Identifying tropical cyclones and determining and defining cyclones and their paths have been challenging topics among climatologists. In one of the first studies on extraterrestrial cyclones, the frequency of cyclonic and anticyclonic centers as well as the destruction centers of these atmospheric systems in the Northern Hemisphere were studied (Smagorinsky, 1950). Some preliminary studies have shown that the North Atlantic, especially its west and east, the North Pacific and the Mediterranean Basin, especially during the cold period of the year, are the main cyclonic centers in the Northern Hemisphere (Withaker and Horn, 1984). In early studies, a cyclone was manually identified and routed from synoptic maps, which was a time-consuming task (Flocas et al., 2010); However, due to the introduction of computers into the world of science, automated and machine methods caused cyclones to be identified and routed objectively and intuitively on digital maps (Ulbrich et al., 2009). Routing cyclones can be a useful tool for classifying them based on their size (Rudeva and Gulev, 2007), their physical properties (Blender et al., 1997), and the degree of disturbances they cause in the atmosphere. Due to the lack of a single scientific definition of extraterrestrial cyclones, a large number of identification and routing methods have been developed (Neu et al., 2013).
The first objective method for Mediterranean cyclones was introduced by Alpert et al. (1990). In their study, they used the ECMWF data based on monthly data of a 5-year period (1982–1987) to analyze the cyclone frequencies and cyclonic tracks with a low temporal and spatial resolution (12-hour temporal resolution and 2.5\(^\circ\) spatial resolution). For this purpose, the minimum level of geopotential height of 1000 hPa was introduced with an interpolation approach to identify surface cyclones. For routing, the elliptical search area was used to determine the cyclone in the next step, the main axis of which was determined using a wind vector of 700 hPa. A slope gradient of 0.5 hPa at 500 km was defined as the criterion for cyclone detection in four directions (north, south, east, and west).
Blender et al. (1997) used a two-step approach to identify mid-latitude cyclones and cyclone paths in the North Atlantic. The authors first considered the minimum altitude geopotential pressure of sea level as a cyclone. This minimum pressure zone should have occurred at least in a 3*3 grid range with a spatial resolution of 1.1* 1.1 and also, the next criterion was the average positive altitude slope, which was a slope of 200m per 1000 km. After identifying the cyclones and clustering them by the k-mean method, the cyclones were clustered into three types of cyclone tracks. The first type was stationary cyclones that did not have a clear and long path and their field of activity was mostly around the place of their formation. The second group of cyclones was identified as North-eastward Cyclones. The general path for them to move was from the formation place in the North Atlantic to the Northeast, and finally, the zonal cyclones, which moved east and west with a shorter spatial and temporal range, and seemed to have limited regional activity.
In another study, Sinclair (1997) used ECMWF rotation data to intuitively identify and route two-hemisphere cyclones and showed that they are formed and intensified in the Northern Hemisphere, near the east coast of the seas in Asia and North America. In the Southern Hemisphere, cyclones are formed in mid-latitudes and on the oceans off the east coast of South America and Australia, and they decay in high latitudes. Mehmet et al. (2004) studied the variability of cyclonic paths in Turkey and showed that the dominant cyclonic paths were five paths with the highest frequency of cyclones occurring in winter. Picornell et al. (2001) identified and routed western Mediterranean cyclones for the period 1995–1999 with a high-resolution (0.5֯). To eliminate weak and small systems, they set the criterion of pressure slope of 0.5 hectopascals per 100 kilometers in 6 directions.
Trigo (2006) in a study on the number and intensity of cyclones in Europe and the North Atlantic showed that cyclones in the Azores and the Mediterranean had a decreasing trend and they had an increasing trend at higher latitudes. Romem et al. (2007) showed that only 13% of the cyclones entered the Mediterranean, while 87% were generated in the Mediterranean Basin. According to the authors, the entering cyclones originated in three different regions: the Sahara Desert (6%), the Atlantic Ocean (4%), and Western Europe (3%). It should be pointed that most of the cyclones that enter the Mediterranean Basin from the Sahara Desert reach the eastern coasts of the basin, while a small percentage of Atlantic cyclones reach the eastern Mediterranean Basin and none of the cyclones from Western Europe entering the Mediterranean basin, are able to reach the eastern coasts of the basin.
