Trends of Extreme Temperature Over the Lake Urmia Basin, Iran, During 1987-2014

The variability of temperature extremes has been the focus of attention during the past several decades and had a great influence on the hydrologic cycle. A long-term, high-quality daily maximum (TX) and minimum temperature (TN) of seven stations was used to determine the spatial and temporal characteristics of extreme temperature events in Lake Urmia Basin in Iran during 1987 to 2014. The RClimdex was used to calculate 16 extreme temperature indices recommended by the Expert Team on Climate Change Detection and Indices (ETCCDI) for this study and the Mann-Kendall test was employed to assess the trend. All the temperature-based indices show patterns consistent with a general warming trend. The results revealed statistically significant changes in important temperature indices over the study area during the past three decades. On the whole, cold indices, including cool days, cool nights, ice days, frost days and cold spell duration index significantly decreased by -3.07, -2.27, -1.8, -1.53 and -0.16 days/decade, respectively. In contrast, warm indices such as warm days, warm nights, summer days, tropical nights, warm spell duration index, and growing season length significantly increased by 2.99, 3.34, 3.3, 3.06, 2.63 and 1.79 days/decade. Minimum TX, maximum TX, Minimum TN and maximum TN increased significantly by 1.00, 1.76, 0.36 and 1.01 °C/decade. Furthermore, the magnitudes of the trends in cold/warm days are larger than those in cold/warm nights, which indicate that the trends in minimum temperature extremes are more rapid than in maximum temperature extremes. Strong relationships between the annual mean temperature and the extreme temperature indices were detected in this study.