Active and Break Phases in the South American Monsoon System

Charles Jones Institute for Computational Earth System Science, University of California, Santa Barbara, Santa Barbara, California

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Leila M. V. Carvalho Department of Atmospheric Sciences, Institute of Astronomy and Geophysics, University of São Paulo, São Paulo, Brazil

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Abstract

The South American monsoon system (SAMS) refers to the austral summer season features of deep convective activity and large-scale circulation. This study examines intraseasonal variations in the low-level wind circulation in the Amazon and their modulating effects on active and “break” phases in SAMS. Daily averages of outgoing longwave radiation (OLR), NCEP–NCAR reanalysis, and gridded rainfall station data in Brazil are used from 1 November to 28 February 1980–99. The direction of wind anomalies (10–70 days) in the Rondônia State, Brazil, is used to classify periods of westerly (W) and easterly (E) low-level wind regimes. Composites of W regime show low-level wind anomalies crossing the equator southward and closing in a cyclonic anomalous circulation off the coast of Argentina and Uruguay. Broad areas of enhanced convection and rainfall are observed in central and southeast Brazil. Suppressed convection is observed over the Bolivian Altiplano and in northern South America. In contrast, in the E regime, opposite patterns are observed in the low-level circulation, convection, and rainfall anomalies. The duration of active (W regimes) and break (E regimes) periods are quite similar, with median values of 4 and 5 days, respectively. Further investigation showed that the region of convection and rainfall anomalies over Venezuela and northwest Brazil is observed only in the 10–30-day band. Comparison of the results shown here with previous studies indicates the importance of intraseasonal variations in the activity of SAMS.

Corresponding author address: Dr. Charles Jones, Institute for Computational Earth System Science, University of California, Santa Barbara, Santa Barbara, CA 93106. Email: cjones@icess.ucsb.edu

Abstract

The South American monsoon system (SAMS) refers to the austral summer season features of deep convective activity and large-scale circulation. This study examines intraseasonal variations in the low-level wind circulation in the Amazon and their modulating effects on active and “break” phases in SAMS. Daily averages of outgoing longwave radiation (OLR), NCEP–NCAR reanalysis, and gridded rainfall station data in Brazil are used from 1 November to 28 February 1980–99. The direction of wind anomalies (10–70 days) in the Rondônia State, Brazil, is used to classify periods of westerly (W) and easterly (E) low-level wind regimes. Composites of W regime show low-level wind anomalies crossing the equator southward and closing in a cyclonic anomalous circulation off the coast of Argentina and Uruguay. Broad areas of enhanced convection and rainfall are observed in central and southeast Brazil. Suppressed convection is observed over the Bolivian Altiplano and in northern South America. In contrast, in the E regime, opposite patterns are observed in the low-level circulation, convection, and rainfall anomalies. The duration of active (W regimes) and break (E regimes) periods are quite similar, with median values of 4 and 5 days, respectively. Further investigation showed that the region of convection and rainfall anomalies over Venezuela and northwest Brazil is observed only in the 10–30-day band. Comparison of the results shown here with previous studies indicates the importance of intraseasonal variations in the activity of SAMS.

Corresponding author address: Dr. Charles Jones, Institute for Computational Earth System Science, University of California, Santa Barbara, Santa Barbara, CA 93106. Email: cjones@icess.ucsb.edu

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  • Carvalho, L. M. V., C. Jones, and M. A. F. Silva Dias, 2002: Intraseasonal large-scale circulations and mesoscale convective activity in the tropical South America during the TRMM-LBA campaign. J. Geophys. Res., in press.

    • Search Google Scholar
    • Export Citation
  • Charba, J. P., A. W. Harrell III, and A. C. Lackner III, 1992: A monthly precipitation amount climatology derived from published atlas maps: Development of a digital database. NOAA TDL Office Note 92-7, 20 pp.

    • Search Google Scholar
    • Export Citation
  • Cifelli, R., W. A. Petersen, L. D. Carey, and S. A. Rutledge, 2001: Radar observations of kinematics, microphysical, and precipitation characteristics of two MCSs in TRMM-LBA. J. Geophys. Res., in press.

    • Search Google Scholar
    • Export Citation
  • Cressman, G. P., 1959: An operational objective analysis system. Mon. Wea. Rev., 87 , 367374.

  • Duchon, C. E., 1979: Lanczos filter in one and two dimensions. J. Appl. Meteor., 18 , 10161022.

  • Figueroa, S. N., P. Satyamurty, and P. L. da Silva Dias, 1995: Simulations of the summer circulation over the South American region with an eta coordinate model. J. Atmos. Sci., 52 , 15731584.

