Oxygen isotopic variations in rainfall proxies such as tree rings and cave calcites from South and East Asia have been used to reconstruct past monsoon variability, mainly through the amount effect: the observed 18O depletion of rain with increasing amount, manifested as a negative correlation of the monthly amount of tropical rain with its δ18O, both measured at the same station. This relation exhibits a significant spatial variability, and at some sites (especially North-East and peninsular India), the rainfall proxies are not interpretable by this effect. We show here that relatively higher 18O-depletion in monsoon rain is not related necessarily to its amount, but rather, to large scale organized convection. Presenting δ18O analyses of ~654 samples of daily rain collected during summer 2012 across 9 stations in Kerala, southern India, we demonstrate that although the cross correlations between the amounts of rainfall in different stations is insignificant, the δ18O values of rain exhibit highly coherent variations (significant at P = 0.05). Significantly more 18O-depletion in the rain is caused by clouds only during events with a large spatial extent of clouds observable over in the south eastern Arabian Sea.

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[1] δD v and δ 18 O v of~70 water vapor samples collected at 6 and 25 m above sea level over the Bay of Bengal (BoB) during July-August 2012 are reported. This helps characterize the isotopic signature of monsoon vapor. No significant vertical variation is observed in δD v , δ 18 O v , or deuterium excess (defined as δD-8δ 18 O); δD v and δ 18 O v are significantly correlated (r = 0.92) at each height; the deuterium excess values do not, because the variation of δD v and δ 18 O v relative to their uncertainties is much larger than that of the latter. The temporal variations of δD v and δ 18 O v correlate well with air temperature rather than sea surface temperature. The control of normalized humidity on deuterium excess is less prominent. While the distribution of water vapor isotopologues over the BoB is primarily determined by the ocean surface conditions, they are significantly altered by laterally advected vapor from rain en route during the monsoon. Citation: Midhun, M., P. R. Lekshmy, and R. Ramesh (2013), Hydrogen and oxygen isotopic compositions of water vapor over the Bay of Bengal during monsoon, Geophys. Res. Lett., 40,[6324][6325][6326][6327][6328]

Understanding the factors that control the variability of oxygen isotopic ratios (δ18O) of Indian Summer Monsoon (ISM) rainfall (δ18Op) is of vital importance for the interpretation of δ18Op derived from climate proxies (e.g., speleothem and tree ring cellulose) of this region. Here we demonstrate the importance of moisture transport pathways on spatiotemporal variations of ISM δ18Op using a new set of daily observations from central and northern India and previously reported data aided by simulations from an isotope‐enabled General Circulation Model. 18O‐depleted rain events are characterized by a higher number of air parcel back trajectories through the Bay of Bengal branch of moisture transport, while those through the Arabian Sea branch are associated with 18O enriched rain events. This effect is observed on intraseasonal to interannual timescales in the long‐term observations at New Delhi as well. Thus, the shift in moisture transport regimes must be considered when interpreting δ 18Op from climate proxies of the ISM region.