واكاوي روند تاوۀ قطبي در نيمكرۀ شمالي تحت شرايط تغيير اقليم

Analysis of the Polar Vortex Trend in the Northern Hemisphere under Climate Change Conditions

تغيير روند سامانه¬هاي كلان مقياس اقليمي همچون تاوه قطبي، نقش بسزايي در تغيير اقليم سطح زمين دارد. در اين پژوهش براي نيل به اين هدف، از داده¬هاي دوباره واكاوي شده ارتفاع ¬ژئوپتانسيل تراز مياني جو از مركز ملي جو و اقيانوس¬شناسي ايالات‌متحده آمريكا استفاده شده است. اين داده¬ها داراي تفكيك مكاني 5/2×5/2 درجه قوسي و به صورت ميانگين روزانه برداشت گرديده است. دوره آماري اين پژوهش از سال 1948 تا 2007 ميلادي براي نيمكره شمالي بوده و شامل 144×36 ياخته است. براي مقايسه ميانگين¬ها، از آزمون تي- تست براي دو گروه وابسته و جهت بررسي روند تاوه قطبي از آزمون روند كندال تاو استفاده گرديد. نتايج پژوهش نشان داد كه تاوه قطبي در ماه¬هاي ارديبهشت، خرداد، تير، مرداد، شهريور و اسفندماه، داراي روند كاهشي (منفي) در سطح معني¬داري 01/0 است. البته علل كاهش سطح تاوه قطبي در نيمه سرد سال و به‌ويژه در بهمن¬ماه و اسفندماه نيز قابل تأمل است. همچنين در تمام ماه¬هاي مورد بررسي، سطح تاوه قطبي كاهش يافته و سير نزولي داشته كه عموماً اين سير كاهشي، در نيمۀ گرم سال بيشتر از نيمه¬سرد سال است. اين تغييرات باعث ناهنجاري در الگوهاي آب‌وهوايي مناطق مي¬گردد.

Introduction Today, climate change problem is considered as one of the most common scientific subject, even as social and political issues, in fact it is not novelty issue. Principally, because the change and movement are one of the component in macro scale climatic systems, discovering the legitimacy of these systems, which mainly is cyclic, it is can be consider as a tool which can help to predict future behavior of the system. The polar vortex (PV) is defined as a large-scale upper-level cyclonic circulation in the middle and upper troposphere which is centered on the polar region. Measurement undertaken, observational investigation and modeling indicate the oscillation cycles of polar vortex. There are two effective mechanisms for oscillation cycles of polar vortex that could be divided into two categories: internal and external. The internal mechanism is included investigation of oscillation cycles of polar vortex by applying thermal and wavy forcing, which is independent of time and leads to identify the effect of the stratosphere on the troposphere. External mechanism is included investigation of oscillation cycles of polar vortex by applying thermal and wavy forcing, which is depends on time and leads to identify the effect of the troposphere on the stratosphere. Trend changes on the macro scale climatic system, such as polar vortex has a significant role on climate change of the surface. Variability in the atmospheric circulation at the hemispheric-scale can have a direct impact on variations in the surface environmental parameters, such as temperature, precipitation, and pollutant transport. One indicator of the behavior of the hemispheric-scale circulation is the polar vortex. The polar vortex can be used for study the holistic hemispheric-scale sub-tropical circulation because it represents the characteristics of long-wave, upper-level ridge-trough configuration around the entire (northern or southern) hemisphere at a given point in time. The PV is defined as the large-scale upper-level cyclonic circulation in the middle and upper troposphere centered on the polar region. The most fundamental question addressed in this research is whether the northern hemisphere polar vortex (NHPV) exhibits long-term trends in area over the 1948-2007 periods. Therefore, the first part of this study answers the question: How can we characterize the long-term trend of the area of the NHPV? It is hypothesized that the global warming signal would be associated with a reduction in area in the NHPV over time as the cold pool of air over the poles shrinks. 2. Material and Methods In this research, daily mean data of 500 hPa geopotential height were used from the National Center for Environmental Prediction & National Center of Atmospheric Research (NCEP/NCAR) during the period of 1948 to 2007. Their spatial resolution is 2/5 * 2/5 degree with 36 * 144 pixels. The 500 hPa geopotential height (i.e., constant pressure surface) was selected for analysis for several reasons. First, since the 500 hPa level is in the middle of the atmosphere, it represents characteristics of both the lower and upper atmosphere. Also, most steering of mid-latitude systems occurs at the 500 hPa level and geostrophic flow occurs at this level as well. A specific isohypse(contour) is selected to represent the NHPV for each month in 500 hpa level (table 1). Table (1): specific isohypse of polar vortex for each month in 500 hpa level Month Jan Feb Mar Apr May Jun July Aug Sep Oct Nov Dec Specific isohypse(gpm) 5460 5460 5520 5580 5640 5700 5700 5760 5640 5580 5520 5460 Choi & et al. (2009) Monthly polar vortex area is calculated with Grads software. This research utilizes a statistical approach to determine the trend in the northern hemisphere (NH) polar vortex. For comparison of area means, used from Paired-Samples T Test and for the detection of polar vortex area trend used from Kendall's tau trend test in SPSS software. Paired-Samples T Test is one of the statistical parametric tests to compare the mean and variance of the two dependent groups in a region. 3. Results and Discussion By doing the Kendall tau test in SPSS, it was determined a reduction in the polar vortex area in all months. Polar vortex area has a decreasing trend during May, June, July, August, September and March which is significant at 0.01 level. The biggest reduction trend is related to August and lowest to December. The results show that the polar vortex during the whole period is declining, but this decline was higher during summer months. The most of decline in the polar vortex area is observed in August that the rate of about 93,000 square kilometers per year. The decline rate has accelerated in the second half of period. Generally, the decline of polar vortex area caused by macro-scale climate change and planetary scale, That certainly global warming and consequently advancement of subtropical high pressure will be one of the components. The results of Paired-Samples Test showed that the mean difference of polar vortex area is negative for all months and this amount represents a reduction in polar vortex area during thirty years of second period compared to the first period. 4. Conclusion The change in configuration of polar vortex (trough and ridge) and transformation from zonal to meridional as a result of reducing of meridional pressure gradient caused regional climatic anomalies such as the change in the type and amount of precipitation and temperature. Generally, the decline of polar vortex area caused by macro-scale climate change and planetary Scale, that certainly global warming and consequently advancement of subtropical high pressure will be one of the effective components