Simmonds et al. (2008), studying the behavior and characteristics of cyclones in the Arctic (the domain north of 70°N), investigated the climate change in the region in relation to changes in cyclone behavior and showed that the highest density of cyclones was in Norway and Svalbard and the Barents and Kara Seas. They also found that the number of cyclones increased more in winter than in summer. In a study conducted by Neu et al. (2013) with fifteen different methods in the two hemispheres to identify and route cold season cyclones, the authors concluded that the number of cyclone centers in the Northern Hemisphere during the 20-year statistical period (1989–2009) had a significant upward trend. While the number of deep cyclones has been decreasing, and in some methods, this trend has been significant. However, they showed that the number of paths, their lifespan, and the intensity of cyclones may vary, depending on the type of algorithm used. This discrepancy in the results may indicate that there is no comprehensive and universal definition of a cyclone. As another important result of their work, one can claim that a particular algorithm does not have a higher priority than other approaches.
In another work by Kelemen et al. (2014) on cyclone formation in the Mediterranean region, the authors showed a significant increase in cyclone formation in the region during the statistical period of 1981–2000. Flaounas et al. (2014) used the relative rotation data at 850 hPa of the European Medium-Term Forecast Center, with a 6-hour temporal resolution and 1.5 degrees spatial resolution during the period 1989–2009, to identify and route northern hemisphere cyclones in the winter. In the algorithm used, they considered the center of the cyclone as the maximum relative rotation or the maximum wind speed, or the minimum sea surface pressure. Some studies have also examined the relationship between cyclones and severe weather events such as heavy rainfall and strong winds (Reale and Lionello, 2013). Winter cyclones associated with heavy rainfall originate from different parts of the Mediterranean basin. If heavy rainfall occurs in the northwestern part of the Mediterranean basin, the existing cyclones are mostly from the North Atlantic. In the eastern part of the Mediterranean, cyclones form in the Mediterranean Sea itself, mainly with Cypriot origin, while in the southern part of the sea, heavy rainfall occurs due to cyclones forming in North Africa. The authors also showed that heavy rainfall is associated with the negative phase of North Atlantic fluctuations and the East-Atlantic West-Russia pattern.
Ramis et al. (1998) studied the heavy rains and floods that occurred on 9 and 10 October 1994 in Catalonia (Spain) and showed that the cyclone formed in the western Mediterranean with the orographic forcing led to more than 400 mm of rain and consequently heavy flooding in southern Catalonia. Later, Campins et al. (2006) studied the relationship between Mediterranean cyclones and heavy rainfall and strong winds. They found a strong link between cyclonic centers in the Balearic Islands in southern Italy and the coasts of Catalonia on the Iberian Peninsula, and extreme events such as heavy rainfall and strong winds in the region.
Bech et al. (2011) investigated floods and storms that occurred on the 2nd of November 2008 in the southern coast of Catalonia (NE Spain). The authors considered flooding during a day, which totaled more than 100 mm, and there was 40 mm precipitation in half an hour, as a tornadic flood, and concluded that this hazard was associated with a cyclone and strong low-pressure formed in the Iberian Peninsula. Some researchers have also pointed to frontless low-pressures that extend from the Red Sea and Sudan to the eastern Mediterranean and southwest Asia, causing torrential rains in these areas (Alpert et al., 2004).
In Iran, few studies have been carried out on cyclones. The first study was conducted by Alijani (1987). He investigated the relationship between the spatial distribution of cyclonic routes in the Middle East and high-level air systems. The results of this study showed that the effect of high-level systems on the dispersion of cyclonic routes in the Middle East is much greater and more important than the role of roughness as well as the pattern of surface pressure. Irannejad et al. (2009) investigated the effect of the annual frequency of cyclones of Mediterranean cyclonic centers on the annual rainfall of Iran in the statistical period of 1960–2002. The results of their study showed that except for the southeastern, eastern, and parts of central Iran, the annual rainfall in other parts of the country is significantly affected by the frequency of cyclones in these centers.
Hejazizadeh and Sedaghat (2009) identified the routes of the Middle East cyclones in the cold period of the year digitally and during the period of 1993–2002 and identified the five main routes and showed that the highest frequency of cyclones enters from the west and northwest of Iran. Another noteworthy result is that the most important route is the Central West originating in Cyprus. Khosravi et al. (2010) studied the temporal and spatial position of cyclones affecting Iran on a monthly and seasonal scale for one year and concluded that the peak period of Mediterranean cyclone activity was December and February and in winter, the Italian cyclonic centers transfer to the eastern half of the sea and Cyprus, Syria and Turkey.
