Advances in Geosciences www.adv-geosci.net 1680-7340 1680-7359 22 4th EGU Alexander von Humboldt Conference "The Andes: Challenge for Geosciences" 2009 10.5194/adgeo-22-67-2009 http://www.adv-geosci.net/22/67/2009/ http://www.adv-geosci.net/22/67/2009/adgeo-22-67-2009.html http://www.adv-geosci.net/22/67/2009/adgeo-22-67-2009.pdf 67 72 2009-10-13 A quantitative evaluation of the role of the Argentinean Col and the Low Pressure Tongue East of the Andes for frontogenesis in the South American subtropics H. M. J. Barbosa hbarbosa@if.usp.br J. M. Arraut Instituto de Física, Universidade de São Paulo, Rua do Matão, 187, São Paulo SP 05508-090, Brazil Centro de Ciências do Sistema Terrestre, Instituto Nacional de Pesquisa Espaciais, Av. dos Astronautas, 10758, São José dos Campos SP 12227-000, Brazil Previous studies have found the South American subtropics to exhibit high climatological frontogenesis in equivalent potential temperature during the austral summer. An important contribution to this pattern is given by frontogenesis over the Argentinean Col (AC), which separates the Northwestern Argentinean Low (NAL) from transient troughs to the south of it. The NAL and the Low Pressure Tongue east of the Andes (LPT) promote efficient transport of Amazonian humidity to the subtropics during the incursion of transient disturbances over the continent. The convergence of this strong warm and humid flow with mid-latitude air brought into the subtropics by the disturbance occurs preferentially in the neighborhood of the AC. The main difficulty in quantifying the contribution of the NAL, AC and LPT structure to frontogenesis in the South American subtropics is the automatic detection of the AC and LPT. In this paper an algorithm developed to this end is briefly presented and applied to obtain statistics on the role of these structures in frontogenesis. Six-hourly data from ECMWF ERA-40 Reanalysis over 21 austral summer periods (December–March) is used. Occurrences of the AC are highly concentrated between 34–39° S and 66–69° W, being present in this region in 42% of the time instants analyzed. The spatial average of the positive values of the frontogenesis over this region was calculated for each time step as a measure of intensity and histograms were built for the cases when the AC was and was not found inside this region. Mean, median and mode are larger for the distribution of cases with the presence of the AC. In addition, we present the frequency of occurrence of the AC as a function of the frontogenesis, showing that it grows with the intensity of the frontogenesis, rising above the 0.955 quantile. We have not found any correlation between the AC frequency and the frontolysis intensity. Acton, F S.: Numerical methods that work, Mathematical Association of America, Washington, corrected edition edn., 1990. Arraut, J M.: Fronts and frontogenesis during summer: geometrical and dynamical aspects and the influence over rainfall on the South American subtropics, Ph.D. thesis, Centro de Previsão de Tempo e Estudos Climáticos – INPE, Rodovia Presidente Dutra Km 40 Cachoeira Paulista, São Paulo, Brasil, online available at: prefixhttp://urlib.net/sid.inpe.br/mtc-m17@80/2007/12.19.10.53, 2007 (in Portuguese). Arraut, J M. and Barbosa, H. J M.: Large Scale Features Associated with Strong Frontogenesis in Equivalent Potential Temperature in the South American Subtropics East of the Andes, Adv. Geo., this special volume, 2009. Bolton, D.: The Computation of Equivalent Potential Temperature, Mon. Weather Rev., 108, 1046–1053, 1980. Ferreira, L.: Causes and Variability of the Northwestern Argentinean Low and Impacts Over Local Circulation Patterns, Ph.D. thesis, Universidade de Buenos Aires, 2008 (in Spanish). Kodama, Y.-M.: Large-Scale Common Features of Subtropical Precipitation Zones (the Baiu Frontal Zone, the SPCZ and the SACZ) Part I: Characteristics of Subtropical Frontal Zones, J. Meteorol. Soc. Jpn, 70, 813–835, 1992. Lichtenstein, E R.: La depresion del noroeste argentino (The northwestern Argentinian low), Ph.D. thesis, Departamento de Ciencias de la Atmosfera, Ciudad Universitaria (1428) Buenos Aires, Argentina, 1980. Ninomiya, K.: Characteristics of Baiu Front as a predominant subtropical front in the summer northern hemisphere, J. Meteorol. Soc. Jpn, 62, 880–894, 1984. Petterssen, S.: Weather analysis and forecasting, McGraw-Hill, New York, 2nd edn., 1956. Press, W H., Flannery, B P., Teukolsky, S A., and Vetterling, W T.: Numerical recipes in Fortran 77: the art of scientific computing, Cambridge University Press, Cambridge, 2 edn., 1992. Seluchi, M E., Saulo, C., Nicolini, M., and Satyamurty, P.: The Northwestern Argentinean Low: A study of two typical events, Mon. Weather Rev., 132, 2361–2378, 2003.