Cloudiness Fluctuations Associated with the Northern Hemisphere Summer Monsoon

Abstract

The broad-scale fluctuations of cloudiness over the Eastern Hemisphere during the northern summer monsoon were investigated by using daily satellite mosaic pictures taken from June 1 to September 30, 1973. Spectral analysis revealed two dominant periodicities, of around 40 days and around 15 days. Cross-spectral, time-sectional, time-lag correlation and phase-lag vector analysis were applied to reveal the characteristics of these two modes in the time-space field. The fluctuation of 40-day period shows marked northward movement of cloudiness from the equatorial zone to the mid-latitudes (around 30°E) over the whole Asian monsoon area, and southward movement over Africa and the central Pacific. The northward movement is most apparent over the India-Indian Ocean sector. The fluctuation of this mode is associated with the major “active”-“break” cycle of the monsoon over the whole Asian monsoon area. The fluctuation of 15-day period shows similar features to that of 40-day period, but includes two clockwise rotations, one over India and Southeast Asia and the other over the western Pacific. A southward movement from the equatorial zone to the Southern Hemisphere middle latitudes is also prominent to the east and west of Australia. The fluctuation of this mode seems to correspond with the movements of equatorial, monsoon (or tropical), and westerly disturbances. It is also suggested that the fluctuation of 40-day period may be closely connected with the global-scale zonal oscillation in the equatorial zone and that of 15-day period may exist as a result of meridional wave interactions.