6 th European Conference on Severe Storms (ECSS 2011) , 3 - 7 October 2011, Palma de Mallorca, Balearic Islands, Spain TORNADO AND WATERSPOUT CLIMATOLOGY IN BRAZIL Lucí Hidalgo Nunes 1 , Laura De Bona 1 , Daniel Henrique Candido 1 1 UNICAMP, Department of Geography, R. João Pandiá Calógeras, 51 – 13.075-020, Campinas, SP, Brazil, luci@ige.unicamp.br, lauradebona@yahoo.com, daniel.candido@ige.unicamp.br (Dated: 22 August 2011) I. INTRODUCTION The appropriate conditions for tornadic storms can be found in different places, and although there are more reports for the USA, Canada, northern Europe, Australia and South Africa, southern Brazil has been recognized as a tornado-prone area (http://www.spc.noaa.gov/faq/tornado/). Some authors have studied specific tornado and waterspout episodes in Brazil, among which Silva Dias and Grammelsbacher (1991), Massambani et al. (1992), Antonio (1996), Held et al. (2004), Marcelino et al. (2006), Nunes et al. (2008) and Candido et al. (2009). However, no consistent information base for these phenomena is kept that could aid identify sites and periods of highest incidence and recognize which characteristics of atmospheric and of other nature are usually associated with tornadoes and waterspouts in Brazil. In order to better understand these atmospheric perturbations that take place mainly in southern Brazil, this study aims to provide an overview of these phenomena during the period 1991 to 2010. II. DISCUSSION AND RESULTS Some studies on tornadoes and waterspouts recorded in Brazil stress out that some relief features are fundamental in the formation of tornadoes, such as the hill chains and steeper relief in São Paulo State, and the ridges in the Rio Grande do Sul and Santa Catarina states. Nunes et al. (2008) also observed that the trajectory of many tornadoes in central-southern Brazil follows river courses, from which they obtain energy. Thus, as observed in the United States by Allaby (2004), in sectors of central-southern Brazil the influence of the Subtropical Jet Stream is perceived. It oscillates according to Rossby waves, following the advance of the high-pressure systems associated with the polar masses (Nunes et al., 2008). In these cases, the jets generate wind shear and a more significant atmospheric movement, aspects that favor the formation of potentially tornadic, severe storms. This analysis focuses on the period between 1991 and 2010, when information became more detailed. For comparison, FIG. 1 presents proven tornadoes and waterspouts occurrences in Brazil between 1967 and 2010. An increasing trend in the number of reported events can be noted, probably due to the facility to document these episodes and to the growth of urbanization and population in many areas of Brazil. Nevertheless, it is important to underline that prior to the 1990s tornadoes and waterspouts were most likely underreported. This raise was more significant from 2005 on, but as no atmospheric alterations occurred that could be associated with this increase, the raise is probably related to the increase of records rather than the events themselves. In the period of analysis (1991 to 2010) 205 events were identified, 167 being tornadoes and 38 waterspouts. FIG. 2 shows the monthly distribution of the occurrences 0 5 10 15 20 25 30 35 40 45 50 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 Events FIG 1: Yearly distribution of events from 1967 to 2010 – in gray, events prior to 1967-2010 (Organized by Laura De Bona) and that the highest concentration of these phenomena took place in the hottest period of the year, between September and March. The month of May also corresponds to a significant number of episodes, which can be associated with the high instability during this period of atmospheric transition. 35 22 25 10 16 7 6 6 17 18 22 20 1 0 5 10 15 20 25 30 35 40 January February March April May June July August September October November December No date Number of Events FIG 2: Monthly distribution of events (1991-2010) (Organized by Laura De Bona) In terms of spatial distribution, 16 Brazilian states recorded tornadoes or waterspouts between 1991 and 2010, a considerable number when taking into account that the federation has 26 states and a Federal District (FIG. 3). The atmospheric conditions in southern Brazil are particularly favorable to the formation of these phenomena, once 81% of the episodes occurred in the southeastern and southern regions. In these sectors the reach of the Jet Stream, the topography, the distribution of the drainage and humidity are decisive contributors to the occurrence of these disturbances. As in the United States, most of them are associated with the meeting of tropical and extra-tropical systems. Besides the favorable physical conditions, southern Brazil states concentrate an important part of the income and population of the country, which aggravates the social-economic consequences generated by these disturbances. Another relevant fact is that in these states tornadoes and waterspouts occurred in different municipalities, signaling that many localities have the conditions to trigger these phenomena.