Global trends of tropopause folds in recent decades

对流层顶折卷是平流层-对流层物质交换的主要机制.本研究采用三维标记方法,利用1979-2020年间的ERA5再分析数据,分析了对流层顶折卷的时空变化特征及机理.研究结果表明,全球对流层顶折卷的多年变化趋势存在明显的季节性和区域性差异.总体上,对流层顶折卷呈增加趋势,其中北半球的春季,夏季和冬季,对流层顶折卷主要沿副热带急流区呈现明显增加的趋势.进一步分析表明,对流层顶折卷发生频率的增加与大气斜压性增大导致的锋生增强有关,而经向位温梯度的增大使得次级环流增强,也可能促进了对流层顶折卷活动的增多.

Role of Stratospheric Processes in Climate Change: Advances and Challenges

In this review,instead of summarizing all the advances and progress achieved in stratospheric research,the main advances and new developments in stratosphere-troposphere coupling and stratospheric chemistry-climate interactions are summarized,and some outstanding issues and grand challenges are discussed.A consensus has been reached that the stratospheric state is an important source of improving the predictability of the troposphere on sub-seasonal to seasonal(S2S)time scales and beyond.However,applying stratospheric signals in operational S2S forecast models remains a challenge because of model deficiencies and the complexities of the underlying mechanisms of stratosphere-troposphere coupling.Stratospheric chemistry,which controls the magnitude and distribution of many important climate-forcing agents,plays a critical role in global climate change.Convincing evidence has been found that stratospheric ozone depletion and recovery have caused significant tropospheric climate changes,and more recent studies have revealed that stratospheric ozone variations can even exert an impact on SSTs and sea ice.The climatic impacts of stratospheric aerosols and water vapor are also important.Although their quantitative contributions to radiative forcing have been reasonably well quantified,there still exist large uncertainties in their long-term impacts on climate.The advances and new levels of understanding presented in this review suggest that whole-atmosphere interactions need to be considered in future for a better and more thorough understanding of stratosphere-troposphere coupling and its role in climate change.

The Influence of Meridional Variation in North Pacific Sea Surface Temperature Anomalies on the Arctic Stratospheric Polar Vortex

This study examines the dependence of Arctic stratospheric polar vortex(SPV)variations on the meridional positions of the sea surface temperature(SST)anomalies associated with the first leading mode of North Pacific SST.The principal component 1(PC1)of the first leading mode is obtained by empirical orthogonal function decomposition.Reanalysis data,numerical experiments,and CMIP5 model outputs all suggest that the PC1 events(positive-minus-negative PC1 events),located relatively northward(i.e.,North PC1 events),more easily weaken the Arctic SPV compared to the PC1 events located relatively southward(i.e.,South PC1 events).The analysis indicates that the North PC1-related Aleutian low anomaly is located over the northern North Pacific and thus enhances the climatological trough,which strengthens the planetary-scale wave 1 at mid-to-high latitudes and thereby weakens the SPV.The weakened stratospheric circulation further extends into the troposphere and favors negative surface temperature anomalies over Eurasia.By contrast,the South PC1-related Aleutian low anomaly is located relatively southward,and its constructive interference with the climatological trough is less efficient at high latitudes.Thus,the South PC1 events could not induce an evident enhancement of the planetary-scale waves at high latitudes and thereby a weakening of the SPV on average.The Eurasian cooling associated with South PC1 events(positive-minus-negative PC1 events)is also not prominent.The results of this study suggest that the meridional positions of the PC1 events may be useful for predicting the Arctic SPV and Eurasian surface temperature variations.

Different Impact of Central Pacific and Eastern Pacific El Nio on the Duration of Sudden Stratospheric Warming

The NCEP-NCAR reanalysis dataset and the HadlSST dataset （1959-2014） are used to analyze the impact of two types of E1 Nino events, i.e., eastern Pacific E1 Nino （EP-E1 Nino） and central Pacific E1 Nino （CP-E1 Nino） events, on the duration of major and minor sudden stratospheric warmings （SSWs） in Northern Hemisphere winter （November to February）. Al- though the frequency of major and minor SSWs during different types of E1 Nino shows no distinct differences, the duration of both major and minor SSWs during CP-E1 Nino is shorter than that during EP-E1 Nino. The spatial distribution of geopo- tential height anomalies preceding major SSWs resembles the western Pacific （WP） teleconnection pattern, while the spatial distribution of geopotential height anomalies preceding minor SSWs bears similarity to the Pacific-North America （PNA） teleconnection pattern. An enhancement of the strength of both wavenumber 1 and wavenumber 2 is found before major SSWs. Before minor SSWs, wavenumber 1 is also strengthened, but wavenumber 2 is weakened. The analysis also reveals that EP-E1 Nino tends to induce positive phases of PNA and WP teleconnections, while CP-E1 Nino induces negative-phase WP teleconnection. As the positive phases of the PNA and WP teleconnections are related to the strengthening of wavenum- bet 1, EP-E1 Nino causes an enhancement of wavenumber 1 in the high-latitude upper troposphere and an enhancement of the upward wave flux in the high-latitude stratosphere, accompanied by a negative anomaly in Eliassen-Palm flux divergence in the subpolar stratosphere, which accounts for the longer SSW duration during EP-E1 Nino than during CP-E1 Nino.

