Orographic Influence of the Rocky Mountains on the Winter Circulation over the Contiguous United States

Part II. Synoptic-scale (Short-period) Disturbances

Abstract

Some of the characteristic features of synoptic-scale disturbances over and around the Rocky Mountains were investigated by using 3.5-7.0 day filtered u, υ, T and Φ data at eight levels (the surface, . 850, 700, 500, 300, 200, 100, and 50mb) for the 1978-79 winter (1 December 1978 to 28 February 1979). Composites of each meteorological variable at every level and grid point were made with respect to changes in the first eigenvector (empirical orthogonal function) coefficients for 3.5-7.0 day filtered meridional wind data at 300mb.
The composite charts of 300mb winds and geopotential heights reveal that troughs and ridges in this region propagate eastward systematically, with phase speeds of approximately 7(12)ms-1 to the west (east) of the Rocky Mountains. These high mountains exert a strong orographic influence on the movement of low-level disturbances. For example, when anticyclonic (cylonic) cells approach the California coast, low-level northerlies (southerlies) flow down (up) the southern slopes of the American Rockies and enhance the local, low-level divergence (convergence) field. Consequently, the anticyclonic (cyclonic) perturbations tend to move along the southern (not northern) periphery of the American Rockies, where they become most intense. However, the intensity of these low-level disturbances decreases sharply as the enhanced divergence (convergence) field weakens away from the high mountains.
At 850mb, strong cyclo- and anticyclogenesis occurs near the eastern periphery of the Rockies. The orographically induced, small-scale, anticyclonic (cyclonic) cells then propagate southward and merge with major anticyclonic (cyclonic) systems at the southeastern border of the American Rockies. When this merging occurs, anomalous low-level, northerly (southerly) surges originate from cold central Canada (the warm Gulf of Mexico) and flow across Texas. Pronounced northward sensible heat fluxes over the central U.S. are associated with this surge activity and indicate that there is a large increase in eddy available potential energy in the area. This region is also characterized by a strong upward geopotential flux into the stratosphere. In contrast, the vertical geopotential flux over the Rockies is downward and contributes to the maintenance of low-level, eddy kinetic energy against frictional dissipation over these mountainous regions.