تأثير نابهنجاري‌هاي ماهانه الگوهاي گردشي جو در رخداد دوره‌هاي خشك وتردر غرب و شمال غرب ايران

Influence of Monthly Atmospheric Circulation Patterns Anomalies on with Wet/Dry conditions in the West and Northwest of IRAN

طي سال‌هاي اخير، رخداد دوره‌هاي خشك و طولاني در مناطق مختلف كشور، بويژه منطقه غرب و شمال غرب، سبب تنش‌هايي در بخش‌هاي مختلف از جمله منابع آب شده و خسارات قابل توجهي را به بخش كشاورزي و منابع طبيعي وارد نموده است. لذا مطالعه عوامل مؤثر بر رخداد دوره‌هاي خشك و تر، از جمله نابهنجاري‌هاي ماهانه الگوهاي همديدي از اهميت زيادي مي‌تواند برخوردار باشد. بر اين اساس در اين تحقيق جهت بررسي تأثير الگوهاي گردشي جو بر دوره‌هاي خشك وتر ماه‌هاي سرد سال در غرب و شمال غرب ايران، داده‌هاي بارش بارندگي ده ايستگاه همديدي در يك دوره 30 ساله(2010-1981) براي ماه‌هاي اكتبر تا آوريل استفاده شد. آنگاه ماه‌هاي خشك و تر، بر اساس شاخص نمره استاندارد(Z) تعيين گرديد، سپس در طول دوره 30 ساله، ماه‌هايي كه ميانگين نمايهZ  داراي مقادير بيشينه و يا كمينه بود نقشه‌هاي همديدي آنها در سه تراز، فشار سطح دريا (SLP)، 850 و 500 هكتوپاسكالي بررسي و تحليل شد، در نهايت الگوهاي ماهانه و نابهنجاري‌هاي ميدان فشار تراز سطح دريا و ميدان ارتفاع ژئوپتانسيل تراز 500 هكتوپاسكال(H500) تعيين گرديد. نتايج حاصل نشان داد كه: 1 تقويت پرفشار بر روي اروپا وتعميق ناوه در روي مديترانه، افزايش بارندگي در غرب و شمال غرب ايران را به دنبال دارد. 2 تقويت پرارتفاع در نواحي مركزي و شرقي ايران و كج شدن ناوه موج غربي به حالت شمال شرقي جنوب غربي، سبب افزايش بارش در غرب و شمال غرب ايران مي‌گردد. 3 تقويت كم فشار در روي مديترانه و سودان و ادغام آنها و تقويت پرفشار در روي اروپا با افزايش بارش در منطقه مورد مطالعه همراه خواهد بود. 4 افزايش نابهنجاري مثبت SLP و H500 از شمال درياي سرخ و روي نيمه شرقي مديترانه تا شمال اروپا همراه با نابهنجاري‌هاي منفي در نيمه غربي مديترانه سبب دوره‌هاي خشك شديد مي‌شود.

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 quasipermanent 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 sealevel pressure (SLP) patterns and dry or wet monthly precipitation conditions over Turkey is significant in winter and nonexistent 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 largescale winter midtropospheric circulation on Greek precipitation were investigated. They concluded that the spatial distribution of winter precipitation over Greece was related to the eastern North AtlanticEuropean midtropospheric 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 30year 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 (NCEPNCAR) reanalysis dataset. The grid data with 2.5°×2.5° resolution was selected for the area between 20°W 110°E and 1080°N for seven months (OctoberApril) over a 30year period (19812010). 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 longterm(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 28). Each of these patterns (Figures 28) 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#039;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 OctoberMarch months through a long term consideration (19812010), 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 negativeositive abnormalities. The extreme drought and wet spells usually are occurred. Establishment of the negative/(positive) abnormalities over a zone between 25ºN45ºN/(20ºN45ºN) 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  45ºN75ºN as well as 10ºE100ºE/(20ºN45ºN (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 ZonalMeridional (a composite status). In this composite status the pressureheight 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.