Record Arctic Ozone Loss in Spring 2020 is Likely Caused by North Pacific Warm Sea Surface Temperature Anomalies

Record ozone loss was observed in the Arctic stratosphere in spring 2020.This study aims to determine what caused the extreme Arctic ozone loss.Observations and simulation results are examined in order to show that the extreme Arctic ozone loss was likely caused by record-high sea surface temperatures(SSTs)in the North Pacific.It is found that the record Arctic ozone loss was associated with the extremely cold and persistent stratospheric polar vortex over February-April,and the extremely cold vortex was a result of anomalously weak planetary wave activity.Further analysis reveals that the weak wave activity can be traced to anomalously warm SSTs in the North Pacific.Both observations and simulations show that warm SST anomalies in the North Pacific could have caused the weakening of wavenumber-1 wave activity,colder Arctic vortex,and lower Arctic ozone.These results suggest that for the present-day level of ozone-depleting substances,severe Arctic ozone loss could form again,as long as certain dynamic conditions are satisfied.

Treatment of LBCs in 2D Simulation of Convection over Hills

A series of idealized model simulations are analyzed to determine the sensitivity of model results to different configurations of the lateral boundary conditions (LBCs) in simulating mesoscale shallow convection over hilly terrain. In the simulations with steady thermal forcing at the model surface, a radiation condition at both boundaries is the best choice under high wind conditions, and the best results are produced when both the normal velocities and the temperature are treated with the radiation scheme in which the phase speed is the same for different variables. When the background wind speed is reasonably small, the LBC configuration with either the radiation or the zero gradient condition at both boundaries tends to make the numerical solution unstable. The choice of a constant condition at the inflow boundary and a radiation outflow boundary condition is appropriate in most cases. In the simulations with diurnal thermal forcing at the model surface, different LBC schemes are combined together to reduce spurious signals induced by the outflow boundary. A specification inflow boundary condition, in which the velocity fields at the inflow boundary are provided using the time-dependent results of a simulation with periodic LBCs over a flat domain, is tested and the results indicate that the specification condition at the inflow boundary makes it possible to use a smaller model domain to obtain reasonable results. The model horizontal domain length should be greater than a critical length, which depends on the domain depth H and the angle between gravity wave phase lines and the vertical. An estimate of minimum domain length is given by , where N and U are the background stability and wind speed, respectively, Lx is the typical gravity wavelength scale, and Zi is the convective boundary layer (CBL) depth.

Tropical Stratospheric Forcings Weaken the Response of the East Asian Winter Temperature to ENSO

The El Niño-Southern Oscillation(ENSO)plays a critical role in predicting the winter surface temperature over East Asia.Numerous studies have attempted to improve the seasonal forecasting skill in view of the combined effects of ENSO and oceanic-tropospheric factors.However,high uncertainty and notable challenges still exist in using the ENSO to predict the surface temperature.Here,we showed that tropical stratospheric forcings(Quasi-Biennial Oscillation,QBO)could disrupt the response of the surface temperature to ENSO.The response of the East Asian surface temperature to El Niño/La Niña events evidently weakened in winter during the westerly/easterly phase of the QBO.This disruption has shown an increasing trend in recent decades,limiting the usefulness of ENSO alone as a seasonal predictor of the surface temperature.The modulation of the QBO on the East Asian surface temperature is achieved mainly by affecting subtropical zonal winds and North Pacific wave activity.Our analyses suggest that the QBO is a nonnegligible predictor in improving seasonal forecasts of the East Asian surface temperature,even comparable to the ENSO.

Zonally Asymmetric Temperature Trends near the Northern Middle and High Latitude Stratopause during Winter

The temperature trend near the stratopause is rarely evaluated owing to the limited long-term observations of global temperature. In this study, the spatial patterns of the temperature trends near the northern stratopause are investigated by using satellite and reanalysis datasets. Our analysis reveals a zonally asymmetric temperature trend pattern near the northern mid-to-high latitude stratopause during January, and this pattern underwent an evident transition around the 2000s. From 1980 to 2003, there was a cooling trend in the Western Hemisphere and a warming trend in the Eastern Hemisphere. In contrast, a reversed zonally asymmetric temperature trend pattern existed in the east–west direction from 2003 to 2020. Although the warming trends are statistically insignificant, they contrasted with the overall cooling trend in the upper stratosphere due to ozone depletion and an increase in well-mixed greenhouse gases in recent decades. The zonally asymmetric temperature trends were induced by the transition in the intensity of quasi-stationary planetary wavenumber 1(wave 1) near the stratopause. The increasing(decreasing) trend of the intensity of wave 1 enhanced(weakened) its meridional temperature advection near the stratopause before(after) the 2000s;consequently, a zonally asymmetric temperature trend pattern exists in the east–west direction near the stratopause. The transition in the intensity of the stratospheric wave 1 around the 2000s is most likely caused by the transition in the intensity of wave 1 activity in the troposphere.

