THE EFFECT OF MODEL BIAS ON THE EQUATORWARD PROPAGATION OF EXTRATROPICAL WAVES

The effect of model bias on the equatorward propagation of extratropic al waves in a GCM simulation is assessed within the context of simple wave-guiding principles. The modes representing these waves are identi fied through the use of empirical orthogonal function analysis perform ed on the 200-mb zonal wind filtered to retain variations between 6 an d 30 days. The temporal evolution and vertical structure of these mode s are examined through the use of time-lagged composite analysis. The differences between observed and simulated wave propagation is examine d in relationship to the theoretical wave-guiding properties associate d with the observed and simulated time-mean flow. The utility of simpl e wave-guiding theory for describing the observed and modeled wave pro pagation is assessed. The model bias in the time-mean flow is closely associated with the differences in the propagation of the transient wa ves. The excessively strong wave-guiding properties associated with th e simulated Pacific jet appear to inhibit the proper meridional propag ation of wave energy into the tropical central and eastern Pacific. In the simulation, waves that do propagate into the tropical Pacific eit her dissipate or are reflected near the equator, while in the observat ions, wave energy propagates into the Southern Hemisphere. On the othe r hand, the wave guiding by the subtropical and midlatitude jets over the Atlantic is weaker in the simulation than in the observations. In this region, wave energy propagates primarily into the Tropics in the simulation, while some of the observed wave energy is reflected toward the east and northeast over the Atlantic and northern Africa. The loc ations of the theoretical critical lines and wave guides of the time-m ean flow, although based on many simplifying assumptions, are remarkab ly consistent with the propagation characteristics of these waves.