Sistema de monção da América do Sul: início e fim da estação chuvosa e sua relação com a Zona de Convergência Intertropical do Atlântico / The south american monsoon system: onset and demise of the rainy season and its relationship with the Atlantic Intertropical Convergence Zone

AUTOR(ES)
DATA DE PUBLICAÇÃO

2010

RESUMO

Physical, dynamic and thermodynamic aspects related to the South American Monsoon System (SAMS) are investigated for the 1979-2006 period. A new methodology to determine the onset and demise dates of the rainy season (ONR and DER, respectively) related to the SAMS is developed using the field of antisymmetric outgoing longwave radiation in relation to the equator (emph{AOLR}). Thus, an index for monitoring these dates is found. The sign change from positive to negative (negative to positive) ofthe Aemph{OLR} averaged in space indicates the ONR (DER) for the SAMS regions. Composites of several variables are done for pentads around the ONR and DER. These composites show coherent features of the atmospheric circulation, providing the physical reasoning for the method application. The dominant OLR modes are found for the tropical American area and adjacent Atlantic Ocean, through the Empirical Orthogonal Functions (EOF) analysis for the 1979-2006 period. In this analysis, the OLR data is filtered in the 0.4-1.2 year scale using the wavelet analysis. The results provide three dominant modes: monsoon, Intertropical Convergence Zone (ITCZ) and symmetric modes, with the first two modes varying in an annual scale, and the latter one in a semiannual scale. The evolving aspects ofthese modes are analyzed by correlating (lagged and simultaneously) the principal component of each mode and the filtered OLR time series. The symmetric mode evolves independentlyand depicts the transition from dry to wet season and vice-versa, while the monsoon and ITCZ modes evolve into each other within a period of one year. The relationship between the SAMS and Atlantic ITCZ found on the corre1ation analysis is investigated through the moisture and heat budgets. While dry (wet) conditions remain in the ITCZ, indicated by negative (positive) anomalies of the vertical advection of q, positive (negative) anomalies of the residue term, positive (negative) anomalies of the adiabatic term, and negative (positive) anomalies of the diabatic term, the convective activity does not start (fmish) in the SAMS related areas. The end of the dry (wet) conditions by lag 20 (56) pentads in the ITCZ area is indicated by maximum positive (negative) anomalies oflocal rates of q and T changes. That is when wet (dry) conditions start to appear in the west-central Brazil and in the South Atlantic Convergence Zone. Therefore, these results might have important implications in modeling studies and monitoring activities of the SAMS and ITCZ.

ASSUNTO(S)

calor atmosférico south america radiação de onda longa intertropical convergence zone monsoon system umidade atmosférica sistema de monção atmospheric heat zona de convergência intertropical atmospheric moisture américa do sul long wave radiation

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