عنوان مقاله [English]
Climate change and global warming may have direct effects on climate extreme values and temporal as well as spatial variations of these events. Alterations in natural and human communities caused by meteorological extreme events are more significant than those caused by climatic averages. These extreme events widely draw public attention, and are particularly foregrounded by governments and academic communities (An et al. 2019). Given the noticeable consequences of climate extremes, the Intergovernmental Panel on Climate Change (IPCC) organized a team of experts to investigate the challenges caused by extreme events and measure extreme indices (Houghton et al. 2001, Peterson et al. 2002). This team suggested 27 indices to investigate and measure climate extremes. These indices have globally drawn the attention of atmospheric sciences researchers, and many studies have been conducted based on these indices to investigate both past and future events. The literature review indicated that spatial and temporal variations of extreme climate events related to both the past and future have been sufficiently investigated by certain researchers abroad. However, extreme climate events in Iran have been rarely examined. The few studies to investigate the events related to temperature and precipitation extremes in the selected region using temperature and precipitation data and synoptic stations located in Western and Southwestern Iran.
Materials and Methods
The regions that have been investigated in the present study are Ilam, Lorestan, Khuzestan, Chaharmahal and Bakhtiari, Kohgiluyeh and Boyer-Ahmad, Bushehr, and Fars provinces in Western and Southwestern Iran. These provinces cover 28/9924 square kilometers comprising 17/6% of the whole country area. The data used in this study include average precipitation, and maximum as well as minimum daily temperatures at 28 synoptic stations located in Western and Southwestern Iran in a common statistical period of 30 years from 1988 to 2017. After selecting the stations, the Run Test was used for all the stations and precipitation parameters, and minimum were measured. Subsequently, a matrix of the daily precipitation data and minimum. as well as maximum daily temperatures for was prepared. Finally, extreme indices (26 precipitation and temperature indices as suggested by the expert group CCL/CLIVAR) were measured using programming in the context of the MMATLAB software environment, and the variation process in every index was examined using the Mann-Kendall test. Then, the required maps and diagrams were prepared.
Results and Discussion
The investigation of temporal variations in the occurrence of warm indices from 1988 to 2017 in Western and Southwestern Iran using the Mann-Kendall test (with a reliability level of 95%) indicated that the total trend of warm indices such as warm nights, warm days, the number of summer days, and the number of tropical nights for most of the stations has been increasing. However, the examination of temporal variations in the occurrence of cold indices using the Mann-Kendall test (with the reliability level of 95%) showed that the total trend of cold indices such as cold days, cold nights, and the number of frost days was declining.
The significant finding revealed by the general investigation of the total warm and cold extreme indices in the regions covered in the present study is the prevalence of the warming trend over the examined statistical era. The results of the maximum of one-day precipitation amount and the maximum of five-day precipitation amount indices were also indicative of remarkable precipitation rates in the regions based on these indices. The average was 177 mm (in Kuhrang station). The average was 347 mm (in Kuhrang station). The trend of temporal variations in these two indices was also insignificant in the majority of stations in the regions corresponding to the R99p, R95p, R20mm, PRCPTOT, and CWD indices.
The investigation and analysis of the extreme indices trend revealed that the occurrence of warm extreme events was increasing, while the occurrence of cold extreme events was decreasing in the areas covered in this study. One of the main reasons behind these phenomena has been progressive global warming, particularly since the late 1990s. The results of the present study concerning temperature extreme events confirm previous findings (by Zhang et al. 2005, Alexander et al. 2006, Zhao et al. 2012, Varshavian et al. 2011, Miri and Rahimi 2015, Karimi et al. 2018) stated in some studies that investigated temperature extreme indices. The above-mentioned researchers have also highlighted the increasing trend of warm extreme indices and the declining trend of cold extreme indices in their studies. According to the results obtained by the investigation of the frequency of the occurrence and trend of extreme precipitation indices in the areas covered in this study, it can be asserted that the total precipitation rate has been declining corresponding to the majority of Iranian provinces. However, maximum precipitation rates have been declining from 1988 to 2017 despite exhibiting noticeable extreme amounts. Hence, it could be stated that extreme precipitation events increase, whereas the duration of the wet season shortens. Moreover, the declining trend of the wet days' index and the increasing trend of successive dry days' index in Western and Southwestern Iran could be indicative of the gradual intensification of water scarcity. The results of the present study concerning precipitation extreme events largely confirm the findings reported in corresponding studies (such as Klein Tank et al. 2006, I'm et al. 2010, Jones et al. 2012, Koozegaran and Mousavi Baigi 2015) that emphasized the increased extreme precipitation rate and the decreased amount of total precipitation rate.