Westward-Propagating Synoptic-Scale Disturbances and the ITCZ

Guojun Gu RSMAS/MPO, University of Miami, Miami, Florida

Search for other papers by Guojun Gu in
Current site
Google Scholar
PubMed
Close
and
Chidong Zhang RSMAS/MPO, University of Miami, Miami, Florida

Search for other papers by Chidong Zhang in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

In an attempt to understand the dynamics of the intertropical convergence zone (ITCZ), this study explores the extent to which the ITCZ is causally related to zonally propagating synoptic-scale disturbances. The ITCZ, measured by its mean convection, is represented by mean outgoing longwave radiation (OLR). Synoptic-scale disturbances, measured by their deep convective signals, are represented by the spectral power of the OLR that is significantly above its red-noise background. Time-mean spatial distributions as well as annual and interannual variability of the ITCZ are compared with those of synoptic-scale disturbances, which are dominated by westward-propagating signals. In general, they match each other well in their mean distributions and annual cycles. But, in detail, discrepancies between the two fields exist, some of them substantial. The maximum disturbance activity tends to be located at the polar side of the ITCZ. The seasonal cycles of the two share many similarities, but the variations in the intensity and latitudinal locations of the disturbances are greater than those of the ITCZ. On interannual timescales, their relationship is even more limited. Comparisons are also made between the observations and theories relating the ITCZ and westward-propagating synoptic-scale disturbances. The results suggest that the observed ITCZ does not owe its existence to zonally propagating synoptic-scale disturbances, in the sense that it would still exist in the absence of the disturbances. But the similarities in their means and annual cycles imply that the disturbances alone can result in an ITCZ resembling the observed one in many respects. The observations, on the other hand, are consistent with the theories that view the dynamical instability of the ITCZ as a cause of some westward-propagating synoptic-scale disturbances.

Corresponding author address: Mr. Guojun Gu, 202 Monell Building, IRI, P.O. Box 1000, 61 Route 9W, Palisades, NY 10964. Email: ggu@rsmas.miami.edu

Abstract

In an attempt to understand the dynamics of the intertropical convergence zone (ITCZ), this study explores the extent to which the ITCZ is causally related to zonally propagating synoptic-scale disturbances. The ITCZ, measured by its mean convection, is represented by mean outgoing longwave radiation (OLR). Synoptic-scale disturbances, measured by their deep convective signals, are represented by the spectral power of the OLR that is significantly above its red-noise background. Time-mean spatial distributions as well as annual and interannual variability of the ITCZ are compared with those of synoptic-scale disturbances, which are dominated by westward-propagating signals. In general, they match each other well in their mean distributions and annual cycles. But, in detail, discrepancies between the two fields exist, some of them substantial. The maximum disturbance activity tends to be located at the polar side of the ITCZ. The seasonal cycles of the two share many similarities, but the variations in the intensity and latitudinal locations of the disturbances are greater than those of the ITCZ. On interannual timescales, their relationship is even more limited. Comparisons are also made between the observations and theories relating the ITCZ and westward-propagating synoptic-scale disturbances. The results suggest that the observed ITCZ does not owe its existence to zonally propagating synoptic-scale disturbances, in the sense that it would still exist in the absence of the disturbances. But the similarities in their means and annual cycles imply that the disturbances alone can result in an ITCZ resembling the observed one in many respects. The observations, on the other hand, are consistent with the theories that view the dynamical instability of the ITCZ as a cause of some westward-propagating synoptic-scale disturbances.

Corresponding author address: Mr. Guojun Gu, 202 Monell Building, IRI, P.O. Box 1000, 61 Route 9W, Palisades, NY 10964. Email: ggu@rsmas.miami.edu

Save
  • Bates, J. R., 1970: Dynamics of disturbances on the intertropical covergence zone. Quart. J. Roy. Meteor. Soc., 96 , 677701.

  • Bates, J. R., 1972: Tropical disturbances and the general circulation. Quart. J. Roy. Meteor. Soc., 98 , 116.

  • Chang, C. P., 1970: Westward propagating cloud patterns in the tropical Pacific as seen from time composite satellite photographs. J. Atmos. Sci., 27 , 133138.

    • Search Google Scholar
    • Export Citation
  • Chang, C. P., 1973: A dynamical model of the intertropical convergence zone. J. Atmos. Sci., 30 , 190212.

  • Charney, J., 1971: Tropical cyclogenesis and the formation of the intertropical convergence zone. Mathematical Problems of Geophysical Fluid Dynamics, W. H. Reid, Ed., Lectures in Applied Mathematics, Vol. 13, American Mathematical Society, 355–368.

    • 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 , 38903900.

    • Search Google Scholar
    • Export Citation
  • Duvel, J. P., 1990: Convection over tropical Africa and the Atlantic Ocean during northern summer. Part II: Modulation by easterly waves. Mon. Wea. Rev., 118 , 18551868.

    • Search Google Scholar
    • Export Citation
  • Ferreira, R. N., and W. H. Schubert, 1997: Barotropic aspects of ITCZ breakdown. J. Atmos. Sci., 54 , 261285.

  • Gruber, A., 1974: Wavenumber–frequency spectra of satellite-measured brightness in the Tropics. J. Atmos. Sci., 31 , 16751680.

  • Gruber, A., and A. F. Krueger, 1984: The status of the NOAA outgoing longwave radiation dataset. Bull. Amer. Meteor. Soc., 65 , 16751680.

    • Search Google Scholar
    • Export Citation
  • Gu, G., and C. Zhang, 2001: A spectrum analysis of synoptic-scale disturbances in the ITCZ. J. Climate, 14 , 27252739.

