Impact Of Arctic Sea Ice Changes On The Stratospheric Polar Vortex And Their Mechanism

With the global warming,the temperature in the Arctic is increasing at a rate nearly4 times as fast as the global average temperature increase rate and the sea ice in the Arctic is melting rapidly in these years.The sea ice is one of the most important atmospheric general circulation forcing factor on the underlying surface,the anomaly of the sea ice is related to the characteristics of the stratospheric polar vortex closely,such as the strength and location shift.It is of great significant to study the influence of the Arctic sea ice changes on the stratospheric polar vortex to understand the weather and climate effects of stratospheric polar vortex anomalies well.The sea ice concentration is from the National Snow and Ice Data Center(NSIDC)and the ERA5 reanalysis data is maily used in this thesis.In the past 30 years(during 1990 to 2019)the spatial and temperal variation of the sea ice concentration are analyed.And the physical process of the influence of water vapor on the changes of the Arctic sea ice in autumn is analyed,the sources of the water vapor in the most significant sea ice change areas are indentified quantitatively.On this basis,this study also analyzes the process of the Arctic sea ice changes regulate the stratospheric and tropospheric dynamic coupling according to influence the tropospheric system,and then have a significant effect on the stratospheric polar vortex strength,location shift and the breakdown time.To be specific,the effect of the North Atlantic blocking anomaly on the stratospheric polar vortex strength caused by Arctic sea ice changes is analyzed and their interaction mechanism is discussed.And it is reveals that the Arctic sea ice changes can lead to the significant temperature increasing in the local troposphere and then cause wave fluxes anomaly which can give rise to the stratospheric polar vortex position shift.In addition,from the diagnostic analysis of the energy budget balance equation,the low frequency response process of tropospheric polar vortex to Arctic sea ice changes is revealed and then the influence of the low frequence anomaly signal of tropospheric polar vortex on the breakdown time of the strapospheric polar vortex is analyzed.The main conclusions of this thesis can be summarized as follows:1.The surface air temperature increases which caused by the increase of Arctic water vapor among the atmosphere in autumn,this process can strengthen the Arctic Amplification and promotes the rapid melting of Arctic sea ice.The water vapor in the most significant sea ice decreasing areas mainly comes from the mid-latitude ocean.The sea ice concentration(SIC)shows a decreasing trend in each month and the SIC is decrease most sinifictantly in the autumn when the SIC is the lowest from 1990 to 2019.The Barents Sea and Kara Sea(B-K),East Siberian Sea and Chukchi Sea(ES-C)have the largest standard deviation and linear trend of SIC.By using AIRS satellite observation data,ERA5 reanalysis data and Sixth International Coupled Model Comparison Programme(CMIP6)model data,they all show that the water vapor increases significantly and the downward long-wave radiation of surface is more sensitive to the change of water vapor in autumn.The increase of water vapor in the Arctic in autumn strengthens the greenhouse effect,causing the strengthening of the downward long-wave radiation of surface which can then increase the surface air temperature.Moreover,the water vapor warming in autumn played an important role in the amplification of Arctic temperature,because of the water vapor warming rate in the Arctic region at 0.86 ℃/10 a.Arctic warming caused by water vapor can further reduce Arctic sea ice,with the effect lasting 2 months.The quantitative analysis of atmospheric water vapor in the areas with the most significant variability of Arctic sea ice(B-K Seas and ES-C Seas)shows that the main water vapor source is water vapor flux transport from the mid-latitude ocean.Water vapor in the North Atlantic transports into the B-K Seas through the Norwegian Channel,and water vapor over the Northeast Atlantic transports into the ES-C Seas,and local evaporation makes a small contribution to water vapor in atmosphere.In the past 30 years,both external water vapor transport and local evaporation are increasing that increase water vapor in the B-K and ES-C resulting in more temperature increasing,which forms a positive feedback process between increase water vapor and decreased sea ice concentration.2.The decrease of Arctic sea ice in autumn increases the cumulative days of the North Atlantic blocking activity,which reduces the strength of the stratospheric polar vortex in winter by regulating the vertical propagation of planetary waves.There is a significant positive correlation between SIC in autumn and the stratospheric polar vortex strength in winter,and the positive correlation between SIC and the stratospheric polar vortex strength in February is the most significant.With the influence of the decrease of Arctic sea ice in autumn,the geopotential height in the North Atlantic region of the troposphere increased significantly in winter,and the cumulative days of blocking occurrence in this region increased significantly from December to January during 1990 to 2019,which lead to the positive anomaly of the stratospheric geopotential height over the Arctic,as the weakening of the stratospheric polar vortex strength.The results show that when the SIC anomaly is less in autumn,the vertical upward planetary waves are strengthened in January and February of the next year,and the strength of the stratospheric polar vortex