اثر موقعیت ناوه و پشته بادهای غربی بر گستره خشکسالی های ایران (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
خشکسالی های متوالی سال های اخیر در ایران اهمیت تحلیل ساختارهای جوّی و اثر آن را بر الگوهای بارش و خشکسالی افزایش داده است. هدف از پژوهش حاضر بررسی موقعیت و تغییرات مکانی اجزای کلیدی گردش جوّ و سامانه های همدید منطقه از جمله پشته ها و ناوه های جوّی در ارتباط با گستره خشکسالی های ماهانه ایران است. در این مطالعه از داده های ژئوپتانسیل تراز 500 هکتوپاسکال و بارش داده های ERA5 با دقت 25/0 درجه از سال 1979 تا 2021 استفاده شد. درصد مساحت خشکسالی های ماهانه (سپتامبر تا می) با شاخص RAI در پنج طبقه محاسبه و موقعیت محور و حد جنوبی و شمالی ناوه ها و پشته ها در گستره ای از غرب ایران تا شرق اقیانوس اطلس استخراج و ترسیم شد. نتایج نشان داد که محور ناوه در خشکسالی های فراگیر از 30 درجه شرقی در سپتامبر و می به 10 درجه شرقی در سایر ماه ها و محور پشته در تمام ماه ها در محدوده مرکز تا غرب ایران و در مقابل، محور ناوه ها در ماه های فاقد خشکسالی بین 30 تا 40 درجه شرقی و محور پشته ها در ماه های فاقد خشکسالی از 0 تا 20 درجه شرقی مستقر بوده است. حد جنوبی ناوه ها در خشکسالی های فراگیر در شمال آفریقا و حد شمالی پشته ها در محدوده روسیه تا آسیای میانه مشاهده شد. میزان جابه جایی محور پشته ها بین ماه های خشکسالی فراگیر و فاقد خشکسالی (حدود 50 درجه) بسیار بیشتر از تغییر موقعیت محور ناوه ها بوده است. جابه جایی حد جنوبی ناوه ها و به خصوص حد شمالی پشته ها، کاهش بارش و افزایش خشکسالی را در ماه های دسامبر، ژانویه و مارس (در پژوهش های قبلی به طور مشخص به آن اشاره شده است) توجیه می کند؛ به طوری که در این ماه ها به ترتیب پشته ها و ناوه ها جابه جایی شرق و غرب سو داشته است. این شاید نمایانگر تغییر رژیم بارش ایران متأثر از تغییر اجزای گردش جوّ منطقه در بستر گردش جوّ سیاره ای باشد.Impact of Westerly Winds Troughs and Ridges on Droughts Extent in Iran
The consecutive droughts experienced in Iran over recent years have highlighted the need to analyze atmospheric structures and their effects on precipitation patterns and drought conditions. This study investigated the location and spatial variations of key atmospheric circulation components, particularly synoptic systems, such as ridges and troughs, in relation to the extent of monthly droughts across Iran. Using ERA5 data, which included 500 hPa geopotential heights and precipitation data with a spatial resolution of 0.25°, the analysis covered the period from 1979 to 2021. Monthly drought extent percentages (September to May) were calculated using the Rainfall Anomaly Index (RAI) and categorized into 5 levels. The axes, as well as the southern and northern limits of ridges and troughs, were extracted and mapped from western Iran to the eastern Atlantic. The results indicated that during periods of widespread drought, the trough axis shifted from 30°E in September and May to 10°E in other months, while the ridge axis remained centered over central to western Iran throughout the year. Conversely, in months without drought, the trough axis was located between 30° and 40°E and the ridge axis spanned from 0° to 20°E. Notably, during widespread droughts, the southern limit of troughs was observed over North Africa, while the northern limit of ridges extended from Russia to Central Asia. The displacement of ridge axes between drought and non-drought months (approximately 50°) was significantly greater than the shifts observed in the trough axes. This displacement, particularly of the southern trough limit and the northern ridge limit, accounted for the decreased precipitation and increased drought conditions observed in December, January, and March. The eastward and westward shifts of ridges and troughs might indicate a change in Iran’s precipitation regime influenced by variations in regional atmospheric circulation within the broader context of planetary circulation. Keywords: Ridge, Trough, Synoptic Systems, Westerly Winds, Drought, Iran. IntroductionWesterly winds are a vital component of mid-latitude atmospheric systems, playing a crucial role in transferring weather conditions across different regions. In the northern hemisphere during winter, these winds generate synoptic systems that can extend from several hundred to several thousand kilometers, forming 3 primary long waves: the long wave of eastern America, eastern Asia, and the Mediterranean. Research suggests that a key convergence point for the westerlies at the 500 hPa level is situated over the Mediterranean Sea. The strong meridional flow of these winds, particularly in winter, significantly influences the weather patterns of the affected areas.Moreover, the Mediterranean region is impacted by air masses originating from the Atlantic Ocean, resulting in a northward influx toward the western Mediterranean. The Mediterranean long wave is shaped by the interactions between subtropical high pressure and the polar vortex with its seesaw pattern reflecting meridional movements. These atmospheric flows characterized by deep troughs and strong ridges have a profound effect on weather conditions and precipitation across various regions. Spatial variations in the Westerly winds depend on the fluctuations in the range and depth of pressure systems, which are crucial for the transport of air masses and moisture. These changes can lead to significant precipitation anomalies and contribute to widespread droughts. In Southwest