Global scale palaeoclimate reconstruction of the middle Pliocene climate using the UKMO GCM: initial results

This paper outlines the results of a new climate modelling study for the middle Pliocene (ca. 3 Ma). The work was carried out using the UK Meteorological Office GCM (UKMO, Version 3.0) which is a grid point model using a grid of 2.5° in latitude by 3.75° in longitude. The model utilised the new PRISM2 2°×2° enhanced data set of boundary conditions for the middle Pliocene supplied by the U.S. Geological Surveyʹs Pliocene Interpretations and Synoptic Mapping Group (PRISM). By comparison with the present, the model simulation predicted a 1.9°C annual mean warming over the globe. Warming was at its greatest in high latitudes and resulted in a reduced equator-to-pole temperature gradient of 6°C and a reduction in the general circulation of the atmosphere. Annual mean values for total precipitation (mm/day) increased by 6% with a minor increase in the high cloud component. In low and equatorial regions, temperature decreases by a maximum of 8°C (e.g. over East Africa), whilst precipitation increases, associated with a broadening of the Hadley Cell, promoted greater evaporation and further cooling. Comparison of our results with previous modelling studies, which have examined the character of the middle Pliocene climate, reveal a broad pattern of agreement between the models, but with significant differences observed in detail. These differences are caused by: (a) the greater spatial resolution and different physical parameterisations used in the UKMO GCM compared to other models and (b) variations in the PRISM2 data set compared to the original 8°×10°, or the PRISM1 2°×2° data set used in earlier modelling studies. Comparison of the simulated palaeoclimate to available geological data shows a broad accordance. However, disparities do exist when examining the apparent strengthening of upwelling suggested by some geological data, against the simulated reduction in strength of the general circulation of the atmosphere predicted by the UKMO GCM. Furthermore, the well-documented uncertainty of the geological record concerning the extent of the East Antarctic ice sheet during the middle Pliocene makes the overall accuracy of the imposed PRISM2 boundary conditions, and hence the predictions of the UKMO GCM, difficult to truly assess.