جابه‌جايي مرز شمالي پُرارتفاع جنب‌حاره در نصف‌النهارهاي متفاوت ايران

Displacement of upper envelope sub-tropical high over Iran

نوار همگرايي درون‌حاره‌اي (ITCZ) در فصل‌هاي گوناگون سال داراي جابه‌جايي نصف‌النهاري است كه موجب جابه‌جايي نوار پُرارتفاع جنب حاره (STH) مي‌شود. هدف از اين پژوهش تعيين موقعيت مرز شمالي نوار STH عبوري از روي ايران در مقياس ماهانه در دوره 1948-2010 است. داده‌هاي ماهانه ارتفاع ژيوپتانسيل تراز 500 هكتوپاسكال از بازكاوي NCEP براي نه نصف‌النهار ايران به فاصله °5/2 استخراج شد. با مشاهده پشت سر هم تعداد 756 نقشه هواشناسي، پربند ارتفاعي5840 ژيوپتانسيل متر درحكم مرز شمالي نوار STH انتخاب شد و سپس سري زماني ماهانه موقعيت آن به‌‌كمك برنامه‌نويسي در محيط گرادز (GrADS) صورت گرفت. نتايج نشان داد كه به‌طور ميانگين كمترين عرض جغرافيايي موقعيت نوار STH در ماه ژانويه در مدار N°18 و بيشترين آن در ماه اوت در مدار N°39 مشاهده مي‌شود؛ به‌طوري‌كه جابه‌جايي آن روي نصف‌النهارهاي عبوري از روي ايران به 21 درجه مي‌رسد. ميانگين‌هاي اقليمي STH در دوره 1981-2010 نسبت به دوره 1951-1980 حدود N°8/2 به سمت شمال به تقريب در همه ماه‌هاي سال جابه‌جا شده‌اند. لذا آزمون مان-كندل در مقياس ماهانه و سالانه صورت گرفت و نشان داد كه روند افزايشي معني‌دار در سطح 01/0 با ميانگين تقريب شيب 07/0 در دوره مورد بررسي وجود دارد كه اين بيانگر سيگنال تغييرات اقليمي گردش جوّي در ايران است.

According to General Circulation Model (GCM), zonal thermal belts are 1- Inter-Tropical Convergence Zones (ITCZ) around eqator; 2- Sub Tropical High (STH) belt around 30 degree latitude; 3- Sub Polar Low (SPL) belt around high latitudes. The belt of Inter-Tropical Convergence Zone (ITCZ) displaces in meridional path, about 5° over the oceans and up to 40° over continents, during the seasons of a year. The position of Sub-Tropical High (STH) belt is also affected by ITCZ movement. STH displacement may change the area covered by the westerly Baroclinic Waves (BW) in temperate regions. The position of the STH is an important issue for changing precipitation regime and onset of precipitation events in Iran. The goal of this research is to determine the position of STH belt over Iran in monthly scale during 1948-2010 period undertaking its meridional displacement. Geopotential height data was extracted from NCEP/NCAR reanalysis with 2.5 degrees resolutions (42.5 °E, 45 °E, 47.5 °E, 50 °E, 52.5 °E, 55 °E, 57.5 °E, 60 °E and 62.5 °E) for nine meridians over Iran ranging from 42.5°E to 62.5°E. Goal of this research consist of: 1- Position determination of Sub Tropical High (STH) belt in monthly scale passing over Iran in 1948 - 2010 period; 2- Evalution of meridional displacement of STH position over Iran in 1948 - 2010 period in order to assess climate changes. By consecutive observations of 756 of 500 hPa monthly maps in GrADS (Grid Analysis and Display System) scope, 5840 gpm contour was indicated as the STH indicator. It is because the southern area of 5840 gpm contour, almost covered by northern latitude of the system while the northern area is occupied by the westerly waves during monthly round maps. This result agrees with previous studies. The strip of northern latitude is determined by 20 m width ranging from 5830 gpm to 5850 gpm. Monthly time series of STH position (unit: degree of northern latitude) was then detected using GrADS programming. Results show that the position of STH is between 10°N and 47.5°N as the most extremes in winter and summer respectively. For the long term means, the minimum northern latitude was averagely observed in January placed on 18°N zone, while maximum happened in August crossing 39 °N zone during the investigated period. Its meridional displacement then reaches 21° over Iran averagely. Moreover climatic means of STH positions during 1981 - 2010 period with respect to 1951-1980 period were migrated approximately 2.8° northward. It is concluded that STH position was moved toward higher latitudes, about 2.8 degrees, in recent decade. The non-parametric Mann - Kendall trend test was applied on time series of STH position in monthly and annual scale. Results showed generally rising trends under 0.01 significance level, with 0.07 slope approximations during 1948 - 2010 period. It demonstrated the signal of climatic variability of atmospheric circulation over Iran. It is suggested that the time series of Sub Tropical High position be as the input of climate prediction models yielding temperature and precipitation as well as drought study.