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Convection over Tropical Africa and the Atlantic Ocean during Northern Summer. Part II: Modulation by Easterly Waves

Jean Philippe DuvelLaboratoire de Météorologie Dynamique du C.N.R.S., Ecole Polytechnique, Palaiseau, France

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Abstract

Using Meteosat data and European Centre for Medium-range Weather Forecast (ECMWF) analyses, we examine easterly waves and their relation with the cloudiness over West Africa and the tropical Atlantic Ocean for three summers (June, July, August and September 1983-85). Spectral analysis of the low-level meridional wind in the 2.8–5.1 day band reveals maximum wave amplitude near the West African coast at 20°N. During August and September the wave amplitude is larger than during June and July and a secondary maximum appears around 7.5°N.

Composites of the mean structure of the wave and the associated cloud modulation reveal consistent relationship between observed cloudiness and ECMWF analyses. The phase of the wave modulation of the cloudiness is strongly dependent on the geographical location in response to changes in the mean climatological conditions. This phase varies however grossly in four large land and ocean regions, centered at 7.5° and 17.5°N, respectively, for which we derive vertical cross sections of the wave modulation of the atmospheric state and of the vertical distribution of clouds for the summer of 1985.

For land and oceanic regions around 7.5°N, the larger deep convective activity at and ahead of the wave trough is well related to the maximum low-level convergence and high-level (200 mb) divergence. At latitudes near 17.5°N over the Saharo-Sahelian region, the deep convection has a primary maximum ¼ wavelength east of the trough, and a secondary maximum cast of the ridge. At and ahead of the trough axis there is highly suppressed cloud condition over Saharo-Sahelian regions consistent with a strong shallow dry convection described by ECMWF analyses. For oceanic trade regions near 17.5°N, the cloudiness is maximum during the phase of maximum southerly wind. This study also shows that the wave modulation of analyzed temperature and moisture profiles is in reasonable agreement with previous results and the observed cloud modulation.

Abstract

Using Meteosat data and European Centre for Medium-range Weather Forecast (ECMWF) analyses, we examine easterly waves and their relation with the cloudiness over West Africa and the tropical Atlantic Ocean for three summers (June, July, August and September 1983-85). Spectral analysis of the low-level meridional wind in the 2.8–5.1 day band reveals maximum wave amplitude near the West African coast at 20°N. During August and September the wave amplitude is larger than during June and July and a secondary maximum appears around 7.5°N.

Composites of the mean structure of the wave and the associated cloud modulation reveal consistent relationship between observed cloudiness and ECMWF analyses. The phase of the wave modulation of the cloudiness is strongly dependent on the geographical location in response to changes in the mean climatological conditions. This phase varies however grossly in four large land and ocean regions, centered at 7.5° and 17.5°N, respectively, for which we derive vertical cross sections of the wave modulation of the atmospheric state and of the vertical distribution of clouds for the summer of 1985.

For land and oceanic regions around 7.5°N, the larger deep convective activity at and ahead of the wave trough is well related to the maximum low-level convergence and high-level (200 mb) divergence. At latitudes near 17.5°N over the Saharo-Sahelian region, the deep convection has a primary maximum ¼ wavelength east of the trough, and a secondary maximum cast of the ridge. At and ahead of the trough axis there is highly suppressed cloud condition over Saharo-Sahelian regions consistent with a strong shallow dry convection described by ECMWF analyses. For oceanic trade regions near 17.5°N, the cloudiness is maximum during the phase of maximum southerly wind. This study also shows that the wave modulation of analyzed temperature and moisture profiles is in reasonable agreement with previous results and the observed cloud modulation.

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