• Arnason, G., 1963: The stability of nongeostrophic perturbations in a baroclinic zonal flow. Tellus, 15 , 205229.

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

  • Burpee, R. W., 1972: The origin and structure of easterly waves in the lower troposphere of north Africa. J. Atmos. Sci., 29 , 7790.

  • Burpee, R. W., 1974: Characteristics of the North African easterly waves during the summers of 1968 and 1969. J. Atmos. Sci., 31 , 15561570.

    • Search Google Scholar
    • Export Citation
  • Carlson, T. N., 1969a: Synoptic histories of three African disturbances that developed into Atlantic hurricanes. Mon. Wea. Rev., 97 , 256276.

    • Search Google Scholar
    • Export Citation
  • Carlson, T. N., 1969b: Some remarks on African disturbances and their progress over the tropical Atlantic. Mon. Wea. Rev., 97 , 716726.

    • Search Google Scholar
    • Export Citation
  • Chang, C. B., 1993: Impact of desert environment on the genesis of African wave disturbances. J. Atmos. Sci., 50 , 21372145.

  • 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
  • Chen, T-C., 2003: Maintenance of summer circulations: A planetary-scale perspective. J. Climate, 16 , 20222037.

  • Chen, T-C., 2005: Maintenance of the midtropospheric North African summer circulation: Saharan high and African easterly jet. J. Climate, 18 , 29432962.

    • Search Google Scholar
    • Export Citation
  • Chen, T-C., , and W. E. Baker, 1986: Global diabatic heating during FGGE SOP-1 and SOP-2. Mon. Wea. Rev., 114 , 25782589.

  • Cook, K. H., 1999: Generation of the African easterly jet and its role in determining West African precipitation. J. Climate, 12 , 11651184.

    • Search Google Scholar
    • Export Citation
  • Eady, E. T., 1949: Long waves and cyclone waves. Tellus, 1 , 3352.

  • Fink, A. H., , D. G. Vincent, , P. M. Reiner, , and P. Speth, 2004: Mean state and wave disturbances during phase I, II, and III of GATE based on ERA-40. Mon. Wea. Rev., 132 , 16611683.

    • Search Google Scholar
    • Export Citation
  • Fjortoft, R., 1950: Application of integral theorems in deriving criteria of stability for laminar flows and for the baroclinic circular vortex. Geofys. Publ., 17 , 5. 152.

    • Search Google Scholar
    • Export Citation
  • Grist, J. P., 2002: Easterly waves over Africa. Part I: The seasonal cycle and contrasts between wet and dry years. Mon. Wea. Rev., 130 , 197211.

    • Search Google Scholar
    • Export Citation
  • Grist, J. P., , S. E. Nicholson, , and A. L. Barcilon, 2002: Easterly wave over Africa. Part II: Observed and modeled contrasts between wet and dry years. Mon. Wea. Rev., 130 , 212225.

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

  • Källberg, P., , A. Simmons, , S. Uppala, , and M. Fuentes, 2004: The ERA-40 archive. ERA-40 Project Report Series 17, 35 pp. [Available online at http://www.ecmwf.int/publications/library/ecpublications/_pdf/era40/ERA40_PRS17.pdf/.].

  • Kalnay, E., , and M. Cai, 2003: Impact of urbanization and land-use change on climate. Nature, 423 , 528531.

  • Kwon, H. J., 1989: A re-examination of the genesis of African waves. J. Atmos. Sci., 46 , 36213631.

  • Landsea, C. W., 1993: A climatology of intense (or major) Atlantic hurricanes. Mon. Wea. Rev., 121 , 17031713.

  • Lau, K-H., , and N-G. Lau, 1990: Observed structure and propagation characteristics of tropical summertime synoptic scale disturbances. Mon. Wea. Rev., 118 , 18881913.

    • Search Google Scholar
    • Export Citation
  • Lorenz, E. N., 1955: Available potential energy and the maintenance of the general circulation. Tellus, 7 , 157167.

