Oscillation of 300 mb Anticyclone and Summer Monsoon Over North East India

Global position of the monthly mean Anticyclone (AC) time series, 300 mb aloft in the North-East Indian subcontinent over the range 0-45°N,60-130°E, were identified from the NCEP/ NCAR reanalysis dataset (1958-1998). Seasonal averagelatitude and longitude of the AC centres in the time series show two extreme complementary characters; 10.34°N (min), 102.27°E (max)in March and 29.08°N (max), 86.48°E (min) in August. Similar characters were also observed in the four decadal periods 1958-`67, 1968-`77, 1978-`87, and 1988-`97. It is observed that the AC centre oscillates along alinear track (R2 = +0.9096)from the western flank of Cambodia (10°N,104°E) to the southern part of Tibetan Plateau (TP) with its centre at 28°N,87°E. Impacts of Southern Oscillation (SO) and Indian Ocean Dipole (IOD) events on the 300 mb AC time series were also investigated. Strong positive (negative) correlation of SO (IOD) with latitude was observed whereas low negative (positive) correlation was found for longitude of the AC timeseries. The monthly mean rainfall dataset, obtained from the India Meteorological Departmentover eight subdivisions in North-East India (NEI) were investigated in relation to the 300 mbAC time series. Very high and direct positive correlations (more than 81%) were observed between the latitude of 300 mb AC centres in the time series and that of the rainfall over the nineNEI subdivisions.The correlations with longitude were comparatively weak and negative(less than 30%). The analysis shows that highest rainfall during summer monsoon (Jun-July-Aug-Sep)starts exactly (onset of summer monsoon) when the 300 mb AC griddles in the 23°-29°N latitude belt over the North-East Indian subcontinent. During rest of the year (Oct to May) 300 mb AC centre hovers on the lower latitude and the rainfall is lowest.