Intraseasonal Variability of the Saharan Heat Low and Its Link with Midlatitudes

Fabrice Chauvin Centre National de Recherche Météorologique, Météo-France, Toulouse, France

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Romain Roehrig Centre National de Recherche Météorologique, Météo-France, Toulouse, France

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Jean-Philippe Lafore Centre National de Recherche Météorologique, Météo-France, Toulouse, France

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Abstract

The Saharan heat low (SHL) is thought to be a key feature of the West African monsoon, and its variations during the summer season have not yet been systematically assessed. To characterize the intraseasonal variations of the SHL, real and complex empirical orthogonal function analyses were applied to the 850-hPa potential temperature field over northern Africa and the Mediterranean, using NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) reanalysis results. A robust quasi-propagative mode was highlighted over North Africa and the Mediterranean. This mode consists of two phases. The west phase corresponds to a maximum temperature over the coast of Morocco–Mauritania, propagating southwestward, and a minimum between Libya and Sicily, propagating southeastward. The east phase corresponds to the opposite temperature structure, which propagates as in the west phase. A lag-composite analysis revealed that this SHL mode was preceded by large-scale, midlatitude, intraseasonal fluctuations of the atmosphere. The southward penetration of a Rossby wave disturbance over Europe and North Africa generates modulations of the three-dimensional atmospheric structure. The low-level ventilations and harmattan-like circulation are particularly impacted, as are the subtropical westerlies and the polar jets in the upper troposphere. The west phase is concomitant with an enhanced convective signal over the Darfur region, which propagates westward, as far as the middle of the Atlantic, at a speed similar to that of the well-known African easterly waves.

The SHL appears to be a bridge between the midlatitudes and the West African monsoon, which may offer promising sources of predictability over the Sahel on an intraseasonal time scale.

Corresponding author address: Fabrice Chauvin, CNRM-GAME, URA 1357, Météo-France, 42 Ave. G. Coriolis, 31057 Toulouse CEDEX 01, France. Email: fabrice.chauvin@meteo.fr

Abstract

The Saharan heat low (SHL) is thought to be a key feature of the West African monsoon, and its variations during the summer season have not yet been systematically assessed. To characterize the intraseasonal variations of the SHL, real and complex empirical orthogonal function analyses were applied to the 850-hPa potential temperature field over northern Africa and the Mediterranean, using NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) reanalysis results. A robust quasi-propagative mode was highlighted over North Africa and the Mediterranean. This mode consists of two phases. The west phase corresponds to a maximum temperature over the coast of Morocco–Mauritania, propagating southwestward, and a minimum between Libya and Sicily, propagating southeastward. The east phase corresponds to the opposite temperature structure, which propagates as in the west phase. A lag-composite analysis revealed that this SHL mode was preceded by large-scale, midlatitude, intraseasonal fluctuations of the atmosphere. The southward penetration of a Rossby wave disturbance over Europe and North Africa generates modulations of the three-dimensional atmospheric structure. The low-level ventilations and harmattan-like circulation are particularly impacted, as are the subtropical westerlies and the polar jets in the upper troposphere. The west phase is concomitant with an enhanced convective signal over the Darfur region, which propagates westward, as far as the middle of the Atlantic, at a speed similar to that of the well-known African easterly waves.

The SHL appears to be a bridge between the midlatitudes and the West African monsoon, which may offer promising sources of predictability over the Sahel on an intraseasonal time scale.

Corresponding author address: Fabrice Chauvin, CNRM-GAME, URA 1357, Météo-France, 42 Ave. G. Coriolis, 31057 Toulouse CEDEX 01, France. Email: fabrice.chauvin@meteo.fr

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