On precursors of South American cyclogenesis

Abstract

Cyclogenesis over the southern region of South America is studied in relation to the evolution of dynamic and thermodynamic fields over the continent, using storm tracking and derived composite analysis over a 25 yr period. Results show that, irrespective of the season, there is always a moist-entropy reservoir northwest of the cyclone formation region. One day before cyclone formation, moist entropy over North Argentina is anomalously high and increasing, due to an intensification of the northerly flow along the eastern flank of the Andes, peaking at the time of cyclogenesis. The new cyclone is fed by warm and moist air coming from the continent but, as it intensifies and moves eastwards, imposes an anomalously southerly flow reducing the moist-entropy anomaly over South America. The presence of the Andes Cordillera plays a major role controlling the location of the main cyclone formation area in the region by (i) channelling the warm moist northerly flow in the lower troposphere and (ii) inducing cyclonic circulation in the lee side, when upper troughs travelling eastwards move over the mountain range at the middle latitudes.

Climatology of extratropical cyclones over the South American–southern oceans sector

“…The algorithm implemented to detecting cyclones in the SA sector follows quite closely that described by Blender et al (1997) and Trigo et al (1999) and later adapted to SH by Mendes et al (2007).…”

“…Moreover, the spatial distribution and inter-seasonal variability of cyclogenesis are consistent with the main mechanisms associated with cyclone development frequently suggested for this area, namely baroclinic instability of the westerlies and orographic induced cyclone development in the lee side of the Andes Cordillera (Gan and Rao, 1991;Gan and Rao, 1994;Sinclair, 1995;Bonatti and Rao, 1997). Recently, Mendes et al (2007) showed that cyclone formation over SA is characterized by some remarkable features: (i) this large number of events originates in a rather localized area; (ii) the cyclogenesis process is preceded by an anomalous flow over the continent, associated with the transport of warm and moist air into the cyclogenesis region and (iii) cyclogenesis occurs in all seasons with similar mean anomalies but with somewhat different statistics.…”

“…Blender et al, 1997;Serreze et al, 1997;Mendes et al, 2007): a cyclone's trajectory is determined by computing the distance to cyclones detected in the previous chart and assuming the cyclone has taken the path of minimum distance; if the nearest neighbor in the previous chart is not within an area determined by imposing a maximum cyclone velocity of 33 km/h in the westward direction and of 90 km/h in any other, then cyclogenesis is assumed to have occurred. We stress that these thresholds were determined empirically by observing cyclone behavior in SLP charts.…”

Estudo de caso de ciclone extratropical sobre a América do Sul: sensibilidade das análises

“…Trigo et al, 1999) Outros pesquisadores, enfocando apenas a América do Sul (AS), observaram que esta região é propícia para a formação de ciclones extratropicais (e.g. Taljaard, 1967;Necco, 1982;Satyamurty et al, 1990;Gan e Rao, 1991;Sinclair, 1994; e mais recentemente, Mendes et al, 2007), devido ao fato desta área ser uma região altamente baroclínica e possuir uma fonte de energia e entropia favorável a formação deste sistema.…”

Contribuição da interação troposfera-estratosfera nas ciclogêneses em superfície sobre a América do Sul

“…Grande parte destes ciclones resulta da instabilidade do escoamento de oeste, chamada de instabilidade baroclínica e que possui forte dependência do gradiente meridional de temperatura, e da indução de desenvolvimento ciclônico a leste dos Andes (Gan e Rao, 1991). Outros fatores também contribuem para a formação dos ciclones, como, por exemplo, o transporte de umidade da região Amazônica para as médias latitudes (em direção à costa leste da América do Sul) e a propagação de um cavado de altos níveis sobre a parte sul dos Andes (Mendes et al, 2007;Reboita, 2008 (Holton, 2004).…”