Wintertime westward-traveling planetary-scale perturbations over the Euro-Atlantic region

The features of the wintertime westward-traveling planetary scale perturbat ions over the Euro-Atlantic region are examined through the use of space-ti me spectral analysis applied to the 500 hPa geopotential height field. The intention is to understand the nature of these phenomena and the performanc e of climate models. Data from both ECMWF re-analyses and a simulation from the ARPEGE general circulation model are used. Westward -traveling planeta ry scale transients are found over the region as local perturbations resemb ling Rossby normal modes, with a maximum power over the Eastern Atlantic. T he westward-traveling planetary scale transients north of 40 degreesN have periods larger than 20 days. South of this latitude, wave periods are shift ed to a band around 10 days, so that they can be related to subtropical tra nsient waves. The atmospheric model used, like other models which exhibit r easonable mean climatic properties, tend to have less overall intraseasonal variability than observed. Nevertheless, the model is able to capture most of the features of the westward-traveling low-frequency transients. The di fferences in basic state, partially produced by scale interactions, would l ead to the generation of westward-traveling waves in the model distinct fro m the observed. However, it is suggested that the improvement of the presen t model version with regard to previous model versions is due to a better s imulation of the time-mean state. The reasonable simulation of the synoptic -scale variability south of 50 degreesN, and thus of its barotropic forcing on the basic state, may also help to explain the realistic westward-travel ing transients in the model.