Atmospheric factors controlling the stable isotopes (δ18O and δ2H) of the Indian summer monsoon precipitation in a drying region of Eastern India

Abstract

The Eastern part of India experiences a rapid reduction in rainfall in the past 20 years (1997–2017), which has several implications to the livelihood of the people and the ecosystems of the region. Stable isotopic analysis of precipitation along with extensive backward trajectory modelling was conducted in this drying region to understand the factors controlling the stable isotopic composition of precipitation and source and origin of atmospheric moisture for precipitation during the Indian Summer Monsoon (ISM). The drivers responsible for the isotopic variations in precipitation were evaluated using their statistical relations with temperature, amount of rainfall and Relative Humidity (RH). Additionally, Outgoing Longwave Radiation (OLR) data from NOAA satellites were used to evaluate the effect of convective activities on isotopic variations in precipitation. The pathways of moisture transport for each rain event were derived using backward trajectory model (HYSPLIT4.0) based on NCEP data and the atmospheric circulation patterns were evaluated using NCAR reanalysis data. Major sources of moisture causing rainfall during the ISM period in the drying region were evaluated using cluster analysis of the backward wind trajectories. The isotopic content in precipitation showed a weak “temperature and amount effects”; however, a moderate correlation was observed with relative humidity of the region. It is inferred that the relatively high isotopic depletion observed in the monsoon rains was not related to the amount of rainfall, but rather, to large scale convective processes. It is found that the rainwater isotopic content during the ISM appears to be controlled by the dynamics of moisture rather than the hydrometeorological conditions of the individual precipitation events. A Local Meteoric Water Line (δ2H = 7.86 δ18O + 8.81) was developed for Eastern India using the isotopic data of rainwater samples. Backward air mass trajectory modelling, cluster analysis of wind trajectories, atmospheric circulation patterns and lower D-excess suggest that the origin of moisture in the drying region of Eastern India was predominantly from the Arabian Sea and the contribution from the Bay of Bengal was insignificant.