In response to the altered boundary conditions imposed in the Pliocene sensitivity experiment, the HadAM3 model predicts reduced surface temperatures compared to the Pliocene control over wide regions of the USA during both winter and summer seasons (Figure 7). The magnitude of this cooling varies spatially with severest cooling (>5ºC) concentrated over the western United States, associated with the western cordillera. However, areas in close proximity to the coast, peninsulas (e.g., Florida) and bays (e.g., Hudson Bay) do not exhibit a cooling trend. This is due to the warmer SST imposed from the PRISM2 data set. This model suggests that in these regions the simulated warming observed from the Pliocene – present-day control experiment are solely attributable to the warmer Pliocene SST. Conversely, the increase in Northern Hemisphere terrestrial ice and snow cover combined with the regional specified increase (~50%) in the elevation of the North American western cordillera primarily drove the cooler temperatures over western and central parts of the USA observed in the Pliocene sensitivity experiment. Therefore, the primary forcing mechanisms for the simulated warmer conditions in the Pliocene – present-day control experiment are reduced Northern Hemisphere terrestrial ice and snow cover (via an ice albedo feedback) and the lowered elevation of the western cordillera of North America.

Overall, the HadAM3 model predictions for the Pliocene sensitivity experiment suggest a pattern of reduced precipitation levels over the USA for the Pliocene sensitivity experiment (Figure 8). The largest reductions are observed during the winter (DJF) season in a latitudinal range corresponding to the westerly wind belt. This reduction in precipitation is generated by a number of feedback mechanisms. Increased Northern Hemisphere snow and ice cover, combined with the increased elevation of the western cordillera of North America, reduces surface temperatures, evaporation, and the moisture available for precipitation. Enhanced terrestrial ice cover and lower surface temperatures, which occur generally in the Northern Hemisphere, weaken the Icelandic low and Azores high-pressure systems. This has the effect of weakening the moisture-bearing westerly wind belt in the Pliocene sensitivity experiment (Figure 9). Therefore, increased precipitation levels simulated for the Pliocene – present-day control experiment for the region result from reduced snow and ice cover in the Northern Hemisphere and the reduced elevation of the western cordillera of North America.