Continental-Scale Circulation in the UKHI GCM

Abstract

The ability of the U. K. Meteorological Office High Resolution General Circulation Model (UKHI GCM) to simulate atmospheric circulation of continental scales, and the influence on circulation of doubled concentrations of greenhouse gases simulated by the same model, are investigated. The 500-hPa geopotential heights are examined over the European and North American continental-scale domains. Two aspects of circulation are considered: circulation types and modes of variability. The principal component analysis is employed as the analysis technique. The UKHI GCM is shown to have several characteristic deficiencies in simulating continental-scale circulation, which are consistent with the analyses of circulation in other models. 1) The simulated jets are too strong and shifted southward. 2) The simulated meridional circulation is too weak. 3) The simulated circulation types are more persistent than the actual. 4) The model reproduces low-frequency (teleconnection) patterns only to a limited extent. The synoptic-frequency patterns are simulated better than the low-frequency ones. There is no evidence of the UKHI GCM being more noisy than the observed. Doubling of CO2 concentrations leads to the zonalization of circulation, whereas the synoptic-frequency variability does not seem to be considerably affected. Since the climate change response of the 500-hPa height field (i.e., the difference between the 2 ? CO2 and control runs) is approximately of the same magnitude as the model error (the difference between the control run and actual circulation), the changes in circulation due to the enhanced greenhouse effect as simulated by the model should be interpreted with great caution.