A Statistical Study of Ultra-Long Waves

Abstract

Zonal harmonic analyses were applied to 5-day mean heights, 25-day mean heights, departures from 25-day mean heights, depatures from the normal and tendency fields on the 500-mb level. It was found that the first and second harmonics for transient waves at 70° and 60°N generally move westward. The mean westward phase velocities are about a quarter of the wave-length per five days, i. e., about 20° and 10° longitude per day for the first and second harmonics respectively.
The westward movement was not pronounced at lower latitudes mainly due to the small amplitudeof transient waves. The frequency distributions of phase velocities of transient waves for the third harmonic are nearly uniform and the third harmonic does not reveal mean westward or eastward movements. The fourth harmonic moves eastward with a mean phase velocity of about 6° longitude per day at 70° and 60°N. On the contrary, the fourth harmonic at low latitudes has a tendency to move westward in summer.
The mean phase velocities of the first two harmonics are nearly the same for each season, and the amplitudes of transient waves are large in winter and small in summer.
As the present study is based on the data for 20 years or more, namely more than 1460 cases for each harmonic, the results obtained are statistically significant. Several examples of travelling waves were presented for the first four harmonics at 60°N.