Dust induced changes on the West African summer monsoon features

Abstract

Dust generation and transportation from North Africa are thought to modulate the West African Monsoon (WAM) features. In this study, we investigated the relationship between the Saharan Air Layer located above Atlantic Ocean (OSAL) and WAM features, including Monsoon flow, African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ) over West Africa using the RegCM4 regional model at 30 km grid resolution. Two sets of experiments with and without dust load were performed between 2007 and 2013 over the simulation domain, encompassing the whole of West Africa and a large part of the adjacent Atlantic Ocean. An intercomparison of the two simulations shows that dust load into the atmosphere greatly influences both the wind and temperature structure at different levels, resulting in the observed changes in the main features of the WAM system during summer. These changes lead to a westward shift with a slight strengthening of AEJ core over tropical Atlantic and weakening of both TEJ and monsoon flux penetration over land. In addition, despite running the RegCM4 with prescribed sea surface temperature, a correlation has been found between Aerosol Optical Depths in OSAL and WAM dynamics suggesting a mechanistic link between dust and WAM well reproduced by RegCM4.

Effect of the African greenbelt position on West African summer climate: a regional climate modeling study

“…It is clear that the performance of the RegCM4 appears in line with previous work on West Africa using either the RegCM4 with different configurations (Sylla et al 2015;Saini et al 2015;Adeniyi 2017;Diallo et al 2016;Kebe et al 2016;N'Datchoh et al 2018) or other regional climate models (Diallo et al 2012Sylla et al 2013;Hagos et al 2014;Gbobaniyi et al 2014;Druyan and Fulakeza 2016;Klutse et al 2016). In addition, the RegCM4 simulates accurately the magnitudes/locations of the main WAM circulation features associated with the summer precipitation (not shown).…”

“…The first year (2000) was considered as a spin-up of the land surface fields and excluded from the analysis. The chosen domain is large enough to represent the main WAM features and allow for interactions between the WAM and extra-tropical systems as well as to reduce initial and lateral boundary conditions (LBCs) problems N'Datchoh et al 2018). Note that the meteorological initial and 6-hourly LBCs necessary to force RegCM4 were taken for all experiments from the 1.5°× 1.5°longitude/ latitude ERA-Interim gridded reanalysis (Dee et al 2011), corresponding to the third generation of European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis product.…”

“…A list of the physical options available in RegCM4 is provided in Giorgi et al (2012). Based on previous studies over different African sub-regions (e.g., Mariotti et al 2014;Bamba et al 2015;Li et al 2015;Diallo et al 2015Diallo et al , 2016Diallo et al , 2018N'Datchoh et al 2018;Tall et al 2017) as well as a series of preliminary experiments, the following options were selected in this study: the radiation scheme from the NCAR Community Climate Model version 3 (CCM3; Kiehl et al 1996), the nonlocal vertical diffusion scheme of Holstag et al (1990) to represent the boundary layer processes, the sub-grid explicit moisture (SUBEX) resolvable scale precipitation scheme of Pal et al (2000), the ocean flux scheme of Zeng et al (1998), and the cumulus convection scheme of Grell et al (1994) with the Fritch-Chappell closure assumption (FC; Fritsch and Chappell 1980) to represent the convective precipitation.…”

“…The recent version of the regional climate model version 4.4 (RegCM4.4; hereafter referred to as RegCM4) N'Datchoh et al 2012N'Datchoh et al , 2018Diallo et al 2016Diallo et al , 2018Diasso and Abiodun 2017) Grell et al 1994). RegCM4 is an improved version of RegCM3 (Pal et al 2007) with multiples upgrades in the model physics parameterizations.…”

“…The remote forcings are mainly from different ocean basins as global SST anomalies have been shown to influence West African climate variability (Giannini et al 2003;Pomposi et al 2016). The impacts of aerosols especially dusts from the Saharan and Sahelian regions impact on West African climate variability have been highlighted (Prospero and Lamb 2003;Konaré et al 2008;N'Datchoh et al 2012N'Datchoh et al , 2018Marcella and Eltahir 2014). Continental and land surface forcing are considered as local forcing impacting the West African climate through rapid biophysical processes of energy budget, water cycle, carbon, and momentum exchange and the long-term interactions which arise from fundamental changes in vegetation cover (Zeng et al 1999;Hagos et al 2014;Boone et al 2016).…”