اندركنش نوسان مديترانه شرقي بر تغييرات زماني بارش غرب ايران

Investigating the Eastern Meditrranean Oscillation Impacts on Precipitation in West of Iran

تغييرات بلندمدت و پيش بيني بارش وابسته به تغييرات سيگنال‌هاي اقليمي جوي - اقيانوسي است. با توجه به اين موضوع، بررسي ارتباط بارش ايستگاه‌هاي غرب كشور و نوسان مديترانه شرقي مهم‌ترين هدف اين مطالعه مي‌باشد. هاتزاكي و فلوكاس با استفاده از روش‌هاي تحليل همبستگي و تحليل مؤلفه‌هاي اصلي (PCA) بر اساس سري‌هاي زماني ماهانه و فصلي، الگويي را كه دو هسته‌ي آن در شرق مديترانه و شمال شرق اقيانوس اطلس و در ترازهاي 300 و 500 هكتوپاسكال قرار دارد شناسايي نمودند كه الگوي مديترانه شرقي (EMO) نام‌گذاري گرديد. براي شناسايي رابطه ميان بارش غرب ايران و نوسان مديترانه شرقي، مقادير اين شاخص با داده‌هاي نرمال شده بارش ايستگاه‌هاي غرب كشور با استفاده از تحليل همبستگي پيرسون، روند خطي و پلي نوميال، مقايسه و مورد بررسي قرار گرفت. نتايج مطالعه نشان مي‌دهد كه بارش در ايستگاه‌هاي نيمه جنوبي منطقه موردمطالعه در فاز مثبت از 5 تا 41 ميلي‌متر افزايش و در فاز منفي از 4 تا 20 ميلي‌متر كاهش داشته است. هم‌چنين نيمه شمالي برعكس نيمه جنوبي در فاز مثبت با كاهش بارش و در فاز منفي با افزايش بارش تا حداكثر 8 ميلي‌متر روبرو بوده است. هم‌چنين تأثير الگوي مديترانه شرقي بر بارش غرب ايران در فصل بهار ضعيف‌تر از فصل زمستان بوده است.

Introduction The term 'teleconnection pattern' refers to the statistically significant negative correlation of recurring and persistent circulation anomalies between two or more geographical areas that could be or may not be adjacent. A pattern between the EM and northeastern Atlantic was identified in the correlation matrices. This pattern was predominantly identified in the upper troposphere,in winter with negative correlations of -0.62 to -0.67. More specifically, at the isobaric level of 500 hPa,a maximum negative correlation (teleconnectivity center) was found between 25 W, 52.5N in the northeastern Atlantic and 22.5 °E, 32.5°N in the EM. More specifically, the Positive Phase indicates that the EM PI value is greater than the mean index value, EMPI, suggesting that the difference between the geopotential heights of the two centers is diminished, while during the negative phase this difference is increased, taking into account that the geopotential heights of the northern pole are naturally lower than the heights of the southern pole. It can be seen that during the negative phase a strong negative anomaly prevails over the NW Atlantic, characterized by increased cyclonic (counterclockwise) circulation. On the other hand, a shallow positive anomaly forms over the EM and northern Africa, implying clockwise circulation around it. This pattern causes an increased southwesterly anomaly flow toward the central Mediterranean and strong westerlies across the middle Atlantic. On the contrary, the positive phase is characterized by a strong anticyclonic anomaly over the NW Atlantic, associated with the intensification of the Atlantic anticyclone and warm air advection, while a cyclonic anomaly predominates over the EM. Material and Methodology The EMP index (EMPI) was defined as follows: EMPI = gpm {25 ow, 52.5°N} -gpm(22.5 °E, 32.5°N) where gpm is the mean monthly geopotential height of the grid point representing each pole. It can be clearly seen that the values of the winterstandardized anomalies between the two poles are reversed, verifying the existence of a seesaw pattern.ln general, an intense positive anomaly of one pole is accompanied by a corresponding negative anomaly of the other. The index was then calculated for each season and for each month and standardized as follows: the difference between the index defined above and its seasonal longterm average divided by its standard deviation: zi = (EMPI i- EMPI)/o where the EM PI is the index value of the year i, and EM PI is its long-term average and a the corresponding standard deviation for the period 1981-2010. In order to differentiate the two situations, the Negative and the Positive Phase of the EMP are defined when zi < -0.5 and zi > 0.5, respectively. Because the eastern mediterranean index data has been gathered in standardized form, the normalized precipitation data (Z Scores) was used and correlation test was fulfilled on them. Discussion According to this analysis, the precipitation and eastern Mediterranean oscillation in March have the highest correlation coefficient, which Kermanshah,Khoramabad and Saghez stations were meaningful at 0.01 level,while other stations were meaningful at 0.05 level, so this relationship was reverse and strong. The correlation of Saghez in January and those of Khoramabad in December were meaningful at 0.01 level.while the of Shahre Kord, Kermanshah and Oromieh in April with the mentioned oscillation were meaningful at 0.05 level. In winter, with the increaseing of negative values of standardized data of EMO, increase and with positive phases the precipitation decrease. The severity of this reduction with respect to the severity of increasing in negative phases has a milder slope.The winter correllations with the mentioned oscillation in positive phases for all the stations except Ahwaz and Dezful were not meaningfui.While this corellation in negative phases only for Ahwaz,Tabriz and Oromieh stations was not meaningfui.The rest,except the above mentioned stations,are meaningful at 0.05 level. The decrease of precipitation the northern part of the region has been more than its southern part. In negative phases the positive shallow flow forms over Eastern mediterranean and north of Africa, which creates the anticyclonic circulation and the southern flow to west in mediterranean, and the warm weather spreads to surroundings.This increases from the minimum 0.02° in Saghez station to maximum 0.85 with respect to average statistical periods, which leads to emerging unusual winter warm waves. The increases from 0.01° to 1 °c as weii.Such an increase,has had winter abundance periods in the region.Aiso,the increasing of has been more in north than south.Therefore,it concluded that the warm and cold periods in the under study months and seasons compeletly agree with positive and negative phases of eastern mediterranean oscillation (EMO) alternatively, such as the warm years of 1990 and 1994 which had negative phases and the cold years of 1981,1992,and 2005 which were in positive phases. Conclusion The study results show that ther is a negative correlation between the stations precipitation of the north of the under study region and the above mentioned oscillation,and ther is a strong positive correlation in southern stations,that most of them are meaningful at 0.05 level. Also, this study proves that stations precipitation in southern half of the under study region in positive phases increases from 5 to 41 mms and in negative phases decreases from 4 to 20 mms.But, contrary to southern half, in the northern half the rainful increases up to 8 mms in negative phases and precipitation decreases in posetive phases.