Analysis of gravity wave activity during stratospheric sudden warmings in the northern hemisphere

Due to the significant changes they bring to high latitude stratospheric temperature and wind,stratospheric sudden warmings(SSWs)can have an impact on the propagation and energy distribution of gravity waves(GWs).The variation characteristics of GWs during SSWs have always been an important issue.Using temperature data from January to March in 2014−2016,provided by the Constellation Observing System for Meteorology,Ionosphere and Climate(COSMIC)mission,we have analyzed global GW activity at 15−40 km in the Northern Hemisphere during SSW events.During the SSWs that we studied,the stratospheric temperature rose in one or two longitudinal regions in the Northern Hemisphere;the areas affected extended to the east of 90°W.During these SSWs,the potential energy density(E_(p)of GWs expanded and covered a larger range of longitude and altitude,exhibiting an eastward and downward extension.The E_(p)usually increased,while partially filtered by the eastward zonal winds.When zonal winds weakened or turned westward,E_(p)began to strengthen.After SSWs,the E_(p)usually decreased.These observations can serve as a reference for analyzing the interaction mechanism between SSWs and GWs in future work.

The Possible Influence of Stratospheric Sudden Warming on East Asian Weather

By analyzing the linkage of the Northern Annular Mode （NAM） anomaly to the East Asian jet and the East Asian trough during Stratospheric Sudden Warming （SSW）, the influence of SSW on East Asian weather is studied. The results show that the East Asian jet is strengthened and the East Asian trough is deepened during SSW. With the downward propagation of SSW, the strengthened East Asian jet and the East Asian trough would move southward, expand westward and gradually influence the area of north and northeastern China. This implies that the winter monsoon tends to be enhanced over East Asia during SSW.

Sub-seasonal to Seasonal Hindcasts of Stratospheric Sudden Warming by BCC_CSM1.1(m):A Comparison with ECMWF

This study focuses on model predictive skill with respect to stratospheric sudden warming(SSW) events by comparing the hindcast results of BCC_CSM1.1(m) with those of the ECMWF's model under the sub-seasonal to seasonal prediction project of the World Weather Research Program and World Climate Research Program. When the hindcasts are initiated less than two weeks before SSW onset, BCC_CSM and ECMWF show comparable predictive skill in terms of the temporal evolution of the stratospheric circumpolar westerlies and polar temperature up to 30 days after SSW onset. However, with earlier hindcast initialization, the predictive skill of BCC_CSM gradually decreases, and the reproduced maximum circulation anomalies in the hindcasts initiated four weeks before SSW onset replicate only 10% of the circulation anomaly intensities in observations. The earliest successful prediction of the breakdown of the stratospheric polar vortex accompanying SSW onset for BCC_CSM(ECMWF) is the hindcast initiated two(three) weeks earlier. The predictive skills of both models during SSW winters are always higher than that during non-SSW winters, in relation to the successfully captured tropospheric precursors and the associated upward propagation of planetary waves by the model initializations. To narrow the gap in SSW predictive skill between BCC_CSM and ECMWF, ensemble forecasts and error corrections are performed with BCC_CSM. The SSW predictive skill in the ensemble hindcasts and the error corrections are improved compared with the previous control forecasts.

Influence of Major Stratospheric Sudden Warming on the Unprecedented Cold Wave in East Asia in January 2021

An unprecedented cold wave intruded into East Asia in early January 2021 and led to record-breaking or historical extreme low temperatures over vast regions.This study shows that a major stratospheric sudden warming(SSW)event at the beginning of January 2021 exerted an important influence on this cold wave.The major SSW event occurred on 2 January 2021 and subsequently led to the displacement of the stratospheric polar vortex to the East Asian side.Moreover,the SSW event induced the stratospheric warming signal to propagate downward to the mid-to-lower troposphere,which not only enhanced the blocking in the Urals-Siberia region and the negative phase of the Arctic Oscillation,but also shifted the tropospheric polar vortex off the pole.The displaced tropospheric polar vortex,Ural blocking,and another downstream blocking ridge over western North America formed a distinct inverted omega-shaped circulation pattern(IOCP)in the East Asia-North Pacific sector.This IOCP was the most direct and impactful atmospheric pattern causing the cold wave in East Asia.The IOCP triggered a meridional cell with an upward branch in East Asia and a downward branch in Siberia.The meridional cell intensified the Siberian high and low-level northerly winds,which also favored the invasion of the cold wave into East Asia.Hence,the SSW event and tropospheric circulations such as the IOCP,negative phase of Arctic Oscillation,Ural blocking,enhanced Siberian high,and eastward propagation of Rossby wave eventually induced the outbreak of an unprecedented cold wave in East Asia in early January 2021.

