Observed Heatwaves Characteristics and Variability over Saudi Arabia

Heatwaves are prolonged periods of excessively hot weather, which can have signi�cant impacts on human health, agriculture, and the environment. Climate change has been linked to an increase in the frequency, intensity, and duration of heatwaves. As the global average temperature rises, heatwaves are becoming more common and more severe. The Arabian Peninsula is warming at a faster rate as compared to the globe in the recent decades. In this paper the mild, moderate, severe, and extreme heatwaves de�ned by 85th, 90th, 95th and 99th percentile respectively are analyzed over Saudi Arabia using historical daily maximum and minimum temperature observations for the period 1985–2021. The large number of mild heatwaves are observed all over Saudi Arabia while extreme heatwaves are dominant in the northwestern region. Moderate and severe heatwaves are observed less in both the Red Sea and Arabian Gulf coastal regions. The heatwaves are intense in the northern and central areas as compared to other regions of the country. Heatwaves frequency, intensity and length in Saudi Arabia are in increasing trends, along with the increase in the heatwaves season length. The heatwaves frequency and intensity are observed large during the ENSO La Nina and neutral phases along with NAO negative phase as well as IOD negative and neutral phases. However, further investigation is required to see the occurrence of heatwaves in different climate zones over Saudi Arabia at various seasons and their teleconnection to largescale circulations.


Introduction
The increase in heatwaves frequency and intensity is one of the greatest risks related to climate to humanity, since the turn of the 21st century Change (IPCC) Special Report on Extreme Events also shows that frequency of very cold/hot days decreased/increased globally while heatwaves will be more frequent, longer, and intense in the 21st century (IPCC, 2013).The IPCC Sixth Assessment Report (AR6) also concluded that, globally, hot extremes (including heatwaves) have increased in frequency and intensity while cold extremes have declined since 1950 (IPCC, 2021).
According to global climate estimates, summer heat extremes in the Middle East and North Africa will likely become far more intense (Zittis et al. 2021).Maximum and minimum temperatures are the major controllers of heatwaves phenomena.Even under low greenhouse gas emission scenarios, it is anticipated that the Middle East would experience especially marked changes in mean temperature in the 21st century, along with less dramatic but still signi cant changes in precipitation (IPCC, 2013).It is found that the regional warming follows a different seasonal response, with the boreal summer being predicted to see a substantially greater temperature increase than the rest of the year (Lelieveld et al. 2016).In recent decades, Saudi Arabia likewise saw rising temperatures that were in accordance with the general trend.Almazroui  Because climate extremes, particularly the heatwaves have impacted human and environmental systems in different regions of the globe in recent decades, therefore one of the key scienti c questions is to understand detail behavior of heatwaves for a region in the recent past.This will allow to explain the behavior of possible heatwaves in the future periods under different climate change scenarios.
Furthermore, the investigation of past heatwaves offers a better understanding of the areas affected by the heat, allowing relevant government authorities to create a better preparedness plan for the potentially dangerous effects of this natural calamity.Therefore, the main focus of this study is on the variability of heatwaves basic components that include frequency, intensity, duration, and timing based on observed daily maximum and minimum temperatures station data in Saudi Arabia.Mild, moderate, severe, and extreme heatwaves, based on percentage-based standards are calculated.The results of this study will be useful for disaster risk managers, agriculturalists, agronomists, hydrologists, and national hydrometeorological organizations for sustainable development in the various socio-economic sectors of the country as a whole.

Study area
The climate in Saudi Arabia is predominantly desert, with temperatures that can reach extremely high levels during the day and drop signi cantly at night.The country has hot and dry summers and cool winters with occasional rain.The coastal regions along the Red Sea and Persian Gulf have a milder climate due to their proximity to water.Saudi Arabia covers about 80% of the Arabian Peninsula (Fig. 1 1).There are no permanent rivers or lakes across the country.Agriculture in Saudi Arabia is limited due to the country's arid climate and lack of water resources.Some of the crops grown in Saudi Arabia include dates, wheat, barley, vegetables, and fruits such as grapes and citrus.Livestock farming, including the breeding of camels, sheep, and goats, is also an important part of the country's agriculture sector.These crops are all irrigation-dependent and negatively impacted by climatic changes (Alkolibi 2002).According to Hui Cao et al. (2015), Saudi Arabia is also susceptible to dust storms and other dustrelated phenomena such as dust in suspension and blowing dust.The majority of the weather stations are situated at various airports over Saudi Arabia (Fig. 1, Table 1).One of the biggest sand deserts in the world, Rub Al-Khali (Empty Quarter), has no meteorological installation.

