Upper tropospheric equatorial waves in ECMWF analyses

Abstract

Tropical wind fields from the European Centre for Medium-range Weather Forecasts operational analyses are analysed to search for equatorially trapped waves in the upper troposphere. Data are studied for 1980-87. Zonal wind spectra show a predominance of power at low zonal wave numbers and low frequencies; these data do not show distinct spectral peaks corresponding to eastward-propagating Kelvin waves or westward-propagating equatorially-trapped Rossby waves. the zonal wind spectra do show episodic evidence for westward-propagating 5-day zonal wave-1 features with maxima over the equator; these are likely related to Rossby normal-mode oscillations previously reported. Meridional wind spectra show a predominance of westward-propagating power in the 6- to 10-day-period range, primarily concentrated in zonal waves 4-7. Enhanced meridional wind activity at the equator occurs episodically in time, with events persisting for one to several months duration. Wave packets are longitudinally localized, with preferred occurrence over the eastern Pacific Ocean. Most cases show evidence of meridional wind maxima in the tropics, suggesting a degree of equatorial trapping. the horizontal structure of these oscillations is that of mixed Rossby-gravity waves, although they are not necessarily centred directly over the equator. These waves have largest amplitudes in the upper troposphere, showing weak coherence with the surface; they appear to be fundamentally distinct from fast 4- to 5-day Rossby-gravity waves observed in the western Pacific. A detailed study of a group of intense Rossby-gravity waves observed over the eastern Pacific during August-September 1985 is presented; the reality of these features is confirmed in direct rawinsonde observations. Correlations are found between bursts of equatorial wave activity and enhanced meridional momentum fluxes from the subtropics, suggesting extratropical forcing as at least one mechanism for their generation. the waves are also strongly coupled to the upper tropospheric zonal wind field, which likely accounts for their longitudinal and temporal confinement.