Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa: Statistical Analysis and Seasonal Variations

Peter Knippertz Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany

Search for other papers by Peter Knippertz in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

In several case studies, tropical–extratropical interactions (TEIs) have been shown to contribute to the transition-season precipitation in subtropical northwest Africa. Such TEI situations are characterized by a moisture source in the Tropics, a midlevel moisture transport into the subtropics to the east of an upper-level trough, and precipitation generation over northwest Africa through upper-level divergence and orographic effects in the Atlas Mountains.

In this paper, an automatic algorithm to identify TEI episodes on the basis of a 20-yr (December 1978–November 1998) climatology of 4-day backward trajectories starting at 400 hPa over northwest Africa, calculated from ECMWF (re-) analysis, is introduced. Twelve-hourly precipitation reports from 36 synoptic stations in northwest Africa are used to investigate the climatological relevance of TEI situations for different seasons. Results show that the region with the highest relative importance of TEIs is the semiarid southern foothills of the High Atlas (up to 40% of the annual precipitation amount). Relevance clearly decreases toward the much wetter Atlantic coast. TEI contributions are largest in the transition seasons, when TEI situations are most distinct, and in summer, when TEI situations are most frequent. It is suggested to consider TEIs in future studies on the observed and modeled precipitation variability of the region around the Atlas chain.

Corresponding author address: Peter Knippertz, Dept. of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, 1225 W. Dayton Street, Madison, WI 53706. Email: pknipp@aos.wisc.edu

Abstract

In several case studies, tropical–extratropical interactions (TEIs) have been shown to contribute to the transition-season precipitation in subtropical northwest Africa. Such TEI situations are characterized by a moisture source in the Tropics, a midlevel moisture transport into the subtropics to the east of an upper-level trough, and precipitation generation over northwest Africa through upper-level divergence and orographic effects in the Atlas Mountains.

In this paper, an automatic algorithm to identify TEI episodes on the basis of a 20-yr (December 1978–November 1998) climatology of 4-day backward trajectories starting at 400 hPa over northwest Africa, calculated from ECMWF (re-) analysis, is introduced. Twelve-hourly precipitation reports from 36 synoptic stations in northwest Africa are used to investigate the climatological relevance of TEI situations for different seasons. Results show that the region with the highest relative importance of TEIs is the semiarid southern foothills of the High Atlas (up to 40% of the annual precipitation amount). Relevance clearly decreases toward the much wetter Atlantic coast. TEI contributions are largest in the transition seasons, when TEI situations are most distinct, and in summer, when TEI situations are most frequent. It is suggested to consider TEIs in future studies on the observed and modeled precipitation variability of the region around the Atlas chain.

Corresponding author address: Peter Knippertz, Dept. of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, 1225 W. Dayton Street, Madison, WI 53706. Email: pknipp@aos.wisc.edu

Save
  • Druyan, L. M., P. Lonergan, and J. Cohen, 1997: Case studies of African wave disturbances in gridded analyses. Mon. Wea. Rev., 125 , 25202530.

    • Search Google Scholar
    • Export Citation
  • Fink, A. H., and P. Knippertz, 2003: An extreme precipitation event in southern Morocco in spring 2002 and some hydrological implications. Weather, 58 , 377387.

    • Search Google Scholar
    • Export Citation
  • Fink, A. H., and A. Reiner, 2003: Spatio-temporal variability of the relation between African Easterly Waves and West African Squall Lines in 1998 and 1999. J. Geophys. Res., 108 .4332, doi:10.1029/2002JD002816.

    • Search Google Scholar
    • Export Citation
  • Geb, M., 2000: Factors favouring precipitation in North Africa: Seen from the viewpoint of present-day climatology. Global Planet. Change, 26 , 8596.

    • Search Google Scholar
    • Export Citation
  • Gibson, J. K., P. Kallberg, and S. Uppala, 1997: ERA description. ECMWF Re-Analysis Project Report Series, No. 1, ECMWF, Reading, United Kingdom, 71 pp.

    • Search Google Scholar
    • Export Citation
  • Hurrell, J. W., and H. van Loon, 1997: Decadal variations in climate associated with the North Atlantic Oscillation. Climatic Change, 36 , 301326.