Responses of Arctic sea ice to stratospheric ozone depletion

The Arctic has experienced several extreme springtime stratospheric ozone depletion events over the past four decades,particularly in 1997,2011 and 2020.However,the impact of this stratospheric ozone depletion on the climate system remains poorly understood.Here we show that the stratospheric ozone depletion causes significant reductions in the sea ice concentration(SIC)and the sea ice thickness(SIT)over the Kara Sea,Laptev Sea and East Siberian Sea from spring to summer.This is partially caused by enhanced ice transport from Barents-Kara Sea and East Siberian Sea to the Fram Strait,which is induced by a strengthened and longer lived polar vortex associated with stratospheric ozone depletion.Additionally,cloud longwave radiation and surface albedo feedbacks enhance the melting of Arctic sea ice,particularly along the coast of the Eurasian continent.This study highlights the need for realistic representation of stratosphere-troposphere interactions in order to accurately predict Arctic sea ice loss.

Comparisons of AGRI/FY-4A Cloud Fraction and Cloud Top Pressure with MODIS/Terra Measurements over East Asia

Fengyun-4 A(FY-4 A),the second generation of China’s geostationary meteorological satellite,provides high spatiotemporal resolution cloud products over East Asia.In this study,cloud fraction(CFR)and cloud top pressure(CTP)products in August 2017 derived from the Advanced Geosynchronous Radiation Imager(AGRI)aboard FY-4 A(AGRI/FY-4 A)are retrospectively compared with those from the Moderate Resolution Imaging Spectroradiometer(MODIS)aboard Terra(MODIS/Terra)over East Asia.To avoid possible errors in the comparison caused by the lower temporal coverage of MODIS/Terra products compared to that of AGRI/FY-4 A over the same region and to account for time lags between observations of the two instruments,we construct datasets of AGRI/FY-4 A CFR and CTP to match those of MODIS/Terra in each scan over East Asia in August 2017.Results show that the CFR and CTP datasets of the two instruments generally agree well,with the linear correlation coefficients(R)between CFR(CTP)data of 0.83(0.80)regardless of time lags.Though longer time lags contribute to the worse consistency between CFR(CTP)data derived from observations of the two instruments in most cases,large CFR/CTP discrepancies do not always match with long time lags.Large CFR discrepancies appear in the vicinity of the Tibetan Plateau(TP;28°–45°N,75°–105°E).These differences in the cloud detection by the two instruments largely occur when MODIS/Terra detects clear-sky while AGRI/FY-4 A detects higher values of CFR,and this accounts for 61%of the CFR discrepancy greater than 50%near the TP.In the case of CTP,the largest discrepancies appear in the eastern Iranian Plateau(IP;25°–45°N,60°–80°E),where there are some samples with long time lags(20–35 min)and fewer daily data samples are available for computing monthly means compared to other regions since there are many clearsky data samples there during the study period.

Increasing trend of lightning activity in the South Asia region

Lightning is an important natural source of wildfires and oxynitride,and hence significantly influences ecological systems and atmospheric chemistry.Here,we choose South Asia,an important region for global water reallocation and global climate changes,to examine lightning variations based on the longest existing lightning dataset from the OTD/LIS observations.We identify a clear increase in lightning density in the research region,increasing at a rate of 0.096 fl km^(-2)a^(-1)over the last two decades.Multiple linear regression analysis is adopted to identify the main influencing factors among ten potential thermodynamic or microphysical factors and the crucial areas contributing to the increases in lightning.The surface latent heat flux along the west coast of the Indian subcontinent is the largest contributor,explaining52%of the lightning variance and contributing to a 0.025 fl km^(-2)a^(-1)increase.The sea surface temperature in the Arabian Sea,the convective available potential energy(CAPE)over the northwestern Indian subcontinent,and the wind shear along the northwestern coast also make important contributions to the lightning increase,indicating that the thermodynamic effects overwhelm the microphysical effects on lightning activity over the South Asia region.

A commentary of“Antarctic ozone layer is gradually recovering”in remarkable discoveries from 2020 in Nature

In the mid-1980s,scientists discovered a spring atmospheric ozone hole over Antarctica,revealing the threat of human-made ozone-depleting substances to the atmosphere.The Antarctic ozone hole located 10 to 20 km above sea level,also affects the circulation of the atmosphere in the southern hemisphere,which in turn affects the global climate.One of its most noticeable effects is that the westerly jet in summer begins to move to the poles.The westerly jet is a planetary-scale atmospheric circulation phenomenon;there are several jet zones on the Earth.The 1987 Montreal Protocol and its subsequent amendments banned the production and use of ozone-depleting substances.Therefore,the concentration of ozone-depleting substances in the atmosphere is declining,and the ozone layer has shown preliminary signs of recovery.The study by Banerjee et al.pointed out that ozone hole-related effects on circulation and climate have ceased since the ozone layer began to recover[1].While others had noticed this trend of cessation of ozone hole effects before,Banerjee and others officially attributed it to the impact of the Montreal Protocol for the first time.