OSCILAÇÃO MADDEN-JULIAN E SEUS IMPACTOS NAS CHUVAS SOBRE A AMÉRICA DO SUL

Anderlan Henrique Batista Siqueira

Meteorologista, COMERC Energia

A previsão do tempo e do clima é essencial para a sociedade. No entanto, prever fenômenos atmosféricos e o regime de precipitação sobre a América do Sul não é tarefa fácil.

O sistema elétrico brasileiro (SEB), cuja matriz é essencialmente de origem hidrelétrica, depende de uma melhor previsão de precipitação tanto diária quanto semanalmente. É possível, contudo, ter uma noção do que pode ocorrer utilizando-se um prognóstico climático que leva em conta as escalas mensal e sazonal. Cada escala temporal pode ser associada a fenômenos distintos que mudam as condições de tempo em determinada região. Por exemplo, as frentes frias, ao entrar na região sul do País, mudam as condições de tempo entre dias e semanas. Esses fenômenos podem ser governados por outros de escala planetária. Este artigo busca estudar um desses fenômenos, a oscilação Madden-Julian (OMJ).

A OMJ é uma perturbação tropical que se propaga do Oeste para o Leste em torno da região tropical do globo, com um ciclo de 30 a 60 dias. Seus impactos são amplos e atingem os padrões de precipitação nas regiões tropicais e extratropicais, a circulação atmosférica e a temperatura da superfície (HIGGINS e MO, 1997; HIGGINS e SHI, 2001).

Há a evidência de que a OMJ influencia o ciclo El Niño-Oscilação Sul. Embora não cause o El Niño ou La Niña, ela pode contribuir para acelerar o seu desenvolvimento e intensidade. Sobre a América do Sul, a OMJ pode ajudar ou inibir a formação de nuvens e, por consequência, aumentar ou diminuir a quantidade de chuva. A identificação da ocorrência da OMJ se dá por meio do campo da velocidade potencial, a uma pressão atmosférica de 200 MiliBar e a aproximadamente 12 quilômetros de altura.

A figura 1, abaixo, mostra o campo da velocidade potencial e a temperatura do topo das nuvens, em Kelvin. As linhas verdes (valores negativos de velocidade potencial) favorecem a formação de nebulosidade; as linhas marrons (valores positivos de velocidade potencial) desfavorecem a formação de nebulosidade.

Figura 1: Campo da velocidade potencial e temperatura de brilho do topo das nuvens para o dia 7 de julho de 2015.

Fonte: CPC/NCEP/NOAA.
Fonte: CPC/NCEP/NOAA.

Nota-se no dia 7 de julho de 2015 a predominância de valores negativos (linhas verdes) e alta nebulosidade sobre a região centro-sul da América do Sul. Com essa configuração, a região pode apresentar um aumento de chuva neste dia. Entretanto, alta nebulosidade não significa necessariamente em um aumento das precipitações.

A figura 2, abaixo, apresenta um diagrama esquemático do deslocamento e da evolução da OMJ sobre a região do Oceano Pacífico tropical em direção à América do Sul.

Figura 2: Diagrama esquemático da evolução da Oscilação Madden-Julian sobre o oceano Pacífico Tropical.

Fonte: https://rclutz.wordpress.com/2015/05/11/dynamic-duo-the-ocean-air-partnership/
Fonte: https://rclutz.wordpress.com/2015/05/11/dynamic-duo-the-ocean-air-partnership/

As etapas de evolução da OMJ podem ser definidas e apresentadas na seguinte ordem:

  1. Formação de um sistema de baixa pressão e convecção profunda no oceano Índico, próximo à Indonésia;
  2. O sistema desloca-se para a região da piscina de água quente do Pacífico na costa norte da Austrália, ganha força e o desenvolvimento vertical das nuvens se intensifica (linhas verdes na figura 1);
  3. A oeste e a leste da nuvem de desenvolvimento vertical são formados sistemas de alta pressão que inibem a formação de nebulosidade (linhas marrons na figura 1);
  4. O sistema desloca-se para leste. Ao se aproximar da América do Sul, a alta pressão se intensifica e inibe a formação de nuvens, principalmente na região tropical da América do Sul, sobre o Brasil;
  5. Com a continuidade do movimento para leste, o sistema de baixa pressão passa sobre a América do Sul e favorece a formação de nebulosidade, que pode ou não aumentar a precipitação.

Referências

Higgins, R. W., and S. D. Schubert, 1996: Simulations of persistent North Pacific circulation anomalies and interhemispheric teleconnections. J. Atmos. Sci., 53, 188-207.

Higgins, R. W., and K. C. Mo, 1997: Persistent North Pacific circulation anomalies and the tropical intraseasonal oscillation. J. Climate, 10, 223-244.

Higgins, R. W., J. K. Schemm, W. Shi and A. Leetmaa, 2000a: Extreme precipitation events in the western United States related to tropical forcing. J. Climate, 13,793-820.

Higgins, R. W., and W. Shi, 2001: Intercomparison of the principal modes of interannual and intraseasonal variability of the North American Monsoon System. J. Climate, 14, 403-417.

L’Heureux, M. L. and R. W. Higgins, 2008: Boreal Winter Links between the Madden-Julian Oscillation and the Arctic Oscillation. J. Climate, In press.

Kousky, V. and M. Kayano, 1994: Principal modes of outgoing longwave radiation and 250-mb circulation for the South American sector. J. Climate, 1994, 1131-1143.

Kayano, M. and V. Kousky, 1999: Intraseasonal (30-60 day) variability in the global tropics: principal modes and their evolution. Tellus, 51, 373-386.

Madden, R.A. and P. R. Julian, 1972: Description of global-scale circulation cells in the tropics with a 40-50 day period. J. Atmos. Sci, 29, 1109-1123.

Mo, K. C. and R. W. Higgins, 1998: Tropical influences on California precipitation. J. Climate, 11, 412-430.

Seo, K., W. Wang, J. Gottschalck, Q. Zhang, J. Schemm, W. Higgins and A. Kumar, 2008: Evaluation of MJO Forecast Skill from several statistical and dynamical forecast models, J. Climate, Submitted.

Wheeler, M. and H. Hendon, 2004: An All-Season Real-Time Multivariate MJO Index: Development of an Index for Monitoring and Prediction. Mon. Wea. Rev., 132, 1917-1932.

Zhang, C. and J. Gottschalck, 2002: SST Anomalies of ENSO and the Madden-Julian Oscillation in the Equatorial Pacific. Journal of Climate, 15, 2429-2445.

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