The present study evaluated the dynamic changes in Sudan low systems over six decades (1957–2019), which were classified based on solar cycles. Rainfall days were extracted in January using data from 42 synoptic stations in the south and southwest regions of Iran. Then, the rainy days due to the Sudan low system were separated from the precipitation of the other atmospheric systems using the visual analysis of the maps. The synoptic analysis indicated that the sea level pressure has been decreasing at all sampling stations from the first to the sixth decade. Furthermore, decreasing elevation to −80 m (negative anomaly) in the 500 hPa level atmosphere from the first decade to the sixth decade indicated further atmospheric instability and more strengthening of the Sudan low compared to previous decades. The contribution of advection moisture to the area increased within the region at 700 hPa. This implies that the role of the transferred moisture from the intertropical convergence zone is increasing in the systems transferred from Sudan and in the study area. In general, the share of Sudan low precipitation is increasing in the south and southwest regions of Iran. This result is a positive effect of the climate change on the study area.
This study describes the decadal changes in Sudanese low pressure affecting precipitation days in southwestern Iran.
Sea level pressures have been declining from the first decade to the sixth decade.
Sudan low pressure has intensified over the period under review.
The share of Sudan low pressure in the abundance of rainy days in the southern and southwestern regions of Iran is increasing.
The Sudan low pressure is regarded as a system influencing the rainfall during the cold season in the south and southwest regions of Iran. This sub-tropical system is part of a large-scale atmospheric circulation pattern of tropical origins. In addition, its low-pressure tracks extend to the south of the Red Sea, Sudan, and Ethiopia. These systems frequently cross the above-mentioned areas during the cold season and continue their route from the south to the southwest regions of Iran, causing rainfall in these areas. Acting at low pressures in terrain surface or low altitude in the upper level is the common feature of all these rainy synoptic-dynamical systems.
According to El-Fandy (1948), the history of recognizing the Sudan low in the Middle East and the Red Sea region goes back about 80 years when Ashbel (1938) first described the eastern Mediterranean rainfall. Ashbel concluded that the rainfall in the area was affected by a system which he called the ‘Red Sea low pressure’. In the early 1950s, El-Fandy studied various characteristics of these low-level pressures. For instance, this Egyptian meteorologist found that these pressures are formed in northern Sudan and the Red Sea region, which affect the climate of the North-East African region and the Middle East, thus El-Fandy called such pressures the ‘Sudan low-pressure monsoon’. According to him, the center of the fluctuation of this low pressure is annually variable from Ethiopia to Sudan during the cold period of the year (El-Fandy 1950a, 1950b, 1952) and the westward shift is from September to January; whereas, according to several studies, the eastern one is from February to May (e.g., Tsvieli & Zangvil 2005, 2007; Awad & Almazroui 2016; Awad & Mashat 2019). This period is considered as the first phase of studies on the Red Sea low pressure. In the early 1960s, the structure of the local air circulation in the Red Sea region was further evaluated, following the findings of some studies regarding the impact of the Red Sea air flow on desert locust life and its migration (e.g., Johnson 1963; Pedgley & Symmons 1968). The continuation of these studies led to the discovery of the convergence zone of the Red Sea (Flohn 1965, 1987; Pedgley 1966a, 1966b).
In addition, other researchers selected a new name, the ‘Red Sea Trough’ (RST), for low pressure without front which was from the Red Sea region and Sudan spreading to the eastern Mediterranean and causing heavy rainfalls (Krichak et al. 1997a, 1997b). Different studies widely confirmed the importance of Sudan low system in the Mediterranean climate (Krichak et al. 2000, 2015; Tsvieli & Zangvil 2005; Haggag & Al-Badry 2013; Almazroui & Awad 2016; De Vries et al. 2016). Therefore, the researchers evaluated the role of other atmospheric parameters in identifying and mitigating the Sudan low by increasing meteorological data from the upper tropospheric conditions.
