سازوکارهای انتقال رطوبت دریای عرب در خشکسالی ها و ترسالی های فراگیر ایران (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
پژوهش حاضر بر اساس یک آستانه فضایی، خشکسالی ها یا ترسالی هایی که حدود 75 درصد و بیشتر ایستگاه های مطالعه شده (63 ایستگاه همدید) را در دوره سرد سال (اکتبر-آوریل) و در بازه زمانی 31 ساله (2016-1986) درگیر خود کرده بودند به عنوان خشکسالی ها یا ترسالی های فراگیر تعریف شدند. در این پژوهش از متغیرهای شبکه بندی جوّی همچون ارتفاع ژئوپتانسیل، چرخندگی نسبی، پوشش ابر پایین، پوشش ابر میانی و پوشش ابر بالا (اخذ شده از تارنمای مرکز اروپایی پیش بینی های میان مدت جوّی ECMWF) استفاده و سازوکار های مختلف انتقال رطوبت دریای عرب در زمان وقوع خشکسالی ها و ترسالی های فراگیر ایران نیز بررسی شده است. نتایج نشان داد که در هر فصل الگوهای خاصی در انتقال رطوبت این دریا نقش دارد. در فصل پاییز تأخیر در جابه جایی جنوب سوی کم ارتفاع مونسونی جنوب آسیا باعث می شود که انتقال رطوبت این دریا به داخل ایران مختل و درنهایت، منجر به خشکسالی های فراگیر ایران شود. در فصل زمستان زمانی که پرارتفاع حرارتی آسیایی در قوی ترین حالت خود قرار می گیرد، زبانه غربی آن گسترش و با پرارتفاع دینامیکی اقیانوس اطلس، کل آسیای مرکزی، خاورمیانه و شمال آفریقا را فرا می گیرند. در این شرایط پرارتفاع مستقر بر روی شبه جزیره عربستان، موقعیت غربی تری به خود می گیرد که این موقعیت جدید ازطرفی، باعث اختلال انتقال رطوبت دریای عرب به داخل ایران می شود و از طرف دیگر، زمستان های خشک فراگیر را برای ایران به وجود می آورد. در فصل بهار کم ارتفاع تبت و پرارتفاع اقیانوس اطلس مهم ترین بازیگران اقلیم جنوب غرب آسیا هستند. تقویت زبانه غربی کم ارتفاع فلات تبت و گسترش آن بر روی نیمه شرقی دریای عرب باعث جابه جایی غرب سوی پرارتفاع مستقر بر روی عربستان می شود که نتیجه آن خشکسالی های فراگیر برای ایران خواهد بود.Arabian Sea’s Moisture Transfer Mechanisms in Pervasive Dry and Wet Periods of Iran
To recognize the Arabian Sea’s moisture transfer mechanisms during the occurrence of pervasive drought and wet years in Iran, a standardized precipitation index (SPI) was applied to quantize Iran’s droughts within the format of a monthly scale. Next, based on a spatial threshold, the drought or the wet years that had engaged about 75% or more of the studied stations (63 synoptic stations) within the period of a cold period (October-April) during 30 years (1986-2016) were defined as the pervasive drought or wet years. In the end, using atmospheric gridded variables, the various mechanisms of the Arabian Sea’s moisture transfer were examined during the occurrence of Iran’s pervasive drought and wet years. The results indicated that during winter, the Asian thermal high pressure reaches its strongest state, its western part is expanded, and, together with the Atlantic Ocean’s dynamic high pressure, they capture the whole of central Asia, the Middle East, and northern Africa. Under these conditions, the high pressure established over the Saudi Arabian Peninsula takes a more western position and this new position disrupts moisture transfer from the Arabian Sea into Iran the result of which would be pervasive dry winters for Iran.Keywords: Geopotential Height, Dynamical High Pressure, Thermal Low Pressure, Saudi Arabian Peninsula, Tibetan Plateau, Monsoon.IntroductionDue to the transfer of a large volume of moisture, the Arabian Sea and the Mediterranean Sea are amongst the most important moisture resources of Iran’s precipitations. The Mediterranean Sea, during the early fall and late spring, and the Arabian Sea, as well, as during the other months of the year’s cold periods, are at the top of the most significant moisture resources for supplying Iran with precipitations. The superior role of the Arabian and Mediterranean seas in the supplying of moisture to Iran’s precipitations can stem from factors like their wide vastness and depth in contrast to the other water breadths. Considering the results of the studies during the recent several decades, appropriate knowledge on the identification of the synoptic patterns leading to Iran’s drought and wet years in various temporal and spatial scales is obtained in this regard in Iran’s climatological literature. However, even with such very rich literature, there are still ambiguities regarding the mechanisms of moisture transfer from the adjacent seas, especially the Indian Ocean and Arabian Sea at the time of the occurrence of pervasive drought and wet years. To fill the gap, the present study intends to investigate the synoptic reasons for the translocation of the cyclonic and anti-cyclonic circulation patterns over the Arabian Sea and north of the Indian Ocean at the time of the occurrence of the pervasive drought and wet years in Iran. Materials and MethodsTo recognize the Arabian Sea’s moisture transfer mechanisms during the occurrence of pervasive drought and wet years in Iran, two different databases were used. The first belongs to the monthly precipitation data recorded in 63 synoptic stations for 30 years (1986-2016) and obtained from Iran’s meteorological organization. The second database, as well, is pertinent to the atmospheric gridded variables recorded in the form of monthly monitoring and acquired from the European Center for Medium-Range Weather Forecast (ECMWF). After collecting the data and forming an information bank, a standardized precipitation index (SPI) was applied to quantize Iran’s droughts within the format of a monthly