Observations of large‐scale traveling Rossby waves

Considerable evidence for the existence of large‐scale (zonal wave numbers 1, 2, and 3) traveling Rossby waves, or waves of the second class, has been published. Analyses indicate that these observed waves disperse according to theory, the largest longitudinal and latitudinal scales moving fastest toward the west. Zonal wave numbers 1 and 2 in the geopotential or pressure fields nearly always propagate westward, wave number 3 propagates both eastward and westward, and wave number 4 nearly always propagates eastward. For zonal wave number 1 the largest latitudinal scale disturbances move around the earth in about 5 days. Smaller latitudinal scales range in period from 1 to 3 weeks. The most often reported period is near 16 days. For zonal wave number 2, most frequent periods reported are near 4 and 15 days. Vertical structures of the traveling large‐scale waves that have been studied so far resemble those of external waves with very small phase change with height and some indication of an amplitude increase with height. These large‐scale traveling waves may be important in furthering our understanding of time variations in the general circulation and in efforts to improve the skill of numerical weather prediction models.