مطالعه موردی نقش گرمایش ناگهانی پوشن‌سپهر در تغییرات ازن کلی نیمکره شمالی

Introduction
The ozone is produced and destroyed by photochemical reactions between highly energetic ultraviolet photons and some gas species present in the stratosphere, especially the oxygen. Ozone was identified in the atmosphere by Schonbein (1867) and Chapman (1930) formulated the first set of chemical reactions in an attempt to explain the existence of an ozone vertical structure (Madhu, 2016). Ozone formation starts when a highly energetic photon coming from the sun with wavelength shorter than 242 nm dissociates an oxygen molecule resulting in two atoms of monoatomic oxygen (Langematz, 2019). Then given the high reactivity of atomic oxygen, these atoms quickly react between each other to form ozone. The ozone effectively absorbs the highly energetic ultraviolet radiation. The result of this absorption is the dissociation of ozone in molecular and atomic oxygen for wavelength shorter than 325 nm (Castillon, 2014; Douglass et al, 2014). Ozone is also destroyed through the recombination with atomic oxygen. The set of mechanisms presented above represent the Chapman Cycle. Atmospheric dynamics is known to be a major factor in the variability of stratospheric ozone distribution over the tropics from year to year. There is considerable evidence that the atmospheric total ozone amount is strongly influenced by the stratospheric circulation. Ozone is first formed in the tropical troposphere and then transported to the tropical stratosphere using Brewer-Dobson circulation. This circulation, systematically transports ozone from tropics to the middle and high latitudes (Gerber, 2012). Stratospheric sudden warming (SSWs) is a violent phenomenon in the winter polar region (Kodera, 2006). In a minor warming the temperature gradient reverses over a range of altitude at or below 10hPa and zonal wind at 10hPa is weakens but does not change its direction. When a major sudden stratospheric warming’s events occur, the zonal mean temperature at 10hPa around the polar cap for latitudes north of 60°N suddenly rises and increases by the 25K over period of several days and zonal-mean zonal wind reversal at 10hPa and 60^oN during the winter from November to March (Moradi,1399). In this paper five major SSWs, one minor SSWs and one year without SSWs events were considered to study the variability of total column ozone over the polar cap region.
Materials and methods
In this study we have used the daily mean data from the Modern Era Retrospective Analysis for Research and Applications version 2 (MERRA2) and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) assimilated data. From the MERRA2 data, zonal wind and temperature were obtained at 10hPa and total column ozone from 01 July 1991 to 30 Jun 1992 (1987-1987, 2008-2009, 2010-2011, 2012-20113, 2016-2017 and 2017-2018). The zonal mean temperature (total column ozone) averaged around the polar cap for latitudes north of 60°N (63°N). This is a good measure of the overall temperature and total column ozone content in the polar vortex. The average east-west (zonal) wind speed for 60°N. This is near the peak of the polar jet maximum. The study region covers 0.0° to 357.5° geographical longitudes and 60°N to 90°N geographical latitudes. From the NCEP/NCAR data, zonal wind and temperature daily were presented by a horizontal resolution of 2.5° ×2.5° at 10hPa in region study at different case studies and averaged from 0.0 to 357.5^o geographical longitudes around the 60°N to 90°N geographical latitudes. To study the distribution of total column ozone during the sudden stratospheric warming events, five major SSWs, one minor SSWs and one year without SSWs events were identified (Table 1). To represent the total column ozone variations during the sudden stratospheric warming; the daily total column ozone in this cases are analyzed.
 
Table 1. Characteristics of sudden stratospheric warming




Row


Year


tYPE


Duration (days)


growth
(day)


maturation
(day)


decline (day)






1


2010-2011


Without  SSWs


-


-


-


-




2


1991-1992


Minor


18


-


-


-




3


1986-1987


Major


66


37


17


12




4


2016-2017


12


11


0


1




5


2012-2013


29


6


12


10




6


2017-2018


29


12


2


15




7


2008-2009


46


16


4


26




 
Results and discussion
The results showed that the during the five major SSWs events (one minor SSWs and one year without SSWs evens), total ozone column over polar cap region is increases (decrease) and the anomaly of this quantity is always positive (negative) compared to the long-term average. Furthermore, in during major SSWs events there observed an increase of 29-70 DU in total ozone column from the average value over the pole cap and if the major SSWs is strong total ozone is found to rise by 99-104 DU. The positive anomaly total column ozone in the polar cap is more in growth and maturation of the major SSWs and less in the period of decline. The longer the growth and maturation period and the faster of reduce of the zonal mean zonal flow, the positive anomaly total column ozone is higher. During the period of growth and maturation of major SSWs, the polar vortex and night jet are rapidly weakened, and total column ozone is better transferred from the equatorial stratosphere to the polar cap by the Brewer-Dobson circulation. During the major SSWs that warming of stratosphere, diabetic cooling on the Brewer-Dobson extra tropical down welling branch also weakens. This mechanism also increases the ozone of the polar cap by reducing the rate of ozone depletion.
Conclusion
In this study, the variation of polar cap total column ozone during one minor SSWs case (1991-1992), five major SSWs cases (1987-1987, 2008-2009, 2012-20113, 2016-2017, 2017-2018) and one year without SSWs events (2010-2011) was analyzed over the polar cap region. The results showed that the during the five major SSWs events (one minor SSWs and one year without SSWs evens), total ozone column over polar cap region is increases (decrease) and the anomaly of this quantity is always positive (negative) compared to the long-term average. During the major SSWs that warming of stratosphere, diabetic cooling on the Brewer-Dobson extra tropical down welling branch also weakens. This mechanism also increases the ozone of the polar cap by reducing the rate of ozone depletion.