Stable Isotopic Characterization of Nor’westers of Southern Assam,NE India

Abstract

Stable oxygen and hydrogen isotopic compositions (δ¹⁸O and δD) of rainwater collected between 2009 and 2011 CE from Hailakandi, southern Assam, north-eastern India have been measured to characterize the extreme rainfall phenomena, delineate moisture sources, determine amount effect and assess the prospect of local proxies for paleoclimate studies. The δ¹⁸O values range from +0.7 to −13.8‰ while δD varies between +12 and −98‰. During the pre-monsoon months (March to May), heavy rains and thunderstorms, called nor’westers (Kal Baisakhi in local parlance), occur in some years giving rainfall as much as the monsoons. Strong evaporation and localised convection due to elevated air and sea surface temperature in the region and over the adjacent Bay of Bengal seem to cause nor’wester activity. No amount effect is observed in rain. The d-excess values for nor’wester rains is very different from those of monsoon rain; very low d-excess values (up to −26‰) were observed in 2010 CE and attributed to vapour originated from re-evaporation of freshwater, contributed by the lowland water bodies and rivers. The meteoric water line for the nor’wester rains, unlike that for the monsoon season, is significantly different from the Global Meteoric Water Line (GMWL). During the monsoon, the rain becomes progressively more depleted in the heavier isotopes (D and ¹⁸O), with the lowest δ values occurring in Sep-Oct. Oxygen isotope-based paleoclimatic study in this region appears to be confounded by a strong dependence of the rainwater isotopic composition on the source of moisture, uneven distribution of rains during the monsoon and the pre-monsoon seasons and the absence of amount effect.