The UK Meteorological Office’s HadAM3 GCM has been used to produce a regional climate simulation of the middle Pliocene over the USA that is in broad agreement with available geological palaeoenvironmental evidence. Generally, a warmer, less seasonal and wetter climate is simulated for the USA during the middle Pliocene. However, according to the geological record and outputs from a biome model (BIOME 4), the HadAM3 model appears to misrepresent changes to the intensity of the hydrological cycle in the southern USA. This inconsistency is related to the way in which the HadAM3 model represents changes to the dynamic circulation of the atmosphere (changes in southerly wind strength) during the middle Pliocene. This problem may arise due to an incorrect response of the model to the imposed boundary conditions. Alternatively, incorrectly prescribed boundary conditions (elevation of western cordillera of North America or SST) in the PRISM2 data set could also be capable of generating the data-model inconsistency.

Future work will continue to evaluate the robustness of the HadAM3 model predictions for the middle Pliocene. Specifically, we aim to evaluate the reliability of the model by comparing its outputs to proxy geological data from other regions and perhaps more significantly attempt to model climate variability within the PRISM2 time slice itself.