Romanian Journal of Meteorology, Vol. 9, No. 1-2, 2007 ISSN 1223-1118 SUMMER ANOMALIES IN 2007 IN THE CONTEXT OF EXTREMELY HOT AND DRY SUMMERS IN ROMANIA Aristita Busuioc, Alexandru Dumitrescu, Elena Soare and Alina Orzan National Meteorological Administration, Romania Sos. Bucuresti-Ploiesti 97, 013686 Bucharest, Romania, e.mail: busuioc@meteoromania.ro Abstract: Summer 2007 was among the hottest summers in Romania over the entire observational period 1901-2007. This paper presents a detailed analysis of the surface air temperature and precipitation anomalies recorded during the summer 2007 in comparison with the corresponding values of the first three hottest summers recorded in Romania, in order to understand the mechanisms controlling these events and, more generally, the summer temperature variability in Romania. In order to reach this objective, the time series of the mean air temperature and total precipitation for two observational periods were analyzed: a longer period for 14 stations (1901-2007) and a shorter one (1961-2007) for a complete higher spatial density data set (94 stations for temperature and 104 for precipitation). Non-parametric tests (Mann-Kendall and Pettitt) and empirical orthogonal function analysis were used as statistical tools to identify the main temporal characteristics of regional climate variability (trends and shifts). It was found that, in terms of spatial average over the country, the summer 2007 was similar with that of 1946, considered so far the hottest one over the period 1901- 2006. In order to understand the large-scale mechanisms responsible for these extreme events, the corresponding anomalies of the possible dynamic factors (sea level pressure and geopotential heights at 500 hPa) and thermodynamic factors (specific humidity and temperature at 850 hPa), from the NCEP reanalysis for the period 1961-2007, were analyzed. The obtained results showed that a persistent anticyclonic structure over Romania, associated with very large positive temperature anomalies at 850 hPa, were the main reasons of the very hot summers in Romania. When these large- scale anomalies were additionally associated with a persistent dry air mass at 850 hPa, prolonged and intense droughts were recorded in Romania. A strong agreement between the temporal variability of the summer temperature anomalies in Romania and upper large-scale geopotential and temperature anomalies over the period 1961-2007 was found as well. The signal extracted from the sea level pressure field was not so significant for the analyzed summer extreme events in Romania. 1. INTRODUCTION The global warming is unequivocally accepted by the scientific community, being already identified in the observed data set, as it has been presented in the Fourth IPCC Assessment Report (AR4) (IPCC, 2007). Instrumental observations over the past 157 years show that temperatures at the surface have risen globally, with important regional variations. For the global average, warming in the last century has occurred in two phases, from the 1910s to the 1940s and more strongly from the 1970s to the present. An increased warming rate occurred over the last 25 years, and 11 of the 12 warmest years on record have occurred in the past 12 years (Trenberth et al., 2007). In addition to global warming, on regional scale, several other factors may shape future climate change such as variation in atmospheric circulation and topography (Christensen et al., 2007). Variations in atmospheric circulation influence the climate over large areas both on interannual and longer time scales while the topography modifies the effects of atmospheric circulation at fine geographical scales (Bojariu and Giorgi, 2003). As an example, for Europe, a severe heat wave was recorded during the summer 2003 (Fink et al., 2004), which