| description abstract | This study presents observational results concerning the structure and evolution of high-frequency fluctuations with periods shorter than 6 days in the geopotential height field. The results are based on a statistical analysis of 1000, 500 and 250 mb height fields derived from twice-daily NMC final analyses over the Northern Hemisphere for the 20 winters 1964/65 through 1983/84. The disturbances assume the form of waves, elongated in the meridional direction, with a mean wavelength of 4000 km, a westward tilt with height, and a mean eastward phase propagation of 12?1 5 m s?1. These results support an interpretation in terms of finite-amplitude baroclinic waves whose structure and evolution varies with geographical location. Over the continents, and especially along the eastern slopes of the Rockies and the Tibetan Plateau, the waves show marked departures in structure and evolution from those over the oceans. In particular, we find evidence of a systematic influence of the terrain in steering the disturbances at the 1000 mb level relative to those in the middle and upper troposphere. The disturbances propagate along distinct, zonally oriented waveguides. It is argued that the major baroclinic waveguides of the Northern Hemisphere are associated with maxima in the variance of the highpass-filtered geopotential streamfunction and in the ?teleconnectivity? (a measure of the extent to which the high-frequency fluctuations are wavelike), and that they are oriented parallel to the phase propagation vectors. With only minor exceptions, the waveguides defined on the basis of these three different criteria are mutually consistent and they agree with results presented by Blackmon et a]. Paths of positive and negative 1000 mb height anomalies propagating through the baroclinic waveguides are compared and found to be very similar. However, the paths of the corresponding cyclones and anticyclones on conventional synoptic charts exhibit a sharply contrasting behavior, the former being oriented from southwest to northeast and the latter from northwest to southeast, in agreement with synoptic experience. Differences in the orientation of cyclone tracks, anticyclone tracks and baroclinic waveguides are explained on kinematic grounds, as a reflection of the changes in the climatological mean basic state that the waves encounter as they propagate eastward through the baroclinic waveguides from the eastern continents toward the midoceans. The observed properties of the high-frequency fluctuations appear to be relatively insensitive to the exact form of the response function of the highpass filter, provided that disturbances with periods ranging from 2.5?6.0 days are retained. A 24-hour (two timestep) difference filter appears to be capable of isolating the high-frequency fluctuations for real-time diagnostics. | |