The first study focusing on the low-pressures of the Red Sea region with regard to Iran was an article by Olfat in 1968 (as cited in Mofidi and Zarrin, 2005). In this article, the author points to the low pressures that are formed in North Africa and the Red Sea, which, after crossing Saudi Arabia and the Persian Gulf, are causing precipitation in Iran. Lashkari (2002) studied the routing of Sudanese low-pressure systems entering Iran. He showed that Sudanese systems enter Iran from five major routes and cause rainfall. Farajzadeh et al. (2008) by routing and finding the frequency of rainfall systems in western Iran concluded that the largest number of rainfall systems causing rain in the region were Sudanese systems and Mediterranean-Sudanese integrated systems were of secondary importance. They also concluded that only Sudanese systems have a rainfall of more than 300 mm.
Lashkari et al. (2008) through a synoptic study of the causes of flooding in Golestan province concluded that five types of air masses and pressure systems are effective in precipitation in the southeast of the Caspian Sea, one of which is the Sudanese low pressure system. Azizi et al. (2009) performed a synoptic analysis of heavy rainfall in the west of the country. For this purpose, they had a case study of the precipitations of March 7–15, 2007. The results of the study showed that the low pressure system of the Mediterranean Sea and the Sudanese low pressure tongue caused the rains of this period and the Mediterranean Sea, the Black Sea and the Red Sea have played a role in strengthening these systems on the land surface. In addition, it was found that on the day that the peak rainfall occurred, the Mediterranean low-pressure system entered the country from the northwest and the Sudanese low-pressure system entered from the southwest and they merged.
Parandeh Khozani and Lashkari (2010) made a synoptic study of flooding systems in southern Iran. For this purpose, they selected 20 severe floods and divided them into 4 patterns and categorized them. The results of their study showed that the storms that lead to heavy rains and floods in the region are the result of the strengthening and intensification of the Sudan low-pressure center and the Red Sea convergence region and in some cases, from the merging of the low-pressure center of Sudan with the low-pressure center of the Mediterranean. There have been many studies on the transport of atmospheric moisture, some of which have referred to it as atmospheric rivers. According to Zhu and Newell (1998), one phenomenon related to atmospheric moisture transport is atmospheric rivers (ARs). This phenomenon, in the form of narrow corridors with a depth of 40 km and a width of 500 km, and a length of several thousand kilometers, directly and indirectly, affect more than 90% of water vapor meridian transport.
Salimi and Saligheh (2016) investigated the effect of atmospheric rivers on Iran's climate. The results showed that an average of twelve atmospheric rivers is formed annually, which provides moisture to part of Iran's rainfall. Akbari et al. (2019) investigated the temporal and spatial changes of atmospheric rivers in the MENA region. The results showed that in more than 90% of the cases, the atmospheric rivers of the study area have a southwest-northeast flow. Also, the atmospheric rivers of the MENA region converge at their destination, and air suction has caused the concentration of atmospheric rivers in this region. The results showed that jet streams play a major role both in the production of atmospheric rivers and in their direction and path, and high-pressure centers have a high role in the expansion and low-pressure centers on the destination of atmospheric rivers. Areas such as the western United States, the west of southern Cape of South America, northwestern Europe, the northeastern route from Saudi Arabia to northeastern Iran, Australia, the border between Japan, and the eastern coast of Russia are also regions that are most affected by ARs rainfall.
Salimi et al. studied the origin of the world's atmospheric rivers. According to their results, there are seven main sources of atmospheric rivers in the world, the most important of which is located in Southeast Asia. In terms of monthly distribution, January, September, and October have the highest occurrence of atmospheric rivers. There is little difference in the two hemispheres in terms of the number of atmospheric river occurrences, only the ARs of the southern hemisphere have a longer longitude. In terms of the number of occurrences in the world, in most cases, 4 to 3 atmospheric rivers were observed simultaneously. Also, more than 90% of ARs travel by ocean path (Salimi et al, 2020). Due to the importance of Sudanese atmospheric and low pressure rivers on moisture transport and its effect on rainfall in Iran, in this study, the dynamics and thermodynamics of Sudan thermal low pressure in relation to the atmospheric river were investigated.