Asia, particularly in Iran, the high variability in precipitation often results in severe and prolonged drought conditions. This study aimed to examine the impacts of changes in mid-latitude atmospheric circulation patterns on drought the occurrences in Iran, focusing on the roles of troughs and ridges in influencing precipitation and drought events in this region. Materials & MethodsTo achieve the research objective of examining droughts in Iran with the spatial changes of troughs and ridges, we utilized monthly data on precipitation and geopotential height (in gpm) at the 500 hPa level. This level was chosen because it represented a critical point for westerly winds, where friction was minimal. The data were collected and analyzed for the period from 1979 to 2021. The Rainfall Anomaly Index (RAI) was employed to identify monthly droughts using monthly precipitation data from the ECMWF- ERA5 database. The RAI assessed drought severity and classified it into 5 categories: very severe, severe, moderate, mild, and non-drought. Geographic positions of the trough and ridge axes spanning from 40°W to 70°E and 10°N to 70°N were extracted using ArcGIS software. We calculated the average positions of these axes for each month. Finally, the relationship between the spatial changes of the trough and ridge features and the extent of drought areas in Iran was analyzed using regression analysis. Research FindingsThe results indicated that the axes of troughs and ridges in the Mediterranean wave activity region exhibited significant spatial shifts in relation to drought areas. When drought extent in Iran was limited (less than 20% of the area), the ridge axes were positioned from the central to western Mediterranean, occasionally extending to the eastern North Atlantic. In contrast, during widespread droughts (over 80% of the area), the ridge axes shifted to span from the eastern Mediterranean to western Iran. Conversely, the trough axes were found in the central to western Mediterranean during extensive drought periods, while during less severe droughts, they were located in the eastern Mediterranean up to 40°E longitude. The study also revealed that the southern limit of the troughs—indicative of their depth and southward extension—shifted southward and westward during widespread droughts.The positioning of the ridges and troughs was crucial for determining Iran's precipitation patterns. Specifically, a ridge over the eastern Mediterranean was associated with reduced rainfall and increased drought severity. Changes in the spatial positions of these ridges and troughs significantly impacted drought conditions in Iran. Shifts in the ridge axes toward the eastern Mediterranean and trough axes toward the western Mediterranean correlated with decreased rainfall and a higher incidence of drought in Iran. Conversely, strong ridges over western Europe and the western Mediterranean could enhance precipitation in Iran by facilitating the formation of deep troughs in the eastern Mediterranean. These findings underscored the critical influence of regional atmospheric circulation patterns on Iran's climate, particularly regarding rainfall distribution and occurrence of droughts. Discussion of Results & ConclusionThis study explored the relationship between drought extent in Iran and the spatial positioning of key synoptic atmospheric systems, emphasizing the role of Mediterranean troughs and ridges associated with the westerlies. Analysis of monthly ERA5 precipitation and 500 hPa geopotential height data revealed that the location of these systems significantly influenced drought severity and extent across Iran. Shifts in their position or wavelength within the Mediterranean wave track of the westerlies affected climate and precipitation patterns in Southwest Asia, including Iran. During widespread droughts, Mediterranean troughs shifted eastward toward the Atlantic and western Mediterranean, resulting in the formation of a strong ridges over the eastern Mediterranean. This shift obstructed rain-bearing systems from reaching Iran, exacerbating drought conditions. Conversely, when the trough axis moved toward central Mediterranean areas, the eastern ridge expanded over Iran, further contributing to drought. In periods of limited drought, the trough axis was situated in the eastern Mediterranean and sometimes extended over Southwest Asia, while the ridge axis remained active over the western Mediterranean. These configurations facilitated greater moisture influx into Iran, mitigating drought severity.The findings suggested that shifts in the southern and northern boundaries of the trough and ridge axes closely correlated with monthly drought trends. For example, the westward shift of ridge axes in October, April, and particularly November, alongside the eastward movement of troughs, corresponded with reduced drought extent, allowing westerly winds and moist flows to penetrate Iran. These results provided valuable insights into how drought conditions in Iran were influenced by the positioning of Mediterranean troughs and ridges, which were also affected by broader atmospheric circulation patterns, such as the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO).
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