For instance, Mashat & Awad (2015) identified the importance of an aspect related to the inactive nature of Sudan low on the spread of dust on the Arabian Peninsula. They found that the RST is responsible for the dust in the atmosphere of this area with a warm and dry nature due to the east and southeast flows in the lower troposphere. However, Saaroni et al. (1998) indicated that trough is drawn northwards and activated by moisture, and finally flooded in the eastern Mediterranean Sea if it is accompanied by a high-level tropospheric trough. This is in line with the results of other studies (e.g., Krichak et al. 2000, 2012; De Vries et al. 2013; Haggag & Al-Badry 2013; Yair et al. 2015). Furthermore, Awad & Almazroui (2016) discovered the Sudan low nature and RST using the mean sea level pressure (SLP) for the 1955–2015 winter period. Based on their results, about 96% of the winter trough formed near the two main sources of the south and southeast Sudan. In another study, Awad & Mashat (2019) utilized the mean SLP in order to investigate the climatological studies on Sudan low and RST fall season during 1955–2015. They concluded that 97% of the RST develops in southern Sudan and the Red Sea in the autumn. Furthermore, the amplification of Sudan low with RST in the upper atmosphere, as well as the weakening of the anti-cyclonic systems on the Red Sea, is simultaneous with the Sudan low shallow on the Arabian Peninsula.
Roushangar & Alizadeh (2018) investigated the detection of annual rainfall changes in Iran using the function analysis method. According to the results, the entropy of precipitation in the south decreased as latitude dropped and showed a decreasing trend. They also found a close relationship between longitude and the mean annual entropy in Iran. Javari (2016), in analyzing the trend and homogeneity of precipitation in Iran, stated that there is a great variety of seasonal precipitation patterns in Iran and less spatial coherence has been observed than temporal patterns in seasonal precipitation. He also suggested that seasonal rainfall changes showed a decreasing trend in the central and eastern regions and an increasing trend in the west and north.
Based on the evidence, Olfat (1968) was the first to study Sudan low in the context of Iran. Olfat refers to low pressures which are formed in northeastern Africa and the Red Sea and then pass through Saudi Arabia and the Persian Gulf, and finally, enter Iran and cause rainfall. Additionally, Ghaemi (1970) investigated the role of moisture resources in the West Indian Ocean, as well as the Sudan systems with the jet stream in the precipitation of southern Iran. In another study, Faraji (1981) evaluated the route of low-pressure systems, which caused precipitation over Iran during 1970–1974. He believed that 23% of these entry systems crossed the Red Sea. Lashkari (1996) conducted the most comprehensive study and paid special attention to Sudan low. In this regard, he identified the most important cause of flooding in the southwestern region of Iran as Sudan low and explained how these low-pressure systems form, develop, and spread in this area. In addition, Lashkari (2002) determined the route of Sudan low systems which enter Iran. To this end, he examined 200 Sudan systems during 1969–1989 and demonstrated that they enter Iran through five major routes due to synoptic patterns, leading to rainfall. Investigating the Sudan low by synoptic methods, Mofidi & Zarrin (2005) indicated that the highest frequency regarding the occurrence of these low pressures was first in December and then during the winter. Sayad et al. (2021), in studying the dynamic effects of Sudanese systems on humidity nutrition in Iran, showed that most of the effect of the Sudanese system on Iran's precipitation is during December–February. Also, jet streams have played an important role in strengthening Sudan's low pressure and transferring humidity into the country during the precipitation days by creating atmospheric rivers. Raziei (2018) in the daily and monthly analysis of precipitation regimes in Iran during 1951–2014 showed that the longest dry period occurred in the southern half of Iran. January and winter have the maximum annual precipitation in most of the surveyed stations in Iran, respectively. Also, in most parts of Iran, the concentration of precipitation is high, which indicates the occurrence of the major portion of precipitation days in certain months, and elapsing most of the year without precipitation in Iran. Furthermore, other valuable studies evaluated the impact of Sudan low on the rainfall of Iran to the community of the climatologists (e.g., Lashkari 2003; Alijani et al. 2008; Lashkari et al. 2013, 2018; Movaghari & Khosravi 2014; Akbary 2015; Lashkari & Mohammadi 2015; Parak et al. 2015; Ghaemi et al. 2017). These climatic studies led to the discovery of Sudan low systems as one of the synoptic components derived from the regional-scale circulation pattern in the climate of the cold period in Iran. Precipitation events and atmospheric instability in the Middle East, especially the south and southwest regions of Iran, are generally affected by Sudan systems (Dunkerton & Delisi 1986; Angell 1992; Davis & Benkovic 1992; Harvey & Hitchman 1996; Dayan et al. 2001; Alpert et al. 2004; Ziv et al. 2004).