    • Search Google Scholar
    • Export Citation
  • Gandu, A. W., and J. E. Geisler, 1991: A primitive equations model study of the effect of topography on the summer circulation over tropical South America. J. Atmos. Sci., 48 , 18221836.

    • Search Google Scholar
    • Export Citation
  • Gandu, A. W., and P. L. Silva Dias, 1998: Impact of tropical heat sources on the South American tropospheric upper circulation and subsidence. J. Geophys. Res., 103 , 60016015.

    • Search Google Scholar
    • Export Citation
  • Glahn, H. R., T. L. Chambers, W. S. Richardson, and H. P. Perrotti, 1985: Objective map analysis for the local AFOS MOS Program. NOAA Tech. Memo. NWS TDL 75, 34 pp.

    • Search Google Scholar
    • Export Citation
  • Horel, J. D., A. N. Hahmann, and J. E. Geisler, 1989: An investigation of the annual cycle of convective activity over the tropical Americas. J. Climate, 2 , 13881403.

    • Search Google Scholar
    • Export Citation
  • Jones, C., 1990: An investigation of low-frequency variability of the large-scale circulation over South America. M.S. thesis, Department of Meteorology, University of Utah, 108 pp.

    • Search Google Scholar
    • Export Citation
  • Jones, C., and B. C. Weare, 1996: The role of low-level moisture convergence and ocean latent heat fluxes in the Madden and Julian Oscillation: An observational analysis using ISCCP data and ECMWF analyses. J. Climate, 9 , 30863104.

    • Search Google Scholar
    • Export Citation
  • Jones, C., D. E. Waliser, and C. Gautier, 1998: The influence of the Madden–Julian Oscillation on ocean surface heat fluxes and sea surface temperature. J. Climate, 11 , 10571072.

    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors. . 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77 , 437471.

  • Knutson, T. R., and K. M. Weickmann, 1987: 30–60 day atmospheric oscillations: Composite life cycles of convection and circulation anomalies. Mon. Wea. Rev., 115 , 14071436.

    • Search Google Scholar
    • Export Citation
  • 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
  • Liebmann, B., G. N. Kiladis, J. A. Marengo, T. Ambrizzi, and J. D. Glick, 1999: Submonthly convective variability over South America and the South Atlantic convergence zone. J. Climate, 12 , 18771891.

    • Search Google Scholar
    • Export Citation
  • Madden, R. A., and P. R. Julian, 1994: Observations of the 40–50-day tropical oscillation: A review. Mon. Wea. Rev., 112 , 814837.

  • Maloney, E. D., and D. L. Hartmann, 1998: Frictional moisture convergence in a composite life cycle of the Madden–Julian Oscillation. J. Climate, 11 , 23872403.

    • Search Google Scholar
    • Export Citation
  • Nogués-Paegle, J., and K. C. Mo, 1997: Alternating wet and dry conditions over South America during summer. Mon. Wea. Rev., 125 , 279291.

    • Search Google Scholar
    • Export Citation
  • Nogués-Paegle, J., L. A. Byerle, and K. Mo, 2000: Intraseasonal modulation of South American summer precipitation. Mon. Wea. Rev., 128 , 837850.

    • Search Google Scholar
    • Export Citation
  • Petersen, W. A., S. W. Nesbitt, R. J. Blakeslee, P. Hein, R. Cifelli, and S. A. Rutledge, 2001: TRMM observations of intraseasonal variability in convective regimes over the Amazon. J. Climate, in press.

    • Search Google Scholar
    • Export Citation
  • Silva Dias, M. A. F., and Coauthors. 2000: Rainfall and surface processes in Amazonia during the WETAMC/LBA: An overview. Preprints, Sixth Int. Conf. on Southern Hemisphere Meteorology and Oceanography, Santiago, Chile, Amer. Meteor. Soc., 249–250.

    • Search Google Scholar
    • Export Citation
  • Silva Dias, P. L., W. H. Schubert, and M. DeMaria, 1983: Large-scale response of the tropical atmosphere to transient convection. J. Atmos. Sci., 40 , 26892707.

    • Search Google Scholar
    • Export Citation
  • Vernekar, A. D., V. Thapliyal, R. H. Kripalani, S. V. Singh, and B. Kirtman, 1993: Global structure of the Madden–Julian Oscillations during two recent contrasting summer monsoon seasons over India. Meteor. Atmos. Phys., 52 , 3747.

    • Search Google Scholar
    • Export Citation
  • Zhou, J., and K-M. Lau, 1998: Does a monsoon climate exist over South America? J. Climate, 11 , 10201040.

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