RESUMEN La ocurrencia de eventos secos en la región Sudeste de Brasil (SEB, por sus siglas en inglés) durante el verano (temporada de lluvias) se ha evidenciado en los últimos años, principalmente debido a eventos extremos previos en las temporadas 2013/14 y 2014/15. Los análisis de sequía se suelen realizar con datos mensuales. Aquí nuestra metodología aborda el tema con datos diarios para generar un análisis exhaustivo. Los eventos secos se evaluaron en diferentes subregiones de precipitación homogénea dentro del SEB, durante 37 temporadas diciembre-febrero (DJF, por sus siglas en inglés) y con dos escalas de tiempo diferentes de duración: sinóptica (5-9 días) e intraestacional (≥10 días). Se encontraron dos patrones dinámicos principales distintos para los eventos secos en las partes sur y centro-norte de SEB, respectivamente, pero no se identificaron diferencias importantes en las diferentes escalas de tiempo de ocurrencia. Los eventos del sur se caracterizaron por una cresta estacionaria que actuaba sobre todo el sur de América del Sur, lo que dificultó la aproximación de los sistemas transitorios al SEB del sur. Al mismo tiempo, este patrón mostró una configuración de Zona de Convergencia del Atlántico Sur (SACZ, por sus siglas en inglés) desplazada hacia el norte. En los eventos centro-norte, una alta presión centrada entre las regiones sur y sureste de Brasil se asoció con las condiciones de sequía. También se verificó un desplazamiento anómalo hacia el sur de los sistemas meteorológicos característico del verano sudamericano para estos eventos. Sobre el Atlántico Sur, se identificó una configuración de anomalía SST opuesta entre los eventos del sur y del centro-norte.

ABSTRACT The occurrence of dry events in the Southeast Region of Brazil (SEB) during summer (rainfall season) has been in evidence in the last years, mainly due to previous extreme events in the 2013/14 and 2014/15 seasons. Drought analyses are usually carried out with monthly data. Here our methodology addresses the issue with daily data to generate a thorough analysis. Dry events were evaluated in different homogeneous precipitation sub-regions within the SEB, over 37 December-February (DJF) seasons and with two different timescales of duration: synoptic (5-9 days) and intraseasonal (≥10 days). Two main distinct dynamic patterns were found for dry events in southern and central-northern parts of SEB, respectively, but no significant differences were identified in the different timescales of occurrence. Southern events were characterized by a stationary ridge acting over the whole of southern South America, making the approximation of the transient system to southern SEB difficult. At the same time, this pattern showed a northern-shifted South Atlantic Convergence Zone (SACZ) configuration. In the central-northern events, a high pressure centered between Brazil’s South and Southeast regions was associated with the dryness conditions. An anomalous southward shift of meteorological systems characteristic of the South American summer was also verified for these events. Over the South Atlantic, an opposite SST anomaly configuration was identified between southern and central-northern events.

Resumo A relação entre a Zona de Convergência do Atlântico Sul (ZCAS) e o Vórtice Ciclônico de Altos Níveis (VCAN) foi analisada durante 34 verões austrais, de acordo com as fases positivas, negativas e neutras do El Niño Oscilação Sul (ENOS). Anomalias das componentes zonais e meridionais do vento, foram calculadas com base na climatologia de 1979-2013 no nível de 200 hPa, e submetidos ao filtro de Lanczos nas bandas temporais de 20-90 dias e 200 dias passa baixa, com a intenção de destacar as variabilidades intrassazonal e interanual, através da análise observacional de três composições médias de ZCAS referentes às fases positiva, negativa e neutra do ENOS. As composições filtradas nas bandas intrassazonal e interanual revelam a intensificação do VCAN tanto para os casos de ZCAS-El Niño e de ZCAS-Neutras, enquanto que a variabilidade interanual durante eventos de ZCAS sob anos de La Niña inibem a ação do VCAN próximo do continente a partir de uma circulação anticiclônica na região.

Abstract The relationship between the South Atlantic Convergence Zone (SACZ) and the Upper Tropospheric Cyclonic Vortex (UTCV) was analyzed over 34 summers, with positive, negative and neutral phases of El Niño Southern Oscillation. Zonal and meridional wind components anomalies, calculated based on climatology between 1979-2013 at the level of 200 hPa, were submitted to the Lanczos filter for 20-90 days and 200 days low pass bands, with the intention of highlighting intraseasonal and interannual variability, ensuring three mean compositions of SCAZ referring to the positive, negative and neutral phases of ENSO. The compositions for the intraseasonal and interannual band reveal the intensification of the UTCV for both SACZ-El Niño and SACZ-Neutral, while the interannual variability during SACZ events under La Niña activity inhibit the formation of the UTCV near the continent due to anticyclonic circulation in the area.