اثر سامانه‌‌هاي مديترانه‌‌اي بر خشك‌سالي غرب ايران

Effect of Mediterranean systems on drought in west of Iran

در اين پژوهش تاثير درياي مديترانه و برخي عوامل جوّي بزرگ‌مقياس منطقه‌اي در دوره 1960 تا 2005 بر خشك‌سالي غرب ايران بررسي شده است. در سال‌هاي خشك غرب ايران، افزايش در ميانگين سالانه فشار و كاهش در فراواني چرخندهاي اكثر مراكز چرخندزا مشاهده مي‌شود. به‌علاوه، بي‌هنجاري‌هاي فشار ماهانه نيز در شروع سال‌هاي خشك وجود دارد ولي به‌تدريج با دور شدن از ژانويه كاهش مي‌يابد. الگوهاي بي‌هنجاري فشار نشان‌دهنده تضعيف پُرفشار سيبري و تقويت پرفشار آزورز در زمستان‌‌هاي خشك هستند. كاهش فشار يا بي‌هنجاري منفي ميانگين فشار سطح دريا روي مديترانه، موجب تقويت چرخندزايي و افزايش فراواني چرخندهاي مديترانه مي‌شود كه حاصل آن ايجاد زمستان‌هاي مرطوب در غرب ايران است در زمستان‌هاي خشك، بي‌هنجاري جهت بُردار باد در غرب ايران، به سمت مديترانه و به معني كاهش تندي باد از مديترانه به سمت اين ناحيه از ايران است. به‌علاوه مولفه‌هاي مداري و نصف‌النهاري باد در شمال غرب كشور و سطح مديترانه داراي بي‌هنجاري منفي هستند. ‌اين بي‌هنجاري، موجب بي‌هنجاري منفي رطوبت نسبي روي مديترانه و در نتيجه كاهش شار رطوبت از مديترانه به غرب كشور مي‌شود. از طرفي ديگر، شار رطوبت از درياي سرخ به جنوب و جنوب‌شرق كشور كاهش محسوسي ندارد. توزيع جغرافيايي بي‌هنجاري سرعت قايم در زمستان‌هاي خشك غرب ايران، در سرتاسر مديترانه و غرب ايران منفي است كه معني آن نزول هوا در اين زمستان‌هاي خشك است.

The areas of the world that have Mediterranean style climate are found at about 35 degrees north and south of the equator, on the western sides of continents. The distinctive feature of this climate is the small amount of rain in the summer. Overall, the climate has a moderate amount of rainfall and warm winters and hot summers and. The climate of the Mediterranean region is conditioned by its position in the transition area between the subtropical high pressure belt and the midlatitude westerlies. In addition, the cyclones have been recognized since long as, a major meteorological feature influencing weather and climate in the Mediterranean region, often producing severe weather events. Mediterranean cyclones have also an influence on areas outside the Mediterranean region. Cyclones generated in the Mediterranean influence the weather and climate further east in Asian areaof the region to the east, including Syria, Iraq, Iran and Afghanistan. Important cyclogenesis centers of the Mediterranean consist of East and West Mediterranean, Gulf of Geneva (in North Italy), South Italy and Cyprus. In this research, effects of the Mediterranean conditions and some other large scale atmospheric factors on precipitation of Iran during 1960 to 2005 are studied. For this purpose, effect of mean sea level pressure of Mediterranean cyclogenesis centers and the positions of subtropical high pressure belt (STPB) and the Siberian high pressure on both Mediterranean cyclogenesis and precipitation in Iran are investigated. To dynamically and synoptically study the effects of these factors on precipitation in the west of Iran, sea level pressure, cyclones frequency in the cyclogenesis centers, horizontal and vertical wind, temperature and moisture fields, and temperature and moisture fluxes are analyzed. Results show that precipitation in the west of Iran depends on the mean annual surface pressure, the annual number of cyclones generated in Mediterranean centers and the position of the subtropical high pressure. In dry years, positive mean sea level pressure anomaly and negative cyclone frequency anomaly are observed in all cyclogenesise centers of Mediterranean Sea. Positive anomaly of monthly sea level pressure in cyclogenesis centers over the Mediterranean is greatest in January and gradually decreases in later months. In winter during which the west of Iran is dry, the Siberian and Azores high pressures join, and hence the cyclogenesis centers of the Mediterranean are dominated by a high pressure system. In such conditions, the Siberian high pressure is weaker and the Azores high pressure is stronger than in normal winters. In dry winters, a negative anomaly of relative and specific humidity over the Mediterranean Sea and a negative anomaly in the meridional and zonal wind in the Mediterranean Sea and over the northwest of Iran develop. This leads to a decrease flux of moisture to the west and northwest of Iran. This is consistent with what expected from dominance of Azores high pressure over the Mediterranean region in dry winters. In this situation, moisture flux from Red Sea to the south and southeast of Iran remains unaffected.