As can be seen, many studies have been done on the role of Sudan low pressure in the occurrence of Middle East precipitation, of which most have focused on identifying the synoptic patterns of Sudan low pressure. However, no study has been done on the decade changes in Sudan low pressure over the past half-century and the changes in climatic elements during the presence of Sudan low pressure at the time of the precipitation occurrence. On the contrary, determining this low pressure intensity during the past decades helps to better identify the behavior of this important low pressure in the Middle East, especially the role this plays in precipitation changes in southwestern and southern regions of Iran and in this regard, will also enable more accurate forecasting of Sudan low pressure changes in the future.
MATERIALS AND METHODS
The evaluation of the rainfall related to this low-pressure system is necessary for determining the spatial extent of the Sudan mechanism in Iran. Several studies (e.g., Lashkari 1996, 2002, 2003; Lashkari & Khalilian 2013; Lashkari et al. 2018) considered that the entry of the system into Iran encompasses south and southwest regions. In this regard, the daily precipitation data of 42 stations of the south and southwest regions of the country were prepared by the Meteorological Organization of Iran, which were related to the period from 1957 to 2017. In this period, January was highlighted since extensive rainfalls in the Middle East are attributed to the cold season and January symbolizes the winter conditions (Barreit 1982). The initial range and stations are shown in Figure 1.
As is known, the sun serves as the fundamental source of energy in the climate system of the Earth (Markson & Muir 1980; Reid 2000; Schlegel et al. 2001; Solanki 2002; Tsiropoula 2003) and creates its climatic differences (Haigh 1996). These differences have a significant impact on the general circulation of the atmosphere and the activity of atmospheric systems in the short and long terms (Hoyt 1979; Benestad 2006). Thus, the study period with long-term variations was considered from 9.5 to 11 years based on solar cycles. Table 1 summarizes the decades of the study. Rainfall days were extracted in January using the daily rainfall data of 42 synoptic stations. For these days, SLP and geopotential height data at 1,000 hPa with 2.5° × 2.5° spatial resolution were obtained from the dataset of NCEP/NCAR1 reanalysis project. Additionally, the frame of the reference was provided in 0°–100°E and 10°–55°N latitude belt in the GrADS2 software. The visual analysis of high and low altitude cores and geopotential height at 1,000 hPa pressure level (El-Fandy 1950a; Lashkari 1996, 2002) were considered based on the aim of the study. Accordingly, the approximate locations of activity centers, as well as the range of the formation and displacement of the Sudan system, were initially identified based on the location of the formation of low-pressure and high-pressure cores. Then, the rainy days due to the Sudan system in January were separated from the precipitation of the other atmospheric systems (i.e., Sudan-Mediterranean and Mediterranean systems).
|Decades .||Periods .||Solar cycle .|
|1||April 1954–October 1964||19|
|2||October 1964–March 1976||20|
|3||March 1976–September 1986||21|
|4||September 1986–August 1996||22|
|5||August 1996–December 2008||23|
|6||December 2008–December 2017||24|
|Decades .||Periods .||Solar cycle .|
|1||April 1954–October 1964||19|
|2||October 1964–March 1976||20|
|3||March 1976–September 1986||21|
|4||September 1986–August 1996||22|
|5||August 1996–December 2008||23|
|6||December 2008–December 2017||24|
For these days, SLP, geopotential height, specific humidity, and omega data at the levels of 1,000, 850, 700, and 500 hPa with 2.5° × 2.5° spatial resolution were obtained from the NCEP/NCAR dataset. Using the GrADS software, the numerical values of these parameters were calculated at 14 agent synoptic stations with the highest statistical data over six decades. In addition, for the sum of the days of Sudan low rainfall in each decade, the daily composites of the anomalies of these variables were provided from the NCEP/NCAR dataset.
