• Aiyyer, A., and C. Thorncroft, 2006: Climatology of vertical shear over the tropical Atlantic. J. Climate, 19 , 29692983.

  • Avila, L., and R. J. Pasch, 1992: Atlantic tropical systems of 1991. Mon. Wea. Rev., 120 , 26882696.

  • Berry, G., and C. Thorncroft, 2005: Case study of an intense African easterly wave. Mon. Wea. Rev., 133 , 752766.

  • Berry, G., C. Thorncroft, and T. Hewson, 2007: African easterly waves during 2004—Analysis using objective techniques. Mon. Wea. Rev., 135 , 12511267.

    • Search Google Scholar
    • Export Citation
  • Burpee, R. W., 1972: The origin and structure of easterly waves in the lower troposphere of North Africa. J. Atmos. Sci., 29 , 7790.

  • Charney, J. G., and M. E. Stern, 1962: On the stability of internal baroclinic jets in a rotating atmosphere. J. Atmos. Sci., 19 , 159172.

    • Search Google Scholar
    • Export Citation
  • Diedhiou, A., S. Janicot, A. Viltard, P. de Felice, and H. Laurent, 1999: Easterly wave regimes and associated convection over West Africa and the tropical Atlantic: Results from NCEP/NCAR and ECMWF reanalyses. Climate Dyn., 15 , 795822.

    • Search Google Scholar
    • Export Citation
  • Emanuel, K., 2003: Tropical cyclones. Annu. Rev. Earth Planet. Sci., 31 , 75104.

  • Frank, N. L., 1975: Atlantic tropical systems of 1974. Mon. Wea. Rev., 103 , 294300.

  • Gibson, J. K., P. Kållberg, S. Uppala, A. Hernandez, A. Nomura, and S. Serrano, 1997: ECMWF Reanalysis Project. Rep. Series 1, 71 pp.

  • Goldenberg, S., and L. Shapiro, 1996: Physical mechanisms for the association of El Niño and West African rainfall with Atlantic major hurricane activity. J. Climate, 9 , 11691187.

    • Search Google Scholar
    • Export Citation
  • Goldenberg, S., C. W. Landsea, A. M. Mestas-Nuñez, and W. M. Gray, 2001: The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293 , 474479.

    • Search Google Scholar
    • Export Citation
  • Gray, W. M., 1968: Global view of the origins of tropical cyclones. Mon. Wea. Rev., 96 , 669700.

  • Grist, J. P., and S. E. Nicholson, 2001: A study of the dynamic factors influencing the rainfall variability in the West African Sahel. J. Climate, 14 , 13371359.

    • Search Google Scholar
    • Export Citation
  • Hodges, K., 1995: Feature tracking on the unit sphere. Mon. Wea. Rev., 123 , 34583465.

  • Hodges, K., 1996: Spherical nonparametric estimators applied to the UGAMP model integration for AMIP. Mon. Wea. Rev., 124 , 29142932.

  • Hulme, M., 1992: A 1951–80 global land precipitation climatology for the evaluation of General Circulation Models. Climate Dyn., 7 , 5772.

    • Search Google Scholar
    • Export Citation
  • Kiladis, G., C. Thorncroft, and N. Hall, 2006: Three-dimensional structure and dynamics of African easterly waves. Part I: Observations. J. Atmos. Sci., 63 , 22122230.

    • Search Google Scholar
    • Export Citation
  • Lamb, P. J., 1982: Persistence of Subsaharan drought. Nature, 299 , 4648.

  • Landsea, C., and W. Gray, 1992: The strong association between western Sahelian monsoon rainfall and intense Atlantic hurricanes. J. Climate, 5 , 435453.

    • Search Google Scholar
    • Export Citation
  • Landsea, C., G. D. Bell, W. Gray, and S. B. Goldenberg, 1998: The extremely active 1995 Atlantic hurricane season: Environmental conditions and verification of seasonal forecasts. Mon. Wea. Rev., 126 , 11741193.

    • Search Google Scholar
    • Export Citation
  • Lau, K-H., and N-C. Lau, 1990: Observed structure and propagation characteristics of tropical summertime synoptic-scale disturbances. Mon. Wea. Rev., 118 , 18881913.

    • Search Google Scholar
    • Export Citation
  • Mekonnen, A., C. Thorncroft, and A. Aiyyer, 2006: Analysis of convection and its association with African easterly waves. J. Climate, 19 , 54055421.

    • Search Google Scholar
    • Export Citation
  • Mestas-Nuñez, A. M., and D. B. Enfield, 1999: Rotated global modes of non-ENSO sea surface temperature variability. J. Climate, 12 , 27342746.

    • Search Google Scholar
    • Export Citation
  • Pasch, R. J., and L. Avila, 1994: Atlantic tropical systems of 1992. Mon. Wea. Rev., 122 , 539548.

  • Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. V. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of SST, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res., 108 .4407, doi:10.1029/2002JD002670.