    • Search Google Scholar
    • Export Citation
  • Knippertz, P., 2003: Niederschlagsvariabilität in Nordwestafrika und der Zusammenhang mit der großskaligen atmosphärischen Zirkulation und der synoptischen Aktivität. Ph.D. thesis, Mitteilungen aus dem Institut für Geophysik und Meteorologie der Universität zu Köln, Vol. 152, 136 pp.

    • Search Google Scholar
    • Export Citation
  • Knippertz, P., M. Christoph, and P. Speth, 2003a: Long-term precipitation variability in Morocco and the link to the large-scale circulation in recent and future climates. Meteor. Atmos. Phys., 83 , 6788.

    • Search Google Scholar
    • Export Citation
  • Knippertz, P., A. H. Fink, A. Reiner, and P. Speth, 2003b: Three late summer/early autumn cases of tropical–extratropical interactions causing precipitation in northwest Africa. Mon. Wea. Rev., 131 , 116135.

    • Search Google Scholar
    • Export Citation
  • Lamb, P. J., and R. A. Peppler, 1987: North Atlantic Oscillation: Concept and application. Bull. Amer. Meteor. Soc., 68 , 12181225.

  • McGrath, R., 1989: Trajectory models and their use in the Irish Meteorological Service. Memo. 112/89, Irish Meteorological Service, Dublin, Ireland, 12 pp.

    • Search Google Scholar
    • Export Citation
  • McGuirk, J. P., A. H. Thompson, and N. R. Smith, 1987: Moisture bursts over the tropical Pacific Ocean. Mon. Wea. Rev., 115 , 787798.

  • Nicholson, S. E., 1981: Rainfall and atmospheric circulation during drought periods and wetter years in West Africa. Mon. Wea. Rev., 109 , 21912208.

    • Search Google Scholar
    • Export Citation
  • Nicholson, S. E., B. Some, and B. Kone, 2000: An analysis of recent rainfall conditions in West Africa, including the rainy seasons of the 1997 El Niño and the 1998 La Niña years. J. Climate, 13 , 26282640.

    • Search Google Scholar
    • Export Citation
  • Parker, S. S., J. T. Hawes, S. J. Colucci, and B. P. Hayden, 1989: Climatology of 500 mb cyclones and anticyclones, 1950–85. Mon. Wea. Rev., 117 , 558570.

    • Search Google Scholar
    • Export Citation
  • Quezel, P., and M. Barbero, 1993: Variations climatiques au Sahara et en Afrique séche depuis le pliocène: Enseignements de la flore et de la végétation actuelles. Bull. Ecol., 24 , 191202.

    • Search Google Scholar
    • Export Citation
  • Reed, R. J., D. C. Norquist, and E. E. Recker, 1977: The structure and properties of African wave disturbances as observed during phase III of GATE. Mon. Wea. Rev., 105 , 317333.

    • Search Google Scholar
    • Export Citation
  • Thompson, R. M., S. W. Payne, E. E. Recker, and R. J. Reed, 1979: Structure and properties of synoptic-scale wave disturbances in the intertropical convergence zone of the eastern Atlantic. J. Atmos. Sci., 36 , 5372.

    • Search Google Scholar
    • Export Citation
  • Ward, M. N., P. J. Lamb, D. H. Portis, M. El Hamly, and R. Sebbari, 1999: Climate variability in northern Africa: Understanding droughts in the Sahel and the Maghreb. Beyond El Niño—Decadal and Interdecadal Climate Variability, A. Navarra, Ed., Springer-Verlag, 119–140.

    • Search Google Scholar
    • Export Citation
  • WMO, 1996: Climatological normals (Clino) for the period 1961–1990. WMO 847, Geneva, Switzerland, 768 pp.

  • Wright, W. J., 1997: Tropical–extratropical cloudbands and Australian rainfall: I. Climatology. Int. J. Climatol., 17 , 807829.

  • Ziv, B., 2001: A subtropical rainstorm associated with a tropical plume over Africa and the Middle-East. Theor. Appl. Climatol., 69 , 91102.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 574 65 9
PDF Downloads 280 48 4