Intraseasonal oscillation (ISO) of rainfall during the summer monsoon season (June-August) and its plausible effect on interannual variation (IAV) of total summer monsoon rainfall over Bangladesh are examined using daily rainfall data from 25 rain-gauge stations for 20 years (1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000). Submonthly scale (7-25 days) ISO is a dominant mode of summer rainfall fluctuation over Bangladesh. In the active (suppressed) ISO phases, a cyclonic (anticyclonic) circulation anomaly develops at the low-level troposphere from northeastern India to Bangladesh, and an anticyclonic (cyclonic) circulation anomaly appears over the Bay of Bengal, accompanied by the enhancement of westerly/southwesterly (easterly/southeasterly) moisture flux anomalies over Bangladesh. The IAV of total summer monsoon rainfall is significantly correlated with that of the rainfall variance, mainly in the submonthly scale, suggesting that the ISO activity controls the IAV of the total summer monsoon rainfall. Moreover, the spatial patterns of atmospheric circulation and convection associated with the ISO and the IAV are quite similar to each other around the monsoon trough, especially from northeastern India to Bangladesh. In wet monsoon years, more frequent strong submonthly scale ISOs and a larger number of rainy days related to the active ISO phase are evident compared with that of dry monsoon years. Climatologically, Bangladesh is one of the most predominant areas of submonthly scale ISO; however, 30-60-day ISO is not prominent. These characteristic features probably allow the submonthly scale ISO to modulate the total seasonal rainfall and the spatial patterns of circulation and convection over and near Bangladesh.

The spatial and temporal distributions of thermal instability and vertical wind shear in the Indian subcontinent are evaluated as the important factors for occurrence and development of severe local storms and compared with the climatology of severe local storms across the Indian subcontinent. High thermal instability and vertical wind shear occur in Bangladesh and northeastern India during the pre‐monsoon season (from March to May), peaking in April. There is great potential for severe local storms during the pre‐monsoon season in Bangladesh and northeastern India.

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