Abstract
Behaviour of stratospheric and mesospheric zonal winds in the equatorial region of Asia has been studied using rocket wind data from 921 flights for the period from 1970 to 1993 over Thumba (India) and upper wind data in the lower stratosphere over Singapore (S.E. Asia) from 1970 to 1997. Monthly means of these zonal winds are subjected to spectrum analysis. It is inferred that the quasi-biennial oscillation (QBO) with periodicity of 22 to 30 months and annual cycle with periodicity of 11 to 12 months dominate in the lower stratosphere over Thumba, while the semi-annual cycle is predominant in the upper stratosphere and mesosphere. QBO in the lower stratosphere over Singapore has been observed to be significant with periodicity of 22 to 28 months, whereas the annual cycle has no significance.
 
 There exists a prognostic value between easterly & westerly phases of QBO over Thumba during the antecedent months of January to May and the performance of succeeding summer monsoon rainfall over India with strong easterlies favouring decreased rainfall and westerlies or light easterlies favouring increased rainfall over the country a s a whole. Correlation coefficients are statistically significant between the seasonal rainfall over India and zonal winds at 10 hPa (30 km) during winter months and also zonal winds at 30 hPa (24 km) during pre-monsoon months. However, there is an improvement in correlation if combined circulation of zonal winds at 10 hPa during winter and at 30 hPa during pre-monsoon is considered. Zonal winds at 50 hPa (20 km) and 30 hPa (24 km) over Singapore during monsoon months show diagnostic value with seasonal rainfall over India having significant concurrent correlation coefficients; however winds during winter and pre-monsoon do not show significant correlation with Indian monsoon. Thus, this could be used to monitor the progress of south-west monsoon performance but not in predicting it.
 
 Over Thumba, the mean zonal winds at 30 hPa during ENSO and anti- ENSO years differ significantly in the annual feature.
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