Special Issue Article

A climatological study of transient–mean-flow interactions over West Africa

  1. Stephanie Leroux1,*,
  2. Nicholas M. J. Hall2,
  3. George N. Kiladis3,†

Article first published online: 9 NOV 2009

DOI: 10.1002/qj.474

Issue

Quarterly Journal of the Royal Meteorological Society

Quarterly Journal of the Royal Meteorological Society

Supplement: Advances in understanding atmospheric processes over West Africa through the AMMA field campaign

Volume 136, Issue S1, pages 397–410, January 2010

Additional Information

How to Cite

Leroux, S., Hall, N. M. J. and Kiladis, G. N. (2010), A climatological study of transient–mean-flow interactions over West Africa. Q.J.R. Meteorol. Soc., 136: 397–410. doi: 10.1002/qj.474

Author Information

  1. 1

    LTHE, Universite de Grenoble, France

  2. 2

    LEGOS, Universite de Toulouse, France

  3. 3

    Earth System Research Laboratory, NOAA, Boulder, CO, USA

  1. †

    The contribution of George N. Kiladis to this article was prepared as part of his official duties as a United States Federal Government employee.

*LTHE, BP 53, 38 041 Grenoble Cedex 09, France.

  1. †

    The contribution of George N. Kiladis to this article was prepared as part of his official duties as a United States Federal Government employee.

Publication History

  1. Issue published online: 11 MAR 2010
  2. Article first published online: 9 NOV 2009
  3. Manuscript Accepted: 12 JUN 2009
  4. Manuscript Revised: 11 JUN 2009
  5. Manuscript Received: 13 JAN 2009

Funded by

  • French Ministry of ‘Enseignement superieur et Recherche’

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Keywords:

  • African easterly waves;
  • African easterly jet;
  • transient eddies

Abstract

A statistical study is made of the relationship between intraseasonal variations in mid-tropospheric flow over West Africa and transient-eddy activity during the June-September monsoon season, in order to investigate the interaction between the African Easterly Jet (AEJ), African Easterly Waves (AEWs) and convection. NCEP2 reanalyses are used together with Outgoing Long-wave Radiation (OLR) data from June-September 1979-2007. Intraseasonal variability in the 600 hPa zonal wind is isolated using a 10-120 day filter. The leading Empirical Orthogonal Function (EOF) describes north-south displacements of the jet axis. Episodes of AEW activity over the continent are evaluated in terms of the perturbation kinetic energy (PKE) of the less-than-six-day high-pass filtered wind at 700 and 850 hPa. PKE and OLR are also filtered for the intraseasonal signal. Lag covariance analyses reveal a two-way interaction between the AEJ and synoptic-scale transients. Prior to episodes of enhanced transient activity, the AEJ is strengthened in the jet entrance region. During and following these episodes, the AEJ is strengthened on the northern flank of the jet exit region. E-vector analysis shows that this displacement of the AEJ is consistent with forcing by transients that have an AEW-like structure. Analysis of OLR shows that periods of enhanced transient activity over West Africa are associated with enhanced convection over the region at intraseasonal time-scales, and are correlated with prior episodes of enhanced convection to the east, in the ‘convective trigger’ region identified by Thorncroft et al. Copyright © 2009 Royal Meteorological Society

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