Isotopic investigation of the moisture transport processes over the Bay of Bengal

Abstract

To study the atmospheric moisture transport processes from the source to a receiver region, isotopic analysis of rainwater from Port Blair (as source region), the Andaman Islands, Bay of Bengal has been carried out. In addition to the island site, rainwater collected at three sites on the Indian mainland (as receiver region) namely Nagpur, Kolkata, and Tezpur has also been isotopically analyzed. We present a daily variation of long-term (2012–2015) record of rain isotope from Port Blair to understand the moisture dynamics on a sub-seasonal timescale. It was observed that the Port Blair rainwater oxygen isotope ratios (δ18O) maintain a temporally dependent correlation with the average rainfall over the Core Monsoon Zone (CMZ) of India, though the seasonal rainfall over these two regions appears to be mutually independent. Rain isotopes are controlled more by the moisture dynamics rather than the individual rainfall events. Consequently, the correlation between rainfall over the CMZ and δ18O of Port Blair rain provides conclusive evidence of the transport of the Bay of Bengal (BoB) moisture to the central and the northeast Indian regions during the summer monsoon season. The spatial dependency of Port Blair rain-isotopes with the CMZ rain shows interannual variability and also indicates different pathways of the BoB moisture to the Indian mainland within a given season. The pathways of the source moisture to the receiver are identified by the trajectory analysis with Port Blair being the source area. We observed analogous variation in δ18O of island rain and that of the mainland rain, and δ18O at these two regions is modulated by the monsoon intra-seasonal oscillation (MISO). The correlation between the two isotopic records appears to arise from the propagation of the rain/cloud band from the Bay to the Indian landmass and the associated moisture transport linked to the MISO. Keywords: Monsoon, Rain isotope, Bay of Bengal, Moisture transport