Abstract
The local influence of mountains upon large- and synoptic-scale low-level atmospheric circulations is investigated in this study. The sea-level pressure associated with low-frequency fluctuations exhibit phase propagation of monopolar structures around mountains in an anticyclonic sense, while the corresponding 500 mb height patterns are relatively stationary and evolve in a manner consistent with the concept of Rossby wave dispersion on a sphere. The sea-level pressure patterns in the high-pass filtered data exhibit characteristics of synoptic-scale baroclinic waves and are steered around mountain ranges in an anticyclonic sense, while the corresponding 500 mb height patterns propagate nearly parallel to the time-mean flow in the middle troposphere.
It is hypothesized that the phase propagation of lower tropospheric circulation patterns is a reflection of the conservation of potential vorticity in flows over variable terrain. Most of the observations presented in this study are interpreted as the evidence of topographic Rossby waves in the atmosphere. However, the features observed to the north of the Tibetan Plateau exhibit some characteristics of Kelvin waves induced by the blocking effects of the orography on the lower tropospheric flow.
Because of the strong stratification during wintertime, the steering effect of mountains upon atmospheric circulations is restricted to the lower troposphere. Lower tropospheric waveguides exist in the vicinity of the major mountain ranges in the Northern Hemisphere. These regions are located (i) along the eastern slopes of the Rockies (ii) along the west and north coasts of Greenland, (iii) along the eastern slopes of the mountain ranges in Mongolia and northern China, (iv) to the north and east of the Tibetan Plateau, and (v) to the north of the mountains in northern Iran and Afghanistan.
There appear to be only minor differences between the structure and evolution of cyclonic and anticyclonic circulation anomalies, even though the corresponding sequences of synoptic maps may appear quite different. Deviations of the static stability field associated with the anomalies from the climatological mean static stability field are relatively small in comparison to the mean static stability. These observations suggest that the behavior of these features is relatively linear.
