Interannual variability of summer monsoon rainfall over Orissa in relation to tropospheric circulation features

Abstract

The summer monsoon rainfall over Orissa shows largescale interannual variability due to variation in the characteristics of meteorological parameters like temperature, pressure/geopotential height and wind at surface and in the troposphere. A study was therefore undertaken to find out the characteristic features of surface and upper air meteorological parameters in the troposphere and their relationship with summer monsoon rainfall over Orissa. For this purpose, rainfall data over 31 uniformly distributed stations in Orissa along with the surface meteorological parameters at seven stations and upper air parameters at different standard isobaric levels up to 200 hPa over four stations in Orissa and its neighbourhood during summer monsoon season over a period of 19 years (1980-98) have been considered. The principal objective of this study is to find out the contribution of different meteorological parameters in the spatio-temporal variability of summer monsoon rainfall over Orissa. The interannual variability of rainfall mostly depends on the interannual variation of wind over Orissa and its neighbourhood. Strong northeasterlies over north coastal Orissa along with the southeasterly winds over Gangetic West Bengal and northwesterlies over northwest Orissa, south Orissa and north coastal Andhra Pradesh at the lower and mid-tropospheric levels are favourable for good rainfall over Orissa as a whole. They are also favourable for good rainfall over the western side of the Eastern Ghats and over the central zone, extending from the central part of coastal Orissa to western Orissa. This pattern of wind is also associated with the synoptic disturbances over northwest Bay of Bengal. Hence, the higher/lower number of days of synoptic disturbances over the NW Bay causes excess/deficient rainfall over Orissa. The stronger gradient of mean sea-level pressure and geopotential over the region to the south of the monsoon trough passing through north coastal Orissa at lower and mid-tropospheric level, is favourable for good rainfall over Orissa. Divergence at the upper troposphere (200 hPa) due to a trough in the upper tropospheric westerlies to the right of Visakhapatnam and an east-west ridge to the south of Visakhapatnam and over the NW Bay is favourable for rainfall over Orissa. There is no significant trend in both zonal and meridional winds at the upper troposphere (200 hPa) over the region, supporting insignificant trend in summer monsoon seasonal rainfall over Orissa during 1980-98.