عنوان مقاله [English]
120-day winds of Sistan are considered as one of the significant phenomenon which has a great impact on the morphology and environment of east and southeast of Iran (Figure.1). The common region for these winds is the border of monsoon region in south of Asia which mainly has sunny and cloudless weather during monsoon period. This condition is due to lack of higher humidity divergence accompanied by tangible decrease of the air on the atmosphere (Salighe, 2010). These winds are the most famous advection system in northern hemisphere whose effects are visible in eastern regions of Iran, west and south of Afghanistan, and northwest of Pakistan(Khosravi, 2008).
Data and Methodology
In order to evaluate the role of the winds, data network of Geopotential height of 850 hPa (hectopascal) level during a 19-year period (1993-2012) from May to the end of September, the period of 120-day winds of Sistan, were found. These data were of those revisited data of 2.5*2.5 NCEO/NCAR during 2480 days. Then, factor analysis and clustering tests were applied on data network of Geopotential height to classify map patterns (Yarnal, translated by Masoudian, 2006: 100). As a matter of fact 5 clusters were recognized in this study presented in table 1. Dynamic method was used in GrADS software in order to find humidity flux of each region in the quintuplet patterns.
Northern Wind Pattern (120-day wind of Sistan)
As a matter of fact 120-day winds of Sistan are a part of northern Trade winds which are the most important source of Caspian Sea high pressure. After passing east of Iran, these winds reach Oman Sea and converge with southern Trade winds. Both of them moved toward Indian Subcontinent and finally enter atmospheric monsoon circulation of south of Asia. High pressure of north of Iran is also a tongue of high pressure Azores which is extended over northern regions of Iran and Caspian Sea by Mediterranean and Black sea Basin. Both existing Gang low during hot period of a year in south of Asia and spreading, its tongues over regions of Middle East make Azores high not be able to penetrate the zone in lower levels of atmosphere (from the earth surface to thelevel 850 hPa.). As a result, Azores high has to locate in northern parts especially north of Iran. Analyzing the curves of geo-potential height, figure (2) precisely shows this phenomenon. Gang low not only is weaken among middle levels of atmospheretongue, but also lost its appearance on Iran Plateau and Arabian Peninsula. Therefore, Azores high tongue also can locate in its normal position and appear with maximum pressure on Iran Plateau and Arabian Peninsula. Figure (3) presents the order of synoptic systems in level 500 hPa. of pattern 1. It shows that Gang low has lost its nature in this level, while Azores high tongue obviously is located on Middle East, especially Iran Plateau and Arabian Peninsula. As a matter of fact atmospheric levels of Geopotential height in pattern 1 (figures 2,3, 4) reveal that as we go away from lower levels of atmosphere to middle levels of atmosphere, Gang low gradually is weaken especially over Iran Plateau and Arabian Peninsula. This situation makes Azores high tongue locate in lower latitude. However, in lower levels (earth surface to level 850 hPa.), as a tongue of Gang comes into some parts of Middle East, expanded tongue of Azores high pressure has to locate on higher latitudes than normal latitudes; on north of Iran Plateau and Caspian Sea.Pattern (2) shows the same order as pattern (1), so it will not be repeated here.
In the following, the effect of 120-day winds of Sistan on humidity of the region will be investigated, thus humidity flux is calculated between levels 925-1000 hPa. 850-925 hPa. and 850 -700hPa. Figure (5) shows sum of humidity flux for aforesaid levels of synoptic pattern (1). 120-day winds of Sistan with prevailing north direction in this pattern lead to the formation of a core of humidity flux divergence in east and center of Iran and decrease humidity of the region. As previously mentioned, after passing Iran, Sistan winds reach Oman Sea and north of Indian Ocean, and converge with southern Trade winds. Both of them move toward Indian Subcontinent. In fact, convergence of 120-day Sistan winds (northern Trade winds) and southern Trade winds leads to formation of a strong core of humidity flux convergence on Oman Sea and north of Indian Ocean (figure 5). The sum and average of humidity flux convergence and humidity flux divergence in studied region are presented in table (2).
