18 O depletion in monsoon rain relates to large scale organized convection rather than the amount of rainfall

Abstract

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 18 O depletion of rain with increasing amount, manifested as a negative correlation of the monthly amount of tropical rain with its δ18 O, 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 18 O-depletion in monsoon rain is not related necessarily to its amount, but rather, to large scale organized convection. Presenting δ18 O 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 δ18 O values of rain exhibit highly coherent variations (significant at P = 0.05). Significantly more 18 O-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.