واکاوی روند تاوۀ قطبی در نیمکرۀ شمالی تحت شرایط تغییر اقلیم

تغییر روند سامانه های کلان مقیاس اقلیمی همچون تاوه قطبی، نقش بسزایی در تغییر اقلیم سطح زمین دارد. در این پژوهش برای نیل به این هدف، از داده های دوباره واکاوی شده ارتفاع ژئوپتانسیل تراز میانی جو از مرکز ملی جو و اقیانوس شناسی ایالات‌متحده آمریکا استفاده شده است. این داده ها دارای تفکیک مکانی 2/5×2/5 درجه قوسی و به صورت میانگین روزانه برداشت گردیده است. دوره آماری این پژوهش از سال 1948 تا 2007 میلادی برای نیمکره شمالی بوده و شامل 144×36 یاخته است. برای مقایسه میانگین ها، از آزمون تی-تست برای دو گروه وابسته و جهت بررسی روند تاوه قطبی از آزمون روند کندال تاو استفاده گردید. نتایج پژوهش نشان داد که تاوه قطبی در ماه های اردیبهشت، خرداد، تیر، مرداد، شهریور و اسفندماه، دارای روند کاهشی (منفی) در سطح معنی داری 0/01 است. البته علل کاهش سطح تاوه قطبی در نیمه سرد سال و به‌ویژه در بهمن ماه و اسفندماه نیز قابل تامل است. همچنین در تمام ماه های مورد بررسی، سطح تاوه قطبی کاهش یافته و سیر نزولی داشته که عموماً این سیر کاهشی، در نیمۀ گرم سال بیشتر از نیمه سرد سال است. این تغییرات باعث ناهنجاری در الگوهای آب‌وهوایی مناطق می گردد.

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

1. IntroductionToday, 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 largescale upperlevel 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 hemisphericscale 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 hemisphericscale circulation is the polar vortex. The polar vortex can be used for study the holistic hemisphericscale subtropical circulation because it represents the characteristics of longwave, upperlevel ridgetrough configuration around the entire (northern or southern) hemisphere at a given point in time. The PV is defined as the largescale upperlevel 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 longterm trends in area over the 19482007 periods. Therefore, the first part of this study answers the question: How can we characterize the longterm 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 MethodsIn 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 midlatitude 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 levelMonthJanFebMarAprMayJunJulyAugSepOctNovDecSpecific isohypse(gpm)546054605520558056405700570057605640558055205460Choi & 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 PairedSamples T Test and for the detection of polar vortex area trend used from Kendall 's tau trend test in SPSS software. PairedSamples 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 DiscussionBy 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 macroscale 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 PairedSamples 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. ConclusionThe 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 macroscale climate change and planetary Scale, that certainly global warming and consequently advancement of subtropical high pressure will be one of the effective components.