Spatio-temporal variations of climate variables and extreme indices over Iran during 1986-2015

In order to better understand how climate changes have taken place in Iran, we carried out a comprehensive analysis of the spatio-temporal trends of various climate variables and extreme indices during 1986 to 2015 at the county-level across the country. Additionally, the interannual oscillation of the temperature and precipitation and their related extreme indices were examined throughout the research. In this study, ERA5-Land and AgrERA5 datasets with hourly, daily, and monthly temporal resolutions were aggregated to the county-level to calculate climate extreme indices. Subsequently, different approaches such as the original Mann-Kendall (MK) trend test, MK with block bootstrap modification, MK with variance correction modification, correlated seasonal MK (partial MK), original and seasonal Sen's Slope were implemented to detect the magnitude and the statistical significance of climatic trends for each county. Finally, the continuous wavelet transform was employed for whole country averages to investigate fluctuations and dominant periods of the variables and indices. The reanalysis model datasets offered us two advantages; firstly, it facilitates obtaining data in some regions with sparse weather stations and secondly, it allows us to inquire about some climate variables that were less studied in the literature, for instance, the wind speed, the surface air pressure, the solar radiation, the surface albedo, the runoff, the evaporation, and the skin reservoir content. The results showed a significant increasing trend in the temperature over all counties and a nonsignificant drying trend in the precipitation for almost the whole country. Other climate variables demonstrated more mixed spatio-temporal trends; however, generally, the wind speed and the solar radiation had an upward trend, the runoff, the skin reservoir content, and the surface albedo showed a downward trend, while the surface air pressure and the evaporation trends exhibited a great deal of variety. Furthermore, the hot climate extremes were increased throughout the country whereas the cold extremes and the extreme precipitations were quite in the opposite direction. It is noteworthy that the Continental and the Warm-Temperate climates were more vulnerable compare to the Arid and Semi-Arid Climates. At last, the wavelet power spectrum maps indicated the consistency between the temperature and precipitation and their related extremes and also showed a reduction in the fluctuation of the precipitation and a constant oscillation for temperature over the study period.