Introduction

Heat wave is an extremely important event related to temperature and has a great impact on human health. With the current trend of global warming, heat waves are likely to appear in the future with more frequency and intensity in most areas of the world. Iran has always been exposed to heat waves due to its special climatic conditions. Therefore, forecasting the future heat waves of the country in order to plan the environment and deal with possible risks is essential. The main goal of the current research is to simulate and predict Iran's heat waves based on general circulation models.

Methodology

In this research, to identify heat waves, the "Heat Wave Magnitude Index Daily" (HWMId) was used based on the maximum daily temperatures of 44 synoptic stations in the country in a 31-year period (1985 to 2015). Also, in order to simulate the daily maximum temperature and predict the future heat waves of the country with this index, the data of the CanESM2 GCM under the RCP4.5 scenario between the years 2015 to 2045 has been downscaled with SDSM. The input data of SDSM includes historical data of CanESM2 GCM (1961 to 2005), NCEP reanalysis data (1961 to 2005) and CanESM2 GCM data (2006 to 2100) under RCP4.5 scenario. Based on the HWMId, a heat wave is a wave greater than or equal to three consecutive days with a maximum temperature above the daily threshold in the reference period. The magnitude of the heat wave in each day is calculated based on the maximum temperature of that day and the 25th and 75th percentiles of the annual maximum temperature time series in the reference period, and the magnitude of each heat wave is the sum of the magnitude values of all days of that wave. The waves were extracted in each season, based on the separate percentile threshold of that season in each station. Annual values of each station were also obtained from seasonal values. Finally, annual and seasonal maps of average "magnitude" and "number" of observed and simulated heat waves of the country with HWMId and their time trend charts were prepared and analyzed.

Results

In general, there is a good agreement between observation and simulated maps. The average frequency of predicted annual waves in different regions of the country is between 2 and 12 occurrences, most of which are located on Shiraz, Shahrekord and Omidiyeh stations. The biggest and greatest heat waves in the future are predicted first for summer and then with a significant difference for winter and spring. Different patterns of the spatial distribution of predicted heat waves can be observed in Iran. In spring and summer, as well as on an annual scale, the maximum "magnitude" and "number" of the simulated heat waves are concentrated on the southwest and the western half of Iran and Gorgan station. In autumn, the center of the maximum is located in the inner regions of Iran and it stretches in the form of an oval from the north-west to the south-east of the country. In terms of the relationship with geographical factors, in summer by moving to higher latitudes and in spring by moving to the east of the country, we see a significant decrease in heat waves. The number and magnitude of heat waves in the country will increase in annual and seasonal scales until 2045, and the highest rate of increase is in summer. The most heat waves are predicted for the year 2043 with the number of 10.5 events.

Discussion

Normally, it is expected that the intensity and frequency of heat waves will be higher in the southern regions of the country. But according to the output of the model used in this research, different patterns of seasonal and annual distribution of predicted heat waves can be observed in Iran. This is related to the index used to define heat waves in this research, that is, the HWMId index, which has a percentage basis, and for this reason, the distribution patterns of the phenomenon in the country are out of the uniform and expected state; In such a way that even in the cold seasons of the year, heat waves occur in different regions of the country. For example, winter accounts for 23% of the total heat waves and 26% of the predicted severe heat waves. Nevertheless, summer is still the leading number of predicted heat waves in the country with 41% frequency. Despite the relative concept of HWMId in the definition of heat wave, the relationship between the occurrence of heat waves and some important geographical factors in the country is in accordance with the expectation, so that in the summer by moving to higher latitudes and in the spring by moving to the east of the country, we see a significant decrease in heat waves. Most likely, this reflects the role of external factors in the occurrence of heat waves in Iran, especially the greater impact of the Azores high pressure on the south-west and south of the country, which, of course, requires more studies.

Conclusion

Although heat waves occur in most regions of the country, in all maps, the cores of most heat waves can be seen in the center and southwest of the country around the provinces of Fars, Khuzestan and Hormozgan, and in some cases in the west of the country, which become more frequent and intense in summer. It should be acknowledged that the ability of all models, including the model used in this research, to communicate between all the factors and elements influencing Iran's heat waves, both in terms of time and space, is limited. Therefore, in order to achieve better results, it is suggested to test the ability of other models and scenarios in estimating the country's heat waves.