EVIDENCE OF TRAVELING EXTERNAL ROSSBY WAVES IN THE ECMWF ANALYSES

Ten years of global tropospheric data from the European Centre for Med ium-Range Weather Forecasts analyses were used to obtain a climatology of quasi-stationary waves and transient normal-mode Rossby waves. The data were split up into a mean annual cycle, reflecting the forced fi elds, and a transient part, containing the traveling waves. Data were then projected onto Hough normal modes, yielding a mean annual behavio r of the quasi-stationary fields and time series of expansion coeffici ents for the transient waves. The latter were analyzed by a space-time spectral method independently for each of the four seasons. The Hough normal modes with low zonal wavenumber and low meridional index show clear peaks in the power spectra at theoretically predicted frequencie s. Some modes have a strong seasonality.