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

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. Key Points Spatial variations of amount effect is driven by ratio of ISM to NEM rains Significant amount effect is observed in NEM-dominated region More ¹⁸O depletion of NEM rain is likely due to the cyclonic storms over BoB.