A cut-off low in southern South America: dynamic and thermodynamic processes

Abstract

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)

A new perspective of the climatological features of upper-level cut-off lows in the Southern Hemisphere

“…In terms of displacement, there is a significant seasonal variation of COL zonal mean velocity, they are faster over the higher latitudes, as suggested by F05. These results suggest that COLs move northeastward in the development phase and eastward/southeastward in the decay phase, as a result of the relative vorticity advection mechanism (Godoy et al 2011).…”

“…Similar but weaker behaviour can be seen in South America from winter to spring, when the COL tracks shift from northeastward on the western side of the Andes to eastward/southeastward on the lee side, as observed in F05. The possible mechanism responsible for the northeastward movement in the COL initial stage is the relative vorticity advection at upper levels, whereas the balance between the relative vorticity advection and the divergent terms is important to make the COL a quasi-stationary system during the cut-off and decay stages (Godoy et al 2011).…”

Climatology and numerical case study of moisture sources associated with subtropical cyclogenesis over the southwestern Atlantic Ocean

“…From 5 to 7 March at 0000 UTC, the warm air advection southeastward/eastward of the cyclone strengthen the midlevel ridge, and the cold air advection near 50°W deepens the upper cold‐core low (Figures f–h). The horizontal temperature advection pattern at these times is conducive to the formation of a cutoff low at 500 hPa [ Campetella and Possia , ; Godoy et al , ].…”

The Exceptionally Cold January of 2017 over the Balkan Peninsula: A Climatological and Synoptic Analysis

“…Three days before the cold episode that occurred over the Balkans, a cold-air outbreak formed over Europe, and cold fronts were observed over the Balkans. Such mid-latitude disturbances (e.g., intense cold fronts and cutoff lows) are also associated with severe temperature events over South America [10,11,38]. Subsequently, the 500-hPa height anomalies of January 2017 presented a strong negative pole over the study region, suggesting the formation of a well-organized cutoff low pressure over the Balkans.…”