Atmospheric rivers (ARs) are filamentary systems which perform nearly all of the poleward moisture transport through the midlatitudes. When these systems are lifted---for instance when impinging on local topography---they can produce substantial precipitation which ranges from beneficial to destructive, sometimes providing essential water resources and sometimes leading to devastating floods. With this in mind, it is critical for planning and preparedness purposes to project the response of ARs to climate change. Open questions remain however, highlighting the need for a better understanding of the physical processes behind the AR response to climate change. To address these gaps in understanding, ARs are simulated in an aquaplanet, an idealized global climate model without land, sea ice, topography, or seasonality. A time-invariant ``Baseline'' sea-surface temperature (SST) distribution is prescribed for a reference run, while each test run applies a uniform increase (plus two, four, and six Kelvin) over the Baseline distribution to isolate the response of ARs to warming SSTs and assess sensitivities.
Under SST increases, zonal mean AR occurrence frequency increases everywhere, mostly due to enhancements in AR size. These AR occurrence frequency increases are greatest at higher latitudes as storm tracks shift poleward with SST warming. Zonal and areal means of AR integrated water vapor (IWV) show that AR moisture is enhanced at rates greater than predicted for surface moisture by the Clausius-Clapeyron relation with respect to SST warming. Vertical profiles of relative humidity (RH) and temperature show that this ``super-Clausius-Clapeyron'' IWV increase masks RH decreases at upper levels which are related to upper-tropospheric warming that far outpaces the prescribed SST increases. Zonal and areal means of AR IWV transport (IVT) show lower increases with SST warming than for IWV; a simple linear decomposition of AR IVT into moisture and wind components shows that a slowing of mean AR wind speeds attenuates the IVT response. Zonal mean AR precipitation rates exhibit a complicated response characterized by increases in some latitude bands and decreases in others. A linear decomposition of AR precipitation rates like that performed for AR IVT once again shows a compensatory relationship between AR moistening and weakening dynamics, in this case mid-tropospheric vertical velocities.
Increases in AR size as SSTs warm are examined separately. Cross-sections of AR IVT are approximated with Gaussian functions with three fit parameters: AR background IVT, AR IVT exceedance above the background, and the breadth of the AR IVT profile, defined as the distance through which 95\% of the IVT exceedance occurs. From both the AR IVT cross-sections and their Gaussian approximations are derived four measures of AR width in total. All widths are enhanced with SST warming, mostly as a result of enhanced background IVT statistics and increased AR IVT profile breadth. IVT profile breadth changes are driven mostly by Clausius-Clapeyron moderated moisture increases, though changes to AR wind profiles also play a role.