Spatial variation of amount effect over peninsular India and Sri Lanka: Role of seasonality

Abstract

The relationship between rain amount and rain δ18O of monsoon rain (amount effect) helps to reconstruct past monsoon variability from proxies (e.g., tree rings and speleothems). Analysis of new (and published) data of the δ18O of monsoon rains and vapor at nine stations shows that in regions of distinct seasonality in precipitation (e.g., peninsular India), the noise in such reconstructions can be minimized by a careful selection of sites. Peninsular India receives rain from both the Indian summer monsoon (ISM) and the northeast monsoon (NEM). Significant amount effect is observed only where the NEM rainfall is larger than or comparable to ISM rainfall. This is due to the higher quantity of NEM rain with more depleted 18O relative to ISM rain. NEM rain is more depleted in 18O because of cyclonic activity over Bay of Bengal, and the 18O depletion of Bay of Bengal surface waters due to post‐ISM river runoff.

The Effect of Monsoon Circulation on the Stable Isotopic Composition of Rainfall

“…We also estimated the possible value of δ 18 O of surface water at the head Bay of Bengal from climatological salinity (S) data using the reported δ 18 O‐S relations (Sengupta et al, ; Singh et al, ) and the estimated δ 18 O of surface water varied from −0.2‰ to −0.8‰ (for September). Thus, the evaporation from this freshwater plume cannot contribute to the observed trend in δ 18 O p (Lekshmy et al, ). We observe that over the inland station New Delhi, this trend (Figure ) co‐occurs with the switching of moisture transport pathway from the AS branch to the BoB branch as the ISM season advances.…”

“…The amount effect is the observed negative correlation between the monthly rainfall and its weighted mean 18 O content. Though this relation is quite robust over tropical island stations, it shows a large scatter over continents (Kurita, 2013;Lekshmy et al, 2015). Thus, the role of local and remote meteorological factors that control δ 18 O of precipitation (δ 18 O p ) needs to be explored for a precise interpretation of 18 O-based continental monsoon proxies.…”

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

“…There is a negative correlation between the ISM and precipitation δ 18 O in the monsoonal area (Vuille et al, 2005;Yang et al, 2016). Asian summer monsoon affects the δ 18 O of precipitation through the amount effect (Cai and Tian, 2016;Dansgaard, 1964;Lekshmy et al, 2015). A stronger summer monsoon usually brings more summer rainfall to the southern Himalaya.…”

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).…”