CONICET | Buscador de Institutos y Recursos Humanos

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 (2.5° latitude per 3.75° longitude) and high resolution (0.833 ° latitude per 1.125 ° 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 reanalysis data for the period 1979-1988. We focus in the models simulation of selected variables representative of the mean state and eddy properties which characterize synoptic-scale activity of the SH. Both models reproduce the basic observed patterns of the pressure fields 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 reanalysis, at both resolutions. This overestimation is much less when compared with ERA. The magnitudes of the eddy kinetic, 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 the synoptic-scale disturbances very well, though the amplitude of the disturbances is larger than in the reanalysis. 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 in the lee side of the are slightly overestimated, in agreement with larger eddy momentum flux. The orographic influence of the Andes mountains is better resolved in the high-resolution simulations for the waves emanating from the polar branch of the Pacific storm-track.

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