Abstract
The West African Monsoon (WAM) involves the interaction of multi-scale processes ranging from planetary to cumulus scales, which makes it challenging for coarse resolution General Circulation Models to accurately simulate WAM. The present study evaluates the ability of the high-resolution (sim 25 km) Atmospheric General Circulation Model HiRAM to simulate the WAM and to analyze its future projections by the end of the 21st century. For the historical period, two AMIP-type simulations were conducted, one forced with observed SST from Hadley Center Sea Ice and Sea Surface Temperature dataset and the other forced with SST from the coarse resolution Earth System Model (ESM2M), which is the parent model of HiRAM, i.e. both models have the same dynamical core and similar physical parameterizations. The future projection, using the Representative Concentration Pathway 8.5 and SST from ESM2M is also conducted. A process-based evaluation is carried out to elucidate HiRAM’s ability to represent the key processes and multiscale dynamic features those define the WAM circulation. Compared to ESM2M, HiRAM better represents most of the key circulation elements at different scales, and thus more accurately represents the intensity and spatial distribution of the WAM rainfall. The position of the African easterly jet is considerably improved in HiRAM simulations, leading to the improved positioning of the WAM rainbelt and the two-cell structure of convection. The future projection of the WAM exhibits warming over the entire domain, decreasing precipitation over the southern Sahel, and increase of precipitation over the western Sahara.
Highlights
West Africa, the home for more than 300 million people, is one of the most vulnerable regions to global warming due to its high exposure and low adaptive capacity (Barros et al 2015)
Two different Atmospheric Model Intercomparison Project (AMIP)-type simulations have been conducted for the historical period, one forced with observed SSTs from the Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST) (Rayner et al 2003), and the other forced with SST from ESM2M, the Earth System Model (ESM) developed at Geophysical Fluid Dynamics Laboratory (GFDL), which uses GFDL’s Modular Ocean Model (MOM) (Dunne et al 2012, 2013)
West African Monsoon (WAM) is a complex system comprised of several multiscale processes ranging from planetary to cumulus scales (Hall and Peyrille 2006) and influenced by coastlines (Nicholson 2009) and circulations driven by orography (Sultan and Janicot 2003)
Summary
West Africa, the home for more than 300 million people, is one of the most vulnerable regions to global warming due to its high exposure and low adaptive capacity (Barros et al 2015). Studies suggest that with increasing horizontal resolution, GCMs are better able to explicitly capture mesoscale convective systems (e.g., Zhao et al 2009; Manganello et al 2012), reproduce diurnally forced circulations, represent orographically modulated rainfall (e.g., Boyle and Klein 2010; Lau and Ploshay 2009), and represent extreme precipitation events (Wehner et al 2014). Since they can resolve mesoscale processes, multiscale interactions are better represented in high-resolution GCMs (Gent et al 2010). The summary and a discussion of the results conclude the paper
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