In the previous framework, the anomaly plots were then prepared and interpreted for six decades. It should be noted that the maps and charts were drawn and reviewed in the six decades for all discussed levels although only some maps were presented for the sake of epitome.
RESULTS AND DISCUSSION
Analysis of Sudan low SLP changes
The changes in the conditions of SLP stations in the southwest region of Iran are illustrated in Figure 2(a). As shown, most stations have a downward trend, along with decreasing and increasing oscillation patterns during the first decade to the sixth decade. The highest decreasing slope is observed at the Ahvaz station (Figure 2(a), the red line). This station, with an elevation of about 22.5 m above sea level, is the entry location for the Sudan systems in the southwest region of Iran (Lashkari 1996). As displayed in Figure 2, the above-mentioned station has SLP values between 1,013 and 1,016 hPa during the first decade to the second decade although this amount demonstrates a decrease by about 1–2 hPa from the beginning of the third decade toward the coming decades. The SLP changes are 1,008–1,010 hPa from the end of the third decade to the sixth decade. The SLP has decreased by about 8 hPa in the last two decades (solar cycles 23 and 24) compared to previous decades. Shahrekord, Yasuj, Ilam, and Khorramabad stations have the highest elevation from the sea level, respectively. However, a decreasing slope is found in the SLP at these stations, except for the Yasuj station, from the first decade to the sixth decade. This decrease in pressure is more pronounced than that of the Ahwaz station (Figure 2(a)), which is due to the avoidance of moisture sources, along with high altitude. The Yasuj station is at a higher altitude while it is less distant to the source of the moisture (Persian Gulf) compared to the other three stations. Thus, the SLP values at this station are decreasing and are lower than those of the three mentioned stations (Figure 2(a), the green line). The change in SLP at the coastal Bushehr station occurs at lower values compared to the other stations. Thus, SLP fluctuates in the range of 1,008–1,010 hPa from the middle of the second decade to the end of the fourth decade. Furthermore, SLP decreases by 4 hPa compared to the first decade. Contrarily, SLP increases to 2 hPa in the fifth decade compared to three decades ago. However, SLP values are still lower than the first decade despite the increasing pressure. By the end of this decade, the size of the pressure represents a decrease compared to the previous decade while the pressure level is equal to the third and fourth decades. As displayed in Figure 2(b), the patterns of pressure reduction are similar during the historical process at Fasa, Lamed, and Shiraz stations. Furthermore, the pressure has a decreasing pattern in stations in the south of Iran except for the stations of Qeshm and Kish Island. Pressure changes reach from 1,017 to 1,011 hPa at Jask, Lar, and Bandar Abbas stations from the first decade to the sixth decade although the slope of the pressure reduction is slow. On the contrary, Qeshm and Kish Island have constant pressure changes (between 1,012 and 1,010 hPa) due to their proximity to the Persian Gulf and the surrounding water resources. Qeshm Island demonstrates higher pressure values compared to Kish Island from the first decade to the middle of the fourth decade.
The results further reveal that both stations represent slighter pressure from the end of the fourth decade to the end of the sixth decade compared to previous decades although the pressure reduction is extremely low (about 1 hPa). Finally, the pressure average on Kish Island has been consistent at around 1,009 hPa over the past six decades (Figure 2(c)).
Generally, the mean range of SLP due to Sudan low in the southwest and south regions of Iran was 1,013–1,014 hPa during the first and second decades, while it was 1,011–1,012 hPa in the third to fifth decades. Additionally, its mean value was 1,010 hPa in the sixth decade, indicating a 4 hPa decrease compared to previous decades. In January, the trend line of SLP changes demonstrated a decrease with a gentle gradient. The determination coefficient (0.96) confirms these results (Figure 3).