scale. Next, based on a spatial threshold, the drought or the wet years that had engaged about 75% or more of the studied stations (63 synoptic stations) within a cold period (October-April) during 30 years (1986-2016) were defined as the pervasive drought or wet years. In the end using atmospheric gridded variables, the various mechanisms of the Arabian Sea’s moisture transfer were examined during the occurrence of Iran’s pervasive drought and wet years. Research FindingsDuring fall, the delay in the translocation of southward monsoon low pressure in southern Asia causes its western part to be expanded over the Arabian Sea. This expansion causes dislocation of westward high pressure situated over the Saudi Arabian Peninsula and the moisture transfer from this sea into Iran is resultantly disrupted leading to pervasive droughts in Iran. But, when the South Asian monsoon low pressure is located in a more southern situation, the expansion of its western part is reduced and the high pressure located over the Saudi Arabian peninsula moves to the coasts of the Arabian Sea with a little eastward dislocation. Under such circumstances, the whole Arabian Sea is overwhelmed by this high pressure and its moisture enters Iran through the Saudi Arabian Peninsula’s moisture canal. Nevertheless, Iran’s pervasive drought and wet years during winter are a function of the jigsaw behavior between Asia’s thermal high pressure and the Atlantic Ocean’s dynamic high pressure. When Asia’s thermal high pressure reaches its strongest state, its western part is expanded and, along with the Atlantic Ocean’s dynamical high pressure, spread over the entire Central Asia, the Middle East, and Northern Africa. Under these conditions, the high pressure situated over the Saudi Arabian Peninsula takes a more western position and this new location causes disruption in the moisture transfer from the Arabian Sea into Iran and this ends in pervasive dry winters in Iran.However, during some times when the Asian thermal high pressure is in its weakest state, the western part thereof would not have the spatial expansion it has to. In such a situation, the eastern part of the Atlantic Ocean’s dynamic high pressure is corroborated and expanded. Such an expansion caused the high pressure situated over the Saudi Arabian Peninsula to move eastward and become established over the eastern coasts of the Saudi Arabian Peninsula. This new situation causes the transfer of moisture from the Arabian Sea into Iran to happen easily and the result would be pervasive wet winters for Iran.But, during spring, Tibetan low pressure and the Atlantic Ocean’s high pressure are the most important actors of climate in southwest Asia. Whenever the low pressure of the Tibetan plateau low pressure is strengthened and expanded over the eastern half of the Arabian Sea, the high pressure established over Saudi Arabia would be dislocated westward as a result of which pervasive droughts happen in Iran. However, when the Tibetan Plateau’s low pressure is ameliorated and its western part distances away from the Arabian Sea, the Atlantic Ocean’s dynamic high pressure is corroborated and its eastern part is expanded. This expansion causes the high pressure situated over the Saudi Arabian Peninsula to move eastward with its central core taking place over the western shores of the Arabian Sea under which conditions the whole Arabian Sea would be captured by this high pressure. This high pressure transfers the Arabian Sea’s moisture through the Saudi Arabian Peninsula’s moisture canal into Iran via a clockwise circulation. During the ending months of spring, as well, and with the northward dislocation of the tropical convergence belt and its establishment over southern Asia and also with the establishment of some parts of India’s monsoon low pressure over the eastern half of the Arabian Sea, the moisture transfer from the Arabian Sea into Iran is practically decreased to a minimum. Discussion of Results and ConclusionThe results indicated that certain patterns play roles during every season in this sea’s transferring of moisture. During fall, delay in the southward dislocation of southern Asia’s monsoon low pressure causes the transfer of moisture from this sea into Iran to be disrupted the result of which would be pervasive droughts in Iran. During winter when the Asian thermal high pressure reaches its strongest state, its western part is expanded and, together with the Atlantic Ocean’s dynamical high pressure, it captures the whole of central Asia, the Middle East, and northern Africa. Under these conditions, the high pressure established over the Saudi Arabian Peninsula takes a more western position and this new position disrupts moisture transfer from the Arabian Sea into Iran the result of which would be pervasive dry winters for Iran. During spring, the Tibetan low pressure and the Atlantic Ocean’s high pressure are the most important players in the climate in southwest Asia. Whenever the western part of the Tibetan Plateau’s low pressure is strengthened and expanded over the eastern half of the Arabian Sea, the high pressure established over Saudi Arabia is dislocated westward the result of which would be pervasive droughts for Iran.
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