@article { author = {Ranjbar SaadatAbadi, A. and Panahi, A. and Fatahi, E.}, title = {Influence of Monthly Atmospheric Circulation Patterns Anomalies on with Wet/Dry conditions in the West and Northwest of IRAN}, journal = {Journal of Climate Research}, volume = {1393}, number = {17}, pages = {91-109}, year = {2014}, publisher = {https://www.irimo.ir/}, issn = {2228-5040}, eissn = {2783-395X}, doi = {}, abstract = {Introduction  In the recent years, the West and Northwest regions of Iran, have faced severe and elongated drought problems due to pressure pattern anomalies. The rainy season in the West and North West (WNW) regions normally lasts from October to April. In the summer months, rainfall is a small occurrence in the region due to the predominance of the quasi-permanent subtropical high system. The relationship between precipitation totals and pressure patterns in the regions close to Iran has been recently published. According to a very recent study (Kutiel et al. 2001), the relationship between regional sea-level pressure (SLP) patterns and dry or wet monthly precipitation conditions over Turkey is significant in winter and non-existent in summer. Pressure patterns associated with dry conditions usually show positive SLP departures, and vice versa. There is a strong relationship between pressure patterns associated with wet air. Similar atmospheric variations and relationships have been found for precipitation over the eastern Mediterranean Basin and Greece, especially during winter. In a study done by Maheras et al. 1999, Xoplaki et al. 2000, the influence of the large-scale winter mid-tropospheric circulation on Greek precipitation were investigated. They concluded that the spatial distribution of winter precipitation over Greece was related to the eastern North Atlantic-European mid-tropospheric circulation fields. Materials and methods Rainfall data including monthly totals for the period from 1981 to 2010 (thirty years) in ten stations across the West and Northwest of Iran, were studied (Table 1). It consists of monthly precipitation totals (mm) for October to April in the 30-year period from the meteorological synoptic stations. Also, the grid point data (Sea level Pressure and 500mb) were extracted from the National Center for Environmental Prediction and National Center for Atmospheric Research (NCEP-NCAR) reanalysis dataset. The grid data with 2.5°×2.5° resolution was selected for the area between 20°W -110°E and 10-80°N for seven months (October-April) over a 30-year period (1981-2010). Based on standard score criterion (Z), dry and wet conditions in each month were defined independently for each station (Table 3). For each month the data were standardized as follows:     where: Pi is the monthly rainfall in the year i; P is the long-term(30 years) monthly rainfall average and SD is its standard deviation. Furthermore, for each month and each year, monthly mean standard score (Z) was calculated for the region (Table 4). Then, the synoptical maps of every months having higher (lower) Z profile than the monthly average were investigated and analyzed SLP and 500hPa anomalies as a dry or wet criterion month. Although in this regarding, the monthly mean SLP, 850 and 500hPa geopotential heights (but not presented here) and their anomalies were investigated for a dry or wet criterion month, but in this paper were emphasized on monthly mean SLP and 500hPa anomalies for extreme dry/wet conditions.    Results and Conclusion The data presented in Table 5 are used for selection of the months with extreme precipitation regimes over the WNW area. The SLP and 500 geopotential height composite anomalies are determined for the dry and wet periods (Figures 2-8). Each of these patterns (Figures 2-8) for only two months with extremely high or low Z determined according Table 5 strengthening high pressure on Europe and deepening trough on Mediterranean follows rainfall increase of west and northwest regions in Iran.  