TELECONNECTION PATTERNS ASSOCIATED WITH EXTREME FREQUENCY OF GENERALIZED FROSTS. PART II: ORIGIN AND EVOLUTION OF THE ROSSBY WAVES PROPAGATION PATTERNS IN THE AUSTRAL HEMISPHERE Gabriela V. Müller* and Tércio Ambrizzi University of São Paulo (Brazil) 1. INTRODUCTION The identification of teleconnections and the analysis of their impact on the atmospheric circulation can be very useful for the understanding of anomalous events at many regions of the planet when one assume that local forcing may influence the atmosphere circulation at remote locations. Hoskins and Karoly (1981) based on the theory of stationary Rossby wave dispersion on the sphere, provided a qualitative explanation about the nature of the wave trains and the teleconnection patterns. Most of their analysis was linked to the wave train patterns in the Northern Hemisphere and assuming a zonally varying basic state. Hoskins and Ambrizzi (1993) extended their linear analysis of the barotropic Rossby wave propagation considering longitudinally varying basic states. In this case, they showed a qualitative view of the atmosphere behavior related to the preferential regions of stationary Rossby wave propagation. In particular they demonstrated that a stationary forcing can create a stationary wave pattern around two weeks. One of the most relevant results from the teleconnection pattern studies and it is related to the wave activity in the Austral winter of Southern Hemisphere is the presence of the subtropical and subpolar jets. As demonstrated by Ambrizzi et al. (1995) through simulations * Corresponding author: Department of Atmospheric Sciences. Institute of Astronomy, Geophysics and Atmospheric Sciences. gabriela@model.iag.usp.br using a barotropic model and afterwards by Ambrizzi and Hoskins (1997) using a intermediate global circulation model, these jets can act as waveguides in the atmosphere. This feature is emphasized by the theoretical- observational study described in Müller and Ambrizzi (2006) that used composites of winters with extreme frequency of occurrence of Generalized Frost (GF) in the Wet Pampa, Argentina. In particular for the extreme represented by the winters with maximum frequency of GF (+σ –defined as one standard deviation above the average–), the composite of GF events shows Rossby wave propagation patterns along of the subtropical and subpolar jets reaching the South America (Müller et al., 2005). One interesting feature observed in these previous studies is the phase coincidence of these waves when they reach South America before the GF occurrence. Based on this result it was suggested that the wave phase coincidence is the dynamical mechanism that contribute to the GF events during the period of their maximum occurrence. In order to better understand this mechanism numerical simulations using a baroclinic model were carried out to analyze the origin and evolution of the wave trains responsible for creating the right environment for the occurrence of frosts over the productive region of the Wet Pampa in Argentina. 2. DATA AND METHODOLOGY The numerical simulations presented below were carried out using a baroclinic model (IGCM –Intermediate Global Circulation Model–), which is a good tool to analyze the physical mechanisms of planetary wave propagation 1563 Proceedings of 8 ICSHMO, Foz do Iguaçu, Brazil, April 24-28, 2006, INPE, p. 1563-1567.