    • Search Google Scholar
    • Export Citation
  • Reed, R. J., 1988: On understanding the meteorological causes of Sahelian drought. Persistent Meteo-Oceanographic Anomalies and Teleconnections, C. Chagas and G. Puppi, Eds., Pontificae Academiae Scientiarvm, 179–213.

    • 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
  • Reed, R. J., A. Hollingsworth, W. A. Heckley, and F. Delsol, 1988: An evaluation of the performance of the ECMWF operational system in analyzing and forecasting easterly wave disturbances over Africa and the tropical Atlantic. Mon. Wea. Rev., 116 , 824865.

    • Search Google Scholar
    • Export Citation
  • Schubert, W. H., P. E. Ciesielski, D. E. Stevens, and H-C. Kuo, 1991: Potential vorticity modeling of the ITCZ and the Hadley circulation. J. Atmos. Sci., 48 , 14931500.

    • Search Google Scholar
    • Export Citation
  • Simpson, R. H., N. Frank, D. Shideler, and H. M. Johnson, 1969: Atlantic tropical disturbances of 1968. Mon. Wea. Rev., 97 , 240255.

  • Thorncroft, C., and B. J. Hoskins, 1994a: An idealized study of African Easterly Waves, part I: A linear view. Quart. J. Roy. Meteor. Soc., 120 , 953982.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C., and B. J. Hoskins, 1994b: An idealized study of African Easterly Waves, part II: A nonlinear view. Quart. J. Roy. Meteor. Soc., 120 , 9831015.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C., and K. Hodges, 2001: African easterly wave variability and its relationship to Atlantic tropical cyclone activity. J. Climate, 14 , 11661179.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C., and I. Pytharoulis, 2001: A dynamical approach to seasonal prediction of Atlantic tropical cyclone activity. Wea. Forecasting, 16 , 725734.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C., and Coauthors, 2003: The JET2000 Project: Aircraft observations of the African easterly jet and African easterly waves. Bull. Amer. Meteor. Soc., 84 , 337351.

    • Search Google Scholar
    • Export Citation
  • Uppala, S. M., and Coauthors, 2005: The ERA-40 re-analysis. Quart. J. Roy. Meteor. Soc., 131 , 29613012.

  • Webster, P., and H-R. Chang, 1988: Equatorial energy accumulation and emanation regions: Impacts of a zonally varying basic state. J. Atmos. Sci., 45 , 803829.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 8 8 8
PDF Downloads 6 6 6

West African Storm Tracks and Their Relationship to Atlantic Tropical Cyclones

View More View Less
  • 1 Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York
  • | 2 Environmental Systems Science Centre, University of Reading, Reading, United Kingdom
  • | 3 Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York
Restricted access

Abstract

The automatic tracking technique used by Thorncroft and Hodges has been used to identify coherent vorticity structures at 850 hPa over West Africa and the tropical Atlantic in the 40-yr ECMWF Re-Analysis. The presence of two dominant source regions, north and south of 15°N over West Africa, for storm tracks over the Atlantic was confirmed. Results show that the southern storm track provides most of the storms that reach the main development region where most tropical cyclones develop. There exists marked seasonal variability in location and intensity of the storms leaving the West African coast, which may influence the likelihood of downstream intensification and longevity.

There exists considerable year-to-year variability in the number of West African storm tracks, both in numbers over the land and continuing out over the tropical Atlantic Ocean. While the low-frequency variability is well correlated with Atlantic tropical cyclone activity, West African rainfall, and SSTs, the interannual variability is found to be uncorrelated with these. In contrast, variance of the 2–6-day-filtered meridional wind, which provides a synoptic-scale measure of African easterly wave activity, shows a significant, positive correlation with tropical cyclone activity at interannual time scales.

Corresponding author address: Susanna B. Hopsch, Department of Earth and Atmospheric Sciences, ES-351, University at Albany, State University of New York, Albany, NY 12222. Email: hopsch@atmos.albany.edu

Abstract

The automatic tracking technique used by Thorncroft and Hodges has been used to identify coherent vorticity structures at 850 hPa over West Africa and the tropical Atlantic in the 40-yr ECMWF Re-Analysis. The presence of two dominant source regions, north and south of 15°N over West Africa, for storm tracks over the Atlantic was confirmed. Results show that the southern storm track provides most of the storms that reach the main development region where most tropical cyclones develop. There exists marked seasonal variability in location and intensity of the storms leaving the West African coast, which may influence the likelihood of downstream intensification and longevity.

There exists considerable year-to-year variability in the number of West African storm tracks, both in numbers over the land and continuing out over the tropical Atlantic Ocean. While the low-frequency variability is well correlated with Atlantic tropical cyclone activity, West African rainfall, and SSTs, the interannual variability is found to be uncorrelated with these. In contrast, variance of the 2–6-day-filtered meridional wind, which provides a synoptic-scale measure of African easterly wave activity, shows a significant, positive correlation with tropical cyclone activity at interannual time scales.

Corresponding author address: Susanna B. Hopsch, Department of Earth and Atmospheric Sciences, ES-351, University at Albany, State University of New York, Albany, NY 12222. Email: hopsch@atmos.albany.edu

Save