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Jon E. Ahlquist

Abstract

Three years of twice-daily NMC global operational analyses were projected onto normal mode Rossby waves to produce a climatology of these waves. For zonal wavenumbers. 1 through 4, annual average geopotential amplitudes at 50 kPa are about 5 gpm for the gravest symmetric meridional mode, and 10 and 20 gpm for the next two meridional model although the amplitude for a given time and latitude can greatly exceed the average. Seasonal average amplitudes differ by less than ±25% from the annual average. The modes’ frequencies drift during the course of a year, but this variation is not correlated with season.

Autocorrelations of Rossby wave time series become negligible for lags greater than approximately ten days, which is of the order of the wave period.

For all ten modes examined, geopotential fluctuations exist in both Northern and Southern Hemispheres.

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Jon E. Ahlquist

Abstract

This study addresses first the question of what normal-mode global Rossby waves might exist in the Earth's atmosphere. Then it identifies fourteen of these theoretically predicted waves in the NMC global tropospheric analyses.

Normal modes of linearized global primitive shallow water equations were found given a basic state of latitudinally dependent steady zonal flow. The solutions are free Rossby and gravity waves. Many of the waves' north-south structures are similar to Hough functions, which are the solutions of the simpler problem of free waves in an atmosphere at rest.

By projecting 1200 consecutive days of twice-daily NMC global tropospheric analyses of velocity and geopotential onto idealized three-dimensional, normal-mode Rossby wave structures, time series of wave amplitudes and phases were formed. Spectral analyses of these time series for zonal wavenumbers 1–4 revealed statistically significant peaks at eight out of 25 theoretical Rossby wave frequencies. Six additional woes may exist, but their significance could not be statistically supported because their spectral peaks fell into the red noise portion of the spectra. Periods for the fourteen waves lie between ∼2 and ∼30 days. Excluding the two weakest waves, average amplitudes at the surface range from 0.3 to 2 mb.

Prior to this study, only two normal-mode Rossby waves had been identified with confidence in the troposphere, a zonal wavenumber-1, 5-day wave and a zonal wavenumber-1, 16-day way. This study has identified up to ten more modes which have comparable amplitudes.

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