The Effect of Model Bias on the Equatorward Propagation of Extratropical Waves

Abstract

The effect of model bias on the equatorward propagation of extratropical waves in a GCM simulation is assessed within the context of simple wave-guiding principles. The modes representing these waves are identified through the use of empirical orthogonal function analysis performed on the 200-mb zonal wind filtered to retain variations between 6 and 30 days. The temporal evolution and vertical structure of these modes are examined through the use of time-lagged composite analysis. The differences between observed and simulated wave propagation is examined in relationship to the theoretical wave-guiding properties associated with the observed and simulated time-mean flow. The utility of simple wave-guiding theory for describing the observed and modeled wave propagation is assessed. The model bias in the time-mean flow is closely associated with the differences in the propagation of the transient waves. The excessively strong wave-guiding properties associated with the simulated Pacific jet appear to inhibit the proper meridional propagation of wave energy into the tropical central and eastern Pacific. In the simulation, waves that do propagate into the tropical Pacific either dissipate or are reflected near the equator, while in the observations, wave energy propagates into the Southern Hemisphere. On the other 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 locations of the theoretical critical lines and wave guides of the time-mean flow, although based on many simplifying assumptions, are remarkably consistent with the propagation characteristics of these waves.