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

Supriyo Chakraborty,S Bera,Amey Datye,Sabyasachi Maji,Pramit Kumar Deb Burman,Abhijit Bora,Nitesh Sinha,Nirmali Gogoi,Dipankar Sarma,Abirlal Metya,Dipak K Parua,Charuta Murkute,Krishnakumar Sujith,Subodh K Saha

doi:10.1038/s41612-022-00231-z

Abstract

The intra-seasonal variation in precipitation isotopes shows a characteristic declining trend over northeast India. As of now, no mechanism offers a consistent explanation of this trend. We have performed the isotopic analysis of precipitation (rain) and estimated net ecosystem exchange and latent heat fluxes using an eddy-covariance system in northeast India. Additionally, we have used a diagnostic model to determine the recycled rainfall in this region. We find a strong link between the enhanced ecosystem productivity and isotopic enrichment in rainwater during the premonsoon season. Subsequently, on the advent of monsoon, the Bay of Bengal generated moisture enters this region and depletes the isotopic values. Additionally, the regional-scale convective activities produce periodic lows in the precipitation isotopes. Contrary to the general understanding, our study shows that the internal factors, such as the local land-atmosphere interactions, rather than the external influences, play a significant role in governing the precipitation isotopes in northeast India.

Highlights

A variety of ocean-atmospheric processes controls the isotopic composition of precipitation
The objectives of our study are: What processes control the isotopic variability of precipitation in NEI on an intra-seasonal timescale? Does plant physiology play any role in determining the precipitation isotopic composition? Whether the internal processes or the external factors play a significant role in modulating the precipitation isotope values in this region? We follow a multidisciplinary approach to address these questions
Characteristics of the precipitation isotopic profiles Rainwater oxygen isotopic values for Tezpur and Darjeeling are shown in Fig. 2a, b, respectively

Summary

Introduction

A variety of ocean-atmospheric processes controls the isotopic composition of precipitation. The oxygen isotopic (δ18O) profiles of precipitation on an intraseasonal timescale often show a declining trend in some parts of India, such as Ahmedabad[4], New Delhi, and Mumbai[8], Port Blair[9] in the Bay of Bengal (BoB) during the monsoon season (June to September) Such kind of declining effect is most dramatic in the northeast region of India ( represented as NEI; Fig. 1a). Sinha et al.[11] reported δ18O variations of Tezpur in NEI and observed a declining trend over 10‰ The cause of such depletion in δ18O is believed to be the modulation by ocean-atmospheric processes. The reported Local Meteoric Water Line (LMWL) from NEI typically shows a slope like the Global Meteoric Water Line (GMWL; ~8) but intercept higher than 10‰10,12 This shows that a significant amount of recycled water vapour contributes to the local precipitation[14]. Earlier investigators mostly ignored the origin and role of this recycled water vapour

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Conclusion