The anomaly maps of SLP due to the Sudan low indicate negative anomalies (a decrease in SLP) for six decades in the south and southwest regions of Iran (Figure 4). In the first and second decades, SLP decreases compared to its long-term normal condition from 0 to −2 and −1 to −4 mbar, respectively (Figure 4(a) and 4(b)). The south and southwest regions of Iran in the third decade have a more negative anomaly than the second decade. In addition, the maximum of the negative anomaly of SLP is −5 mbar in the southwest region of Iran (Figure 4(c)). In the fourth decade, SLP has a lower negative anomaly (between −1 and −5 mbar) compared to the last three decades (Figure 4(d)). During the fifth decade, SLP anomalies are about −1 to −6 mbar, representing that the anomalies have increased compared with the last decade (Figure 4(e)). However, SLP over the past few decades reduces from 2 to 8 mbar in the sixth decade (Figure 4(f)). In January, the decrease in SLP is most likely due to a reduction in the intensity and extent of the Azore sub-tropical high pressure and its retreat on Iran (Alipour & Zalnezhad 2018; Toulabi Nejad et al. 2018). The SLP reduction and Azore high-pressure retreat on the south and southwest regions of Iran lead to an increase in the Sudan low power, the intensification of instability, ascendant mechanism, and the creation of suitable conditions for precipitation.
Analysis of the elevation changes of Sudan low
The survey of the diagrams showing geopotential height variations at 1,000–500 hPa levels revealed that the altitude size decreased during the historical trend from 1957 to 2017 (Figure 5). The average height decrease of the Sudan low at the level of 1,000 hPa was about −20 m during the first decade to the sixth decade. Furthermore, decreasing altitude, along with increasing and decreasing oscillations, displayed a relatively steep slope during the historical process (Figure 5(a)).
However, the analysis of height changes at 850 hPa indicated the oscillating pattern with a slower slope compared to lower levels over the six decades. The year-to-year and a decade changes of the Sudan low height were accompanied by a gentle and decreasing slope and the average of the height decrease was about −23 m (Figure 5(b)) during the study period. Based on the evaluation of height changes at 700 hPa, a gentle slope was observed with a reduction trend similar to that of 850 hPa. In this level, the slope of the altitude demonstrated a decline with an average of about −45 m from the first decade to the sixth decade (Figure 5(c)).
The values of geopotential height anomaly at 700 hPa over six decades confirm this result as well (Figure 6). As depicted in Figure 6(a), the height anomaly is between 0 and +10 m in the first decade. This implies that the height of the Sudan low increases by about 10 m in the first decade in the south of Iran compared to the normal condition before 1957 while it remains unchanged in the southwest region of Iran. Contrarily, the height decreases in the south and southwest regions of Iran in the second decade compared to the first decade. The anomaly range is from 0 in the south to −10 m in the southwest (Figure 6(b)). Sudan low altitude is constant during the third decade in the south and southwest regions of Iran compared to the previous decade (Figure 6(c)). In the fourth decade, the height reduction of the Sudan low is significant throughout the area compared to previous decades. The negative anomaly (the height reduction) ranges from −10 to −50 m. The maximum of height reduction (−50 m) is observed in the southwest corner of Iran (Figure 6(d)).
The anomaly analysis of Sudan low altitude in the fifth decade represented that its operation and activity were similar to the fourth decade. In other words, the elevation changes have been steady since the fourth decade while demonstrating a decrease compared to the first and third decades. This indicates the strengthening and increasing performance of Sudan low over the past decades (Figure 6(e)). By the sixth decade, the altitude anomaly was more negative than the previous five decades. Such decreases were observed from −10 to −60 m. As in the fourth and fifth decades, the maximum reduction occurred in the southwest region of Iran with a greater intensity of about −60 m (Figure 6(f)).
The evaluation of the height changes in the middle atmosphere is a strong seal on the results of the lower levels for increasing the positive performance of the Sudan low in the south and southwest regions of Iran. At the level of 500 hPa, the altitude slope is decreasing and strong during the first decade to the sixth decade. Therefore, the average annual changes indicates an approximate decrease of −80 m (Figure 5(g)). In all the expressed levels, the coefficient of determination represents that the validity and accuracy of the results are >0.9 (Figure 5).
