Interannual Variability in the Summer Season Onset, Length and End Dates Across the Iran (1948-2016)

It is very important to study the role of global warming on the variability of summer characteristics in 11 arid and semi-arid climates such as Iran, because of its impact on natural and social systems.The aim 12 of this study is to detect interannual variability in the onset, end and duration of the summer season in 13 Iran. To achieve this goal, three indices are defined: physical, dynamic and synoptic. Daily data of the 14 Earth's surface temperature ( ° C), tropopause level pressure (hPa) and geopotential height at 1000 15 hectopascals (m) are the basis of physical, dynamic and synoptic indices, respectively. Data were 16 extracted from the reanalysis databases for 1948 to 2016 in different domains. A three-variable dataset 17 using regional mean values allowed researchers to determine the start date (onset) of the summer based 18 on the following thresholds: Temperatures above 25 ° C and duration of 10 days (physical index), 19 tropopause level pressure less than 120 hPa and duration of 10 days (dynamic index), geopotential 20 height at 1000 hPa less than 50 m and duration of 10 days (synoptic index). The opposite situation 21 indicates the end of summer. The results showed that the start and end of summer based on physical, 22 dynamic and synoptic indices are May 11 and September 21, June 4 and October 1, May 20 and 23 September 15, respectively. The average length of summer is 134, 120 and 119 days, respectively. 24 Statistically significant long-term trends have been observed for the start and end dates of the summer 25 season. Seasonal variability has accelerated significantly since the late the twentieth century. In short, 26 the summer season in Iran tends to start early and end late. As a result, summer lengths are longer in 27 80 percent of the years after 1981 compared with less than 50 percent from 1948 to 1981.


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Changes in the time and intensity of seasons can have a variety of effects on ecosystems and 33 human society. For more than a century, we have realized that increasing the concentration of 34 two decades, changes in atmospheric CO2 levels have been shown to affect the timing of the 36 annual temperature cycle (Thomson, 2009). The change of seasons is directly related to global 37 warming. The first discussion of the effect of global warming on seasonal cycle change was 38 introduced by Thomson in 1995(Mann & Park, 1996. A slight change in temperature is 39 enough to shift the seasons with the early onset of summer and the delay of the first frost until 40 late autumn. Climate change is expected in the future to lead to shorter winters, early springs, 41 longer summers and late autumns in the mid-range. 42 In determining the boundaries of the seasons, it is important to pay attention to their 43 astronomical, meteorological and temperature definitions (Trenberth, 1983). It is clear that the 44 change of seasons is more related to climate conditions than to astronomical calculations. To  Another approach to examine the quantity and quality of the beginning and end of the summer 55 atmosphere has been a dynamic approach. This view is largely based on the theory that higher 56 heat transfer in summer conditions leads to higher terpopause (Gabriel et al, 1999). Therefore, 57 deep convection in the intercontinental convergence belt or on the intermediate extensions of 58 the continents in the summer continuously solves upward terpopause and deepens the 59 troposphere. During the monsoon summer, terpopause sometimes occurs above 18 km 60 (Mohanakumar, 2008). From low to high latitudes, the height of the terpopause decreases 61 rapidly at the location of the subtropical jet and the polar front. Especially when the jet is strong 62 and Fritsch, 1991).

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Goldreich and Chermoni (2006) used tropopause altitudes above 16 km for at least ten 65 consecutive days on synoptic graphs as a measure of summer length change in the Middle East.

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The results of significant trends showed that summer starts earlier and ends later, and the 67 number of summer days has increased during the study period. With this approach, the 68 structure of large-scale air circulation and summer behavior of subtropical jet in the Middle

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East over a period of 30 years  have been studied to determine the beginning of 70 summer in Iran (Gholipour et al. 2017). In this study, the daily behavior of the subtropical jet, 71 including the flow intensity and the amount of northward movement, has been investigated.

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The results show significant deviations in the early beginning of summer in the Middle East.

