Stable isotopic differences between summer and winter monsoon rains over southern India

Rain drop size and isotopic composition of rain, important parameters that shed light on rain formation processes, are highly sensitive to the ambient weather. We reported earlier a significant correlation between them in individual rain events (with limited sampling), but this is yet to be tested with better, longer term sampling. Here we attempt to do so over a tropical region (i.e. Tirupati, India). Rain samples were collected at short time intervals (< 1 h) to capture even small variations in their stable oxygen (delta O-18) and hydrogen (delta D) isotopic compositions. Isotopic analyses were made using an isotope ratio mass spectrometer, and a disdrometer measured the drop size distribution. Summer rains show a progressive O-18 & D depletion with time, while the winter rains fluctuate about a mean value. We find no definite correlation between the drop size and stable isotope ratios as was reported earlier, based on a smaller number of samples: the complexity of rain formation process and varying ambient weather conditions for individual rain events could be the reason. Further, there is no significant difference between the local meteoric water lines (delta O-18- delta D line) of summer and winter monsoon rains, though the intercepts in both the cases were significantly smaller than global meteoric waterline, suggesting significant strong influence of secondary evaporation. However, the winter rains are more depleted in D & O-18. Paleoclimate proxies such as delta O-18 of cave calcite or teak cellulose form this region need to be interpreted in terms of the relative seasonality of the rainfall rather than the total annual rain.