Data
In order to calculate heatwaves, daily surface air temperature (maximum and minimum) collected by the National Centre for Meteorology (NCM) from 26 stations across Saudi Arabia for the period 1985-2021 is used in this study.The characteristics of all 26 stations across Saudi Arabia with their average climatology (maximum and minimum temperatures and rainfall) over the period are shown in Table 1.In this study, data were analyzed for ve climatic zones in Saudi Arabia (Fig. 1), namely the northern, coastal, interior, highland, and southern regions (Almazroui et al. 2014).A Cressman objective analysis (Cressman 1959) is performed on the station data to arrive at a gridded result that represents the station data.

Heatwave de nition and components
The World Meteorological Organisation states that there is no agreed-upon de nition of heatwaves.The literature has a number of de nitions, measures, and indices that are utilised by the meteorological services and other authorities.Some of these de nitions are derived using regional norms and absolute temperature limits, while others use percentage-based standards.In this study, having minimum and maximum daily temperatures that are higher than the 85th, 90th, 95th, and 99th percentiles of the historical recorded minimum and maximum daily temperatures, respectively, is considered a heatwave event..These subclasses are referred as mild, moderate, severe, and extreme events, respectively which are widely used in heatwave studies (see 1 The signi cant function is sgn(θ), which ranges from − 1 to 1 (Eq.1), and the data values are j and k.The data length is indicated by n. 2 Sample data with the same value as the data values of the p th group supplied by t p are presented by q number.The statistic S is as displayed in (Eq.2) 1 The value of Z is near to normal distributed.

Sen's slope method 2
Sen's slope test estimates the linear time series' trend (change per unit of time) in terms of magnitude (Sen, 1968).
In Eq. 2, the slope and constant are denoted as M and C, respectively, and the function of a linear trend is represented by f(t).Using this method, the trend's size is estimated: According to Eq. 3, the data values are shown as x i and x j at time t i and t j (i > j) respectively.
Sen's slope was used to calculate the trend magnitude (slope) of the aforementioned variables, while the Mann Kendall test was employed to establish the linear trend signi cance (at the 95% level).Both tests offer trends and their importance for each of the observational grids over Saudi Arabia based on historical data.

Maximum and minimum temperature
The spatial distribution of annual mean maximum temperature, minimum temperature climatological for the period 1985-2021 is shown in Fig. 2. The gure shows that, with the exception of Gizan, the mean annual maximum temperature was high in much of western and southern Saudi Arabia while it was low in the region's north and southwest.The minimum temperature remains high in the western Saudi Arabia (Makkah, Jeddah, Madinah, and Yenbu, see Table 1).Overall, the annual mean maximum temperature in Saudi Arabia is observed above 27°C, which reach to about 40°C at some pockets (Fig. 2a, Table 1).The maximum temperature is increasing at the rate of about 0.1-0.2°C/decade, while it reaches to up to about 0.5°C/decade at few pockets in the central and southern regions of the country (Fig. 2b).The annual mean minimum temperature observed below 18°C in almost half part of the country while the rest half experienced above 18°C, only at two regions it exceeded 24°C (Fig. 2c, Table 1).The minimum temperature is increasing at the rate of about 0.3-0.4°C/decade, while it exceeds about 0.5°C/decade at few pockets in the southern region of the country (Fig. 2d).It is evident that over much of Saudi Arabia, the minimum temperature is rising more quickly than the maximum temperature.Saudi Arabia's western region is made up of desolate mountains, whereas the majority of its southern region is made up of a sand desert.Due to the incursion of summer monsoon currents and shifting of the Inter-Tropical Convergence Zone (ITCZ), which causes moisture advection in the lower troposphere, Saudi Arabia's annual rainfall remains high (> 200 mm) in the southwestern parts of the country while remaining very low in some of the country's northern and southern regions (Table 1).(e.g.Lashkari et al. 2021).The severity of heatwaves is increased by terrain features, high temperatures, and minimal rainfall.

Hot days
The climatology of number of hot days (Fig. 3) shows above 25 days/year over most parts of the country and the value reaches to above 40 days/year in some pockets, one in the very north and another to the south of central Saudi Arabia (Fig. 3).The trend analysis show signi cant increase in the number of hot days over the whole region.The hot days frequency is increasing at the rate of about 5 days/decade and is reaching up to 8 days/decade at some places over the country.

