Seasonal differences in tropical intraseasonal variability over South America from the NCEP reanalysis

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Wednesday, 20 January 2010: 4:30 PM
B216 (GWCC)
Fernando Hirata, Georgia Institute of Technology, Atlanta, GA; and C. D. Hoyos and P. J. Webster

Precipitation over tropical South America presents marked intraseasonal variability during the entire year. Summertime variations of rainfall associated with the monsoonal regime and the development of the South Atlantic Convergence Zone are well known. However, the physical mechanisms controlling this variability are still unclear, especially during austral winter. Composites reveal the major features of the intraseasonal signal associated with minimum OLR events over the South American continent for both warm and cold seasons separately. The results indicate remarkable seasonal differences that may help to improve regional weather forecasts and diminish losses associated with extreme floods and droughts. During the warm season, the South American Summer Monsoon imprint is prominent. A global eastward-propagating signal links the South Pacific Convergence Zone, the South American Monsoon and the initiation of a new cycle of the Madden-Julian Oscillation. The composite analysis for the cold season indicate a strong association between maximum intraseasonal convection over South America and the Asian Summer Monsoon. An anomalous extratropical wave pattern acting over the South American sector, resembling a Pacific-South American mode, is apparently related to the triggering of convection over Brazil. Finally, we attempt to explain physically the features highlighted by the analysis.