The Aerial Hydrologic Cycle in a Warming World: Perspectives from Numerical Water Tracers and Implications for the Interpretation of Water Isotopes

Abstract:

How does atmospheric moisture transport change as the globe warms? And how will these changes in moisture transport impact water isotopes? We tackle these questions using numerical water tracers implemented in a state-of-the-art Earth system model, which enable a source-to-receptor analysis of atmospheric moisture transport: aerial water is tagged by region at its evaporative source, and this tag persists through advection and phase changes in the atmosphere, up to the locale where it precipitates. We compare the behavior of these tracers in pre-industrial and CO₂-doubled model experiments, and find robust changes in atmospheric moisture transport when the globe warms. With CO₂-doubling, the distance between moisture source (evaporation) and sink (precipitation) regions increases nearly globally. As this moisture transport length scale expands, evaporated moisture is more likely to diverge from its source region, more likely to travel further within a given ocean basin before precipitating, and more likely to move from one ocean basin to another. Such changes are consistent with theoretical arguments that suggest an increase in the atmospheric moisture residence time as the planet warms. While these results are robust over much of the globe, they only apply to the polar regions in summer, where polar moisture is derived from more equatorward regions as the planet warms. In polar winter, on the other hand, increased local evaporation over regions of sea ice loss overwhelms the increase in moisture sourced from more equatorward regions. We conclude by discussing how greater moisture transport length scales may impact water isotope signals in the extratropics.