Introduction

General circulation of the atmosphere generates planetary large-scale systems, with subtropical high pressure being one of the most stable and yet most extensive and profound systems in the troposphere structure. This system has relatively stable cores in both hemispheres, which, considering the spatial location, are assigned different names. Azores high pressure, Africa high pressure, and ASTAC are among the most important systems in the northern hemisphere. The atmospheric general circulation has a determining role in the formation and development of special climatic features on both regional and local scales. It also affects the atmosphere’s circulation structure in both adjacent and remote climate regions.

Data and Method

In this study, in order to detect the role of the ASTAC core establishment location in the advection of humidity into the Sudan low system, the track of systems, the pattern of development in the Mediterranean trough, and the spatial displacement of the high-precipitation core, the following research procedure was adopted. First, the statistics of the daily precipitation of 30 synoptic stations of the southern half of Iran were taken from the Iranian Meteorological Organization for a Seven-year statistical period. From among various precipitation systems, 40 pervasive and high-rainfall systems were chosen. Besides, the rainfall zone map of these systems was plotted in Arc-GIS 10.4. Then, the atmospheric data of the mentioned years were extracted for all rainy days using http://www.esrl.noaa.gov. Furthermore, using Grads software, the diurnal maps of these years were plotted at levels of 1000 to 500 hPa within the range of western 0° to eastern 80° as well as zero latitude (Equator) up to northern 50°. The ASTAC central core was determined as follows. The point that had the maximum geopotential height in comparison with the adjacent points within the longitude ranges of eastern 30° - eastern 80° and latitudes of northern 10° and northern 35° was considered as the high-pressure central core. The geographical position of the core center was calculated on the map with a precise latitude and longitude. The maximum geopotential height is considered as the most common criterion for determining subtropical high centers. These cores were grouped as four general patterns based on the establishment location. In the next step, the spatial location of the high-precipitation core was determined and then the distance between the high-pressure core and the high-precipitation core was calculated. To determine the humidity advection, the axis of the special moisture chart of 850, 1000, and 700 hPa levels was plotted and the location of the axis of the highest special moisture was drawn. However, due to a relative consistency of the pattern of charts, eventually, the special moisture axis of 850 hPa level has been analyzed here. To determine the role of high-pressure core in the trough spatial displacement and trough axis, using Ncep/Ncar atmospheric data, the geopotential height chart of 850, 700, and 500 hPa levels was drawn. Eventually, the maps of 700 hPa were chosen for plotting the trough axis.

Results and Discussion

As stated in the methodology section, along with various studies conducted on the atmospheric systems in the south of Iran and west of Asia by the author and his research team, we found that the ASTAC core displacement and the stretching pattern of its axis play a significant role in the synoptic patterns of the region. In this region, the role of ASTAC central core displacement on the spatial displacement of precipitation cores, humidity transfer axis, axis of troughs, and their synoptic patterns were emphasized. To this end, based on the position of the establishment of ASTAC cores, four general patterns were identified.

1. The pattern of Hormuz Strait cores

2. The pattern of India and Pakistan cores

3. The pattern of Arabian Peninsula eastern coast cores

4. The pattern of the Arabian Peninsula's southern cores

The summary of the synoptic patterns, humidity track, and source, trough axis, and the distance between the precipitation core and each of the precipitation systems in the four patterns is as follows:

The pattern of the Strait of Hormuz cores

Figure 1 demonstrates the synoptic map of the 700 hPa level representing the pattern of the establishment of ASTAC systems on the Hormuz Strait. As was observed in Table 1, across seven precipitation systems, the ASTAC central core lies within the Hormuz Strait range. Almost in all seven systems in this pattern, the Arabian high has a southwestern-northeastern stretching. Visual investigation of the anticyclone development zone in this pattern at all levels of 1000 to 700 hPa indicated that half of the anticyclone circulation zone lies on warm waters of the Oman and Arab seas. In this state, the humidity of these seas in the anticyclonic circulation and north-northeastern streams is gradually advected over Ethiopia and Sudan. Then, it transfers to western Arabia and Iran through the southern and southwestern streams of the trough front. This high establishment pattern causes the precipitation region to be transferred to the southwest of Iran and develop along a southwestern-northeastern direction over Iran.