Heatwaves frequency
Saudi Arabia observe frequent mild heatwaves (Fig. 4a).The mild heatwaves frequency is more than 5 per year.The occurrence of mild heatwaves is more in the southern, coastal, and northwestern areas of the country, where it reaches to about 8 per year.The severe heatwaves frequency is 2 to 3 per year at some areas in Saudi Arabia, while extreme heatwaves occur only about once per year in the northwestern area (Fig. 4d).In Saudi Arabia, the average number of heatwaves is about 6.3, 3.9, 1.6, and 0.2 per year for mild, moderate, severe, and extreme, respectively (Fig. 5).Overall, heatwaves frequency has increasing trend over the country for all categories.The increase rate is observed about 1.8, 1.7, 0.9 and 0.1 heatwaves per decade for mild, moderate, severe, and extreme heatwaves, respectively.Hence, the frequency of mild and moderate heatwaves is increasing at a faster rate as compared to sever and extreme heatwaves.There is also large inter-annual variability in the occurrence of heatwaves.The extreme heatwaves events were not frequent before the year 2010 (Fig. 5), whereas many extreme heatwaves have occurred in the region in the last decade.

Heatwaves intensity
The trend in the intensity of heatwaves is highest in the northern and central areas of the country (Fig. 6).
It is lowest in the southern and southwestern areas.The observed increase in the intensity of mild and moderate heatwaves is about 40 and 20°C/decade, respectively while it is about 10°C/decade for severe heatwaves.The extreme heatwave trend in the intensity is observed only about 1°C/decade in the northern side of the country.The time sequences of heatwaves intensity show that in Saudi Arabia, the intensity is about 110.9, 52.2, 15.3, and 0.9°C/year for mild, moderate, sever and extreme respectively (Fig. 7)..It is increasing at a rate of about 54.4, 30.1, 10.1 and 0.7°C/decade for mild, moderate, severe and extreme respectively.Hence, the mild heatwaves intensity is increasing at a faster rate in the country as compared to the other categories.This indicates an alarming climatic uncomforting condition for the country.

Heatwaves season and length
The date of the occurrence of heatwaves is shown in Fig. 8.There is a shift in terms of the onset and the ending of the heatwaves over the years.Mild heatwaves often begin to appear during June in Saudi Arabia except for the western area it is in May (spatial plots are not shown).As for moderate heatwaves, it often begins to occur during July in the northern and western areas whereas for the rest of the country it starts in June.For the extreme heatwaves, it often starts during July in the northern, central, and southern areas, whereas in western and hilly areas it starts during June.For the severe heatwaves, it begins to occur in June on the western and hilly areas, while the rest encounter in July.The rst day of the rst heatwave has decreasing trend, while the last day of the last heatwave in the year is showing increasing trend for the mild, moderate, and severe heatwaves (Fig. 8a-c).For the extreme heatwave, the last day of the last heatwave is also showing decreasing trend with a slower rate than the rst day of the rst heatwave.This indicates that heatwaves season length is increasing in Saudi Arabia.
The total length of heatwaves de ned as the cumulative duration of all heatwaves in each calendar year, for mild, moderate, severe and extreme is increasing in Saudi Arabia (Fig. 9).On average, the total length of mild, moderate, severe, and extreme heatwaves is about 30.3, 15.8, 5.3, and 0.4 days, respectively.The total length of heatwaves is increasing at the rate of about 12.6, 8.3, 3.3, and 0.4 days per decade.

