SOME ASPECTS OF VARIANCE SPECTRA OF SYNOPTIC SCALE TROPOSPHERIC WIND COMPONENTS IN MIDLATITUDES AND IN THE TROPICS

Abstract

The primary concern of this paper is an examination of the utility of spectral techniques in detecting wave modes of the large-scale tropospheric circulation. Sample time- and space-spectra of meridional winds in middle latitudes are presented. A maximum in the spatial spectra of the transient waves at hemispheric wave numbers 5?7 is shown, in agreement with similar, previously published spectra. The temporal spectra, unlike the spatial spectra, fail to show any significant features that can be attributed to the passage of baroclinic disturbances. (In this paper, the frequency range considered is limited to 0.03?0.50 per day.) The continuum of the coherence statistic between lower and upper tropospheric wind components is highly significant, but no significant ?peaks? or maxima exist. Because of recent interest in the dynamics of tropical regions, spectral techniques are employed on meridional wind data for five tropical stations with record lengths of from 3 to 7 yr. At 850 mb, the two stations in the west-central Pacific possess spectra with a peak in the 4?5-day period range. Similar spectra for two stations in the Atlantic Ocean and one in Indonesia do not exhibit this feature, and at 200 mb none of the five spectra has any indication of a significant peak at any frequency. The coherence between the two levels is not significant at any frequency, with the possible exception of the 0.20?0.25 per day band at the Pacific stations. As in the case of the midlatitude spectra, the peak in the spatial spectra of the transient 200-mb meridional winds does not possess a counterpart in the temporal spectra. Finally, complex demodulation is used to investigate possible nonstationary aspects of the 4?5-day period wave mode for the Pacific stations. The results suggest nonstationary behavior with an annual modulation but of a sporadic nature. In addition, the demodulation procedure provides information on the questionable significance of the coherence between the lower and upper tropospheric tropical meridional winds.