Northern Hemisphere Sudden Stratospheric Warming and Its Downward Impact in Four Chinese CMIP6 Models

Using the World Meteorological Organization definition and a threshold-based classification technique,simulations of vortex displacement and split sudden stratospheric warmings(SSWs)are evaluated for four Chinese models(BCC-CSM2-MR,FGOALS-f3-L,FGOALS-g3,and NESM3)from phase 6 of the Coupled Model Intercomparison Project(CMIP6)with the Japanese 55-year reanalysis(JRA-55)as a baseline.Compared with six or seven SSWs in a decade in JRA-55,three models underestimate the SSW frequency by~50%,while NESM3 doubles the SSW frequency.SSWs mainly appear in midwinter in JRA-55,but one-month climate drift is simulated in the models.The composite of splits is stronger than displacements in both the reanalysis and most models due to the longer pulse of positive eddy heat flux before onset of split SSWs.A wavenumber-1-like temperature anomaly pattern(cold Eurasia,warm North America)before onset of displacement SSWs is simulated,but cold anomalies are mainly confined to North America after displacement SSWs.Although the lower tropospheric temperature also displays a wavenumber-1-like pattern before split SSWs,most parts of Eurasia and North America are covered by cold anomalies after split SSWs in JRA-55.The models have different degrees of fidelity for the temperature anomaly pattern before split SSWs,but the wavenumber-2-like temperature anomaly pattern is well simulated after split SSWs.The center of the negative height anomalies in the Pacific sector before SSWs is sensitive to the SSW type in both JRA-55 and the models.A negative North Atlantic Oscillation is simulated after both types of SSWs in the models,although it is only observed for split SSWs.

Observational Subseasonal Variability of the PM_(2.5) Concentration in the Beijing-Tianjin-Hebei Area during the January 2021 Sudden Stratospheric Warming

It is still not well understood if subseasonal variability of the local PM_(2.5) in the Beijing-Tianjin-Hebei(BTH)region is affected by the stratospheric state.Using PM_(2.5) observations and the ERA5 reanalysis,the evolution of the air quality in BTH during the January 2021 sudden stratospheric warming(SSW)is explored.The subseasonal variability of the PM_(2.5) concentration after the SSW onset is evidently enhanced.Stratospheric circumpolar easterly anomalies lasted for 53 days during the January-February 2021 SSW with two evident stratospheric pulses arriving at the ground.During the tropospheric wave weakening period and the intermittent period of dormant stratospheric pulses,the East Asian winter monsoon weakened,anomalous temperature inversion developed in the lower troposphere,anomalous surface southerlies prevailed,atmospheric moisture increased,and the boundary layer top height lowered,all of which favor the accumulation of pollutant particulates,leading to two periods of pollution processes in the BTH region.In the phase of strengthened East Asian winter monsoon around the very beginning of the SSW and another two periods when stratospheric pulses had reached the near surface,opposite-signed circulation patterns and meteorological conditions were observed,which helped to dilute and diffuse air pollutants in the BTH region.As a result,the air quality was excellent during the two periods when the stratospheric pulse had reached the near surface.The increased subseasonal variation of the regional pollutant particulates after the SSW onset highlights the important role of the stratosphere in the regional environment and provides implications for the environmental prediction.

