Extreme events of cold and heat waves in the northeastern regions of Iran during the period 2001-2020

Thermal temperature events for a period of 20 years (2001-2020) were extracted based on the data of synoptic meteorological stations in Mashhad, Sabzevar, Quchan, Neishabour, Bojnourd and Gonbad Kavous and the atmospheric patterns leading to these events were investigated. In this period, a total of 158 cases of cold waves and 1,014 cases of heat waves have been recorded in selected stations in the study area.From a statistical point of view, after using the Sen’s slope estimator nonparametric method, it has been shown that there was no significant increase in the intensity of cold waves for none of the stations and the highest trend of decreasing the intensity of cold waves is related to Mashhad station by 0.045 degrees Celsius per year. The number of cold waves during the statistical period has increased in Quchan, Neishabour and Gonbad Kavous stations and has decreased in Mashhad, Sabzevar and Bojnourd stations. The highest decreasing trend of heat waves is related to Quchan station by 0.056 degrees Celsius per year and no sharp increasing trend has been observed for any of the stations and at any of the significant levels. The number of heat waves during the statistical period has increased in Sabzevar, Neishabour and Gonbad Kavous stations and has decreased in Mashhad, Quchan and Bojnourd stations. From the point of view of synoptic analysis, strengthening of two high pressure systems (Siberia and Zinc in Europe) and expansion of orbital tabs of these two systems and creating high pressure belt in latitudes about 35 to 55 degrees north and altitude banding pattern of 500 hPa Northwest of the Aral Sea and the impact of the study area on the active tilt canal can lead to a noticeable decrease in temperature and the occurrence of a cold wave, which is associated with northeast winds of 850 hPa. In contrast, the spread of the European high-pressure system to the northeast of the country, along with the region's impact from the high-altitude subtropical ridge centered on North Africa and increasing the geopotential height of 500 hPa compared to the long-term average of the same period causes heat waves and temperature increases.Thermal temperature events for a period of 20 years (2001-2020) were extracted based on the data of synoptic meteorological stations in Mashhad, Sabzevar, Quchan, Neishabour, Bojnourd and Gonbad Kavous and the atmospheric patterns leading to these events were investigated. In this period, a total of 158 cases of cold waves and 1,014 cases of heat waves have been recorded in selected stations in the study area.From a statistical point of view, after using the Sen’s slope estimator nonparametric method, it has been shown that there was no significant increase in the intensity of cold waves for none of the stations and the highest trend of decreasing the intensity of cold waves is related to Mashhad station by 0.045 degrees Celsius per year. The number of cold waves during the statistical period has increased in Quchan, Neishabour and Gonbad Kavous stations and has decreased in Mashhad, Sabzevar and Bojnourd stations. The highest decreasing trend of heat waves is related to Quchan station by 0.056 degrees Celsius per year and no sharp increasing trend has been observed for any of the stations and at any of the significant levels. The number of heat waves during the statistical period has increased in Sabzevar, Neishabour and Gonbad Kavous stations and has decreased in Mashhad, Quchan and Bojnourd stations. From the point of view of synoptic analysis, strengthening of two high pressure systems (Siberia and Zinc in Europe) and expansion of orbital tabs of these two systems and creating high pressure belt in latitudes about 35 to 55 degrees north and altitude banding pattern of 500 hPa Northwest of the Aral Sea and the impact of the study area on the active tilt canal can lead to a noticeable decrease in temperature and the occurrence of a cold wave, which is associated with northeast winds of 850 hPa. In contrast, the spread of the European high-pressure system to the northeast of the country, along with the region's impact from the high-altitude subtropical ridge centered on North Africa and increasing the geopotential height of 500 hPa compared to the long-term average of the same period causes heat waves and temperature increases.