نقش سامانه‌هاي همديدي گذرا در ترابرد فصلي رطوبت روي ايران

Role of transient synoptic systems on seasonal moisture transport over Iran

در اين پژوهش منابع رطوبت و چگونگي ترابرد رطوبت روي ايران و عوامل موثر بر آنها در فصل­هاي مختلف در يك دوره 30 ساله (1981-2010) بررسي شده است. به اين منظور از داده هاي ميانگين ماهانه cfsr با تفكيك افقي 0/5 درجه براي محاسبه و تحليل كميت هاي ارتفاع ژئو پتانسيل، باد، دما، فشار سطح دريا، آب بارش شو، مولفه هاي چرخشي و واگراي بردار شار رطوبت و توابع جريان و پتانسيل بردار شار رطوبت استفاده شده است.نتايج نشان مي دهد كه در فصل پاييز با استقرار واچرخند جنب حاره روي نوار جنوبي ايران و غرب اقيانوس هند خمش واچرخندي بردار شار رطوبتو ترابري رطوبت از دريايعمان به جنوب ايران وجود دارد. همچنين بررسي بي هنجاري مولفه چرخشي بردار شار رطوبت نشان دهنده ترابرد رطوبت از درياي عمان به جنوب شرق ايران در اثر شكل گيري چرخند گرمايي روي جنوب ايران و ترابرد رطوبت از شرق مديترانه به شمال غرب ايران ناشي از تقويت چرخند شرق مديترانه است. در فصل زمستان، واچرخند جنب حاره روي شرق عربستان موجب خمش واچرخندي بردار شار رطوبت روي درياي عرب، جنوب درياي سرخ و شرق عربستان و در نتيجه ترابرد رطوبت به نواحي غرب و جنوب غرب ايران است. بررسي بي­هنجاري مولفه چرخشي بردار شار رطوبت نشان دهنده ترابرد رطوبت از درياي سرخ به جنوب غرب ايرانبراثر تقويت چرخند شرق آفريقا و ترابرد رطوبت به شمال غرب و سواحل جنوبي درياي خزر ناشي از نفوذ پرفشار از شرق اروپا به درياي مديترانه است. در فصل بهار به علت نفوذ واچرخند جنب حاره روي غرب اقيانوس هند و گردش كلي آب اقيانوس وابسته به نوسان دو قطبي اقيانوس هند (IOD) در عرضهايN2015 خمش واچرخندي بردار شار رطوبت روي غرب اقيانوس هند شكل مي گيرد و ترابرد رطوبت به شبه قاره هند و جنوب شرق ايران قابل توجه است. همچنين خمش چرخندي بي هنجاري بردار شار رطوبت ناشي از شكل گيري چرخند گرمايي روي جنوب شرق ايران، موجب ترابرد رطوبت از درياي عمان به جنوب شرق ايران مي شود و خمش واچرخندي بي هنجاري بردار شار رطوبت ناشي از نفوذ واچرخند روي شرق مديترانه و شمال غرب ايران، موجب ترابرد رطوبت از شرق مديترانه به شمال غرب ايران مي شود. در فصل تابستان با نفوذ واچرخند جنب حاره به عرض هايN3525 و تقويت مونسون هند و گردش كلي آب اقيانوس هند ناشي از دو قطبي اقيانوس هند، ترابرد رطوبت از غرب اقيانوس هند به شبه قاره هند و جنوب شرق ايران مشاهده مي شود. همچنين بر اثر شكل گيري چرخند گرمايي در اين ناحيه بي هنجاري ترابرد رطوبت از درياي عمان به جنوب شرق و شرق ايران مشاهده مي شود.

Materials and Methods An attempt is made to examine the moisture sources and transport over Iran and its relation to other influencing factors during different seasons for a 30 years period (19812010. Using National Center for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data with 0.5 degree horizontal resolution, described by Saha et al., 2010, several fields including geopotential height, horizontal wind and temperature at several vertical levels along with sea level pressure, precipitable water and rotational and divergent components of the moisture flux vector are presented and analyzed.   Results and discussion Results show that, during the autumn, the subtropical high is located over south of Iran and west of Indian Ocean and there is anticyclonic curvature of moisture flux vector and moisture transport from over Oman sea to south of Iran. Examination of the anomaly of rotational component of moisture flux vector indicates that formation of thermal low over south of Iran causes moisture transport form over Oman sea to southeast of Iran and intensification of eastern Mediterranean low causes moisture transport from over east of Mediterranean to northwest of Iran. During winter, subtropical anticyclone located over east of Saudi Arabia causes anticyclonic curvature of moisture flux vector over west of Indian ocean, south of Red sea and east of Saudi Arabia and consequently moisture transport to west and southwest of Iran. Examining the anomaly of rotational component of moisture flux vector indicates that intensification of eastern African low causes moisture transport from over Red Sea to southwest of Iran and eastward migration of high pressure system from eastern Europe to Mediterranean causes moisture transport to northwest and southern coast of the Caspian sea. During Spring, eastward expansion of subtropical anticyclone over west of Indian ocean and also general circulation of Indian Ocean due to Indian Ocean Dipole (IOD) at 1020 N, anticyclonic curvature of moisture flux vector over west of Indian ocean is formed and moisture transport to Indian subcontinent and southeast of Iran is significant. Also, formation of thermal low over southeast of Iran causes cyclonic curvature of moisture flux vector anomaly and moisture transport from over Oman sea to southeast of Iran. Anticyclonic curvature of moisture flux vector anomaly due to expansion of high pressure to east of Mediterranean and northwest of Iran, causes moisture transport from over east of Mediterranean to northwest of Iran. During summer, northward migration of subtropical high to higher latitudes 2535N over Indian subcontinent and Monsoon intensification and Indian ocean general circulation associated with IOD, moisture transport from over west of Indian ocean to Indian subcontinent and then to east of Iran are observed. It is to be noted that during summer, a thermal low pressure region is formed over east of Iran and extended up to south of Alborz and east of Zagross mountain ranges. Coexistence of the thermal low with transported moisture from Indian subcontinent, causes a moisture flux vector anomaly is observed. Moreover, formation of thermal low pressure over this region causes anomaly of moisture transport from over Oman Sea to southeast and east of Iran.   Conclusion   In this research, the important role of the subtropical high pressure in combination with passing synoptic systems on the moisture transport over Iran during different seasons is investigated. It is concluded that during winter the subtropical high pressure has its prominent role in moisture transport from over Arab and Red Seas toward west and southwest of Iran. Moreover western synoptic systems are associated with moisture advection over west and south west of Iran. As such, a moisture convergence is formed over west of Iran During summer the subtropical high pressure is extended over Iran and Indian subcontinent. As such there is a moisture transport towards Indian subcontinent and from there toward south east and east of Iran. And thermal low pressure systems cause moisture convergence and subsequently summer time monsoonal rain fall over south of Iran.