On the 30-40 day oscillations in southwest monsoon : A satellite study

Abstract

A comparative study of the low frequency oscillations during the southwest monsoon seasons of 1986 and 1987 is presented in this paper. The parameter used to study these oscillations is the fractional cloud cover, colder than a threshold temperature over each area of 2.5° Lat. /Long. Covering Arabian Sea, Bay of Bengal and part of Indian Ocean, using INSAT-IB IR data.

 The time-latitude sections Indicate a regular northward movement of the cloud maxima starting from equatorial Indian Ocean to about 30°N with a period of 30-40 days. The phase speed is about 0.9° Lat./day. This data was subjected to the spectral analysis by DFT method. This was computed for each area of 5.0 Lat. in width and 10° Long. in length. The results indicate that (his mode can be seen up to 25° N latitude beyond which it gets obliterated. The period is enhanced to 60 days between 20°& 25° N in 1987, which is a drought year.

Transient systems with the associated convection are seen to be moving westwards north of 5° N with a periodicity of less than 10 days. Comparatively poorer activity of monsoon of 1987 is possibly due to overall less cloudiness and the suppression of transient disturbances activity and their smaller zonal scale.