West African Monsoon: current state and future projections in a high-resolution AGCM

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 (∼ 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 AMIPtype 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.

Impact of increased resolution on long-standing biases in HighResMIP-PRIMAVERA climate models

“…Despite some improvements, large biases remain at higher resolutions, especially in precipitation and cloud cover over the tropis and the upper-troposphere zonal wind at mid-latitudes, for which the benefit from resolution is rather modest. Our results are generally in line with previous modeling work suggesting improvements in biases due to increased resolution (e.g., Mertens et al, 2014;Harlaß et al, 2018;Monerie et al, 2020) or not at all depending on the model and region (e.g., Delworth et al, 2012;Menary et al, 2015;Raj et al, 2019;Bador et al, 2020). The emergence of a consistent warm bias in the Labrador Sea at a high resolution poses the question of what new other biases might appear as resolution increases and highlights the difficult task of removing all biases in models.…”

“…Increased model resolution can alleviate the double ITCZ bias, especially over the Atlantic when the eastern tropical warm bias is reduced (Seo et al, 2006;Delworth et al, 2012;Doi et al, 2012;Harlaß et al, 2018;Song and Zhang, 2020) and when orography or mesoscale systems are better resolved in models (de Souza Custodio et al, 2017;Vannière et al, 2019;Monerie et al, 2020). Strong biases in the ITCZ however still develop in high-resolution models (Gent et al, 2010;McClean et al, 2011;Raj et al, 2019), which might further be reduced through improved convective parametrizations (Zhang et al, 2019) or the use of atmospheric convection-permitting (i.e., storm-resolving) climate models (Klocke et al, 2017]).…”

A tale of two futures: contrasting scenarios of future precipitation for West Africa from an ensemble of regional climate models

“…West Africa, with a fast population growth and an economy reliant on rain-fed agriculture, is one of the regions most affected by climate variability (Sultan andGaetani 2016, Sylla et al 2018). The annual cycle of precipitation over the region is linked to the passage of the West African Monsoon (WAM), which produces annual rainfall up to around 2500 mm (Raj et al 2019). In the Sahel, in particular, the WAM accounts for about 80% of annual precipitation (Steinig et al 2018).…”

“…The mechanisms that control present and future rainfall variability have been investigated by many studies (e.g. Poan et al 2016, Akinsanola and Zhou 2019b, Raj et al 2019: in particular, the analysis of atmospheric moisture fluxes and, in particular, the thermodynamic and dynamic components of the moisture budget has proved particularly helpful in understanding future changes in precipitation at global and regional scale (e.g. Endo and Kitoh 2014, Pomposi et al 2015, Giannini et al 2018.…”

Impacts of Large‐Scale Sahara Solar Farms on Global Climate and Vegetation Cover

“…EC-Earth (version 3.3.1) is a European community ESM which integrates several component models (atmosphere, ocean, sea-ice, and dynamic vegetation) and thus is capable of simulating complex interactions between the atmosphere, the ocean, and the land biosphere. In addition to incorporating a full set of interacting components affecting large-scale climate, EC-Earth employs a relatively high atmospheric spatial resolution that can more accurately represent synoptic precipitation (Raj et al, 2019) and boundary layer conditions (e.g., better resolved topography) (Zheng & Eltahir, 1998), important for capturing key dynamic features of the Sahara regional climate system such as the Western African Monsoon (WAM) strength (Hourdin et al, 2010;Sylla et al, 2010;Xue et al, 2010). Past studies (Hazeleger et al, 2010(Hazeleger et al, , 2012Kageyama et al, 2017) provide an overview of the general performance of an earlier version of this model.…”