High height strengthening in central and eastern regions of Iran and tilting western waves in a northeast to southwest axis, cause rainfall increase in Iran's west and north west regions. Strengthening Sudanese and Mediterranean low pressure, their integration and strengthening high pressure in Europe cause rainfall increase of the region under study. Through strengthening each of Siberian and north Atlantic high pressures in a way affecting the under study region, follows rainfall decrease there. The derived results indicate a decreasing trend for Z index in October-March months through a long term consideration (1981-2010), that confirms expansion of drought over the study area. We have different abnormalities in the pressure cells of sea level (SLP) and 500hPa level through dry and wet periods as following 3 forms: 1. Zonal abnormality extension mostly results in the extreme dry and wet spells over the region. In this condition the abnormalities of pressure and height are mostly extended zonal and based on the position of negative/positive abnormalities. The extreme drought and wet spells usually are occurred. Establishment of the negative/(positive) abnormalities over a zone between 25ºN-45ºN/(20ºN-45ºN) associated with positive/ (negative) abnormalities caused severe dry periods (Jan 1987/(Feb 2002) for example) in the region. Occurrence of the positive/(negative) pressure abnormalities over a zone between  45ºN-75ºN as well as 10ºE-100ºE/(20ºN-45ºN (Apr 2002 and Oct 1987 for example)) caused severe wet periods over the region. 2. Meridional abnormality extension also results in extreme dry and wet periods over the region, as following:  - increase of positive/(negative) pressure and height abnormalities from north side of the Red Sea, eastern parts of the Mediterranean, Iran up to north of Europe/(western parts of the Mediterranean, some parts of northern Atlantic cause the sever dry periods. - establishment of negative/ (positive) abnormalities from northeast of Africa and eastern side of the Mediterranean up to Iran and a zone around 80ºN / (over the west of the mentioned zones) caused extreme rainfalls in the region (December 1991 and March 1998 for example). 3. But we found another situation which shows that the extension of abnormalities is not similar to the both aforementioned status. In this situation the extension of abnormalities is Zonal-Meridional (a composite status). In this composite status the pressure-height abnormalities are not sufficiently extended and correlated, as this situation results in the weak dry/wet periods or normal conditions. In this situation mostly the positive/ (negative) abnormalities are established over eastern side of the Mediterranean Sea, in turn, increase occurrence of the weak dry/ (wet) or normal periods. Regarding the high resolution in the seasonal forecasting of pressure cells in contrast to the precipitation forecasting- the results obtained from this investigation have a potential to enhance the quality of the seasonal forecasting.    }, keywords = {The abnormalities of pressure and height,Dry month,Wet month,The West and North West}, title_fa = {تأثیر نابهنجاری‌های ماهانه الگوهای گردشی جو در رخداد دوره‌های خشک وتردر غرب و شمال غرب ایران}, abstract_fa = {طی سال‌های اخیر، رخداد دوره‌های خشک و طولانی در مناطق مختلف کشور، بویژه منطقه غرب و شمال غرب، سبب تنش‌هایی در بخش‌های مختلف از جمله منابع آب شده و خسارات قابل توجهی را به بخش کشاورزی و منابع طبیعی وارد نموده است. لذا مطالعه عوامل مؤثر بر رخداد دوره‌های خشک و تر، از جمله نابهنجاری‌های ماهانه الگوهای همدیدی از اهمیت زیادی می‌تواند برخوردار باشد. بر این اساس در این تحقیق جهت بررسی تأثیر الگوهای گردشی جو بر دوره‌های خشک وتر ماه‌های سرد سال در غرب و شمال غرب ایران، داده‌های بارش بارندگی ده ایستگاه همدیدی در یک دوره 30 ساله(2010-1981) برای ماه‌های اکتبر تا آوریل استفاده شد. آنگاه ماه‌های خشک و تر، بر اساس شاخص نمره استاندارد(Z) تعیین گردید، سپس در طول دوره 30 ساله، ماه‌هایی که میانگین نمایهZ  دارای مقادیر بیشینه و یا کمینه بود نقشه‌های همدیدی آنها در سه تراز، فشار سطح دریا (SLP)، 850 و 500 هکتوپاسکالی بررسی و تحلیل شد، در نهایت الگوهای ماهانه و نابهنجاری‌های میدان فشار تراز سطح دریا و میدان ارتفاع ژئوپتانسیل تراز 500 هکتوپاسکال(H500) تعیین گردید. نتایج حاصل نشان داد که: 1- تقویت پرفشار بر روی اروپا وتعمیق ناوه در روی مدیترانه، افزایش بارندگی در غرب و شمال غرب ایران را به دنبال دارد. 2- تقویت پرارتفاع در نواحی مرکزی و شرقی ایران و کج شدن ناوه موج غربی به حالت شمال شرقی- جنوب غربی، سبب افزایش بارش در غرب و شمال غرب ایران می‌گردد. 3- تقویت کم فشار در روی مدیترانه و سودان و ادغام آنها و تقویت پرفشار در روی اروپا با افزایش بارش در منطقه مورد مطالعه همراه خواهد بود. 4- افزایش نابهنجاری مثبت SLP و H500 از شمال دریای سرخ و روی نیمه شرقی مدیترانه تا شمال اروپا همراه با نابهنجاری‌های منفی در نیمه غربی مدیترانه سبب دوره‌های خشک شدید می‌شود. }, keywords_fa = {نابهنجاری‌ میدان‌های فشار و ارتفاع,ماه خشک,ماه تر,غرب و شمال غرب}, url = {https://clima.irimo.ir/article_15089.html}, eprint = {https://clima.irimo.ir/article_15089_3fd7a17eadbe7da3fc5fa345997fd876.pdf} }