Observed trend of the minimum temperature and the number of the frost days in the Rio Grande do Sul, State

Abstract

In this study, an exploratory analysis of the observed trend of minimum temperatures as well as an analysis of the number of frost days in the State of Rio Grande do Sul was made. Data were obtained monthly minimum temperature of 18 meteorological stations (period 1936-2000) as well as the daily minimum temperature of ten meteorological stations (period 1945-2005). The results show that there was a coherent and statistically significant increase of the minimum tem- perature in the whole State, in the year as well as in the four seasons. Regarding the year, the linear increase of the period (65 years) was of 1.3°C. Summer was the season that presented the highest increase (1.8°C in 65 years) and Winter was the season with the lowest increase (0.5°C in 65 years). The second half of the series studied (1970-2000) stands out not only by the increasing of the minimum temperature but also by changing the probability distribution. Reducing the number of frost days was higher for the year, Winter and Autumn, but with only 40 %, 40 % and 30 %, respectively, from meteorological stations with statistical significance for the negative sign (reduction of frost). In part at least, this behavior of the minimum temperature (and the number of frost days) in the State is associated with higher frequency (and severity) of the El Niño Southern Oscillation (ENSO) warm events, particularly in the last three decades of the twentieth century. The results found in Rio Grande do Sul, State, are coherent with the ones from other authors who have studied the tem- perature trend and the occurrence of frost in the southeastern region of South America, showing the large-scale character of the trend of those meteorological elements in this region, but also coherent with the hypothesis of the global warming.