The dynamic and thermodynamic processes involved in the life cycle of a cutoff low occurred in March 2007 are studied. These processes are analyzed using the vorticity and thermodynamic equations and a set of analyses generated with the BRAMS model. The main processes that explain the segregation of the subtropical part of the trough are the horizontal advection of cyclonic vorticity at high levels and warm horizontal advection at middle levels, both over the Pacific Ocean extending south to the Patagonia region, building the ridge located upstream of the trough. Increased intensity of the upper level low pressure system is mainly explained by intensification of the ridges down and upstream. The divergence effect is opposed to the horizontal advection of vorticity which explains the stagnation of the cut-off low windward of the Andes. The decay stage is dominated by warm vertical advection. Assuming the conservation of potential vorticity the analysis of parcel trajectories , allowed detecting the entrance of stratospheric air to middle levels of the lower troposphere around the cut-off low. Keywords: cut-off lows life cycle, physical processes RESUMO: UM CASO DE VÓRTICE CICLONICO DESPRENDIDO EM ALTOS NÍVEIS NO SUL DA AMÉRICA DO SUL: PROCESSOS DINÂMICOS E TERMODINÂMICOS Neste trabalho foram estudados os processos dinâmicos e termodinâmicos que envolvem o ciclo de vida de um caso de vórtice ciclônico desprendido em altos níveis, que ocorreu em março de 2007. Tais processos são analisados através dos termos das equações de vorticidade e termodinâmica usando um conjunto de análises geradas com o modelo BRAMS. Os principais processos que explicam o desprendimento da parte subtropical de um cavado são a advecção horizontal da vorticidade anticiclonica em níveis altos e a advecção horizontal de ar quente em níveis médios, ambas sobre o sul do Oceano Pacífico e estendendo-se em direção a região Patagônica, aprofundando a crista localizada corrente acima do cavado. A continuidade do aprofundamento desta crista, juntamente com a crista localizada corrente abaixo do cavado são as responsáveis da intensificação do sistema de baixa pressão. O efeito da divergência se opõe a advecção horizontal da vorticidade, e isso explica a situação estacionaria da baixa desprendida a barlavento da Cordilheira dos Andes. A fase de decaimento é dominada pela advecção vertical de ar quente. A análise das trajetórias das parcelas, considerando a conservação da vorticidade potencial, permitiu observar a entrada de ar estratosférico até os níveis médios da troposfera, em torno do vórtice ciclônico desprendido em altos níveis. Palavras-Chave: vórtice ciclônico desprendido, ciclo de vida, processos dinâmicos e termodinâmicos . 504 Godoy et al. Volume 26(4)
RESUMENEn este trabajo se analiza el ciclo de vida de una baja segregada que afectó el sur de Sudamérica entre el 26 de Marzo y el 2 de Abril del 2007 y se explora la relación existente entre las diferentes etapas de este sistema y la precipitación ocurrida sobre la franja central de Argentina. Además se evalúa si otros procesos coadyuvaron para la ocurrencia de la misma. Las características más relevantes de este sistema son su larga duración y el lento desplazamiento, ya que entre el inicio y el fin de su vida se desplaza sólo 780 km, manteniéndose estacionario, durante las etapas de segregación y madurez, por más de 56 hs frente a la costa central chilena. Esta última característica favorece la formación de un sistema de baja presión sobre el noroeste de Argentina y la persistencia de una corriente en chorro en capas bajas del norte que se extiende desde el centro de Bolivia hasta norte argentino. La masa de aire con características tropicales conducida por esta configuración y la convergencia en niveles bajos favorecieron el desarrollo convectivo. La precipitación acumulada durante estas etapas, en algunas regiones del sur del litoral argentino, superó los 300 mm, produciendo extensas inundaciones en la región. Durante la etapa de decaimiento (96 hs de duración), el sistema se debilita a medida que se desplaza hacia el este y las precipitaciones afectaron las provincias del centro-oeste de Argentina donde la lluvia en 48 horas duplicó el promedio mensual de marzo. Palabras-clave: Baja Segregada, ciclo de vida, impacto en precipitación ABSTRACT: A CASE OF HIGH LEVEL CUT-OFF CYCLONIC VORTEX OVER SOUTHERN SOUTH AMERICA: DESCRIPTION OF THE LIFE CYCLE AND ITS RELATION TO PRECIPITATIONThis paper describes the life cycle of a high level cut-off cyclonic vortex that affected southern South America between 26 March and 2 April 2007, and explores the relationship between the evolution stages of the system and the rainfall over central Argentina. The possibility of other processes contributing to rainfall is also analyzed. The most relevant characteristics of this system were its length and slow displacement: from its development to decay it moved only 780 km. During its segregation and mature stages, it remained stationary for over 56 hours along the coast of central Chile. This latter feature favored the establishment of a low pressure system in the northwest of Argentina and the persistence of a northerly low level jet, from the center of Bolivia to the north of Argentina. The air mass with tropical characteristics driven by this system and the low-level convergence contributed to the development of convection. Accumulated rainfall during those stages, in some regions of central eastern Argentina was above 300 mm, producing significant flooding in the region. During the decay phase (96 hours duration), the system weakened as it moved eastward and precipitation affected the provinces of central-western Argentina, where the 48 h rain was twice the monthly average for March.
Abstract:Storm surges are water-level anomalies linked to meteorological conditions which interact with astronomical tides. This study presents the first approach to the atmospheric circulation patterns in the Río de la Plata estuary (port of Buenos Aires) that cause negative storm surges (NSS) lower than −120 cm.The aim of this work is to make a comprehensive statistical analysis of the NSS and to examine their variability on monthly to interdecadal scales. These events occur with a frequency of 4.64 events per year and take place during the whole year but are more usual during the austral winter. They persist from 10 to 50 h and take more time to reach their maximum depth than to recover.The interdecadal variability of all NSS lower than −120 cm indicates a reduction in the number of cases. However, a detailed analysis shows that the more intense events (lower than −180 cm) increase not only in their number but in their depth as well.NSS are caused by different atmospheric patterns that promote the generation of west component winds. Observational winds recorded at two synoptic stations suggest that the most frequent wind directions are north, northwest and west at the beginning of the events.Storm surges lower than −180 cm are related to a cyclone located in the east of Buenos Aires province. This system intensifies and moves towards the southeast, favouring westerly winds over the Río de la Plata. The interdecadal variability analysis of the deepest surges suggests a deepening of the cyclone since 1970.
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