Abstract
The West African monsoon intraseasonal variability has huge socio-economic impacts on local populations but understanding and predicting it still remains a challenge for the weather prediction and climate scientific community. This paper analyses an ensemble of simulations from six regional climate models (RCMs) taking part in the coordinated regional downscaling experiment, the ECMWF ERA-Interim reanalysis (ERAI) and three satellite-based and observationally-constrained daily precipitation datasets, to assess the performance of the RCMs with regard to the intraseasonal variability. A joint analysis of seasonal-mean precipitation and the total column water vapor (also called precipitable water—PW) suggests the existence of important links at different timescales between these two variables over the Sahel and highlights the relevance of using PW to follow the monsoon seasonal cycle. RCMs that fail to represent the seasonal-mean position and amplitude of the meridional gradient of PW show the largest discrepancies with respect to seasonal-mean observed precipitation. For both ERAI and RCMs, spectral decompositions of daily PW as well as rainfall show an overestimation of low-frequency activity (at timescales longer than 10 days) at the expense of the synoptic (timescales shorter than 10 days) activity. Consequently, the effects of the African Easterly Waves and the associated mesoscale convective systems are substantially underestimated, especially over continental regions. Finally, the study investigates the skill of the models with respect to hydro-climatic indices related to the occurrence, intensity and frequency of precipitation events at the intraseasonal scale. Although most of these indices are generally better reproduced with RCMs than reanalysis products, this study indicates that RCMs still need to be improved (especially with respect to their subgrid-scale parameterization schemes) to be able to reproduce the intraseasonal variance spectrum adequately.
Highlights
More than three hundred million people live in West Africa and this population is expected to double in the two decades
Even though this study showed the high potential of RegCM3 to handle very subtle interactions between the African Easterly Waves (AEWs) and the African Easterly Jet (AEJ), it was quite limited in terms of quantitative evidence, such as the extent to which this synoptic-scale activity determines the monsoon precipitation
Focusing on the climate of the recent past (1989–2008), this paper aims to assess the ability of the regional climate models (RCMs) to represent the West African monsoon (WAM) intraseasonal variability spectrum in comparison with observations
Summary
More than three hundred million people live in West Africa and this population is expected to double in the two decades. The economy of West Africa is mainly based on the rain-fed agricultural and pastoral sectors, so West Africans and, the people of the Sahel, are strongly dependent on the four-month rainfall season that generally occurs between June and September. This dependence seems to have been exacerbated recently, probably due, at least partly, to climate variability and change: local surveys and reports indicate concerns about a later monsoon onset, increasingly severe dry spells and flash floods, destruction of biodiversity, etc. Three main intraseasonal scales of variability have been studied: (1) the 30–90-day scale, which is an MJO- (Madden and Julian Oscillation; Madden and Julian 1971) like activity (Janicot et al 2009), (2) the 10–25-day scale (Sultan et al 2003) involving atmosphere–ocean interactions, and (3) the 1–10day scale, which is driven by meteorological events occurring at synoptic scale to mesoscale, such as the African Easterly Waves (AEWs; Kiladis et al 2006) and Mesoscale Convective Systems (MCSs; Mathon et al 2002), respectively
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