Transient Characteristics of Residual Meridional Circulation during Stratospheric Sudden Warming

The residual meridional circulation derived from the transformed Eulerian-mean thermodynamic equation and continuity equation can be separated into two parts,the slowly varying diabatic circulation and the transient circulation,as demonstrated by others.We calculated and composite-analyzed the transient and diabatic circulation for 14 stratospheric sudden warming(SSW) events from 1979-2002 by using the daily ECMWF reanalysis data.Specifically,the transient residual meridional circulation was calculated both with and without inclusion of the eddy heat transport term in the transformed Eulerian-mean thermodynamic equation to investigate the importance of the eddy heat transport term.The results showed that calculations of transient residual meridional circulation present rapid variations during SSWs,with or without inclusion of the eddy heat transport term.Although the patterns of transient residual meridional circulation with the eddy heat transport term were similar to that without the eddy heat transport term during SSW,the magnitudes in the upper stratosphere and high-latitude regions differed.As for the diabatic circulation,its daily variations were small during SSW events,and its patterns were in agreement with its monthly average.

Comparison of stratospheric evolution during the major sudden stratospheric warming events in 2018 and 2019

Using Modern-Era Retrospective analysis for Research and Applications,Version 2(MERRA-2)data in the northern hemisphere at the 10 hPa level,we compared the stratospheric evolution of temperature and geopotential height during two major sudden stratosphere warming events(SSWs)that occurred in the Arctic winter of 2018 and 2019.In the prewarming period,poleward temperature-enhanced regions were mainly located around 120°E with a displaced vortex and around 120°E and 60°W with splitting vortices.The evolution of geopotential height indicated that these temperature-enhanced regions were both on the western side of high-latitude anticyclones.In the postwarming period,the polar vortex turned from splitting to displacement in the 2018 SSW but from displacement to splitting in the 2019 SSW.Both transitions were observed over the Atlantic region,which may have been caused by anticyclones moving through the polar region.Our findings revealed that the evolution of the anticyclone is important during SSWs and is closely related to temperature-enhanced regions in the prewarming periods and to transitions of the polar vortices in postwarming periods.

The Spatial and Temporal Variability of Global Stratospheric GravityWaves and Their Activity during Sudden StratosphericWarming Revealed by COSMIC Measurements

This study investigates the spatial and temporal variability of global stratospheric gravity waves(GWs) and the characteristics of GW activity during sudden stratospheric warming(SSW) using the GPS radio occultation measurements from the COSMIC mission during September 2006 to May 2013. Corresponding to the COSMIC RO observational window and analysis method,GW potential energy(Ep) with vertical scales no shorter than ～2 km is resolved. It is found that the distributions of GW Ep over 20-30 km and 30-38 km show similar spatial and seasonal variations. The variations of GW Ep with altitude and latitude along the westerly wind are identified in different seasons over 60°-80°W. In the middle and high latitudes,seasonal cycles are distinct in the time-latitude and time-altitude distributions of GW activities,which show larger Ep in winters when westerly wind dominates and smaller Ep in summers when easterly wind dominates. The influence of quasi-biennial oscillation on GW activity is recognized in the tropics. GW Ep enhances closely following the occurrence of minor SSW events; while during major events, GW Ep may not enhance, and sometimes may even weaken,in the regions where reversals of zonal wind occur, probably caused by the filtering impact of the 0 ms^(-1) wind level on the GWs.

Reponses of middle atmospheric circulation to the 2009 major sudden stratospheric warming

In this research, the roles of gravity waves and planetary waves in the change to middle atmospheric residual circulation duringa sudden stratospheric warming period are differentiated and depicted separately by adopting the downward control principle. Ouranalysis shows clear anomalous poleward residual circulation patterns from the equator to high latitudes in the lower winterstratosphere. At the same time, upward mean flows are identified at high latitudes of the winter upper stratosphere and mesosphere,which turn equatorward in the mesosphere and reach as far as the tropical region, and consequently the extratropical region in thesummer hemisphere. The downward control principle shows that anomalous mesospheric residual circulation patterns, includinginterhemispheric coupling, are solely caused by the change in gravity wave forcing resulting from the reversal of the winter stratosphericzonal wind. Nevertheless, both planetary waves and gravity waves are important to variations in the winter stratospheric circulation, butwith opposite effects.

