A Wavenumber-Frequency Analysis of Large-Scale Tropospheric Motions in the Extratropical Northern Hemisphere

Abstract

The geopotential height and temperature at 500 and 300 mb at various northern latitudes are analysed in wavenumber-frequency space, for a summer and a winter season. The power spectra at individual wave-numbers are flattish or peaked at low frequencies, but above a cutoff frequency (which increases with wavenumber from 0.03 to 0.3 cycles per day) they fall off sharply, with a ?3 to ?4 power law for the geopotential height at mid-latitudes. Temperature spectra are less steep than height spectra, and summer spectra less steep than winter spectra. The quadrature spectra show higher eastward phase speeds in the winter than in the summer. Westward-moving ultralong waves at mid-latitudes are visible in the quadrature spectra of height, but not of temperature. By means of quadrature spectral analysis, separate wavenumber spectra are computed for travelling and stationary waves. The stationary-wave spectra of the temperature fall almost monotonically with wave-number, and at mid-latitudes tend to a ?3 power law at high wavenumber. The travelling-wave spectra, on the other hand, are peaked and cut off more steeply at high wavenumbers. In the case of the geostrophic meridional wind, the stationary-wave spectra are peaked too, but at lower wavenumbers than the travelling-wave spectra. All the wavenumber spectra show a shift toward higher wavenumbers and a decrease in total variance in the summer, most pronounced at low latitudes.