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 mi ddle Pliocene (ca. 3 Ma). The work was carried out using the UK Meteorologi cal Office GCM (UKMO, Version 3.0) which is a grid point model using a grid of 2.5 degrees in latitude by 3.75 degrees in longitude. The model utilise d the new PRISM2 2 degrees x 2 degrees enhanced data set of boundary condit ions for the middle Pliocene supplied by the U.S. Geological Survey's Plioc ene Interpretations and Synoptic Mapping Group (PRISM). By comparison with the present, the model simulation predicted a 1.9 degree s 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 o f 6 degrees 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 degrees 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 dif ferences are caused by: (a) the greater spatial resolution and different ph ysical parameterisations used in the UKMO GCM compared to other models and (b) variations in the PRISM2 data set compared to the original 8 degrees X 10 degrees, or the PRISM1 2 degrees X 2 degrees data set used in earlier mo delling studies. Comparison of the simulated palaeoclimate to available geo logical data shows a broad accordance. However, disparities do exist when e xamining the apparent strengthening of upwelling suggested by some geologic al data, against the simulated reduction in strength of the general circula tion of the atmosphere predicted by the UKMO GCM. Furthermore, the well-doc umented uncertainty of the geological record concerning the extent of the E ast Antarctic ice sheet during the middle Pliocene makes the overall accura cy of the imposed PRISM? boundary conditions, and hence the predictions of the UKMO GCM, difficult to truly assess. (C) 2000 Elsevier Science B.V. All rights reserved.