Linkage between the precipitation isotopes and biosphere-atmosphere interaction in northeast India

The intra-seasonal variation of precipitation isotopes in the Indian subcontinent shows a declining trend, especially in the north-east region. As of now, no mechanism offers a consistent explanation of this trend. Rainwater samples have been collected across north-east India during the past 4-5 years. They have been analyzed for the isotopic ratio to investigate the cause of such a trend. The results show that the isotopic values are almost always enriched during the pre-monsoon season, which progressively decreases until the late phase of the monsoon. We propose that pre-monsoon enrichment is caused by enhanced plant productivity and transpiration mediated by abundant solar radiation and water resources. A significant amount of rain driven by thunderstorm activities in March-April helps increase the soil water content, which in turn accelerates the transpiration process providing relatively more number of heavier oxygen and hydrogen isotopes. Eddy-covariance based observation shows that the latent heat flux is maximum during this time. Numerical estimation using a diagnostic model also confirms increased moisture recycling. After the onset of the southwest monsoon in May-Jun, an increased amount of isotopically depleted Bay of Bengal moisture enters this region resulting in a progressive decrease in transpired vapor and hence the isotopic values. Additionally, local to regional scale convective activities also produce isotopically depleted rainwater. These three effects, viz. enhanced transpiration during the pre-monsoon season, increased amount of Bay of Bengal derived moisture during the monsoon season, and the convective activities produce a declining trend in the isotopic values. The study has implications in understanding the role of the biosphere in modulating the monsoon processes.