Spatial and temporal analysis of trends in extreme temperature indices in Iran over the period 1960–2014

The recent years have witnessed a rapid increase in frequency, severity, duration and geographical extent of extreme weather events. Having an area of 1 648 195 km2, Iran spans over a wide latitudinal range and enjoys a diverse terrain in terms of topography and elevation. The combination of this topographic complexity with the effects of extensive water bodies in the north and the south plays a major role in the distribution of temperature over this country. This study analysed the temporal trend of extreme temperature indices observed in 33 synoptic stations across Iran over the period 1960–2014, and then examined their relationship with elevation and latitude. The results showed a significant warming trend in extreme temperature indices of many stations, especially the indices that are based on daily minimum temperature. At low elevations, the trends of indices exhibited the features of a warming climate, but at higher elevations, these indices showed a mixture of positive and negative trends. Examining the relationship between the trends of indices and elevation showed that in the altitudinal band E1 [elevations lower than 23 m above sea level (ASL)], only the trend of growing season length is correlated with elevation; but in the altitudinal band E2 (elevations higher than 700 m ASL), elevation exhibited significant correlation with the trends of percentage of cool nights, frequency of frost days, diurnal temperature range, cold spell duration, percentage of warm nights and frequency of tropical nights. The analysis did not show any meaningful relationship between extreme temperature indices trends with latitude. Considering the high heterogeneity of the trends of indices and the limited number of stations with sufficient data, generalizing the results and predicting the wider trend patterns require further studies using a denser network of stations.