West African monsoon decadal variability and surface-related forcings: second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II)

Abstract

in the Sahel climate system at seasonal to decadal scales. The project's strategy is to apply prescribed observationally based anomaly forcing, i.e., "idealized but realistic" forcing, in simulations by climate models. The goal is to assess these forcings' effects in producing/amplifying seasonal and decadal climate variability in the Sahel between the 1950s and the 1980s, which is selected to characterize the great drought period of the last century. This is the first multi-model experiment specifically designed to simultaneously evaluate such relative contributions. The WAMME II models have consistently demonstrated that SST forcing is a major contributor to the twentieth century Sahel drought. Under the influence of the maximum possible SST forcing, the ensemble mean of WAMME II models can produce up to 60 % of the precipitation difference during the period. The present paper also addresses the role of SSTs Abstract The second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II) is designed to improve understanding of the possible roles and feedbacks of sea surface temperature (SST), land use land cover change (LULCC), and aerosols forcings This paper is a contribution to the special issue on West African climate decadal variability and its modeling, consisting of papers from the West African Monsoon Modeling and Evaluation (WAMME) and the African Multidisciplinary Monsoon Analyses (AMMA) projects, and coordinated by Yongkang Xue, Serge Janicot, and William Lau. Provided Funding information has to be tagged. Electronic supplementary materialThe online version of this article (doi:10.1007/s00382-016-3224-2) contains supplementary material, which is available to authorized users. 3in triggering and maintaining the Sahel drought. In this regard, the consensus of WAMME II models is that both Indian and Pacific Ocean SSTs greatly contributed to the drought, with the former producing an anomalous displacement of the Intertropical Convergence Zone before the WAM onset, and the latter mainly contributes to the summer WAM drought. The WAMME II models also show that the impact of LULCC forcing on the Sahel climate system is weaker than that of SST forcing, but still of first order magnitude. According to the results, under LULCC forcing the ensemble mean of WAMME II models can produces about 40 % of the precipitation difference between the 1980s and the 1950s. The role of land surface processes in responding to and amplifying the drought is also identified. The results suggest that catastrophic consequences are likely to occur in the regional Sahel climate when SST anomalies in individual ocean basins and in land conditions combine synergistically to favor drought.Keywords Sahel seasonal and decadal climate variability · Sahel drought, SST and land forcings · GCM

Intraseasonal variability of low-level moisture transport over West Africa

“…Over Africa, more emphasis frequently is placed on model forecasts because of the limitations imposed by the lack of observations, especially radiosonde observations. However, despite the progress in the WA modeling, there are still some errors in the simulated WAM precipitation events (e.g., Xue 1997;Giannini et al 2003;Hoerling et al 2006Hoerling et al , 2010Lau et al 2006;Yoshioka et al 2007;Caminade and Terray 2010;Agustí-Panareda et al 2010;Xue et al 2010;Meynadier et al 2010b;Bock et al 2011;Martin and Thorncroft 2014;Xue et al 2016). Moreover, biases in both the observations and the models are known to introduce spurious variability and trends into the NCEP-NCAR reanalysis (e.g., Bock and Nuret 2009;Agustí-Panareda et al 2010).…”

Land-Surface Characteristics and Climate in West Africa: Models’ Biases and Impacts of Historical Anthropogenically-Induced Deforestation

“…Such discrepancies have been highlighted by the West African Monsoon Modeling and Evaluation (WAMME) project which inter-compared such models to address issues regarding the role of ocean-land-aerosol-atmosphere interactions on WAM dynamics [41]. The recent phase II of WAMME project provides a basic and better understanding of LULCC forcing on the regional climate of West Africa [42,43]. The employed strategy consists in applying observational data-based anomaly forcing, i.e., "idealized" anomaly in GCM and RCM simulations.…”

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

“…These changes are the results of a combination of both natural and anthropogenic forcings. Several studies have attempted to explain the Sahelian severe droughts using variety of approaches: (i) variability of sea surface temperature (SST; Folland et al 1986;Giannini et al 2003;Paeth and Hense 2004;Pomposi et al 2016;Adeniyi 2017); (ii) land surfaceatmosphere interactions (Charney et al 1977;Wang and Eltahir 2000;Koster et al 2004;Xue et al 2016;Wang et al 2016); and (iii) large-scale atmospheric teleconnections related to the El Niño or North Atlantic Oscillation (NAO) (Bader and Latif 2003;Joly and Voldoire 2009). The West African climate is dominated by the West African monsoon (WAM) system, and monsoon variability explains most of the total annual precipitation fluctuations.…”