Teleconnection patterns orographically induced in model results and from observational data in the austral winter of the Southern Hemisphere

Low frequency disturbances in the Southern Hemisphere are investigated thro ugh an observational study and numerical model experiments. The orography o f the Andes and Antarctica in the Southern Hemisphere are the forcing of nu merical experiments, submitted to zonal climatological austral winter condi tions. The large-scale pattern forced by the Andes orography shows a high l evel wavetrain from the southeast of the mountain to the South Atlantic and two jet streams at high levels. The Antarctic case presents a wavenumber t hree horizontal structure around the hemisphere and three regions of jet st ream. Characteristics of this wavenumber three model results are discussed based on analysis of streamfunction, vorticity and temperature, and related to blocking regions in the Southern Hemisphere. Empirical orthogonal funct ion analysis of the low frequency filtered streamfunction at 250 hPa also s hows, for the austral winter season, a wavenumber three structure and a wav etrain over the South Pacific and the South Atlantic, similar to the Pacifi c South American Pattern (PSA). Parts of the low frequency teleconnection p atterns are identified in both the observational data and the model results , supporting the influence of the orography on the Southern Hemisphere atmo spheric circulation. Analyses of periods with extreme amplitudes of the eig envectors also show the influence of tropical convection on the anomalies o ver South America. Copyright (C) 2000 Royal Meteorological Society.