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Response of the North African summer monsoon to precession and obliquity forcings in the EC-Earth GCM

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Abstract

We investigate, for the first time, the response of the North African summer monsoon to separate precession and obliquity forcings using a high-resolution state-of-the-art coupled general circulation model, EC-Earth. Our aim is to better understand the mechanisms underlying the astronomical forcing of this low-latitude climate system in detail. The North African monsoon is strengthened when northern hemisphere summer insolation is higher, as is the case in the minimum precession and maximum obliquity experiments. In these experiments, the low surface pressure areas over the Sahara are intensified and located farther north, and the meridional pressure gradient is further enhanced by a stronger South Atlantic high pressure area. As a result, the southwesterly monsoon winds are stronger and bring more moisture into the monsoon region from both the northern and southern tropical Atlantic. The monsoon winds, precipitation and convection also extend farther north into North Africa. The precession-induced changes are much larger than those induced by obliquity, but the latter are remarkable because obliquity-induced changes in summer insolation over the tropics are nearly zero. Our results provide a different explanation than previously proposed for mechanisms underlying the precession- and, especially, obliquity-related signals in paleoclimate proxy records of the North African monsoon. The EC-Earth experiments reveal that, instead of higher latitude mechanisms, increased moisture transport from both the northern and southern tropical Atlantic is responsible for the precession and obliquity signals in the North African monsoon. This increased moisture transport results from both increased insolation and an increased tropical insolation gradient.

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Acknowledgments

Computing time was provided by the Royal Netherlands Meteorological Institute (KNMI) and the European Center for Medium-range Weather Forecast (ECMWF). Joyce Bosmans was funded by a “Focus en Massa” Grant of Utrecht University, the Netherlands. The authors thank the editor and two anonymous reviewers for their comments which helped improve this manuscript.

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Author notes

  1. J. H. C. Bosmans

    Present address: Department of Physical Geography, Faculty of Geosciences, Utrecht University, Heidelberglaan 2, 3584 CS, Utrecht, The Netherlands

  2. E. Tuenter

    Present address: Royal Netherlands Meteorological Institute (KNMI), P.O. box 201, 3730 AE, De Bilt, The Netherlands

Authors and Affiliations

  1. Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD, Utrecht, The Netherlands

    J. H. C. Bosmans, E. Tuenter, F. J. Hilgen & L. J. Lourens

  2. Global Climate, Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730 AE, De Bilt, The Netherlands

    J. H. C. Bosmans, S. S. Drijfhout & E. Tuenter

  3. National Oceanographic Centre, European Way, Southampton, SO14 3ZH, UK

    S. S. Drijfhout

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Bosmans, J.H.C., Drijfhout, S.S., Tuenter, E. et al. Response of the North African summer monsoon to precession and obliquity forcings in the EC-Earth GCM. Clim Dyn 44, 279–297 (2015). https://doi.org/10.1007/s00382-014-2260-z

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