  • Lorenz, E. N., 1967: The nature and theory of the general circulation of the atmosphere. WMO 218, Tech. Publication 115, World Meteorological Organization, Geneva, Switzerland, 161 pp.

  • Mass, C., 1979: A linear primitive equation model of African wave disturbances. J. Atmos. Sci., 36 , 20752092.

  • Murakami, M., 1979: Large-scale aspects of deep convective activity over the GATE area. Mon. Wea. Rev., 107 , 9941013.

  • Nicholson, S. E., , B. Some, , and B. Kone, 2000: A note on the recent rainfall condition in West Africa, including the rainy season of the 1997 ENSO year. J. Climate, 13 , 26282640.

    • Search Google Scholar
    • Export Citation
  • Nitta, T., , and Y. Takayabu, 1985: Global analysis of the lower tropospheric disturbances in the tropics during the northern summer of the FGGE year. Part II: Regional characteristics of the disturbances. Pure Appl. Geophys., 123 , 272292.

    • Search Google Scholar
    • Export Citation
  • Norquist, D. C., , E. E. Recker, , and R. J. Reed, 1977: The energetics of African wave disturbances as observed during phase III of GATE. Mon. Wea. Rev., 105 , 334342.

    • Search Google Scholar
    • Export Citation
  • Pytharoulis, I., , and C. Thorncroft, 1999: The low-level structure of African easterly waves in 1995. Mon. Wea. Rev., 127 , 22662280.

  • Reed, R. J., 1979: The structure and behavior of easterly waves over West Africa and the Atlantic. Meteorology over the Tropical Oceans, D. B. Show, Ed., Royal Meteorological Society, 57–72.

    • Search Google Scholar
    • Export Citation
  • Reed, R. J., , A. Hollingsworth, , W. A. Heckley, , and F. Delsol, 1988a: An evaluation 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
  • Reed, R. J., , E. Klinker, , and A. Hollingsworth, 1988b: The structure and characteristics of African easterly wave disturbances as determined from the ECMWF operational Analysis/Forecast System. Meteor. Atmos. Phys., 38 , 2233.

    • Search Google Scholar
    • Export Citation
  • Rennick, M. A., 1976: The generation of African waves. J. Atmos. Sci., 33 , 19551969.

  • Sanders, F., 1984: Quasi-geostrophic diagnosis of the monsoon depression of 5-8 July 1979. J. Atmos. Sci., 41 , 538552.

  • Simmons, A. J., 1977: A note on the instability of the African easterly jet. J. Atmos. Sci., 34 , 16701674.

  • Thorncroft, C. D., 1995: An idealized study of African easterly waves. Part III: More realistic basic states. Quart. J. Roy. Meteor. Soc., 121 , 15891614.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C. D., , 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. D., , 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. D., , and D. P. Rowell, 1998: Interannual variability of African wave activity in a general circulation model. Int. J. Climatol., 18 , 13051323.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C. D., , 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
  • van den Hurk, B. J. J. M., , P. Viterbo, , A. C. M. Beljaars, , and A. K. Betts, 2000: Offline validation of the ERA40 surface scheme. ECMWF Tech. Memo. 295, 42 pp.

  • Wiin-Nielsen, A., , and T-C. Chen, 1993: Fundamentals of Atmospheric Energetics. Oxford University Press, 376 pp.

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Characteristics of African Easterly Waves Depicted by ECMWF Reanalyses for 1991–2000

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  • 1 Atmospheric Science Program, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa
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Abstract

Several interesting characteristics of African easterly waves (AEWs) were observed and investigated by previous studies: two separate propagation paths, genesis mechanisms, restriction of vertical development, and the interaction with the African easterly jet (AEJ). However, some aspects of these characteristics have been neglected: the contrast of the AEW population along the two propagation paths, the AEW genesis mechanism over the Saharan thermal low and the role played by the low-level North African circulation in this mechanism, the dynamical mechanism restricting the vertical development of AEWs, and the synoptic relationship and interaction between the AEJ and the AEWs along the two propagation paths. The ECMWF reanalyses for the 1991–2000 period supplemented with those of 1979 were analyzed to explore these AEW features. Major findings of this effort are the following:

