Sensitivity of Atmospheric River Vapor Transport and Precipitation to Uniform Sea‐Surface Temperature Increases
An idealized climate model is used to examine changes in vapor transport and precipitation by filaments of enhanced vapor transport called “atmospheric rivers” (ARs) with respect to theory. Total column moisture in ARs increases more than anticipated by theoretical predictions conditioned on sea-surface temperature (SST) warming alone, and this unexpectedly large increase masks drying in the mid-upper troposphere. Meanwhile, increases in AR moisture are frequently at odds with weakening AR dynamics: enhancement in vapor transport or precipitation from ARs never exceeds that in AR column vapor. Still, AR heavy precipitation rates always increase in frequency, even in regions where mean AR precipitation rates decrease.
Unexpectedly large enhancement in AR column vapor under warming conditions has been noted in previous literature and hypotheses grounded in theory have been put forth before, but this was the first study to directly evaluate model results with respect to theoretical predictions. The ability to bridge the gap between simulation and theory was facilitated by the use of idealizations such as uniform SST warming, since it allowed for a direct attribution between an isolated climate change forcing and its effect on ARs. Thus these results raise confidence in future projections by providing physical context for observed changes in ARs.
The Community Atmosphere Model version 5 (CAM5) is run in an “aquaplanet” configuration which features a global, non-interactive ocean with a prescribed sea-surface temperature (SST) to isolate the impact of SST warming on AR statistics. Four two-year runs are performed: a Baseline run featuring SSTs similar to present-day climate, and three test runs featuring uniform SST warming over the Baseline (+2, +4, and +6K). ARs are identified with an objective algorithm conditioned on vapor convergence. Changes in vapor transport, precipitation, and other relevant quantities in AR and non-AR grid points are assessed with respect to these SST increases. AR vapor transport and precipitation are decomposed into moisture and wind components to broadly assess the thermodynamic and dynamical contributions to these quantities.