Long term observations on stable isotope ratios in rainwater samples from twin stations over Southern India; identifying the role of amount effect, moisture source and rainout during the dual monsoons

Abstract

The dominant factor that controls the rainwater isotope ratios in the tropics is believed to be the local or instantaneous rainfall amount. This notion was challenged in several recent observations indicating that the stable ratios in rainwater are controlled by other factors such as source water composition and intensity of convective activity. Since amount effect is the basis for paleoclimate reconstruction of rainfall it is important to develop comprehensive knowledge about it at different time scales. Here we investigated the relationship of rainwater δ18O with the local or instantaneous rainfall amount at different time scales at two Indian stations, Thiruvananthapuram (TRV) and Bangalore (BLR), for four years covering the dual monsoon systems [Indian Summer Monsoon (ISM) and Northeast Monsoon (NEM)]. It was observed that the seasonal variations are more pronounced over BLR due to its continental location in the central Southern peninsular India, compared to TRV which is a coastal station. Isotopically enriched samples were found during seasonal high temperature and low relative humidity at the respective stations. The intra-event observations indicated that the ‘amount effect’ at the intra-event scale was significant due to post-condensation evaporation, which was supported by low d-excess values and its inverse correlation with rainfall amount. The correlation between event based δ18O and local rainfall amount for both the stations was weak (r = 0.2 and 0.3) whereas the isotopic signature was sensitive to the convective activity during the monsoon period. Significant negative spatio-temporal correlations of δ18O over the moisture convergence regions were obtained with proxies of convective activity over parts of the Arabian Sea as well as over the regions of moisture pathways associated with synoptic scale disturbances from the Bay of Bengal. We observed that the correlation pattern varies with the seasonal changes of moisture source and transport pathways during the periods of ISM and NEM.