Changes in thermal discomfort indices in Romania and their connections with large-scale mechanisms

We analysed changes in variability characteristics for 2 indices quantifying winter (WCT: wind chill equivalent temperature chart index) and summer (THI: temperature−humidity index) thermal stresses at a high spatial resolution across Romania for the period 1962−2010. The linear trends, and shifts in the mean and main modes of variability were examined using nonparametric tests (Mann-Kendall and Pettitt) and empirical orthogonal function (EOF) analysis. The physical mechanisms responsible for the identified characteristics of variability were examined through canonical correlation analysis (CCA). The results showed a significant upward trend for both indices over the entire country, except for some areas for WCT, with a significant upward shift around the year 1987 for winter and 1985 for summer. A strong increase in the frequency of extremely high THI values was revealed, indicating an increased risk to human health during summers in Romania after 1985. However, the upward trend in WCT would indicate a decreased risk of human health during winters in Romania after 1987. The main large-scale mechanism responsible for the spatial and temporal behaviour of the 2 stress indices was given by the first CCA pair, showing that the increasing trend was mainly explained by the increasing trend in the temperature at 850 hPa covering Romania simultaneously with an increasing trend in zonal circulation (WCT) and an increasing trend in specific humidity at 700 hPa (THI). On a decadal/multidecadal time scale, the variations in the 2 indices are modulated by the North Atlantic Oscillation (winter) and the Atlantic Multidecadal Oscillation (summer).