Heatwaves regional analysis in Saudi Arabia
The analysis of heatwaves implemented over the ve climatic zones (northern, coastal, interior, highland, and southern) in Saudi Arabia discussed here.
The frequency of heatwaves over all climate zones is in increasing trend during the study period with variation in the rate of change from one category to another and one region to another region (Fig. 10).
The frequency of heatwaves in the highland is the highest for mild and moderate, where the average was 7.1 and 4.2 days per year, respectively.The frequency of heatwaves in the southern region is also large for the mild, moderate and severe heatwaves.As for the extreme heatwaves, the occurrence rate is too less at all zones except for northern and highland zones, a few heatwaves observed in the recent years.
The highest rate of the recurrence of moderate, extreme, and severe heatwaves observed during the years 2010 and 2017, whereas the highest number of mild heatwaves observed during 2010, 2018 and 2019.
The occurrence of large number of heatwaves in Saudi Arabia is dominated by ENSO La Nina and neutral phases along with NAO negative phase as well as IOD negative and neutral phases.However, ENSO El Nino phase dominant for the large frequency of heatwaves in the highland.It needs further investigation to understand the relationship and teleconnection of largescale circulations to the occurrence of heatwaves in different climatic zones over Saudi Arabia.
The intensity of mild heatwaves is highest in the northern and central zones of the country, followed by the coastal zone (Fig. 11a).It is lowest in the southern and highland zones.The highest heatwave intensity occurred in the northern zone in 2017, where it reached to 6.7°C, followed by the year 2020 for the same zone where the intensity reached to 6°C.It is worth noting that the most intense 9 heatwaves during the study period occurred in the northern zone.The intensity of moderate heatwaves also increasing in all ve zones (Fig. 11b).The average intensity of the moderate heatwaves is highest in the northern and coastal zones, and lowest in the highland.The most intense moderate heatwaves observed in the coastal zone in 1991, where it reached to 6.9°C, followed by the northern zone, where it reached to 5.4°C.The intensity of the severe heatwaves slightly increasing in the northern zone encounters the most intense severe heatwaves among all zones (Fig. 11c).The top ten intense severe heatwaves in Saudi Arabia occurred in the northern and coastal zones.And most intense severe heatwaves were observed in the coastal zone in the years 1993 and 1997, where the intensity reached to 6.4°C and 6°C, respectively.The intensity of the extreme heatwaves is increasing for all zones (Fig. 11d).The average intensity of extreme heatwaves is highest in the northern and coastal zones, followed by the interior zone and it is lowest in the southern and highland.Looking at the year of largescale circulations, the intensity of heatwaves is related to ENSO La Nina and neutral phases along with IOD negative and neutral phases, while governing by negative NAO.
The length of all four categories of heatwaves is increasing in the study period (Fig. 12).The average cumulative length of the heatwave is large in the northern, interior and southern zones up to 2012.After 2013, it is large for northern and highland.The coastal zone has the lowest cumulative heatwave length.By investigating the time series for each year separately, it is noted that the years 2017 and 2010, respectively have the longest cumulative heatwaves, and this is consistent with the result of analyzing the frequency of heatwaves over the regions for these two years.
In the country, the average length of mild heatwaves is about 30 days, which reached to about 65 days for the country and about 82 days in the northern zone in 2017 (Fig. 12a).The fact is that the mild heatwaves often begin in June for all zones except for the coastal zone it begins in May.The length of moderate heatwaves follows the pattern of mild heatwaves, however the average length of the moderate heatwaves of the country is about 16 days while it reached to about 47 days for the country and about 65 days in the northern zone in 2017 (Fig. 12b).The moderate heatwaves often begin during July in the northern and coastal zones whereas for the rest of the zones it starts in June.The length of severe heatwaves for the country is about 5 days, which reach to about 24 days and 43 days in 2017 respectively for country and northern zone (Fig. 12c).The severe heatwaves occur during June in the coastal and highland, while the rest encounter it occurs during July.The average length of extreme heatwaves is below 1 day for the country, and it is above 3 days only in 2010, 2017, and 2020 (Fig. 12d).
The extreme heatwave often starts from July in the northern, interior, and southern zones, while in costal and highland it starts from June.The long total length of heatwaves in Saudi Arabia observed during the ENSO La Nina, negative NAO, and negative IOD phases.However, detail study on the occurrence of various heatwaves in different climate areas over Saudi Arabia in various seasons and their teleconnection to largescale circulations is required using observed and climate models simulations.

