چکیده:
دراین پژوهش، گرمایش ناگهانی پوشنسپهر، با استفاده از دادههای باز تحلیلی NCEP/NCAR، در دوره آماری2020-1948 مورد بررسی قرار گرفت. نتایج تحلیل نشان داد که فراوانی رخداد گرمایش ناگهانی پوشنسپهر، در ماه فوریه با 17 درصد، بیش از سایر ماهها میباشد. پس از محاسبهی شدت گرمایشهای آشکار شده، مشخص شد که درگرمایش 2018-2017، میانگین مولفهی مداری باد به 48- متر بر ثانیه رسید و مقادیر منفی این کمیت20 روز ادامه داشته است؛ این گرمایش به عنوان شدیدترین گرمایش ناگهانی پوشنسپهر در دورهی آماری مورد مطالعه شناسایی شده است. میزان همبستگی بین تغییرات مؤلفهی مداری باد با زمان شروع گرمایش پایانی در تمام سالهای تحت بررسی 6/0- میباشد و بدین معناست که هرچه انحراف معیار دادههای مؤلفهی مداری باد بیشتر باشد، پایان فصل سرد و گرمایش پایانی زودتر فرا میرسد. میزان همبستگی فاصلهی دو گرمایش زمستانه و گرمایش پایانی با شدت گرمایش اصلی 8/0- میباشد و نشان دهندهی ارتباط قوی و معکوس بین این دو پارامتر می باشد و نشان میدهد هر چه گرمایش پوشنسپهر زمستانه (اصلی) شدیدتر باشد، گرمایش پایانی زودتر رخ میدهد و فاصلهی دو گرمایش اصلی و پایانی کمتر میشود.
IntroductionIn this research, sudden stratospheric warming has been investigated. The stratosphere, along with the troposphere, plays an active role in determining the climate of the Earth's surface. Sudden stratospheric warming refers to a large-scale winter disturbance that significantly affects the temperature and circulation in the middle atmosphere. One of the goals of this research is to investigate and discover the relationship between the changes in the speed of the zonal component of the wind and the occurrence of the two sudden winter major warming and the final warming.MethodologyIn order to further understand the phenomenon of sudden stratospheric warming, the average zonal wind component at the pressure level of 10 hPa on the longitude of 60° N was investigated for a 73-year period (1948-2020). NCEP/NCAR reanalysis data have been used to reveal this phenomenon. And from the zonal component of the wind at the level of 10 hPa on the 60° N, from zero to 360 degrees, which has 144 points with a spatial resolution of 2.5 degrees, a zonal average was taken; then in each month of the year for the number of years, the average zonal component of the wind has been averaged (measured) again. This study has been limited to the occurrence of this phenomenon only in the cold period of the year; it is because this phenomenon occurs only in cold seasons. The calculations were done by using Excel and MATLAB software. The criterion for detecting sudden stratospheric warming is the negative value of the average zonal component of the wind, and its intensity is considered based on the amount of this component going below zero and the number of consecutive days when this quantity has negative values. In order for the sudden warming of the stratosphere to occur, the speed of the zonal component of the wind must decrease and, as a result, the temperature must increase. Moreover, in the warming of March, the researcher did her best to allocate a time gap of at least 20 days with the final warming. Pearson's correlation test has been used to correlate the changes in the wind component with the final warming event.Results and DiscussionInvestigating and understanding the changes in the zonal wind speed will shed light on many factors. The speed of the zonal component of the wind is not the same throughout the year, but due to the changes in the angle of the sun's rays, this component also changes. The highest speeds of this component are in January with 44.05 m/s and in December with 39.94 m/s. The average speed of the zonal component of the wind is 21.29 m/s in October, 33.91 m/s in November, 36.31 m/s in February, and 23.34 m/s in March. In April, the speed of the currents is greatly reduced, and in some years, in the second half of the month, the wind currents blow in an eastward direction. The overall average wind speed in this month is 5.27. From May onwards, wind currents blow to the east; in other words, they become negative. And this means that the warm season has begun and the final warming has occurred. The duration of the effect of wind speed changes on the amount of temperature changes was obtained by running Pearson correlation calculations between these two components. In addition to simultaneous correlation, the delayed correlation was also used. The degree of simultaneous correlation was measured to be -0.73 and has been the most related to the applied delays. These calculations show that the effects of reducing the speed and changing the direction of the wind, at the same time, have the greatest effect on increasing the temperature, and this effect decreases as the time interval increases. After examining and extracting warming that occurred, by drawing a two-line graph for the two components of wind and temperature for all the years under investigation, the intensity of this phenomenon was also investigated.Identification of 36 sudden stratospheric warmings of the major type and determination of their intensity was carried out. The intensity of this phenomenon is different in each occurrence. The maximum drop in wind speed, along with the number of days below zero, is the criterion for measuring the intensity of sudden stratospheric warming. In the year between 2017 and 2018, the most severe warming occurred with the negative direction of the zonal wind at the rate of -48.8 and remaining in a negative state for 20 days. The most likely occurrence of a sudden warming of the major type is related to February. The intensity of warming that occurred in each month shows a direct relationship with the amount of zonal wind speed. January ranks first in terms of warming intensity, with a rate of -20.5 m/s; Seemingly, March has had the slightest warming with a rate of -7.1 m/s. The annual fluctuation in the zonal component of the wind at the beginning of the final warming is a significant value. This relationship was explored by calculating the correlational strength between the standard deviation of the speed of zonal wind and the starting day of the major warming. The degree of this correlation in all years under study (in years both with and without warming) is -0.6 which shows a moderately strong inverse relationship. And it means that the higher the standard deviation of the wind speed, the earlier ending of cold season and the occurrence of final warming. The commencement of the final warming, or in other words, the end of the cold season in years with sudden warming, was calculated as follows: the correlation between the intensity of sudden stratospheric warming and the interval time of the major warming and the final warming was measured; with a rate of -0.8, this correlation indicates a rather strong but inverse one between the two figures – it also states that the higher the intensity of the sudden warming, the sooner the final warming will occur, and the shorter the time interval of the two warmings will be.ConclusionChanges in the zonal wind speed at the level of 10 hPa on the longitude 60° have a relatively strong relationship with factors such as sudden stratospheric warming, the intensity of sudden warming, the percentage of sudden warming, and the time of final warming. The zonal wind speed in each month depends on the angle of the sun rays. This has caused the possibility of sudden stratospheric warming to be different each month and also affects the intensity of the warming. With a zonal wind speed of 44.05 m/s, January has the most intense warming in terms of polar vortex destruction, which changes the direction of the wind orbit by 20.5 m/s. The degree of correlation between zonal wind changes and the occurrence of final warming indicates a strong but inverse relationship between the two. That is, the higher the standard deviation of the zonal wind speed, the sooner the final warming will arrive. Finally, the relationship between the two winter major warmings and the final warming can be expressed as: in years when sudden stratospheric warming occurs, the greater the intensity of the warming is, the earlier the final warming occurs.