Based on the examination of the altitude anomaly at a level of 500 hPa, Sudan low activity was weak (a positive anomaly) in the south of Iran during the first decade to the third decade. However, in the southwest region of Iran, it was constant and slightly stronger (a negative anomaly) during the first decade to the third decade (Figure 7(a)–7(c)). Furthermore, the decrease in height became more noticeable as in the lower levels and the altitude reduction occurred between −10 and −60 m. Additionally, the maximum of the negative anomaly with more intensity was observed in the southwest region of Iran as in the lower levels (Figure 7(d)). Thus, Sudan low activity demonstrated an increase in the fourth decade compared to previous decades. The status of Sudan low performance in the fifth decade was equal to that of the fourth decade (Figure 7(e)).
In the sixth decade, Sudan low activity was more intense compared to the last two decades. As shown in Figure 7(f), Sudan low height anomaly is between −10 and −80 m in this decade and the maximum of the negative height anomaly is in the southwest region of Iran as in the lower levels. Therefore, the reduction in the Sudan low height from the past to the present increases the activity of this system in January in the south and southwest regions of Iran.
Analysis of specific humidity changes in Sudan low
Changes in the specific humidity (Shum) average of the Sudan systems are surveyed for the levels of 1000, 850, and 700 hPa during the climate decades. The mean values of Shum distribution are nearly constant at a level of 1,000 hPa throughout all climate decades. In addition, the range of changes is between 4 and 9 g/kg (as mentioned earlier, 1,000 hPa Shum maps are not displayed because of briefing).
At the level of 850 hPa, the minimum of moisture indicates a slight increase during the second decade (1.93 g/kg) compared to the first decade (1.66 g/kg). However, the maximum of moisture is constant and the difference is only related to its distribution (Figure 8(a) and 8(b)). The moisture core in the second decade further enhances about 0.5 g/kg in the southwest (stations of Khuzestan and Ilam provinces) and south (stations of the south of Fars province) of Iran (Figure 8(b)). Furthermore, humidity value changes are in the range of 2–5.7 g/kg from the third decade to the sixth decade and it only increases about 0.5 g/kg in the southwest region of Iran (Figure 8(c)–8(f)).
At the level of 700 hPa, the Shum changes demonstrate that moisture varies between 1 and 3.8 g/kg during the historical process from the first decade to the sixth decade (Figure 9). The values of Shum increase equally in the second to fourth decades (Figure 9(b)–9(d)) compared to the first decade (Figure 9(a)). At this level, this measure of increases is observed in the coastal Bushehr station and the south coast (Island Stations) that is contrary to the 850 hPa level at which the increase is found in the southwest (Figure 9(b)–9(d)). Furthermore, a slight increase is detected in the maximum and minimum of Shum in the last two decades compared to the previous decades. This amount of increase is observed in the coastline of Bushehr station and the upper latitude like Yasuj and Dogonbadan stations (Figure 9(e) and 9(f)).
Analysis of the omega changes in Sudan low
Figure 10 illustrates the anomaly maps of omega cross-section for 48°–64°E longitude (the south of Iran) and the latitude of 35°N from the surface of the Earth to the level of 100 mbar for the six decades. During the first decade, the condition is stable in the southwest region of Iran from the Earth surface to the upper levels of the atmosphere (positive anomaly, 0 to +0.04 P/s). In southern Iran, the anomaly is positive at lower levels (<800 hPa) and there are conditions for an air convergence at the ground level. However, the anomaly is negative and weak in the upper levels (from 850 to 200 hPa), leading to divergence conditions and atmospheric instability. The pressure with a value of −0.08 Pa/s decreases more than the other levels in the middle of the atmosphere (Figure 10(a)).
