18O 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 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.

Decreasing Indian summer monsoon on the northern Indian sub-continent during the last 180 years: evidence from five tree-ring cellulose oxygen isotope chronologies

“…The removal of the heavier isotopes during the condensation process results in the oxygen isotopic depletion of the water vapor. The greater the total amount of precipitation, and the stronger the convection, the more the oxygen isotopic composition of the rainwater is affected by depletion (Lekshmy et al, 2014;Vuille et al, 2003), and this signal is reflected in tree-ring δ 18 O values. In addition, monsoonrelated factors (e.g., upstream rainout process) other than the "amount effect" may affect precipitation δ 18 O significantly (Vuille et al, 2005).…”

Isotopic composition of daily precipitation along southern foothills of the Himalayas: impact of marine and continental sources of atmospheric moisture

“…The isotopic patterns of precipitation in the tropics are expected to be different from the sub-tropics and temperate regions due to large scale convection systems, cyclonic storms and multitude of vapour sources (e.g. Midhun et al, 2013;Lekshmy et al, 2014;Lekshmy et al, 2015). Consequently, the well-established isotope effects such as amount effect, temperature effect 20 and altitude effect are not clearly visible in precipitation isotope data sets available for the Indian subcontinent (Deshpande and Gupta, 2012;Deshpande et al, 2010;Warrier et al, 2010).…”

Effects of changes in moisture source and the upstream rainout on stable isotopes in precipitation – a case study in Nanjing, eastern China

“…Recent studies have revealed the importance of variability in moisture sources (Peng et al, 2010;Xie et al, 2011) and large-scale convective activities (Vimeux et al, 2011;Tremoy et al, 2012;Kurita, 2013;Moerman et al, 2013;Lekshmy et al, 2014;He et al, 2015) in controlling precipitation δ 18 O in monsoon regions. Strong convection at source regions tends to produce more precipitation, causing heavy isotopes to preferentially condense from vapor, leading to lower values of downstream precipitation δ 18 O.…”