The pattern of India and Pakistan cores

Figure 2 represents this synoptic pattern at the level of 700 hPa. In this pattern, the high-pressure central core is deployed in the northwestern coast of India or the southeast of Pakistan. Considering the eastward displacement of the ASTAC central core and the synoptic pattern governing such systems, the eastern Mediterranean trough takes a different form compared to other systems. As can be seen in Figure 2A, the trough is very deep. Furthermore, in all cases, the trough finds northeastern-southwestern stretches. The concentration of troughs axis is on western Iraq and the northwestern of the Arabian peninsula, which, in comparison to Hormuz Strait group systems, has moved eastward by around 5-6° of latitude. The southern end of troughs usually extends down and below the northern 10° latitude, which, in comparison to the previous group, suggests greater depth of troughs and their considerable southward extension.

The pattern of eastern coast cores of Arabian Peninsula

Figure 3 demonstrates the synoptic pattern of stations where the central high core of Arabia lies over the eastern coast of the Arabian Peninsula. In this state, the high central core has around 3-5° eastward displacement compared to the Strait of Hormuz. However, its mechanism of action is very similar to the synoptic pattern of the Strait of Hormuz. In this state, again the anticyclone circulation pattern across all middle and lower layers of the troposphere is such that the whole or a major part of the anticyclonic circulation zone lies over Arab and Oman warm waters. The stretching pattern of the system’s axis is northeastern and southwestern.

The pattern of the Arabian Peninsula's southern cores

Figure 4 demonstrates this synoptic pattern. In this pattern, the ASTAC central core lies over the southern Arabian Peninsula and generally over Yemen and the Gulf of Aden. The stretching pattern of a high-pressure cell is generally a stretched oval shape with a northeastern-southwestern stretch. Thus, considering the pattern of the development of the ASTAC , the eastern Mediterranean troughs are very diverse in terms of depth of penetration and position of the axis. In the northeastern-southwestern stretch pattern of the high-pressure cell, the Mediterranean trough has a completely eastward displacement and lies over southern Iraq. In this state, they also have a relatively good depth. However, in the western-eastern stretch pattern of the high-pressure cell, the Mediterranean trough is completely westward, and the axis lies over western Cyprus.

Conclusion

ASTAC , as an important and influential system in the atmospheric circulation of the region, plays a significant role in supplying humidity for the incoming systems. Moreover, this system has an undeniable effect on the track of the Sudan system over Iran and the countries of the region. This research showed that the central core of this high lies over four regions around Arab and the Oman seas. The deployment of cores in any region determines the movement track of precipitation systems and the advection of humidity into them. In all patterns, precipitations occur in the northwestern region of the high-pressure core. The distance between the high-precipitation core and high-pressure core is 1051 km on average. The moisture sources of all precipitation systems were primarily located over the Arabian Sea and Oman Sea. After accumulating moisture over southern Sudan and Ethiopia, this moisture was transported from the western flank of the ASTAC through southerly flows across the Arabian Peninsula toward Iran. The moisture flux maps clearly demonstrated the dominant role of the Arabian Sea and Oman Sea in supplying moisture for precipitation systems entering from the south. These findings are inconsistent with the results reported by Rafiani et al. (2014); however, they are consistent with the results of Karimi et al. (2019), Lashkari and Mohammadi (2019), Khosravi et al. (2017), Kumar et al. (2008), and Lindsay and Daberton (1993). This study demonstrated that the location of the ASTAC core plays a key role in determining the pathways of precipitation systems entering Iran and neighboring regions from the south. When the high-pressure core is positioned over the Strait of Hormuz, precipitation systems tend to move toward western Arabia, Kuwait, southern Iraq, and southwestern Iran. In contrast, when the high-pressure core shifts toward northwestern India and southern Pakistan, the Sudan Low pathway extends across the western Gulf of Aden, central Saudi Arabia, and southern Iran. When the ASTAC core is located near the eastern coast of the Arabian Peninsula, the pathway of precipitation systems becomes more variable and extensive, allowing Sudan Low systems to follow different trajectories across the Arabian region and enter Iran between the Strait of Hormuz and Khuzestan. Under this configuration, the spatial extent of precipitation over Iran also increases. In the pattern characterized by the location of the high-pressure core over southern Saudi Arabia, precipitation is generally restricted to Saudi Arabia and the southern half of Iran.