Abstract
General circulation models still show deficiencies in simulating the basic features of the West African Monsoon at intraseasonal, seasonal and interannual timescales. It is however, difficult to disentangle the remote versus regional factors that contribute to such deficiencies, and to diagnose their possible consequences for the simulation of the global atmospheric variability. The aim of the present study is to address these questions using the so-called grid point nudging technique, where prognostic atmospheric fields are relaxed either inside or outside the West African Monsoon region toward the ERA40 reanalysis. This regional or quasi-global nudging is tested in ensembles of boreal summer simulations. The impact is evaluated first on the model climatology, then on intraseasonal timescales with an emphasis on North Atlantic/Europe weather regimes, and finally on interannual timescales. Results show that systematic biases in the model climatology over West Africa are mostly of regional origin and have a limited impact outside the domain. A clear impact is found however on the eddy component of the extratropical circulation, in particular over the North Atlantic/European sector. At intraseasonal timescale, the main regional biases also resist to the quasi-global nudging though their magnitude is reduced. Conversely, nudging the model over West Africa exerts a strong impact on the frequency of the two North Atlantic weather regimes that favor the occurrence of heat waves over Europe. Significant impacts are also found at interannual timescale. Not surprisingly, the quasi-global nudging allows the model to capture the variability of large-scale dynamical monsoon indices, but exerts a weaker control on rainfall variability suggesting the additional contribution of regional processes. Conversely, nudging the model toward West Africa suppresses the spurious ENSO teleconnection that is simulated over Europe in the control experiment, thereby emphasizing the relevance of a realistic West African monsoon simulation for seasonal prediction in the extratropics. Further experiments will be devoted to case studies aiming at a better understanding of regional processes governing the monsoon variability and of the possible monsoon teleconnections, especially over Europe.
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Acknowledgments
This study has been mainly supported by the French National Research Agency in the framework of the IRCAAM (Influence Réciproque des Climats d’Afrique de l’ouest, du sud de l’Asie et du bassin Méditerranéen, http://www.cnrm.meteo.fr/ircaam/) project. Thanks are also due to the AMMA (African Monsoon Multidisciplinary analysis, http://www.amma-international.org) project funded by the European commission sixth framework program. The authors are very grateful to Michel Déqué, Virginie Lorant and Christophe Cassou for their support in using the Arpege-Climat model and the tools for weather regime analysis. The authors are also grateful to the two reviewers for their thoughtful and constructive reviews, which helped improving the manuscript.
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This paper is a contribution to the special issue on West African Climate, consisting of papers from the African Multidisciplinary Monsoon Analysis (AMMA) and West African Monsoon Modeling and Evaluation (WAMME) projects, and coordinated by Y. Xue and P. M. Ruti.
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Bielli, S., Douville, H. & Pohl, B. Understanding the West African monsoon variability and its remote effects: an illustration of the grid point nudging methodology. Clim Dyn 35, 159–174 (2010). https://doi.org/10.1007/s00382-009-0667-8
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DOI: https://doi.org/10.1007/s00382-009-0667-8