Isotopic Signatures of Precipitation: Linking Tropospheric and Surface Processes in India's Core Monsoon Zone

The monsoon system is a dynamic and complex component of the atmospheric water cycle, profoundly impacting weather, climate, and human activities. A variety of meteorological observations are used to understand the monsoon system. The isotopic technique provides a unique perspective on moisture dynamics, enhancing our understanding of the monsoon system. The isotopic signature of precipitation is shaped by numerous geographical and environmental variables, making only select regions suitable for in-depth monsoon isotopic studies. The central Indian region, a pivotal monsoon zone, exhibits distinct characteristics ideal for studying monsoon dynamics. Key features include the passage of the monsoon trough- a modified Intertropical Convergence Zone, and frequent low-pressure systems (LPS) from the northern Bay of Bengal, contributing significantly to summer monsoon rainfall. Notably, rainfall variability in central India shows an out-of-phase relationship with northeastern India. Furthermore, central Indian rainfall strongly correlates with the All-India Summer Monsoon Rainfall, serving as a reliable proxy. Despite its potential, the isotopic technique remains underutilized in this core monsoon zone (CMZ: approximately defined by an area 18-28^oN, 65-88^oE) for monsoon research.

We report a multi-year (2016-2021) precipitation isotopic record obtained from Sagar, a site in the CMZ of India. We explore the relationship between isotopic signatures and regional-scale atmospheric processes mediated by diabatic heating and its vertical distribution pattern, the LPSs, moisture source dynamics, monsoon trough variability, and other meteorological conditions. We also examine the role of recycled rainfall in modulating the precipitation isotopic variability.

We have computed the diabatic heating profiles over India's CMZ. The calculated heating profiles are strongly associated with the monsoon rainfall variability expressed through a precipitation index over the CMZ. We observed a strong association between precipitation isotopic depletion and tropospheric heating. Our analysis reveals that LPSs significantly influence rainfall isotopic values through their origin, trajectory, and intensity. These systems and associated convective activity yield depleted isotopic signatures. A strong inverse relationship exists between LPS intensity and corresponding precipitation isotopic values.

Terrestrial evaporation, leading to substantial recycled rainfall, plays a pivotal role in modulating precipitation isotopic variability. A notable inverse correlation exists between precipitation isotopes and recycled rainfall. The isotopic depletions resulting from diabatic heating, LPSs, and recycled rainfall collectively manifest the amount effect, highlighting a common link among these processes.

The out-of-phase isotopic patterns observed in central and northeastern India mirror the region's dipolar rainfall variability, rendering the CMZ an optimal location for proxy-based reconstructions of past rainfall variability.