On the evaluation of the representation of mid-latitude transients in the Southern Hemisphere by HadAM2B GCM and the impact of horizontal resolution

Abstract

An evaluation of the Hadley Centre atmospheric general circulation model, HadAM2b, is presented, focusing on the ability of the model to simulate Southern Hemisphere (SH) transient disturbances. An assessment is also made of the effect of changing resolution. Standard resolution (2.5º latitude by 3.75º longitude) and high resolution (0.833º latitude by 1.25º longitude) AMIP (Atmospheric Model Intercomparison Project) runs in which the models are forced with observed monthly varying sea surface temperature (SST) are compared with NCEP and ERA re-analysis data for the period 1979-1988. We focus on the model’s simulation of selected variables representative of the mean state and eddy properties which characterise synoptic-scale activity of the SH. Both models reproduce the basic observed patterns of the pressure field and upper level jetstream quite well. Overall, the changes due to increasing resolution do not represent an improvement in the simulated climate. The transient eddy activity is reasonably well simulated, though overestimated, relative to the NCEP re-analysis, at both resolutions. This overestimation is much less when compared with ERA. The magnitudes of the eddy kinetic energy, momentum fluxes and heat fluxes in the latitudes of the storm tracks increase with resolution, though some regional features are better simulated with the high-resolution model. Both models capture the structure and time evolution of synoptic-scale disturbances very well, though the amplitude of the disturbances is larger than in the analysis. Both models also capture the north-eastward propagation of the waves in the vicinity of the Andes, though the NW-SE tilt of the disturbances on the lee side are slightly overestimated, in agreement with larger eddy momentum flux. The orographic influence of the Andes mountains is better resolved in the high-resolution simulation for the waves emanating from the polar branch of the Pacific storm-track.