Wintertime Stratospheric Anomalies-Part II: Sudden Warmings

The process of stratospheric sudden warmings from development of planetary waves to.the sudden cooling after reversal of mean zonal circulation will be studied with the primitive equations of heat and momentum balances. It will be explained that the sudden warmings may occur only in the polar regions of winter stratosphere where zonal mean temperature decreases poleward. The heating rate in the order of major warmings is produced by developed planetary waves in the stratospheric breaking layers. The particular perturbation structure characterized by large amplitude of wave 1 together with minimum of wave 2 discovered by Labitzke (1977) is crucial for initiation of major warmings. The cooling by the same mechanism can be produced in the regions with reversed mean temperature gradient.

Verification of Subseasonal-to-Seasonal Forecasts for Major Stratospheric Sudden Warmings in Northern Winter from 1998/99 to 2012/13

This study reports verification results of hindcast data of four systems in the subseasonal-to-seasonal(S2S)prediction project for major stratospheric sudden warmings(MSSWs)in northern winter from 1998/99 to 2012/13.This report deals with average features across all MSSWs,and possible differences between two MSSW types(vortex displacement and split types).Results for the average features show that stratospheric forecast verifications,when further averaged among the four systems,are judged to be successful for lead times around 10 d or shorter.All systems are skillful for lead times around 5 d,whereas the results vary among the systems for longer lead times.A comparison between the MSSW types overall suggests larger forecast errors or lower skill for MSSWs of the vortex split type,although the differences do not have strong statistical significance for almost all cases.This limitation is likely to at least partly reflect the small sample size of the MSSWs available.

Analysis of the variations in the strength and position of stratospheric sudden warming in the past three decades

利用ERA-Interim再分析资料统计分析了1920–2016年期间北半球33次强平流层爆发性增温(SSW)事件和22次弱增温事件的强度和位置变化特征。发现SSW主要发生在二月,其纬向平均温度的最大经向梯度变化在1979–8319年和1998–2011年呈上升趋势,1984–1997年和2012–2016年呈下降趋势,但是整体呈现略微下降的线性趋势,并且强SSW事件的强度和持续时间的变化特征相似。一些SSW事件几乎同时在100 hPa到10 hPa上出现,一些SSW事件却首先出现在10 hPa上再向低层下传。另外,我们还发现这55个SSW事件的最大增温中心主要位于欧亚大陆30°E到120°E之间,这或许跟过去三十年中欧亚大陆的极涡转移有关。

Kinetic and Available Potential Energy Transport during the Stratospheric Sudden Warming in January 2009

The local features of transient kinetic energy and available potential energy were investigated using ECMWF （European Centre for Medium-Range Weather Forecasts） Interim Reanalysis data for the stratospheric sudden warming （SSW） event of January 2009. The Western Europe high plays important roles in the propagation of the energy from North America to Eurasian. When the Western Europe high appeared and shifted eastward, energy conversions increased and energy propagated from North America to Eurasian as a form of interaction energy flow. The baroclinic conversion between transient-eddy kinetic energy （Ke） and transient-eddy available potential energy （Ae） and the horizontal advection of geopotential height were approximately one order of magnitude less than Ke and Ae generation terms. So, these terms were less important to this SSW event.

Variation in Brewer–Dobson Circulation During Three Sudden Stratospheric Major Warming Events in the 2000s

As the strongest subseasonal atmospheric variability during boreal winter, three remarkable sudden stratospheric major warming(SSW) events in the 2000 s are investigated in terms of the Brewer–Dobson circulation(BDC) response. Our study shows that the changes of cross-isentropic velocity during the SSWs are not only confined to the polar region, but also extend to the whole Northern Hemisphere: enhanced descent in the polar region, as well as enhanced ascent in the tropics. When the acceleration of the deep branch of the BDC descends to the middle stratosphere, its strength rapidly decreases over a period of one to two weeks. The acceleration of the deep branch of the BDC is driven by the enhanced planetary wave activity in the mid-to-high-latitude stratosphere. Different from the rapid response of the deep branch of the BDC, tropical upwelling in the lower stratosphere accelerates up to 20%–40% compared with the climatology, 20–30 days after the onset of the SSWs,and the acceleration lasts for one to three months. The enhancement of tropical upwelling is associated with the large-scale wave-breaking in the subtropics interacting with the midlatitude and tropical Quasi-Biennial Oscillation–related mean flow.

