بررسي شار رطوبت از درياهاي مجاور در دوره‌هاي مرطوب و خشك فصل زمستان نيمه جنوبي كشور

Studying of moisture flux during wet and dry periods over southern part of Iran from adjacent Seas

در اين پژوهش، نقش منابع آبي مجاور روي بارش فصل زمستان نيمه جنوبي كشور با استفاده از داده‌هاي ماهانه بارش 183 ايستگاه سينوپتيك ، اقليمي و باران سنجي سازمان هواشناسي كشور و سازمان آب براي دوره 2005-1974 مورد بررسي قرار گرفت. توزيع مكاني نابهنجاري بارش براي ماه‌هاي دسامبر، ژانويه و فوريه با استفاده از روش تابع متعامد تجربيEOF شناسايي شده و مؤلفه‌هاي اول و دوم كه بيش از 66 درصد واريانس بارش را تعيين مي‌كنند براي مطالعه تعيين شدند. براي شناسايي الگوهاي همديدي و ديناميكي رژيم بارش هر منطقه نقشه‌هاي تركيبي نابهنجاري دماي سطح آب، شار رطوبت و فشار سطح دريا براي دوره‌هاي مرطوب و خشك محاسبه و ترسيم شد. الگوي نابهنجاري دماي سطح آب پهنه‌هاي آبي نيمه جنوبي كشور در دوره مرطوب و خشك نشان داد كه تغييرات دماي سطح آب نقش مهمي در نوسانات بارش دارد به طوري كه نابهنجاري دماي سطح آب براي مؤلفه‌هاي اول EOF كه بيش از 50 درصد واريانس كل بارش را در نيمه جنوبي كشور تبيين مي‌كند در دوره مرطوب (خشك) مثبت (منفي) است از آنجائيكه گردش جو حاكم در منطقه نقش مهمي در انتقال رطوبت به جنوب ايران دارد در دوره مرطوب رطوبت از درياي عرب، درياي سرخ و خليج عدن به جنوب ايران منتقل مي‌شود و در دوره خشك از جنوب ايران خارج مي‌گردد. نابهنجاري دماي سطح آب درياي عرب براي مد دوم EOF  مثبت مي‌باشد اما تغييرات دمايي در دوره خشك برجسته نيست. جابه جايي پر فشار عربستان به سمت راست (چپ) در دوره‌هاي مرطوب (خشك) رطوبت بيشتري را از درياي عرب به نواحي جنوبي – شرقي (غربي) كشور انتقال مي‌دهد.

Introduction        The ocean has an important role on climate variability. Most of the water that evaporates from oceans is precipitated back into them and continents. Because of its low annual mean precipitation, Iran is commonly regarded as an arid country. So it is important to understand the processes that govern on the moisture transport in atmosphere and how they are related to precipitation over Iran. Therefore the goal of the study is identifying the role of the adjacent Seas on the winter precipitation over the southern part of Iran. Materials and Methods        Monthly precipitation data from 183 stations in the southern part of Iran were made available by the Iranian Meteorological Organization and Iranian water resource institute. The SST data used in the present study is obtained from the Hadley centre sea ice and sea surface temperature. It is monthly data with 1ºx1º horizontal resolution. The NCEP/NCAR reanalysis data is used for wind, specific humidity, geopotential height and sea level pressure. For all data, we adopted the period from December 1974 to February 2005. To capture the dominant mode of interannual variability, we have applied the EOF analysis to deterended monthly precipitation anomaly data from 183 stations. To characterize the dynamical features related to each precipitation regime composites of sea surface temperature, moisture flux and SLP were computed for wet and dry years. Wet (dry) years correspond to those years with the principle component of the first EOF mode for each month above 0.8 (below 0.8) standard deviation. Results and discussion        Regional spatial precipitation on variability was identified by Empirical Orthogonal Function (EOF). The first 2 components represent more than 66% of total variance. In the first mode of EOF which represent 50% of total variance the positive (negative) sea surface temperature anomaly is in the Arabian Sea during wet (dry) years. The southeasterly moisture flux anomaly over the Arabian Sea turns anticyclonically and transport more moisture to the southern part of Iran from Arabian Sea, Red Sea, and Persian Gulf during wet years. On the other hand, the moisture flux has northerly anomaly over Iran during the dry years, which results in reduced moisture supply from the south. In the second mode of EOF the sea surface temperature anomaly over the Arabian Sea is positive in the wet years but it is not dominant for dry years. The displacement of Arabian anticyclone to eastward (westward) in wet (dry) years transport more moisture from Arabian Sea, Red Sea to south and east (west) part of Iran. Conclusion        The results of this study showed that the Sea surface temperature variation over adjacent Seas causes the precipitation variation over southern part of Iran. The main source for the winter precipitation in the south part of Iran is Arabian Sea at the lower level and Red Sea at the middle level. In almost all precipitation regimes the trough and Arabian anticyclone have an important role to transfer moisture from southern water bodies to the southern part of Iran.