Abstract
Although a significant number of studies have evaluated the trends in different characteristics of precipitation in Iran, the trends in precipitation indicators related to bioclimate are still not explored. The 0.5° spatial resolution gauge-based gridded monthly precipitation data of global precipitation climatology centre (GPCC) for the period 1901–2016 was used in this study for the evaluation of the geographical distribution of the trends of bioclimatic precipitation indicators of Iran. The trends in the indicators due to global warming were estimated using modified Mann-Kendall (MMK) trend test which can estimate unidirectional trend by separating the natural variability in climate. Obtained results were compared with that found using classical Mann-Kendall (MK) test. Besides, gridded temperature data of climate research unit (CRU) was used to identify the warm/cold periods at each grid point to assess the trends in precipitation during warm/cold periods, considering a wide spatial variation in the onset time of different seasons in Iran. The results revealed that many of the trends in some of the precipitation indicators obtained in earlier studies were due to natural fluctuation of climate. Annual precipitation in Iran was found decreasing only in the northwest semi-arid region at a rate of − 12.1 to − 14.05 mm/decade, while the precipitation in the wettest month was found increasing in a large area in the southwest semi-arid region at a rate of 3.1 to 5.3 mm/decade. The most significant changes were observed in precipitation seasonality, which was found to increase in 22.4% area, mostly in the central dry and northeast semi-dry regions and decrease in 11.3% area, mostly in the northern wetter region. The study indicates that the long-term natural variability in large-scale atmospheric phenomena that influences the precipitation of Iran may be the cause of many significant changes observed in precipitation in previous studies.
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