According to Figure 10(b) in the lower levels, omega anomaly is positive and the stability of the air can be observed all over the south of Iran during the second decade. On the contrary, the negative anomaly is found in the upper levels of the atmosphere and increasing air instability occurs, especially in the southwest of Iran. The magnitude of the air instability in the middle atmosphere reaches about −0.16 Pa/s. This rate of change indicates the increase of unstable intensity in the southwest of Iran compared to the first decade. During this decade, weak positive (convergence-air stability) and weak negative (divergence-air instability) anomalies are observed in the lower atmosphere (1,000–800 hPa) and the upper levels of 850 hPa in the south of Iran, respectively (Figure 10(b)). In the third decade, the omega anomaly represents negative abnormality at the levels of 850–300 hPa over a wider geographical area (the entire study area). As in the last decade, strong core instability with a value of −0.16 Pa/s is found in the southwest of Iran as well (Figure 10(c)). Regarding the fourth decade, the stability and instability of the atmosphere are the same as the third decade. The only difference is related to the value of negative anomaly which decreased to −0.04 Pa/s in the southwest of Iran compared to a decade ago (Figure 10(d)). The deployment of the strong core of negative omega in the southwest of Iran indicates greater instability of the atmosphere in the fifth decade compared to the past decades in this region. Additionally, the magnitude of this strong anomaly is 0.21 Pa/s in the middle level of the atmosphere (Figure 10(e)). In the sixth decade, a strong convergence (positive anomaly) at lower levels (1000–800 hPa), as well as severe divergence and ascending air (negative anomaly) at higher levels (850–500 hPa), is observed throughout the study area. In addition, the strong core of instability is stretched from the southwest to the south of Iran with −0.16 Pa/s value in the middle of the atmosphere compared to previous decades. During this decade, the Sudan low becomes stronger across the study area (Figure 10(f)).
SUMMARY AND CONCLUSION
The evaluation of pressure structure, humidity, and the other dynamic parameters of Sudan systems in six solar cycles revealed that the SLP decreased at all sample stations from the first decade to the sixth decade in the south and southwest regions of Iran. The SLP reduction occurred in the range of 4–8 hPa at all stations. Furthermore, the study of the SLP anomaly indicated −2 to −8 mbar decreases over the six decades. This means that transitional Sudan systems in the study area were strengthened over the course of six decades.
From the first decade to the sixth decade, the increase in the negative values of height anomaly in the middle of the atmosphere was a sign of more unstable atmosphere and further activation of the Sudan system compared to previous decades. Accordingly, the height at the level of 500 hPa decreased to −80 m, which indicates more strengthening of the Sudan system in the study area.
The contribution of moisture advection demonstrated no changes in the area at a level of 1,000 hPa while it increased inside the study area at a level of 700 hPa. Accordingly, most of the moisture transferred from the Arabian and Oman Seas is related to the lower layers of the troposphere (<850 hPa). This implies that the role of the transferred moisture from the intertropical convergence zone is increasing in the systems transferred from Sudan and in the study area. Furthermore, the increase of the advection moisture inside the Sudan systems has had an effective role in reinforcing and further developing the Sudan systems in the study area. Additionally, previous studies have indicated the reason for the Shum increase at the 700 hPa level at this area. At this level, unlike the lower levels, the Eastern Mediterranean had no role in providing the moisture. On the contrary, the tropical regions of northeast Africa were considered as the major sources in the divergence of the Red Sea moisture, which is the amplification enhanced by passes through Saudi Arabia, the Persian Gulf and in the form of the Southwest currents, moisture advection occurred at the area under study (Movaghari & Khosravi 2014). On the contrary, based on the results of Lashkari et al. (2018), the Arabian Sea and the Indian Ocean can be regarded as another factor in increasing the moisture advection at the 700 hPa into the Sudan system if high altitude deployed on the Arabian Peninsula becomes stronger.
The analysis of omega cross-section represented that the air ascent with more intensity has come to pass in the middle of the atmosphere during the last two decades. Therefore, the intensity of the precipitation of the Sudan low has increased during the recent years and the rainfalls that occur are stormier than before.
Finally, it should be noted that the quota of the Sudan low is increasing in the rainfall of the south and southwest regions of Iran. This increase in the quota of the participation of these systems in the rainfall of the study area indicates that the climate of this region is changing and new requirements should be considered in the planning policies in the study area. Thermodynamics and synoptic structure of this system are accompanied by severe convective precipitation, along with flooding in some cases that is an effect of climate change in the south and southwest regions of Iran. Therefore, new solutions should be considered to adapt with this climate change and to use such a rainfall optimally and keep it away from possible damages.
DATA AVAILABILITY STATEMENT
All relevant data are available from https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html.
National Centers for Environmental Prediction/National Center for Atmospheric Research.
Grid Analysis and Display System.