Eastern Wind Pattern
The other clusters (3, 4, and 5) have different order from 120-day Sistan winds which are introduced as eastern wind pattern. Unlike clusters (1) and (2), in these clusters (table 1) the wind direction is not northern; in other words, the winds blow with prevailing east direction in east and northeast of Iran, however southeast of Iran experience mild weather at the same time. As synoptic order of pressure system and humidity flux system are mainly the same, pattern (3) will be analyzed precisely. The order of synoptic systems of level 850 hPa. in pattern (3) is presented in figure (5). This map reveals that the contrast between high pressure of north and Gang low differs from northern wind pattern, as on the one hand,the strength and breadth of Gang low increase, while on the other hand the strength and breadth of Azores high tongue (high pressure in north of Iran) decrease. In fact, this condition makes most regions of Iran Plateau in lower levels of atmosphere (1000 hPa, 925 hPa and 850 hPa.) be dominated by Gang low. Besides, this order of synoptic systems eliminates 120-day wind conditions of Sistan and make eastern wind conditions in east and northeast of Iran. Since the orders of synoptic systems of levels 925 hPa. and 1000 hPa are the same as level 850 hPa. they will not be presented here.
The orders of synoptic systems in middle levels are different, as in level 700 hPa. Azores high tongue comes to Iran Plateau by Arabian Peninsula (figure 7). This layer of atmosphere is a transition layer from dominance of low pressure pattern in lower layers to high pressure pattern in middle levels and upper atmosphere. Moreover, in level 500 hpa. Azores high tongue dominates Iran Plateau and Arabian Peninsula with more power and breadth. The orders of synoptic systems of clusters 4 and 5 are the same as cluster 3.
The sum of humidity flux divergence and humidity flux convergence of pattern 3 are presented in figure (9). In this figure, the core of humidity flux divergence, which covers eastern half and center of Iran, is omitted and a core of humidity flux convergence covers east and southeast of Iran. It can be said that both penetration of Gang low into Iran and lack of 120-day winds provide special conditions in which the zone of humidity flux convergence in north of Indian Ocean moves to southeast of Iran leading to moisture condensation.
In this study 2 patterns of synoptic systems of warm period in east and southeast of Iran were recognized. First pattern (northern wind pattern) makes 120-day winds of Sistan (cluster 1 and 2). In contrast to Gang low tongue, when high pressure of north of Iran and Caspian Sea are in strong mode, it provides the conditions for 120-day winds of Sistan. On the other hand,in contrast to Gang low tongue increasing its influence and spread over Iran Plateau, when the aforesaid high pressure rollbacks of north of Iran and it is weakened, 120-day winds of Sistan stop and second pattern (eastern wind pattern) starts. In this pattern the winds with prevailing east direction cover east and northeast of Iran (clusters 3, 4,and 5). High pressures of Caspian Sea and north of Iran are a tongue of Azores subtropical high pressure which has to move abnormally to higher latitudes due to coming Gang low into lower atmosphere layer. Since Gang low is an inter-tropical convergence zone moving abnormally to higher latitudes in south of Asia, 120-day winds of Sistan are part of northern Trade winds which are flowing from subtropical high pressure (Azores high tongue in north of Iran) to Gang low in south of Asia (inter-tropical convergence zone). After converging with southern Trade winds on north of Indian Ocean, they move toward Indian Subcontinent. 120-day winds of Sistan exclude the entranceof moisture from Oman Sea and Indian Ocean into southeast of Iran (figure 5). However, as 120-day winds of Sistan stop, a core of humidity flux is formed on southeast of Iran providing the entrance of moisture of water areas into southeast of Iran (figure 9). Generally, weakening of Azores subtropical high will help to provide rainfall conditions in southeast by 2 ways: on the one hand, as Azores high pressure is weakened, the influence of decent factors of this high pressure air in levels 700 hPa. and 500 hPa. decreases. As a result ascent conditions are provided in the zone, but on the other hand the weakening of subtropical high pressure in lower levels of atmosphere (1000 hPa to 850 hPa.) also makes expanded Azores tongue weaken and rollback over north of Iran and Caspian Sea leading to stop 120-day Sistan winds. This phenomenon provides appropriate condition to inject moisture from Oman Sea and Indian Ocean to southeast of Iran.