Interannual variability of middle atmosphere and Indian summer monsoon

The interannual variability of the mean zonal wind and the mean temperature, obtained from the rocketsonde data for January (symbolizing winter), of the middle atmosphere over the Northern Hemisphere for the 6-year period, 1970-1972 and 1974-1976 has been investigated. The stations considered for the study are Thule, Poker Flat, Fort Churchill, Wallops Island, Cape Kennedy, Fort Sherman, Kwajalein and Ascension Island. The study has shown marked interannual variation in the zonal wind northward of 35-40°N. In the stratosphere (25-45 km), the westerly zonal wind reached its maximum during 1970, 1971 and 1975 near 35-40°N whereas it reached its maximum during 1972, 1974 and 1976 near 60°N, where the polar night jet is located. At stratopause level (50 km), the temperature gradient between equator-middle latitude and the polar region showed interannual variability. The gradient was stronger during 1970, 1971 and 1975 than during 1972, 1974 and 1976. The rainfall activity of the following summer monsoon (June-September) over India during the 6-year period of study has also been examined. The monsoon was weak to normal during 1972, 1974 and 1976 when the maximum westerly flow was strong, stable and located at its normal position (near 60°N), and the equator-middle latitude to pole temperature gradient at stratopause level was weak. The monsoon was normal to strong during 1970, 1971 and 1975 when the maximum westerly flow was weak, diffuse and shifted south-ward (35-40°N) and the equator-middle latitude to pole temperature gradient at stratopause level was strong