Tokyo, Japan –Did you know that we can track the journey of a drop of water through space and time? Water is made of hydrogen and oxygen, and sometimes these atoms are slightly heavier than usual. These heavier forms are called isotopes. As water evaporates or moves through the atmosphere, the amount of these isotopes changes in predictable ways. This can act as a fingerprint, allowing researchers to trace the movement of water at global scales. This information can then be used with hydrological modeling, allowing scientists to interpret extreme weather events, such as storms, flooding and droughts, and to predict weather changes due to climate change.
Climate models have been developed that include isotopic processes. However, it is extremely difficult for one individual climate model to accurately simulate water circulation. Now, in a study published by Journal of Geophysical Research: Atmospheres, the team at the Institute of Industrial Science, The University of Tokyo have applied a technique called an ensemble, which uses multiple models at the same time. The ensemble includes eight isotope-enabled climate models and spans a 45-year period from 1979 to 2023. All the models were run using the same wind and sea-surface temperature data, allowing researchers to test individual model physics as well as the performance of the ensemble mean against climate observations.
"Changes in water isotopes reflect shifts in moisture transport, convergence, and large-scale atmospheric circulation. Although we know, at a simple level, that isotopes are affected by temperature, precipitation and altitude, the variability of current model simulations makes it difficult to interpret the results," said Professor Kei Yoshimura, one of the senior authors of the study, who advised on several of the isotope-enabled climate models participating in the project. "We are delighted that our ensemble mean values capture the isotope patterns observed in global precipitation, vapor, snow, and satellite data much more successfully than any of the individual models."
Examining changes over the past 30 years, the ensemble simulations captured a general increase in atmospheric water vapor associated with warming temperatures and a strong link with large-scale interannual climate phenomena such as the El Niño-Southern Oscillation, the North Atlantic Oscillation, and the Southern Annular Mode. These climate systems drive multi-year variability in global water availability, affecting billions of people worldwide.
"Ensembles offer a nuanced modeling approach that reduces divergence between individual models. This approach allows us to separate the effects of how each model represents water cycle processes from differences arising from individual model structures," said Dr. Hayoung Bong, alumnus of the Institute of Industrial Science, The University of Tokyo, now at NASA Goddard Institute for Space Studies.
This study marks a world-first, bringing together multiple isotope-enabled climate models in a unified framework and producing an ensemble that closely matches observations.
"Importantly, the research advances our ability to interpret past climate variability and provides a stronger foundation for understanding and predicting how the global water cycle and the weather it shapes will respond to continued global warming," said Professor Yoshimura.
