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O balanco do vento termico indica a relacao entre o gradiente meridional de temperatura e o cisalhamento vertical do vento, o que explica a existencia do jato de altos niveis – chamado de corrente de jato, ou simplesmente jato – em latitudes medias e altas. E esperado que a posicao do jato corresponda a condicoes de instabilidade em superficie. O fenomeno climatico ENOS (El Nino-Oscilacao Sul) provoca uma alteracao no gradiente de temperatura em baixos niveis, pois muda o padrao normal de temperatura da superficie do mar do Pacifico Tropical leste. Assim, o presente trabalho usa dados de reanalise II do NCEP (1981-2010) para analisar como que as tres diferentes fases do ENOS (El Nino, La Nina e neutra) modificam o padrao medio climatologico de balanco do vento termico no continente sulamericano. Primeiramente, os resultados mostraram boa associacao entre os nucleos do jato e as regioes de maior gradiente de temperatura em 850 hPa. Nos periodos de La Nina, o jato apresentou-se mais intenso no inverno sobre o continente. Ja em anos de El Nino observou-se jato mais intenso em latitudes medias sobre o Pacifico, na costa oeste do continente, principalmente no verao e primavera, indicando que o incremento de temperatura na superficie do mar no Pacifico acarreta em maior gradiente de temperatura em latitudes medias e, consequentemente, em uma corrente de jato mais intensa. A B S T R A C T The thermal wind balance shows the relation between meridional gradient of temperature and vertical wind shear, explaining the existence of the upper-tropospheric jet – called jet stream, or simply jet – in high and mid-latitudes. It is expected that jet position is associated to instability conditions in surface. The climate phenomenon ENSO (El Nino-South Oscillation) causes a perturbation in the low levels temperature gradient, since that changes the normal pattern of sea surface temperature of eastern tropical Pacific ocean. Thus, this work employs NCEP II reanalyses dataset (1981-2010) to analyze how the three phases of ENSO (El Nino, La Nina and neutral) modify the climatic pattern of thermal wind balance in the South American continent. First, the results showed good association between jet streaks and the regions with highest temperature gradient at 850 hPa. During La Nina periods the jet stream had become more intense in the winter, over the continent. On the other hand, during El Nino, a more intense jet was observed in the mid-latitudes over Pacific, on the west coast of the continent, mainly in summer and spring, indicating that the increase in sea surface temperature in Pacific entails a higher mid-latitudes temperature gradient and, therefore, a more intense jet stream. Keywords: jet stream, meridional gradient of temperature, El Nino, La Nina

revista brasileira de geografia física

Comportamento climático do balanço do vento térmico na América do Sul de acordo com os eventos ENOS: Estudo preliminar (Climatic behavior of thermal wind balance in South America according to ENSO events: Preliminary study)