Simple solutions for the summer shallow atmospheric circulation over North Africa

Abstract : The spring-to-summer shallow atmospheric circulation over North Africa is driven by two desert heat lows (DHLs), one over Sahara (SHL) and one over Arabia (AHL), with cyclones embedded in the convective layer, capped by mid-tropospheric anticyclones. The aim of this work is to produce a minimal theoretical framework for these dynamical features in terms of Kelvin, mixed and planetary mode solutions to a two-layer Matsuno-Gill model with a lower Rayleigh frictional layer dynamically coupled to an almost frictionless upper layer. Results show that a DHL drives a Walker-like cell with rising air over the low and subsiding air to its west, with the lower-level cyclone to the east of the mid-tropospheric anticyclone; the latter being bounded to the south-west by an unstable easterly jet. These features, related to frictional differences between the two layers, are enhanced by ambient easterly winds. In spring, the stronger SHL drives a cell with rising air over West Africa and subsiding over the Atlantic. The weaker AHL drives a cell with rising air over Arabia and subsiding over East Africa. In summer, the stronger AHL drives a cell with rising air over Arabia and subsiding over Libya, and the weaker SHL drives a cell with rising air over West Sahara and subsiding over the Atlantic. The concurrent contributions of easterly winds, monsoonal low level convergence, and seasonal airborne dust warming strengthen the SHL, making its winds stronger than those of the AHL. In addition, a third order expansion of the Coriolis parameter strengthens the zonal winds and weakens the meridional winds. These contributors strengthen anticyclones over the East Mediterranean and to the west of Gibraltar. The ability of the model in representing the main features of the North African atmospheric circulations suggests a potential for better understanding climate dynamics in connected regions (Mediterranean, Tropical Atlantic).
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Giovanni A. Dalu, Marco Gaetani, Christophe Lavaysse, Cyrille Flamant, Amato T. Evan, et al.. Simple solutions for the summer shallow atmospheric circulation over North Africa. Quarterly Journal of the Royal Meteorological Society, Wiley, 2018, 144 (712), pp.765-779. ⟨10.1002/qj.3246⟩. ⟨insu-01706761⟩

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