بررسي روند تغييرات پُرفشار سيبري و تاثير آن بر ميدان‌‌هاي هواشناختي در بازه زماني 1948تا 2008

The trend of the Siberian high pressure and its impacts on the meteorological fields during 1948-2008

يكي از مهم‌ترين سامانه‌هاي جوّي در فصل زمستان نيمكره شمالي و در منطقه اوراسيا كه نقش مهمي در تغيير كميت‌هاي جوّي دارد، پُرفشار سيبري است. در اين مقاله، روند خطي تغييرات سالانه شدت مركز اين سامانه و تاثير آن روي برخي از كميت‌هاي هواشناختي در مناطق فراحاره‌اي نيمكره شمالي در يك دوره 60 ساله (از 1948 تا 2008) با استفاده از داده‌هاي NCEP/NCAR بررسي شد. نتايج نشان مي‌دهد كه فشار تراز دريا در مركز پُرفشار سيبري در ابتداي اين دوره با روند ملايم hPa 10/1 در هر دهه تقويت شده، اما از اوايل دهه هفتاد به بعد، با روند hPa 12/0- در هر دهه تضعيف شده است. ضريب همبستگي بين شاخص پُرفشار سيبري و دماي دومتري از سطح زمين در منطقه مركزي پُرفشار سيبري در طول دوره مورد بررسي برابر با 46/0- است كه نشان‌‌دهنده روند افزايشي دما با آهنگ 13/0 درجه سلسيوس در هر دهه بر روي اين منطقه است. همچنين با محاسبه شاخص‌هاي دورپيوندي، مشاهده مي‌شود كه تاثير اين سامانه روي متغيرهاي جوّي بسيار فراتر از محدوده استقرار آن است. ارتباط بسيار نزديك با مانسون زمستاني شرق آسيا، افزايش شرايط مناسب براي چرخندزايي روي درياي مديترانه و تقويت فرارفت هواي گرم از شمال افريقا تا شرق درياي مديترانه و از آنجا تا شمال اروپا از جمله اين تاثيرات هستند. به‌‌علاوه، نتايج حاكي از آن است كه پُرفشار سيبري يك پديده محدود به سطوح پايين جوّ نيست و با جريان‌هاي هوا در سطوح فوقاني جوّ نيز در ارتباط است. براي نمونه، مي‌‌توان به تقويت جت جنب‌حاره‌اي در شرق آسيا و درياي مديترانه، وجود يك قطار موج از شرق اقيانوس اطلس تا غرب اقيانوس آرام به شكل مقادير مثبت و منفي ضرايب همبستگي بين شاخص پُرفشار سيبري و ارتفاع ژيوپتانسيلي سطوح مياني و بالاتر جوّ اشاره كرد.

One of the most important atmospheric systems in the Eurasian continent during the Northern Hemisphere winter season is the Siberian high pressure. In this study, using the NCEP/NCAR reanalysis data for winters of 1948-2008, trend of changes in the intensity of the high system center and its effects on some meteorological fields are investigated. The results show that the 2m height Mean Temperature in central Siberian high region were -17.7 ?C during the study period and the Mean Sea Level Pressure (MSLP) were over 1030hPa. The MSLP had a minor linear positive trend of 1.10hPa/dec (hPa per decade) at the beginning of the 60-years period. Also, a weak negative linear trend of -0.12hPa/dec occurs in early 1970s. There is a meaningful correlation (-0.46) between the Mean Temperature averaged on influence area of the central Siberian high and the Siberian High Index (SHI) during the study period, as in most cases enhancement (weakening) of the high center accompanied with cooling (warming) in the region. Calculations indicate a trend of 0.13 ?C/dec warming for the area. The results show that the pressure gradient causes strong north monsoonal currents and thus cold advection toward far-east or east-Asia winter monsoon. Calculated correlation coefficients between the SHI and some meteorology fields indicate that the enhanced Siberian high provides suitable conditions for cyclogenesis over the Mediterranean Sea and development of warm advection from north Africa to east Mediterranean and then to north Europe. Another result indicates that when anti-cyclones over the Siberia form and develop, they act as a barrier to eastward movement of the extra-tropical cyclones. This leads the systems to move to higher latitudes and thus fewer cyclonic system pass from the Siberia and this reduce warm advection over the Siberia. Furthermore, calculated tele-connection correlations show that the system affects atmospheric variables beyond its established region. Enhancement of pressure in the system normally leads to a strong pressure gradient between east Siberian high and west Aleutian high. Correlation coefficients between the SHI and some meteorological fields in the extra-tropical regions of the north hemisphere indicate that the Siberian high could exert impacts on meteorology fields beyond its source region. Examples include a strong relationship between the SHI and the East Asian winter monsoon, enhancement of the suitable conditions for cyclogenesis over the Mediterranean Sea and warm advection intensification in North Africa toward the east Mediterranean Sea and north Europe. Beyond these features, there is intensification of the subtropical jet over the East Asia and the eastern Mediterranean Sea as well as the existence of a wave train in the form of positive and negative correlation coefficients between the SHI and the Geopotential height (GPH) field in middle and upper troposphere. These findings suggest that the Siberian high should not be considered simply as a local low-level phenomenon, and it exerts significant impacts on middle and upper-level circulations.