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Ann. Geophys., 26, 1379-1390, 2008
www.ann-geophys.net/26/1379/2008/
© European Geosciences Union 2008
The role of the observed tropical convection in the generation of frost events in the southern cone of South America
G. V. Müller1, T. Ambrizzi2, and S. E. Ferraz3
1Centro de Investigación Científica y de Transferencia Tecnológica a la Producción (CICYTTP/CONICET), Diamante, Argentina
2Departamento de Ciências Atmosféricas, Universidade de São Paulo, Brazil
3Departamento de Física, Universidade Federal de Santa Maria/CRSPE-INPE, Brazil
Abstract. Based on previous results obtained from observations and linear wave theory
analysis, the hypothesis that large-scale patterns can generate extreme cold
events in southeast South America through the propagation of remotely
excited Rossby waves was already suggested. This work will confirm these
findings and extend their analysis through a series of numerical experiments
using a primitive equation model where waves are excited by a thermal
forcing situated in positions chosen according to observed convection
anomalies over the equatorial region. The basic state used for these
experiments is a composite of austral winters with maximum and minimum
frequency of occurrence of generalized frosts that can affect a large area
known as the Wet Pampas located in the central and eastern part of Argentina.
The results suggest that stationary Rossby waves may be one important
mechanism linking anomalous tropical convection with the extreme cold events
in the Wet Pampas. The combination of tropical convection and a specific
basic state can generate the right environment to guide the Rossby waves
trigged by the tropical forcing towards South America. Depending on the
phase of the waves entering the South American continent, they can favour the
advection of anomalous wind at low levels from the south carrying cold and
dry air over the whole southern extreme of the continent, producing a
generalized frost in the Wet Pampa region. On the other hand, when a basic
state based on the composites of minimum frosts is used, an anomalous
anticyclone over the southern part of the continent generates a circulation
with a south-southeast wind which brings maritime air and therefore humidity
over the Wet Pampas region, creating negative temperature anomalies only
over the northeastern part of the region. Under these conditions even if
frosts occur they would not be generalized, as observed for the other basic
state with maximum frequency of occurrence of generalized frosts.
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