Divergent Circulations on the 30 to 50 Day Time Scale

Abstract

A brief review of some of the recent results on the 30 to 50 day time scale is presented in this paper. We have examined the divergent circulations on the time scale of 30 to 50 days during the FGGE year. The present study is based on two different data sets. These are the FGGE IIIb analysis from the European Center for Medium Range Weather Forecasts (ECMWF) and the Florida State University's analysis over the monsoon region during the FGGE year. The analysis clearly identifies a planetary-scale divergence wave that traverses around the globe eastward throughout the FGGE year. Its speed of eastward propagation is around 8° longitude day⁻¹. The amplitude of this wave is largest during the summer season over the monsoon region and the western Pacific Oceans. The amplitude decreases somewhat as the wave traverses across the eastern Pacific and Atlantic Oceans. This wave appears to modulate the monsoon activity such that active, inactive spells seem to bear a close relationship to the divergence on this time scale. A planetary-scale sea level pressure wave accompanies this divergence wave and is also presented. Ale regional higher density data shows a meridionally-propagating divergence wave that moves from the equatorial latitudes towards the Himalayas in the monsoon region. The two sets of analysis (global and regional) clarify this dichotomy about the zonal versus the meridional phase propagation of the divergent circulations on this time scale. Another aspect of this study relates to the phase locking of two families of low frequency waves during the breaks (inactive spells) of the monsoon. Besides the eastward-propagating planetary-scale waves on the 30 to 50 day time scale, a 10 to 20 day westward propagating wave has been noted to influence the monsoon activity. The simultaneous arrival of ridges (or high pressure) of thew two families of low frequency waves during breaks is an interesting phase locking phenomenon. Similar phase locking of troughs of sea level pressure are noted during the active spells of monsoon. Another important question relates to the energetics on this time scale; i.e., am the 30 to 50 day divergent circulations thermally direct? blew calculations am performed in a frequency domain and confirm the thermally-direct circulation.