Editorial Type:
Article
Article Type:
Research Article

On the Instability of the African Easterly Jet and the Generation of African Waves: Reversals of the Potential Vorticity Gradient

Jen-Shan Hsieh Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York

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Kerry H. Cook Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York

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Print Publication:
01 Jul 2008
DOI:
https://doi.org/10.1175/2007JAS2552.1
Page(s):
2130–2151
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Abstract

The relationship between the generation of African easterly waves and instability growing in regions with reversed potential vorticity gradients is studied using a regional climate model. Results indicate that the convective generation of potential vorticity (PV) due to the meridional and vertical gradients of diabatic heating in the upper and lower troposphere causes a vertically elongated PV anomaly on the southern flank of the African easterly jet. This PV maximum at 9°N in the midtroposphere, together with a PV minimum near 15°N at lower levels because of dry convection over the Sahara, reverses the meridional PV gradient between 9° and 15°N, which suggests that the zonal flow may be unstable in this region. Analysis of the seasonal mean Eliassen–Palm flux for African waves indicates that wave energy generated convectively through baroclinic overturning in the upper troposphere propagates downward and triggers barotropic conversions south of the jet and baroclinic conversions below and north of the jet.

The barotropic conversion of the jet initiates primarily outside of the region of strengthened reversed potential vorticity (q) gradients, suggesting that this barotropic conversion is a result of convectively induced eddies extracting energy from the zonal flow rather than the release of zonal kinetic energy to the waves in the unstable region. In contrast, the residual barotropic conversion occurs inside the region of reversed q gradients during the waves’ decaying stage when ITCZ convection weakens. The baroclinic instability in the unstable region becomes distinguishable from that due to surface temperature gradients when the surface heat flux is weak, a condition under which the African easterly jet better acts as an internal jet. Thus, this analysis indicates that the shear instability of the jet occurs to sustain the waves at the decaying stage rather than to initiate the waves, since it does not appear strong enough to reenergize the waves.

* Current affiliation: Department of Oceanography, Texas A&M University, College Station, Texas

Corresponding author address: Jen-Shan Hsieh, Department of Oceanography, Texas A&M University, College Station, TX 77843. Email: jsh@ocean.tamu.edu

Abstract

The relationship between the generation of African easterly waves and instability growing in regions with reversed potential vorticity gradients is studied using a regional climate model. Results indicate that the convective generation of potential vorticity (PV) due to the meridional and vertical gradients of diabatic heating in the upper and lower troposphere causes a vertically elongated PV anomaly on the southern flank of the African easterly jet. This PV maximum at 9°N in the midtroposphere, together with a PV minimum near 15°N at lower levels because of dry convection over the Sahara, reverses the meridional PV gradient between 9° and 15°N, which suggests that the zonal flow may be unstable in this region. Analysis of the seasonal mean Eliassen–Palm flux for African waves indicates that wave energy generated convectively through baroclinic overturning in the upper troposphere propagates downward and triggers barotropic conversions south of the jet and baroclinic conversions below and north of the jet.

The barotropic conversion of the jet initiates primarily outside of the region of strengthened reversed potential vorticity (q) gradients, suggesting that this barotropic conversion is a result of convectively induced eddies extracting energy from the zonal flow rather than the release of zonal kinetic energy to the waves in the unstable region. In contrast, the residual barotropic conversion occurs inside the region of reversed q gradients during the waves’ decaying stage when ITCZ convection weakens. The baroclinic instability in the unstable region becomes distinguishable from that due to surface temperature gradients when the surface heat flux is weak, a condition under which the African easterly jet better acts as an internal jet. Thus, this analysis indicates that the shear instability of the jet occurs to sustain the waves at the decaying stage rather than to initiate the waves, since it does not appear strong enough to reenergize the waves.

* Current affiliation: Department of Oceanography, Texas A&M University, College Station, Texas

Corresponding author address: Jen-Shan Hsieh, Department of Oceanography, Texas A&M University, College Station, TX 77843. Email: jsh@ocean.tamu.edu

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  • Andrews, D. G., and M. E. McIntyre, 1976: Planetary waves in a horizontal and vertical shear: The generalized Eliassen-Palm relation and the mean zonal acceleration. J. Atmos. Sci., 33 , 2031–2048.

    • 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 , 77–90.

  • Charney, J. G., and M. E. Stern, 1962: On the stability of internal baroclinic jets in a rotating atmosphere. J. Atmos. Sci., 19 , 159–172.

    • Search Google Scholar
    • Export Citation

  • Cook, K. H., 1999: Generation of the African easterly jet and its role in determining West African precipitation. J. Climate, 12 , 1165–1184.

    • Search Google Scholar
    • Export Citation

  • Dickinson, M., and J. Molinari, 2000: Climatology of sign reversals of the meridional potential vorticity gradient over Africa and Australia. Mon. Wea. Rev., 128 , 3890–3900.

    • Search Google Scholar
    • Export Citation

  • Dickinson, R., 1973: Baroclinic instability of an unbounded zonal shear flow in a compressible atmosphere. J. Atmos. Sci., 30 , 1520–1527.

    • Search Google Scholar
    • Export Citation

  • Dickinson, R., and F. J. Clare, 1973: Numerical study of the unstable modes of a hyperbolic-tangent barotropic shear flow. J. Atmos. Sci., 30 , 1035–1049.

