A 50km resolution GFDL AM4 well captures many aspects of observed atmospheric river (AR) characteristics including the probability density functions of AR length, width, length-width ratio, geographical location, the magnitude and direction of AR mean vertically integrated vapor transport (IVT) with the model typically producing stronger and narrower ARs than the ERA-Interim reanalysis results. Despite significant regional biases, the model well reproduces the observed spatial distribution of AR frequency and their variability in response to large-scale circulation patterns such as the El Niño - Southern Oscillation (ENSO), the Northern/Southern hemisphere Annular Mode (NAM/SAM), and the Pacific North American (PNA) teleconnection pattern. For global warming scenarios, in contrast to most previous studies which show a large increase in AR length and width and therefore the occurrence frequency of AR conditions at a given location, this study shows only a modest increase in these quantities. However, the model produces a large increase in strong ARs with the frequency of Category 3-5 ARs rising by roughly 100-300%/K. The global mean AR intensity as well as AR intensity percentiles at most percent ranks increases by 5-8%/K, roughly consistent with the Clausius-Clapeyron scaling of water vapor. Finally, the results point out the importance of AR IVT thresholds in quantifying modeled AR response to global warming.