Time series of winter AR totals in the Historical (grey lines) and AMIP (black lines) runs in the (a) bcc-csm1-1 model, (b) CanESM2/AM4 model, (c) CNRM-CM5 model, (d) GFDL-ESM2G model, and (e) NorESM1-M model

<p><strong>Figure 2.</strong> Time series of winter AR totals in the Historical (grey lines) and AMIP (black lines) runs in the (a) bcc-csm1-1 model, (b) CanESM2/AM4 model, (c) CNRM-CM5 model, (d) GFDL-ESM2G model, and (e) NorESM1-M model. The light blue shading represents the range of AR totals in the five reanalyses (20CR, CFSR, ERAIN, MERRA and NCEP–NCAR). Panel (f) contains the average number of ARs per winter (using a model-dependent IVT threshold) with the corresponding 2.5th–97.5th sampling confidence intervals (reanalyses are in blue). Panel (g) shows the average winter 2 m surface temperature (red points; right <em>y</em>-axis) over the North Atlantic region (20° N–60° N; 60° W–0°) and the IVT threshold (black points; left <em>y</em>-axis) from 1980 to 2005 for the CMIP5 models and ERAIN, and panel (h) shows the average number of ARs per winter (using a fixed IVT threshold) with the corresponding 2.5th–97.5th sampling confidence intervals (reanalyses are in blue).</p> <p><strong>Abstract</strong></p> <p>Within the warm conveyor belt of extra-tropical cyclones, atmospheric rivers (ARs) are the key synoptic features which deliver the majority of poleward water vapour transport, and are associated with episodes of heavy and prolonged rainfall. ARs are responsible for many of the largest winter floods in the mid-latitudes resulting in major socioeconomic losses; for example, the loss from United Kingdom (UK) flooding in summer/winter 2012 is estimated to be about $1.6 billion in damages. Given the well-established link between ARs and peak river flows for the present day, assessing how ARs could respond under future climate projections is of importance in gauging future impacts from flooding. We show that North Atlantic ARs are projected to become stronger and more numerous in the future scenarios of multiple simulations from five state-of-the-art global climate models (GCMs) in the fifth Climate Model Intercomparison Project (CMIP5). The increased water vapour transport in projected ARs implies a greater risk of higher rainfall totals and therefore larger winter floods in Britain, with increased AR frequency leading to more flood episodes. In the high emissions scenario (RCP8.5) for 2074–2099 there is an approximate doubling of AR frequency in the five GCMs. Our results suggest that the projected change in ARs is predominantly a thermodynamic response to warming resulting from anthropogenic radiative forcing.