    • Search Google Scholar
    • Export Citation

  • Diedhiou, A., S. Janicot, A. Viltard, and P. de Felice, 1998: Evidence of two regimes of easterly waves over West Africa and the tropical Atlantic. Geophys. Res. Lett., 25 , 2805–2808.

    • Search Google Scholar
    • Export Citation

  • Edmon, H. J., B. J. Hoskins, and M. E. McIntyre, 1980: Eliassen-Palm cross sections for the troposphere. J. Atmos. Sci., 37 , 2600–2616.

    • Search Google Scholar
    • Export Citation

  • Eliassen, A., 1983: The Charney-Stern theorem on barotropic-baroclinic instability. Pure Appl. Geophys., 121 , 563–572.

  • Eliassen, A., and E. Palm, 1961: On the transfer of energy in stationary mountain waves. Geofys. Publ., 22 , 1–23.

  • Erikson, C. O., 1963: An incipient hurricane near the West African coast. Mon. Wea. Rev., 91 , 61–68.

  • Estoque, M. A., and M. S. Lin, 1977: Energetics of easterly waves. Mon. Wea. Rev., 105 , 582–589.

  • Ferreira, R. N., and W. H. Schubert, 1997: Barotropic aspects of ITCZ breakdown. J. Atmos. Sci., 54 , 261–285.

  • Frank, N. L., 1970: Atlantic tropical systems of 1969. Mon. Wea. Rev., 98 , 307–314.

  • Green, J. S. A., 1970: Transfer properties of the large-scale eddies and the general circulation of the atmosphere. Quart. J. Roy. Meteor. Soc., 96 , 157–185.

    • Search Google Scholar
    • Export Citation

  • Hall, N. M. J., G. N. Kiladis, and C. D. Thorncroft, 2006: Three-dimensional structure and dynamics of African easterly waves. Part II: Dynamical modes. J. Atmos. Sci., 63 , 2231–2245.

    • Search Google Scholar
    • Export Citation

  • Holton, J. R., 1971: A diagnostic model for equatorial wave disturbances: The role of vertical shear of the mean zonal wind. J. Atmos. Sci., 28 , 55–64.

    • Search Google Scholar
    • Export Citation

  • Hsieh, J-S., and K. H. Cook, 2005: Generation of African easterly wave disturbances: Relationship to the African easterly jet. Mon. Wea. Rev., 133 , 1311–1327.

    • Search Google Scholar
    • Export Citation

  • Hsieh, J-S., and K. H. Cook, 2007: A study of the energetics of African easterly waves using a regional climate model. J. Atmos. Sci., 64 , 421–440.

    • Search Google Scholar
    • Export Citation

  • Kiladis, G. N., C. D. Thorncroft, and N. M. J. Hall, 2006: Three-dimensional structure and dynamics of African easterly waves. Part I: Observations. J. Atmos. Sci., 63 , 2212–2230.

    • Search Google Scholar
    • Export Citation

  • Kwon, H. J., 1989: A reexamination of the genesis of African waves. J. Atmos. Sci., 46 , 3621–3631.

  • Lipps, F., 1970: Barotropic stability and tropical disturbances. Mon. Wea. Rev., 98 , 122–131.

  • Mekonnen, A., C. D. Thorncroft, and A. Aiyyer, 2006: Analysis of convection and its association with African easterly waves. J. Climate, 19 , 5405–5421.

    • 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 FATE. Mon. Wea. Rev., 105 , 334–342.

    • Search Google Scholar
    • Export Citation

  • Pytharoulis, I., and C. D. Thorncroft, 1999: The low-level structure of African easterly waves in 1995. Mon. Wea. Rev., 127 , 2266–2280.

    • Search Google Scholar
    • Export Citation

  • Rennick, M. A., 1976: The generation of African waves. J. Atmos. Sci., 33 , 1955–1969.

  • 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 , 1493–1509.

    • Search Google Scholar
    • Export Citation

  • Shea, D. J., K. E. Trenberth, and R. W. Reynolds, 1992: A global monthly sea surface temperature climatology. J. Climate, 5 , 987–1001.

    • Search Google Scholar
    • Export Citation

  • Simmons, A. J., 1977: A note on the instability of the African easterly jet. J. Atmos. Sci., 34 , 1670–1674.

  • Thorncroft, C. D., and B. J. Hoskins, 1994a: An idealized study of African easterly waves. I: A linear view. Quart. J. Roy. Meteor. Soc., 120 , 953–982.

    • Search Google Scholar
    • Export Citation

  • Thorncroft, C. D., and B. J. Hoskins, 1994b: An idealized study of African easterly waves. II: A nonlinear view. Quart. J. Roy. Meteor. Soc., 120 , 983–1015.

    • Search Google Scholar
    • Export Citation

  • Thorncroft, C. D., and M. Blackburn, 1999: Maintenance of the African easterly jet. Quart. J. Roy. Meteor. Soc., 125 , 763–786.

  • Willmott, C. J., C. M. Rowe, and Y. Mintz, 1985: Climatology of the terrestrial seasonal water cycle. J. Climatol., 5 , 589–606.

  • Zhang, D-L., and R. A. Anthes, 1982: A high-resolution model of the planetary boundary layer—Sensitivity tests and comparisons with SESAME-79 data. J. Appl. Meteor., 21 , 1594–1609.

    • Search Google Scholar
    • Export Citation
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