  • Hack, J. J., W. H. Schubert, D. E. Stevens, and H. C. Kuo, 1989: Response of the Hadley circulation to convective forcing in the ITCZ. J. Atmos. Sci., 46 , 29572973.

    • Search Google Scholar
    • Export Citation
  • Hayashi, Y., 1982: Space–time spectral analysis and its applications to atmospheric waves. J. Meteor. Soc. Japan, 60 , 156171.

  • Hess, P. G., D. S. Battisti, and P. J. Rasch, 1993: The maintenance of the intertropical convergence zone and the large-scale circulation on a water-covered earth. J. Atmos. Sci., 50 , 691713.

    • Search Google Scholar
    • Export Citation
  • Holton, J. R., J. M. Wallace, and J. A. Young, 1971: On boundary layer dynamics and the ITCZ. J. Atmos. Sci., 28 , 275280.

  • Kuo, H. L., 1978: A two-layer model study of the combined barotropic and baroclinic instability in the Tropics. J. Atmos. Sci., 35 , 18401960.

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

    • Search Google Scholar
    • Export Citation
  • Li, T., and S. G. H. Philander, 1996: On the annual cycle of the eastern equatorial Pacific. J. Climate, 9 , 29862998.

  • Liebmann, B., and C. A. Smith, 1996: Description of a complete (interpolated) outgoing longwave radiation dataset. Bull. Amer. Meteor. Soc., 77 , 12751277.

    • Search Google Scholar
    • Export Citation
  • Lindzen, R. S., 1974: Wave-CISK in the Tropics. J. Atmos. Sci., 31 , 156179.

  • Lindzen, R. S., and A. Y. Hou, 1988: Hadley circulations for zonally averaged heating centered off the equator. J. Atmos. Sci., 45 , 24162427.

    • Search Google Scholar
    • Export Citation
  • Matthews, A. J., and G. N. Kiladis, 1999: Interaction between ENSO, transient circulation, and tropical convection over the Pacific. J. Climate, 12 , 30623086.

    • Search Google Scholar
    • Export Citation
  • Mitchell, T. P., and J. M. Wallace, 1992: The annual cycle in equatorial convection and sea surface temperature. J. Climate, 5 , 11401156.

    • Search Google Scholar
    • Export Citation
  • Molinari, J., D. Knight, M. Dickinson, D. Vollaro, and S. Skubis, 1997: Potential vorticity, easterly waves, and tropical cyclogenesis. Mon. Wea. Rev., 125 , 26992708.

    • Search Google Scholar
    • Export Citation
  • Molinari, J., D. Vollaro, S. Skubis, and M. Dickinson, 2000: Origins and mechanisms of eastern Pacific tropical cyclogenesis: A case study. Mon. Wea. Rev., 128 , 125139.

    • Search Google Scholar
    • Export Citation
  • Nitta, T., Y. Nakagomi, Y. Suzuki, N. Hasegawa, and A. Kadokura, 1985: Global analysis of the lower tropospheric disturbances in the Tropics during the northern summer of the FGGE year. Part I: Global features of the disturbances. J. Meteor. Soc. Japan, 63 , 119.

    • Search Google Scholar
    • Export Citation
  • Philander, S. G. H., D. Gu, D. Halpern, G. Lambert, N-C. Lau, T. Li, and R. C. Pacanowski, 1996: Why the ITCZ is mostly north of the equator. J. Climate, 9 , 29582972.

    • Search Google Scholar
    • Export Citation
  • Pike, A. C., 1971: Intertropical convergence zone studied with an interacting atmosphere and ocean model. Mon. Wea. Rev., 99 , 469477.

    • Search Google Scholar
    • Export Citation
  • Raymond, D. J., 2000: The Hadley circulation as a radiative-convective instability. J. Atmos. Sci., 57 , 12861297.

  • Salby, M. L., H. Hendon, K. Woodberry, and K. Tanaka, 1991: Analysis of global cloud imagery from multiple satellites. Bull. Amer. Meteor. Soc., 72 , 467480.

    • 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 , 14931509.

    • Search Google Scholar
    • Export Citation
  • Tai, K. S., and Y. Ogura, 1987: An observational study of easterly waves over the eastern Pacific in the northern summer using FGGE data. J. Atmos. Sci., 44 , 339361.

    • Search Google Scholar
    • Export Citation
  • Tomas, R. A., and P. J. Webster, 1997: The role of inertial instability in determining the location and strength of near-equatorial convection. Quart. J. Roy. Meteor. Soc., 123 , 14451482.

    • Search Google Scholar
    • Export Citation
  • Waliser, D. E., and C. Gautier, 1993: A global climatology of the ITCZ. J. Climate, 6 , 21622174.

  • Waliser, D. E., and R. C. J. Somerville, 1994: Preferred latitudes of the intertropical convergence zone. J. Atmos. Sci., 51 , 16191639.

    • Search Google Scholar
    • Export Citation
  • Wang, B., 1994: On the annual cycle in the tropical eastern central Pacific. J. Climate, 7 , 19261942.

  • Wheeler, M., and G. N. Kiladis, 1999: Convectively coupled equatorial waves: Analysis of clouds and temperature in the wavenumber–frequency domain. J. Atmos. Sci., 56 , 374399.

    • Search Google Scholar
    • Export Citation
  • Xie, S-P., and S. G. H. Philander, 1994: A coupled ocean–atmosphere model of relevance to the ITCZ in the eastern Pacific. Tellus, 46A , 340350.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 300 49 8
PDF Downloads 124 30 0