, Volume 38, Issue 1-2, pp 22-33

The structure and characteristics of African easterly wave disturbances as determined from the ECMWF operational analysis/forecast system

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Gridded data from the ECMWF archives are analyzed spectrally in order to determine the structure and characteristics of the African wave disturbances observed during August–September 1985. Major findings are:

  1. The most prominent peaks in the 850 mb meridional wind spectra occurred at periods of 3–5 days. Co-spectrum analysis revealed that the 3–5-day oscillations were produced by waves of approximately 2500 km wavelength that travelled westward at speeds of about 8 m s−1.

  2. Patterns of vorticity variance in the 3–5-day frequency band indicated that the waves originated primarily in two regions, one centered near 10°E and 22°N and the other centered near 10°E and 12°N, and that the storm tracks emanating from these regions merged into a single track, located between 15° and 20°N, over the Atlantic.

  3. The energetics of the waves, inferred from covariance patterns of temperature, vertical velocity and meridional and zonal winds, indicated that the waves formed as a result of the joint baroclinic/barotropic instability of the African Easterly Jet. There was evidence that wave-CISK enhanced the growth of the waves that followed the southerly storm track.

The findings are in good agreement with those found in previous studies. It is concluded that the ECMWF operational analysis/forecast system gives a realistic portrayal of African easterly waves.

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