Austral Teleconnection Patterns Associated to Generalized Frosts

MULLER GABRIELA V; AMBRIZZI TÉRCIO

Trieste, Italia

Conferencia; Conference on Teleconnections in the Atmosphere and Oceans; 2008

International Centre for Theoretical Physics (ICTP)

The identification of teleconnection patterns and the analysis of their effect on thehorizontal structure of the atmospheric circulation can be very useful to understand anomalousevents in many regions of the world. Previous observational studies of the mean atmosphericcirculation leading to generalized frosts (GF) in central Southern South America, suggested thatspecific large scale patterns can be associated to the frequency of occurrence of these eventsthrough the propagation of Rossby waves remotely excited. This hypothesis is tested through ateleconnection analysis for austral winters which present an extreme frequency of occurrence ofGF in southeastern South America. Rossby wave propagation regions are identified for twobasic states given by the composition of winters with maximum and minimum frequency of GFoccurrence, during the 1961–1990 period.Based on the stationary wavenumber (Ks) it is possible to identify regions where theRossby wave propagation is permitted and those where it will be inhibited (Ks = 0), highlightingthe importance of the upper level jets as waveguides. The differences found indicate that thelocation of the wave generation and its later evolution are conditioned by the basic state.Results are validated through a baroclinic model, which simulates the Rossby wave patternsresponsible for the teleconnection. Numerical experiments confirm that the principal waveactivity takes place inside the subtropical and polar jets. In particular, for the basic state withmaximum frequency of GF occurrence, the wave trains propagating inside the subtropical andpolar waveguides merge just before entering the continent, in agreement with the observationsprior to the occurrence of GF events. This configuration favors the development of an intensesouth wind anomaly with large meridional extension which results in the intensification ofanticyclonic circulation in southern South America, with cold dry air advection from the south,dropping the temperature in the center and east of the southern South American continent. Aconceptual model will be presented summarizing all these results.