decreases the most significantly in February.Compared with the Arctic sea ice less years,in the less sea ice and the high frequency North Atlantic blocking years,the upward propagation of planetary waves is more stonge in January and February,and the uploading of 1 wave is dominant,while the uploading of 2 wave is strengthened in December,and the strength of stratospheric polar vortex is weakened more obviously.This means that,under the background of decreasing Arctic sea ice,the cumulative days of blocking activity in the North Atlantic increase,which further strengthens the weakening effect of Arctic sea ice anomaly in autumn on the strength of stratospheric polar vortex in winter,and further weakens the strength of stratospheric polar vortex.3.The decrease of sea ice concentration in the Barents Sea,Cara Sea and Laptev Sea in autumn will lead to the increase of local tropospheric temperature,which will increase the vertical wave flux and the horizontal wave flux propagating from the Arctic to Asia,so as to shift the position of the stratospheric polar vortex to Asia in winter.The changes in the boundary,center and relative area of the winter stratospheric polar vortex indicate that the winter stratospheric polar vortex shifted to Asia and away from North America during 1990-2019,and shifted to Europe during 1990-2009 and away from Europe during 2010-2019.The sea ice changes in Barents Kara Sea and Laptev Sea(BK-L)during September to November were most significantly correlated with the shift of the stratospheric polar vortex to Asia.Reanalysis data and numerical model simulation results show that the reduction of sea ice density in autumn weakens the stability of tropospheric atmospheric over the Arctic region in winter,and the corresponding vertical wave flux is strengthened.Moreover,the horizontal wave flux anomaly transmitted southeast from the B-K-L sea to Asia is generated,and the wave energy over Asia is concentrated,which lead position of the winter stratospheric polar vortex is shifted to Asia.In addition,the decrease of B-K-L sea ice in autumn increases the geopotential height over the Arctic and decreases the geopotential height over Asia in winter.The meridional gradient of atmospheric meridional thickness between high and low latitudes is weakened,and the meridional circulation in middle and high latitudes is weakened while the meridional circulation is strengthened.The strengthened meridional circulation is conducive to the further southward propagation of wave flux,and the weak westerly winds are conducive to the vertical upward propagation of waves.The adjustment of circulation field is conducive to the propagation of the wave from B-K-L sea to Asia,and the shift of the stratospheric polar vortex to Asia.When the stratospheric polar vortex shift towards Asia,the frequency and intensity of extreme cold events increase in the north of Asia and the temperature is low,while the frequency of extreme cold events decreases in North America and the temperature is high.4.The decrease of Arctic sea ice caused the decrease of energy conversion of tropospheric polar vortex from mean flow field and high frequency disturbance field to low frequency disturbance field,and the low frequency signal of tropospheric polar vortex anomaly inhibited the transmission of planetary wave 1 from January to February.This dynamic process inhibited the premature breakdown time of the stratospheric polar vortex caused by planetary waves increased by the sea ice decrease.In the troposphere,the polar vortex is weak in intensity and large in acreage,and it exists all year round,which is strongest in winter and weakest in summer.In the stratosphere,the polar vortex is strong and small in acreage.It weakens to breakdown in spring,which corresponds to the westerly wind in the stratosphere turning to easterly wind.In summer,the stratosphere is controlled by anticyclone.During the seasonal evolution of tropospheric polar vortex,the low frequency energy of tropospheric polar vortex was obtained in autumn,and the intensity of tropospheric polar vortex was enhanced.In winter,the low frequency energy reached the maximum,and the intensity of tropospheric polar vortex was also the strongest.In the following spring,the low frequency energy of tropospheric polar vortex was released,and the tropospheric polar vortex was weakened.The results of the diagnostic analysis of the energy budget balance equation show that,compared with the period with more Arctic sea ice(1990-2004),both the mean flow field transformed to the low frequency kinetic energy of tropospheric polar vortex and the mean flow field transformed to the low frequency potential energy is weaker than that in the period with less Arctic sea ice(2005-2019).At the same time,the conversion of high frequency disturbance energy to low frequency disturbance energy at synoptic scale is also reduced,which inhibits the maintenance and development of tropospheric polar vortex.In other words,the decrease of Arctic sea ice weakens the low frequency signal of tropospheric polar vortex.From 1990 to2019,the breakdown time of the stratospheric polar vortex tended to delay,the breakdown time of the stratospheric polar vortex delayed by 4 days.When the Arctic sea ice is less,the low frequency signal of tropospheric polar vortex is weakened in January and February,which inhibits the transmission of planetary wave 1 to the stratosphere,which is not conducive to the transformation of the stratosphere from winter type to summer type,and the dynamic effect of the underlying Arctic sea ice anomalies was one of the reasons for the delay.with less Arctic sea ice compared with the period with more Arctic sea ice.