Simple solutions for the summer shallow atmospheric circulation over North Africa

Dalu, G.; Gaetani, Marco; Lavaysse, Christophe; Flamant, Cyrille; Evan, Amato T.; Baldi, Marina

doi:10.1002/qj.3246

Cited by 3 publications

(2 citation statements)

References 58 publications

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“…Ref. [36] analyzed this circulation structure and highlighted the strengthening of the AHL in summer and its impact in terms of a vertical zonal circulation structure associating mean lift over Arabia and mean subsidence over Libya. Therefore, after various tests, we choose, following the example of the SHL, an index that we conveniently call AHL, by the geographical average of the air temperature at 850 hPa over the domain 25˚E-35˚E; 18˚N-30˚N.…”

Section: Definition Of Shl and Ahl Indexesmentioning

confidence: 99%

Influence of Continental Atmospheric Forcing on the Decadal Variability of the West African Monsoon

Famien,

Djakouré,

Youan

et al. 2024

ACS

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The West African Monsoon (WAM) is characterized by strong decadal and multi-decadal variability and the impacts can be catastrophic for the local populations. One of the factors put forward to explain this variability involves the role of atmospheric dynamics, linked in particular to the Saharan Heat Low (SHL). This article addresses this question by comparing the sets of preindustrial control and historical simulation data from climate models carried out in the framework of the CMIP5 project and observations data over the 20 th century. Through multivariate statistical analyses, it was established that decadal modes of ocean variability and decadal variability of Saharan atmospheric dynamics significantly influence decadal variability of monsoon precipitation. These results also suggest the existence of external anthropogenic forcing, which is superimposed on the decadal natural variability inducing an intensification of the signal in the historical simulations compared to preindustrial control simulations. We have also shown that decadal rainfall variability in the Sahel, once the influence of oceanic modes has been eliminated, appears to be driven mainly by the activity of the Arabian Heat Low (AHL) in the central Sahel, and by the structure of the meridional temperature gradient over the inter-tropical Atlantic in the western Sahel.

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Section: Definition Of Shl and Ahl Indexesmentioning

confidence: 99%

Influence of Continental Atmospheric Forcing on the Decadal Variability of the West African Monsoon

Famien,

Djakouré,

Youan

et al. 2024

ACS

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“…Dalu et al [60] showed that the strength and position of SAC are tightly related to the SHL dynamics. The SHL drives a Walker-like subsiding to the west, which in turn reinforces the SAC.…”

Section: Shl and Sac Positions And Intensitymentioning

confidence: 99%

Cloud Cover over the Sahara during the Summer and Associated Circulation Features

et al. 2021

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Over the Sahara in summer, the activity of the Saharan thermal low pressure system (SHL), which is linked to the West-African monsoon dynamics and the mid-latitude circulation, is modulated by dust concentration and water-vapor transport. In this context, the role of clouds over western Sahara remains under-investigated. Using Meteosat-Second-Generation geostationary satellite data, for the first time the variability of cloud occurrence over Sahara by type in summer, at diurnal, daily and intra-seasonal time scales for the 2008–2014 period is documented. Using European Center for Medium-range Weather Forecasting (ECMWF) Reanalysis (ERA) Interim (ERAI) reanalysis, cloud cover occurrences are characterized in terms of regional circulation patterns and moisture balance. We show that, over West-Sahara and Hoggar, mid-top clouds are the most frequent cloud-type in summer. Their summit reaches between 500 hPa and 400 hPa and lies just above the top of the Saharan Atmospheric Boundary Layer (SABL). During the rest of the year, high-top clouds are the most frequent. The variations in the spatial distribution of mid-top cloud occurrence coincide with the seasonal displacement and strengthening of the SHL and, in the mid-troposphere, of the Saharan anticyclone. Mid-top clouds occur most frequently when, at large scale, mass and humidity converge in the lower SABL due to heating on an extensive surface, and diverge in the upper SABL. Their diurnal cycle, with minimal frequency around 10 UTC and maximum in the evening, is consistent with the diurnal development of the Saharan Convective-Boundary-Layer. The frequency of high cloud increases when anticyclonic circulations at mid-level and upper-level retreat to the southeast and upper-level trough from mid-latitudes can penetrate more southwards.

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A Simple Conceptual Model for the Heat Induced Circulation over Northern South America and Meso-America

Hernandez

Sánchez

2020

Atmosphere

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The physical description of the atmosphere’s general circulation over Northern South America and Meso-America deserves a more comprehensive explanation. This work presents the Pacific coast of Colombia as the rainiest place on Earth, with annual rainfall averaging 5000 to 13,000 mm, and record values as high as 13,159 mm for the location of Puerto López (77∘14′ W, 2∘50′ N). Using information from the ECMWF ERA-40 Atlas and ERA-Interim Reanalysis, we describe the existence of a concentrated diabatic heating source due to condensation and the main features of its related circulation over Northern South America and Meso-America. For simplicity, we used the analytical solution of the Phlips-Gill Model to diagnose the main flow patterns. Results show that the diabatic source over western Colombia generates equatorial trapped Rossby-Kelvin waves, which dominate the low-level circulation. A Kelvin wave explains the low-level easterly flows over the Tropical Atlantic Ocean, the Caribbean Sea, the Venezuelan-Colombian Llanos, and the Northern Amazon Basin. This circulation is analogous to a Walker cell. To the west, two cyclonic flows and strong westerly winds are present in Meso-America and the far eastern Pacific because planetary waves propagate there. A slight asymmetry in the equator’s diabatic heating location is responsible for the intense low-level pressure over Panama. The vertical velocity over the source area induces vortex tube stretching, and zonal mean flow excites a mixed wave and a northward flow.

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Simple solutions for the summer shallow atmospheric circulation over North Africa

Cited by 3 publications

References 58 publications

Influence of Continental Atmospheric Forcing on the Decadal Variability of the West African Monsoon

Influence of Continental Atmospheric Forcing on the Decadal Variability of the West African Monsoon

Cloud Cover over the Sahara during the Summer and Associated Circulation Features

A Simple Conceptual Model for the Heat Induced Circulation over Northern South America and Meso-America

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