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Characteristics of North African 6–9 Day Waves during Summer 1981

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  • 1 Laboratoire de Météorologie Dynamique du CNRS, Ecole Polytechnique, Palaiseau, France
  • | 2 Laboratorie de Météorologie, Université P. et M. Curie, Paris, France
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Abstract

This study applies compositing methods to ECMWF model analyses during July and August 1981 to determine the structure of synoptic scale waves with periods of 6–9 days. Over northern Africa, these ware intermittent in time, are most regular in period and amplitude west of the Greenwich meridian in a narrow longitude band, though there is a suggestion of their existence between 15°E and 0°. The average wavelength is 5000 km and the westward velocity is 6°–7° longitude day−1. The horizontal wind pattern in midtroposphere (700 and 500 hPa) consists of two vortices of opposite signs on either side of 12.5°N, with maximum perturbation of the zonal wind component at this latitude, and maximum perturbation of the meridional wind component at 5°–7.5°N and at 17.5°–20°N. Temperature is modulated by the passage of the waves throughout the troposphere. This structure is very different from that of the 3‐4 day African wave.

Abstract

This study applies compositing methods to ECMWF model analyses during July and August 1981 to determine the structure of synoptic scale waves with periods of 6–9 days. Over northern Africa, these ware intermittent in time, are most regular in period and amplitude west of the Greenwich meridian in a narrow longitude band, though there is a suggestion of their existence between 15°E and 0°. The average wavelength is 5000 km and the westward velocity is 6°–7° longitude day−1. The horizontal wind pattern in midtroposphere (700 and 500 hPa) consists of two vortices of opposite signs on either side of 12.5°N, with maximum perturbation of the zonal wind component at this latitude, and maximum perturbation of the meridional wind component at 5°–7.5°N and at 17.5°–20°N. Temperature is modulated by the passage of the waves throughout the troposphere. This structure is very different from that of the 3‐4 day African wave.

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