تغییر اقلیم در ایران از دیدگاه تغییرات آب قابل بارش، نم ویژه و باد برداری

تغییرات زمستانه اقلیمی ایران و بخش‌هایی از کشورهای همسایه با بررسی تغییرات مکانی زمانی پراسنج های آب قابل بارش و نم ویژه و باد برداری ارزیابی شد. در این راستا، اندازه ماهانه ژانویه تا مارس این پراسنج ها برای دوره 2017-1960 در پیکسل های °2.5 * °2.5 از داده های نسخه دوم بازتحلیل سازمان ملی جوی و اقیانوسی آمریکا فراهم گردید. چند ویژگی آماری (مانند میانگین فصلی) این داده ها نخست برای در دو بازه زمانی 29 ساله (1960 تا 1988 و 1989 تا 2017) و سپس برای پنج دوره ده ساله پیاپی و یک دوره هشت ساله (20172010) با هم مقایسه شد. کمترین اندازه آب قابل بارش و بیشترین اندازه نم ویژه در پهنه کوهستانی شمال باختری، باختر و مرکز کشور دیده شد. در سنجش با بازه1960 تا 1988، بازه زمانی 1989 تا 2017 با کاهش معنی داری در اندازه آب قابل بارش، نم ویژه، بادهای باختر وزان و جنوب وزان به ویژه در گستره جنوب باختری ایران روبرو بود. در بازه زمانی 1960 تا 1969، بیشترین اندازه های مثبت بی‌هنجاری در اندازه این پراسنج ها برای بخش‌های بزرگی از ایران بویژه گستره های باختری و جنوب باختری دیده شد. کمترین اندازه این پراسنج‏ها نیز وابسته به دوره 2009-2000 بود. به جز دوره 1999-1990، اندازه های آب قابل بارش و نم ویژه به گونه‎ای پیوسته برای دهه های پس از 1970 کاهش یافته است. افزایش (کاهش) سرعت بادهای باختر وزان و جنوب وزان، با بیشتر (کم) شدن آب قابل بارش و نم ویژه در بخش‌های بزرگی از ایران همراه بوده است. در سنجش با دهه 1960، در دیگر دهه ها و بویژه دو دهه 2000 و 2017 از سرعت این بادها کاسته شده و در برابر بادهای خشک خاور وزان و شمال وزان چیره می شود.

Climate Change in Iran Based on Changes in Precipitable Water, Specific Humidity and Wind Data

Introduction:Based on the research and assessment carried out during the Climate Change Enabling Activity Project under United Nations Framework Convention on Climate Change (UNFCCC) and using the scenarios proposed by IPCC, it is estimated that if the CO2 concentration doubles by the year 2100, the average temperature in Iran will increase by 1.54.5°C which will cause significant changes in water resources, energy demand, agricultural products and coastal zones. The present study is aimed to investigate the characteristics of climate change in Iran and some parts of the neighboring countries. Identifying the spatiotemporal changes in three atmospheric variables comprising perceptible water (PW), specific humidity (SH) and vector wind (VW, U and V components) over 19602017 was the main themes of the study. Materials and Methods: Monthly values of these variables during wintertime (January to March) were extracted from the CDC/ Reanalysis 2/ NOAA in 2.5 * 2.5 grids for the period of 19602017. The study area locates between 20o to 45o N and 30o to 70o E. After averaging monthly data into seasonal series, as first step, significant changes in the considered series were investigated between two equal periods having 29 years of data (19601988 and 19892017). In the second step, the 58 years of the study period were divided into five successive decades (19602009) and a period with eight years (20102017). The KolmogorovSmirnov (KS) field significant test was used for assessing the spatiotemporal difference between the obtained maps associated with various decades. Results and Discussion: According to Figures 1 and 2, for both of the 29year timescales (19601988 and 19892017), PW was maximum (12 to 17 kg/m2) alongside the northern coasts of the Persian Gulf and the Oman Sea. After this, PW had the highest values over the southern coasts of the Caspian Sea (10 to 12 kg/m2). Oppose to these coastal areas, minimum values of this variable with about 6 to 10 kg/m2 were associated with the Zagros mountains. In general, PW exhibited an inverse relationship with elevation. In contrast to PW, SH  maximized (4.2 to 5 g/kg) over the Zagros ranges and its relationship with elevation was generally positive. The lowest value of the SH data was about 3.5 g/kg suggesting relatively low variation in the SH data within the country. Compared to the 19601988 period, a significant decline was observed in the values of PW and SH in 19892017. Although this decline was obvious over all parts of the country, it was slightly significant for the southwestern (northwestern) districts. Compared to the first half of the study period (19601989), PW (or SH) decreased by about 2.5 kg/m2 (or 0.6 g/kg) in southwestern and 0.3 kg/m2 (or 0.15 g/kg) in northwestern parts of Iran for the recent half (19892017). Differences between wind data during these two timeperiods were mostly either northerly or easterly suggesting a significant decrease in the rainbearing southerly or westerly circulation over 19892017. Anomalies of the nearsurface wintertime winds were mostly found to be southerly or westerly during 19601988 implicating the possibility of moisture transport from the Persian Gulf, the Oman Sea, the Mediterranean Sea, and the Red Sea into the most parts of Iran. Conversely, the anomalies were either northerly or easterly in19892017 suggesting less moisture transport into Iran for this recent period.  In the decadal timescale, maximum values of PW, SH, as well as southerly or westerly circulations, were observed during 19601969. The given results suggest that the enhanced (or suppressed) values of PW and SH are generally harmonized with the strengthened southerly and westerly (or northerly and easterly) wind anomalies. For this period, prevailing of southeasterly winds over the Caspian Sea enhanced or suppressed the measure of PW, SH over the western or eastern coasts of the Sea, respectively. Even though the mentioned atmospheric circulation patterns were generally similar for the 19601969 and 19701979 decades, positive anomalies of PW and SH, as well as the westerly and southerly airflows, were slightly suppressed for the second decade. The anomalies of westerly and southerly winds decreased by about onefifth for 19801989 as compared with that in 19601969 resulting in a significant decrease in the PW and SH data for this decade. Although these anomalies were slightly positive over most parts of Iran, their weakness did not allow significant improvement in the PW and SH values. The period of 20002009 was evaluated as the driest decade of the study period for which the negative anomalies of PW and SH, as well as westerly and southerly circulations, were maximized (in absolute values). In spite of the fact that these undesirable conditions have recovered during the period of 20102017, PW and SH were still very low for this recent period. With the exception of the 19901999 decade, PW and SH have continuously decreased for the decades after 1970. The rainbearing southerly and westerly winds have been gradually replaced with dry northerly or easterly wind during the recent periods. Conclusion: The findings showed that the PW and SH distribution patterns are close together in the 29year periods, the measures were, however, significantly smaller in the second period than in the first. The wind anomalies, which were mostly southerly and westerly in 19601988, have been changed to northerly and easterly in 19892017. Since the southerly and westerly winds play an influential role in moisture transfer to Iran, their reduction in the second period is consistent with the observed decrease in PW and SH. Among the tenyear periods, the highest positive PW and SH abnormalities are associated with the 1960 and 1969 decade. This positive anomaly decreased over the time. Since a positive trend is observed for 20102017, it can be concluded that 20002010 is the driest decade of the study period. The positive anomalies of westerlies (easterlies) and southerlies (northerlies) increased (decreased) the magnitudes of PW and SH.