1. 11th EMS/ 9th ECAM Berlin, Germany September 12–16, 2011 Trends in the frequency of extreme climate events in Latvia as influenced by large-scale atmospheric circulation processes Zanita Avotniece 1, Valery Rodinov 2, Lita Lizuma 1, Agrita Briede 3, Māris Kļaviņš 2, 1 Latvian Environment, Geology and Meteorology Centre, 165 Maskavas str., Rīga, LV-1019, Latvia 2 Department of Environmental Science, University of Latvia, Raiņa blvd. 19, LV- 1586, Rīga, Latvia, e-mail: maris.klavins@lu.lv 3 Department of Geography, University of Latvia, Raiņa blvd. 19, LV- 1586, Rīga, Latvia INTRODUCTION In this study the long-term variability of extreme climate event indicators in Latvia was investigated. To assess the trends of changes of extreme climate events, indexes, such as the number of extremely hot days, frost days or the number of days with heavy precipitation (altogether 11 indexes) were used and compared with the indexes characterizing climate. The trend analysis of long-term changes of extreme climate events demonstrates a significant increase in the number of meteorological events associated with an increased summer temperature and a decrease in the number of events associated with extreme temperature events in winter time. There is also an increase in the number of days with heavy precipitation. The present study is based on daily air temperature and precipitation data series for 10 meteorological stations in Latvia obtained from Latvian Environment, Geology and Meteorology Centre. Amongst the factors influencing the trends of changes of extreme climate events, the character of large-scale atmospheric circulation processes are discussed. An ensemble climate change indices (Table 1), that follow the definitions recommended by the Expert Team on Climate Change Detection and Indices, were derived from daily temperature and precipitation data. Table 1. A list of indices used in this study The visual analysis of indicators of extreme climate events for the last ~ 80 years indicate well expressed trends of changes (Figure A). The results of the analysis (Table 2) show that the changes related to negative temperatures demonstrate a decreasing trend, but many indicators describing positive temperature extremes demonstrate an increasing trend, for example, the annual number of tropical nights, which also showed a record high number in the capital city of Riga during the summer 2010 (Figure B). Also the number of days with heavy precipitation shows an increasing trend of changes. Figures A-B. Trends in the annual number of summer days and consecutive summer days (A) in Liepāja for the period 1923-2010.; The spatial variations in the number of tropical nights (B) in Latvia during the summer 2010. Table 2. Long-term trend statistics of extreme meteorological events CONCLUSIONS There have been significant changes in the extreme climate events in Latvia in the past ~80 years: there has been a significant increase in the number of meteorological events associated with an increased summer temperature and extreme precipitation, and a decrease in the number of events associated with extreme temperature events in winter. Some of the climate indicators are much stronger in the capital city Riga, which could be associated with the effects of the specific urban climate°and the influence of the Gulf of Riga. During the analysis some regularities between the large–scale atmospheric circulations and the weather conditions on the boundary layer were found: the most common synoptic situations for the occurrence of extremely high air temperatures can be found in the conditions of south–westerly, southerly anticyclonic flows and in the cases of the warm sector of a cyclone being situated over the territory – in the conditions of westerly, south–westerly cyclonic flows. Extreme precipitation events during the summertime were mainly associated with cyclonic activity. For the determination of the specific weather conditions favorable for the occurrence of extreme weather events, 18 large-scale atmospheric circulation patterns (computed by European Cooperation in Science and Technology Action 733) for the Baltic Sea region were used. Atmospheric circulation patterns were determined using the (GWT) Grosswetter-types or prototype classification. During the analysis of atmospheric circulation conditions, it was stated, that each of the circulation types can be responsible for extreme heat and heavy precipitation in Riga. Though the most common conditions during days with extremely high air temperatures were found in the cases of SW and S anticyclonic flow (Figures E, F) and SW, S and W cyclonic flow, but heavy precipitation are usually associated with cyclonic activity – W, SW, N cyclonic flow and the air flow in the center of a low pressure area (Figures A, B, D and C respectively). A BC DE F A B Trends in meteorological event time series were analyzed by using non- parametrical Mann-Kendall test, which was applied to each variable at each site at a significance level of p≤0.01. The statistically significant trends are accentuated with bold