Trends and Anomalies in Daily Climate Extremes over Iran during 1961-2010

The analysis of the spatiotemporal trends of temperature and precipitation are important for the policy guidelines of future development and sustainable management of a country. This study presents spatiotemporal variations in temperature and precipitation indices to elucidate climatic change over Iran during last five decades i.e., 1961 -2010 by examining data obtained from30 synoptic stations across the country. Results indicate significant and spatially coherent rising trends in temperature indices. From the results it is clear that the majority of the trends in the annual means of daily maximum temperature, daily minimum temperature, tropical nights, warm nights and warm spell duration index showed strongly positive tendency during the last decades, while the increasing trends in the Central region of Iran was stronger than this in the other areas. Decreasing trends have been found in the diurnal temperature range, Cold spell duration index and Cool nights were found over most regions of Iran and positive trends were found for Growing season length and Warm days over other regions of the country. Cooling trends were found for the southern half of the Zagros Mountain and Shahrekord station. Among the precipitation indices we found positive trends in daily intensity index, heavy precipitation and extreme wet days over most of the country and a significant increase in the heavy precipitation series was observed mostly in Southwestern regions of Iran, as well as along the coasts of the Persian Gulf. We can conclude that the average amount of wet day precipitation has significantly increased across Iran. Overall, these trends were statistically significant mostly over the central areas of the country and there was strong climatic variability in the rest of country, especially arid and semi-arid regions. The northwest region was only region with weak anomalies in studies indices.