Laboratory Simulation of Mechanical Effects of Mountains on the General Circulation of the Northern Hemisphere: Uniform Shear Background Flow

Abstract

The dimensionless governing equations and boundary conditions for the atmosphere are compared with those for a linearly stratified rotating dishpan laboratory experiment; by doing so a set of similarity criteria are determined. Model experiments on the effects of Greenland, the Rocky Mountains and Tibet on a uniform shear zonal flow in the Northern Hemisphere are presented. The laboratory model qualitatively simulates such semipermanent features of the Northern Hemisphere circulation as the Aleutian and Icelandic lows, the ridgesand troughs in the vicinity of the Rocky Mountains and Tibet and the shedding of the "southwest eddy" in the lee of Tibet. Although the background flow is steady, the large-scale disturbances caused by the mountains are unsteady and have an inherent periodicity equal to the time required for a fluid parcel to make a single 'circuit around the dishpan (globe). The strengths of the Aleutian and Icelandic lows, for example, oscillate with this Iiod; the lows are out of phase in the sense that when one is weak the other is strong and vice versa. A number of other correlations between various regions of the flow field are also noted. Removal of the model of Tibet does not greatly affect ihe qualitative nature of the general flow pattern. For example, the Aleutian and Icelandic lows remain as distinct entities, although their strengths and locations are altered somewhat, and the general character of the flow in the vicinity of the Rocky Mountains remains essentially unchanged. On the other hand, removal of the Rocky Mountains eliminates the Aleutian and Icelandic lows as separate entities. The joint effect ofthe Rocky Mountains and Tibet is to deflect the streamlines in the higherlatitudes toward the north, thus causing deeper Aleutian and Icelandic lows and locations of these features which are more similar to observations in the atmosphere than if either of these features is absent. The experiments show in a quite straightforward fashion the important effects mountains have on the formation of some of the semipermanent features of the Northern Hemisphere.