Changes in seasonal mean maximum air temperature in Romania and their connection with large‐scale circulation

Abstract

This paper investigates the temporal and spatial variability of the seasonal mean of maximum air temperature in Romania and its links with the large-scale atmospheric circulation. The Romanian data sets are represented by time series at 14 stations. The large-scale parameters are represented by the observed sea-level pressure (SLP) and geopotential height at 500 hPa (Z500). The period analysed was 1922-98 for winter and 1960-98 for all seasons. Before analysis, the original temperature data were tested to detect for inhomogeneity using the standard normal homogeneity test. Empirical orthogonal functions (EOFs) were used to analyse the spatial and temporal variability of the local and large-scale parameters and to eliminate noise from the original data set. The time series associated with the first EOF pattern of the SLP and mean maximum temperature in Romania were analysed from trend and shifts point of view using the Pettitt and Mann-Kendall tests respectively. The covariance map computed using the Z500 and the seasonal mean of maximum temperature in Romania were used as additional methods to identify the large-scale circulation patterns influencing the local variability.Significant increasing trends were found for winter and summer mean maximum temperature in Romania, with upward shifts around 1947 and 1985 respectively. During autumn, a decreasing trend with a downward shift around 1969 was detected. These changes seem to be real, since they are connected to similar changes in the large-scale circulation. So, the intensification of the southwesterly circulation over Europe since 1933 overlapped with the enhancement of westerly circulation after the 1940s could be the reason for the change in winter mean maximum temperature. The slight weakening of the southwesterly circulation during autumn could be one of the reasons for the decrease in the regime of the mean maximum temperature for autumn seasons. Additionally, the covariance map technique reveals the influence of the North Atlantic oscillation in winter, East Atlantic Jet in summer and Scandinavian (or Euroasia-1) circulation pattern in autumn upon mean maximum air temperature.

Spatial and temporal variability of climate extremes in Romania and associated large‐scale mechanisms

“…Even if some of them show a global influence, their effects are strongest for some regions such as: El Niño-Southern Oscillation around the Pacific Rim and throughout all of North America; Pacific interdecadal variability for the Northern Hemisphere, especially around the Pacific region and North America (Kenyon and Hegerl, 2008). For Europe, the strongest influence is given by the North Atlantic Oscillation (NAO), especially in wintertime (Hurrell and van Loon, 1997;Bojariu and Paliu, 2001;Tomozeiu et al, 2002;Haylock and Goodess, 2004;Cassou et al, 2005;Scaife et al, 2005;Birsan and Dumitrescu, 2014). Folland et al (2009) defined a summer counterpart of NAO called Summer North Atlantic Oscillation (SNAO) that exerts a strong influence on the northern European climate through changes in the position of the North Atlantic storm tracks, having a very important role in generating summer climate extremes (flooding, drought and heat stress) in northwestern Europe.…”

“…Efthymiadis et al, 2011;Ionita et al, 2011), composite maps (e.g. Tomozeiu et al, 2002;Ionita et al, 2013) or through multivariate techniques such as the canonical correlation analysis (CCA) (Haylock and Goodess, 2004;Xoplaki et al, 2004;Della-Marta et al, 2007).…”

“…The present work aims to carry out such a study. Regarding Romania, several studies documented on the analysis of the climate extremes variability (Tomozeiu et al, 2002;Baciu et al, 2004;Busuioc et al, 2007Busuioc et al, , 2010Pȃltineanu et al, 2009;Birsan and Dumitrescu, 2014) but most of them used a limited number of stations/short intervals for analysis or did not enter into the details of the mechanisms controlling their variability. The present study attempts to fill this gap.…”

Variability of extreme temperature events in south–central Europe during the 20th century and its relationship with large‐scale circulation

“…Beyond real spatial and temporal differences, the results likely depend on the number and type of variables, and the quality of the time series. Tomozeiu et al (2002) carried out a thorough investigation of the variability in the seasonal means of maximum daily temperature in Romania, and examined the connections to several variables of the large-scale circulation. They concluded that most of the long-term temperature variability is related to some specific changes of the circulation regime.…”

Large‐scale anomaly patterns associated to temperature variability inside Scarisoara Ice Cave

“…These patterns also prove to be stable when different filtering procedures were applied. Owing to the proximity of NAO and its documented effects on winter climate (mainly precipitations) over northwestern Romania (Mares et al, 2002;Tomozeiu et al, 2002) it was just natural to search for a possible linkage between its variability and climate variability over the Scarisoara Cave region. The NAO is considered a mode of internal climate variability throughout the extratropical Northern Hemisphere that influences atmospheric circulation, temperature and precipitation over a wide region (Hurrell, 1995).…”