Abstract

BERMAN, Ana Laura  and  BISCHOFF, Susana Amalia. Circulation fields associated to extreme showalter indexes in Argentina. Geoacta [online]. 2013, vol.38, n.1. ISSN 1852-7744.

The knowledge of atmospheric conditions in a region is important especially when they are associated with the occurrence of storms and other severe weather phenomena that have socio-economic impacts. A way to identify such atmospheric conditions is calculating indexes of instability. Therefore, in order to know characteristics of the information contained in these indexes, the aim of this study is to investigate the variability of the frequency of extreme daily Showalter Index values in Argentina and its relationships with the regional atmospheric circulation. The analysis is performed considering radiosonde data at the meteorological station Resistencia, Santa Rosa and Comodoro Rivadavia during the period 1973-2005 and defining the extreme values of the index as extreme percentiles. The study in each meteorological station reveals that the frequency of extreme values associated with unstable cases have decreased during the last part of the analyzed period whereas the frequency of stable extreme values has increased. For the cases of extreme stable index in the three considered meteorological stations, the mean patterns of anomalous circulation show a dipolar structure with a cyclonic circulation center on the eastern coast of the continent and an anticyclonic center located to the west over the Pacific Ocean. Such anomalous circulation produces advection of cold and dry air from higher latitudes stabilizing the lower troposphere that result in stability. For extreme cases of unstable index, the anomalous circulation patterns also describe a dipolar structure but with a weak anticyclonic circulation on the eastern coast of the continent and the Atlantic Ocean and an anomalous cyclonic circulation center over the south of the continent. This circulation structure produces advection of warm and wet air from the north which is favorable for convection and the development of instability in the regions. The comparison between the mean patterns corresponding to stable and unstable cases shows a dipolar structure of circulation in both cases but with inverted sign.

Keywords : Radiosonde data; Showalter index; Atmospheric circulation.

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