  1. The population of AEWs along the propagation path north of the AEJ (AEWn) is approximately 2.5 times of that along the propagation path south of the AEJ (AEWs).
  2. The AEWn geneses primarily occur over the three convergent centers and the southwestward extension of the Saharan thermal low. Underneath the midtropospheric Saharan high, the baroclinic instability of a shallow, low static stability environment, which may be triggered by the intrusion of dry northerlies over central North Africa, leads to the AEW genesis.
  3. Continental-scale upward motion along the Saharan thermal low and the cyclonic-shear side of the AEJ maintains positive vortex stretching below the Saharan high and the western part of the Asian monsoon high. These two regions thus form a favorable environment for the development of AEWs within the near-surface troposphere along the Saharan thermal low and the midtroposphere south of the AEJ.
  4. The passage of AEWn (AEWs) across the coastal zone of West Africa is accompanied by a weak (strong) AEJ and weak (strong) Saharan high. The westward propagation and development/maintenance of the two types of AEWs are achieved through vorticity advection by the AEJ, which is the major AEW–AEJ interaction.
These findings will facilitate the search for AEW dynamics and aid in assessing the impact of AEW activity on North African climate change.

* Visiting scientist at the National Central University in Taiwan sponsored by a National Science Council chair

Corresponding author address: Tsing-Chang (Mike) Chen, Atmospheric Science Program, Department of Geological and Atmospheric Sciences, 3010 Agronomy Hall, Iowa State University, Ames, IA 50011. Email: tmchen@iastate.edu

Abstract

Several interesting characteristics of African easterly waves (AEWs) were observed and investigated by previous studies: two separate propagation paths, genesis mechanisms, restriction of vertical development, and the interaction with the African easterly jet (AEJ). However, some aspects of these characteristics have been neglected: the contrast of the AEW population along the two propagation paths, the AEW genesis mechanism over the Saharan thermal low and the role played by the low-level North African circulation in this mechanism, the dynamical mechanism restricting the vertical development of AEWs, and the synoptic relationship and interaction between the AEJ and the AEWs along the two propagation paths. The ECMWF reanalyses for the 1991–2000 period supplemented with those of 1979 were analyzed to explore these AEW features. Major findings of this effort are the following:

  1. The population of AEWs along the propagation path north of the AEJ (AEWn) is approximately 2.5 times of that along the propagation path south of the AEJ (AEWs).
  2. The AEWn geneses primarily occur over the three convergent centers and the southwestward extension of the Saharan thermal low. Underneath the midtropospheric Saharan high, the baroclinic instability of a shallow, low static stability environment, which may be triggered by the intrusion of dry northerlies over central North Africa, leads to the AEW genesis.
  3. Continental-scale upward motion along the Saharan thermal low and the cyclonic-shear side of the AEJ maintains positive vortex stretching below the Saharan high and the western part of the Asian monsoon high. These two regions thus form a favorable environment for the development of AEWs within the near-surface troposphere along the Saharan thermal low and the midtroposphere south of the AEJ.
  4. The passage of AEWn (AEWs) across the coastal zone of West Africa is accompanied by a weak (strong) AEJ and weak (strong) Saharan high. The westward propagation and development/maintenance of the two types of AEWs are achieved through vorticity advection by the AEJ, which is the major AEW–AEJ interaction.
These findings will facilitate the search for AEW dynamics and aid in assessing the impact of AEW activity on North African climate change.

* Visiting scientist at the National Central University in Taiwan sponsored by a National Science Council chair

Corresponding author address: Tsing-Chang (Mike) Chen, Atmospheric Science Program, Department of Geological and Atmospheric Sciences, 3010 Agronomy Hall, Iowa State University, Ames, IA 50011. Email: tmchen@iastate.edu

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