Large scale variability associated with Indian summer monsoon

Abstract

The objective of this study is the investigation of the large scale variability of the atmospheric circulation over the Mediterranean region in relation to the Indian summer monsoon. For this purpose composite anomalies of selected fields at various isobaric levels are analyzed for strong versus weak monsoon years. Gridded, monthly mean data, such as geopotential height, horizontal wind components, vertical velocity and relative vorticity at 300 hPa were used, as obtained from the ERA-40 Reanalysis Data Base, with 2.5 2.5 resolution for the boreal summer (June-September) and for a 44-year period (1958–2001). The standardized Dynamic Indian Monsoon Index by Wang and Fan (1999) was employed to determine the strong and weak monsoon years in the 44-year period. It was found that there are significant differences between strong and weak composites for all fields, especially in the upper troposphere. The results suggest that these differences may be related to the existence of Rossby wave trains as well as to the intensity and the meridional shift of the upper-level jet streams.