Conclusions
The occurrence of heatwaves over Saudi Arabia is analyzed using surface observations at 26 station locations for the period 1985-2021.In the analysis, the mild (85th percentile), moderate (90th percentile), severe (95th percentile), and extreme (99th percentile) are considered for ve climate zones over the country.The occurrence of mild heatwaves is observed all over the country while extreme heatwaves are observed only in the northwestern region.The occurrence of moderate and severe heatwaves is less in the coastal regions.The frequency of heatwaves in Saudi Arabia is about 6, 4, 2, and < 1 days per year for mild, moderate, severe, and extreme, respectively.In recent years, Saudi Arabia has experienced an increase in the frequency and intensity of heatwaves.This can be attributed to global warming and climate change, which are causing temperatures to rise globally Overall, heatwaves frequency and intensity are in increasing trends in different climate zones over Saudi Arabia.The total length of heatwaves in Saudi Arabia is also increasing because the occurrence of the rst/last day of rst/last heatwave is in backward/forward direction.These are the signal of regional warming due to climate change.The in uence of largescale circulations such as ENSO, NAO and IOD are observed on the frequency and intensity of heatwaves in Saudi Arabia.However, further detailed investigations are . The European 2003, Greece 2007, Russian 2010, and Pakistan 2015 are some of the unprecedented heatwave examples in terms of mortality.Heatwaves have a number of negative effects on people's health, agriculture, workplace productivity, the frequency and intensity of wild res, infrastructure, and other living things in different parts of the biosphere (McMichael et al. 2011; Lesk et al. 2016; Thornton et al. 2009; Lobell and Field 2007; Kjellstrom 2016; McEvoy et al. 2012; Rübbelke and Vögele 2011; Barriopedro et al. 2011; Coumou and Rahmstorf 2012; Zittis et al. 2022).Previous studies on predicted global warming and climatic variability, from a regional to a global scale, indicate that hot extremes will become more frequent and intense over the next few decades (e.g.Donat et al. 2013; Fischer and Schär 2010; Russo et al. 2014).The Intergovernmental Panel on Climate et al. (2020) found a 0.63 C per decade increase in the observed national annual average temperature over Saudi Arabia.The ensemble of 31 Coupled Model Intercomparison Project phase 6 (CMIP6) models predicts that the 66% probable ranges of annual-averaged temperature will rise by 1.8-2.7 C and 4.1-5.8C, respectively, for the near (2030-2059) and distant (2070-2099) future time frames under the SSP5-8.5 scenario (Almazroui et al. 2020).The rise of temperature in a region is intimately related to hot extremes, pronounced drought, rainfall variations, and water scarcity.The Middle East region including Saudi Arabia is experiencing an increase in warm days and a decrease in cold days, according to climate indicators related with global warming (Zhang et al. 2005).According to Zittis et al. (2021), over 600 million people-roughly half of the population of MENA-could be exposed to super-and ultra-extreme heatwaves by the end of the twenty-rst century.Therefore, occurrence of heatwaves in the region is interesting to discuss.Numerous large-scale teleconnection patterns have been connected to variations in precipitation and temperature over the region of the Middle East.In the past, there has been a great deal of research on how global circulation patterns connected to ENSO and the North Atlantic Oscillation (NAO) affect the temperature and precipitation over the Arabian Peninsula (e.g.Donat et al. 2014; Al Sena and Anis 2015; Athar 2015; Kang et al. 2015; Ehsan et al. 2017; Almazroui et al. 2019; Donat et al. 2014).Hasanean and Almazroui (2017) investigated how the south Asian monsoon low affected the effect of tropical Atlantic SSTs on the summer temperature over the Arabian Peninsula.The effect of Indian Ocean Dipole (IOD) (Al Sena and Anis 2015; Athar 2015) and CircumGlobal Teleconnection (CGT) (Saeed and Almazroui 2019; Atif et al. 2020) on the Arabian Peninsula climate variability is also extensively studied.Worldwide episodic droughts are most frequently caused by ENSO, which also happens to be the major mode of global climate variability (Neelin et al. 2003).Syed et al. (2022) found that droughts over Saudi Arabia are linked with Paci c Decadal Oscillation (PDO).

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Table 1
List of meteorological stations across Saudi Arabia along with latitude, longitude, elevation, annual average maximum and minimum temperatures with rainfall for the period 1985-2021.
(Mann 1945;Kendall 1975;Gilbert 1987identify trends.The Mann-Kendall test(Mann 1945;Kendall 1975;Gilbert 1987) is a non-parametric statistical test used to determine if there is a monotonic trend (increasing or decreasing) in a time series dataset.It is often used in environmental and hydrological studies to detect trends in climate or water resource data(Kundu etal.2014; Oza and Kishtawal 2014; Zhang et al. 2015.The method used to compute the equation's variance is as follows: (Sha ei et al. 2019.Following traditional heatwave components including frequency, intensity, duration and timing(Sha ei et al. 2019) and in accordance with our previous research, following eight heatwave components that are computed at each station based on the available meteorological data:1.Number of hot days (Days): When the maximum and minimum temperatures exceed predetermined thresholds, it is considered a hot day.2.Frequency of heatwave (waves): Number of distinct heatwaves that occur in a given year.3.Total length of heatwaves (total): The length of all heatwaves combined in a given year.4.Longest heatwave event (longest):The incidence of a heat wave that lasts the longest in a given year. 5. Heatwave intensity: Sum of the cumulative value of daytime and nighttime temperatures above the maximum and minimum temperature threshold respectively during a heatwave.6.First heatwave event (First): the day of the rst heat wave in a certain calendar year.7.Last heatwave event (Last): The day of the last heat wave in a certain calendar year.The examples and further details can be found in Sha ei et al., (2018).2.2 Mann-Kendall signi cance test