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2. Observed downward propagation of zonal wind variations

Daily ECMWF Re-Analyses (ERA-15) from October 1988 to April 1989 (horizontal resolution 2.5Ú with pressure up to 10 hPa) are used to investigate the vertical propagation of variations of the zonal wind averaged over the polar cap north of 60Ú N.

Figure 1: The low-pass (91-day running mean) filtered u_pcm time series and second order polynomial fits for the winter (DJF) of 1988-89 at different levels. t_fitmax is the time at which the polynomial maximizes. The difference between the values of t_fitmax at different levels is a measure for the time lag for variations in u_pcm between these levels.

Figure 2: Correlation coefficient between the low-pass filtered u_pcm time series at 10 hPa and the low-pass filtered time series at all levels, with different time lags.

Figure 1 and 2 show that ultra-low frequency variations (described by the polynomial fits) of u_pcm propagate downward from 10 hPa to 200 hPa in about 20 days and then to the surface in about 5 days.