The global distribution of natural tritium in precipitation simulated with an Atmospheric General Circulation Model and comparison with observations

TL;DR

This study introduces the first GCM simulation of natural tritium in water, validating it against historical data, and demonstrates its potential to improve understanding of atmospheric water cycle dynamics.

Contribution

The paper presents the novel implementation of tritium in a GCM, enhancing water cycle modeling and validation using water isotope diagnostics.

Findings

Accurately captures continental and latitudinal tritium variations.

Reproduces seasonal tritium variability in Antarctica.

Partially reproduces European spring tritium maximum.

Abstract

The description of the hydrological cycle in Atmospheric General Circulation Models (GCMs) can be validated using water isotopes as tracers. Many GCMs now simulate the movement of the stable isotopes of water, but here we present the first GCM simulations modelling the content of natural tritium in water. These simulations were obtained using a version of the LMDZ General Circulation Model enhanced by water isotopes diagnostics, LMDZ-iso. To avoid tritium generated by nuclear bomb testing, the simulations have been evaluated against a compilation of published tritium datasets dating from before 1950, or measured recently. LMDZ-iso correctly captures the observed tritium enrichment in precipitation as oceanic air moves inland (the so-called continental effect) and the observed north-south variations due to the latitudinal dependency of the cosmogenic tritium production rate. The seasonal variability, linked to the stratospheric intrusions of air masses with higher tritium content into the troposphere, is correctly reproduced for Antarctica with a maximum in winter. LMDZ-iso reproduces the spring maximum of tritium over Europe, but underestimates it and produces a peak in winter that is not apparent in the data. This implementation of tritium in a GCM promises to provide a better constraint on: (1) the intrusions and transport of air masses from the stratosphere and (2) the dynamics of the modelled water cycle. The method complements the existing approach of using stable water isotopes.