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The third method for identifying seasons is the synoptic approach. A review of climatic records 74 shows that in each season, there are specific synoptic systems in an area (Lamb, 1972). In each

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(2015) shows that the situation is more worrying than it is. It is predicted that the average 98 annual temperature in Iran by 2100, according to Scenario A2, will increase by 4.5 to 5.5 99 degrees Celsius, and according to Scenario B2, by 3 to 4 degrees Celsius. Assuming that global In this study, the summer season in the territory of Iran has been studied from three aspects.  (Table 1). The threshold of dynamic and synoptic parameters has been determined 120 experimentally. The decrease in the mean values of terpopausal pressure below the threshold 121 of 120 hPa indicates the northward displacement of the subtropical atmosphere from the 122 Iranian plateau and the establishment of the summer atmosphere. This shift, followed by a 123 decrease in terpopause pressure, often occurs over a short period of one to three days.

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Therefore, monitoring the pressure drop of this layer below 120 hPa as the threshold to study Portmanteau test for the first time was offered by Box-Pierce (1970) and its statistics are as follows: Where 154 is the number of observations, is the number of lags included in the test (generally between N/15 155 and N/10) and is the th autocorrelation coefficient between and +1 ; that is, The three rules proposed for the calculation of are as follows, (1) = /4, (2) = √ + 10 and (3) 158 = ln ( ). Simulation studies have shown that ≈ ln ( ) leads to the best power for the test. In the 159 portmanteau test, the independence hypothesis is written as 0 = 1 = 2 = ⋯ = = 0 versus 1 =

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Ljung and Box (1978) proposed a modified version of the portmanteau test as follows: Here, the independence hypothesis is not confirmed if | * | ≥ 2 . In other words, if the probability 164 value of the test statistic is less than 0.05, the independence hypothesis is not confirmed. If the 0 is 165 rejected, the Pearson's correlation coefficient test is used to determine trend characteristics. Pearson's 166 correlation coefficient is denoted by symbol and is obtained from the following equation.
The significance of the is displayed with T and is calculated based on the following statistics: If | | ≥ 1− 2 is holding, then the null hypothesis is rejected, otherwise (| | < 1− 2 ) the null hypothesis 173 is confirmed.

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Applying the conditional function on the data of average temperature zones in the territory of

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For comparative study, the study period was divided into two periods: 1948-1981 and 1982-190 2015. Representation of the average data of temperature zones (physical index) showed that in 191 the second climatic period , the length of the summer season has increased. This 192 increase from 129 days to 141 days was associated with 5 days beginning earlier and 7 days 193 ending later in the second climatic period than in the first climatic period (Figure 3). Unlike 194 the physical index, the dissociation and plotting of fluctuations in the regional mean data of 195 the terpopause pressure (dynamic index) showed that the length of the summer in the second 196 climate period  has been reduced compared to the first climate period .
second climatic period with 6 days beginning later and 7 days ending earlier (Figure 4).

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Also, the study of the fluctuations of the regional mean pressure data at 1000 hPa in the study  (Table 4). The authors declare that they have no known competing financial interests or personal 263 relationships that could have appeared to influence the work reported in this paper.

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Author's contribution 265 The authors declare that the manuscript has been read and approved by all named authors and 266 that there are no other persons who satisfied the criteria for authorship but are not listed. We 267 further confirm that the order of authors listed in the manuscript has been approved by all of 268 us.

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Funding statement 270 The authors received no financial support for the research, authorship, and publication of this 271 article.

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Availability of data and material 273 The datasets used during the current study are available in the Geospatial Interactive Online  293 We confirm that we have given due consideration to the protection of intellectual property 294 associated with this work and that there are no impediments to publication, including the timing 295 of publication, with respect to intellectual property. In so doing we confirm that we have 296 followed the regulations of our institutions concerning intellectual property.

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We understand that the Corresponding Author is the sole contact for the Editorial process. He Iran during 2071-2100 using PRECIS regional climate modelling system. Desert, 20(2), 123-134.       Figure 1 Geographical domains of different indicators for determining the temporal characteristics of the summer season, PI (Physical), DI (Dynamic) and SI (Synoptic). Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 2
Average geopotential height at 1000 hPa, June to September . Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.