The all India monsoon rainfall (IMR) for the summer months, often used as the index of Indian summer monsoon to study its interannual variability, is inadequate as it is based on observations at only land based rain gauge stations. This study presents a more representative index of the Indian summer monsoon precipitation and associated convective heating using merged rain gauge and satellite estimates of precipitation. This index, termed the extended Indian monsoon rainfall (EIMR), covers the Indian continent as well as the northern Bay of Bengal and part of southern China (70°E - 100°E, 10°N - 30°N). We show that the large precipitation over the northern Bay of Bengal with significant interannual variability cannot be ignored in the definition of the Indian summer monsoon and its variability. The June-to-September climatological mean EIMR is found to be larger than that of IMR even thought the former is averaged over a larger area. The dominant mode of the interannual variability of the Indian summer monsoon is associated with an east-west dipole between the EIMR region and the northwestern Pacific monsoon region (110°E - 160°E, 10°N - 30°N) and a meridional dipole between the EIMR region and the equatorial Indian Ocean (70°E - 110°E, 10°S - 5°N). We argue that the interannual variability of monsoon circulation is primarily driven by fluctuations of the convective heating associated with the interannual variability of EIMR is in the form of a regional Hadley circulation.
Based on linear theory, a large scale monsoon Hadley (MH) circulation index is defined as the meridional wind shear anomaly (between 850mb and 200mb) averaged over the same domain as the EIMR region. Using National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalyzed circulation data as well as European Centre for Medium Range Weather Forecasts (ECMWF) operational analyses, we show that both EIMR and MH indices have a dominant quasi-biennial variability, consistent with previous studies of IMR. Teleconnections of IMR, EIMR and MH indices with summer sea surface temperatures (SST) have also been investigated. The results indicate that the southern equatorial Indian Ocean SST has a strong positive correlation with Indian monsoon rainfall and circulation. On the other hand, the correlation with eastern equatorial SST has been weak during the period under consideration primarily due to an almost reverse relationship between the monsoon and El Niño and the Southern Oscillation (ENSO) during the latest seven years.
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last update: 9 October 1997
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