Investigating the Apparent Link between Cosmic Ray Muon Flux, Sudden Stratospheric Warming and Dry Season Rainfall over Jamaica

The Jamaican primary dry season extends from November-April with the driest period being January-March each year. Examination of the rainfall records over several decades reveal months that experienced a sharp increase in rainfall while sometimes they are drier than normal. During this dry season, some of the weather systems that impact the island significantly are cold fronts, highs pressure systems and troughs that migrate from the mainland USA. Major Sudden Stratospheric Warmings (SSW) occur routinely north of 60°N and severely impact weather over the North American continent. The islands of the Greater Antilles also experience inclement weather during this period as weather systems migrate southwards and impact the Caribbean. Severe winter weather due to the increase frequency and intensity of storms related to SSW events over North America is important to the Jamaican economy as tourists escape harsh winters by travelling to the island. Predicting the variability of rainfall during the primary dry season is therefore important as it has significant implications for event planning, tourism and agriculture. Cosmic-ray Muon flux has been found to be positively correlated with the atmospheric effective temperature used to indicate the occurrence of SSW events. Current research investigates the relationship between cosmic-ray Muon flux, SSW events and primary dry season rainfall. Our findings suggest that a significant change in rainfall occur over Jamaica during the primary dry season on average 29 days after the central date or on average 15 days after the SSW events end. Our study also suggests apparent similarities in the behaviour of cosmic-ray Muon flux over Jamaica to those when major SSW occurs at high latitudes. We therefore argue that the Muon detectors at low latitudes have practical use with respect to identifying SSWs and merits further study.

Effects of Sudden Stratospheric Warming Events on the Distribution of Total Column Ozone over Polar and Middle Latitude Regions

In winter the polar stratosphere is extremely cold. During the Sudden Stratospheric Warming events, the polar stratospheric temperature rises concurrently zonal-mean zonal flow weakens over a short period of time. As the zonal flow weakens, the stratospheric circulation becomes highly asymmetrical and the stratospheric polar vortex is displaced off the pole. The polar stratospheric temperature rises by 50°C and the stratospheric circumpolar flow reverses direction in a span of just few days. Sudden Stratospheric Warming (SSW) leads to significant changes in the rate of several chemical reactions which occur in the polar stratosphere. During such events, the dynamical fields in the polar stratosphere completely altered and columnar ozone changed. This study concentrated on the variability of winter polar vortex, meridional temperature gradient and associated changes in the Total Column Ozone (TCO) over the polar and middle latitude regions. It is found that changes in the amount of column ozone are positively correlated with polar lower stratospheric temperature with colder (warmer) temperature correlating with less (high) amount column ozone. But in the middle latitude region we observed negative correlations between ozone concentration and stratospheric temperature. In almost all cases there is sudden increase of ozone concentration over the pole and after few days the value is reduced when the warming effect is weak. During SSW events there observed an increase of 30 DU in TCO from the average value over the pole and if the SSW is strong TCO is found to rise by 50 DU. But in the middle latitude approximately 10 DU increase is noted. From the above results it may be concluded that variability of column ozone depends on dynamic and stratospheric chemistry over the poles and in middle latitude the variability can be attributed to the dynamical aspects. Anomaly of column ozone is higher during sudden stratospheric warming events over both polar and middle latitude region. The meridional temperature gradient reverses first and after two days polar vortex changes its direction or weakens followed by an increase of column ozone over the polar region. An increase of 30° Kelvin in the average temperature value noted over the polar region during sudden stratospheric warming events.

Middle Stratospheric Polar Vortex Ozone Budget during the Warming Arctic Winter, 2002-2003

The ozone budget inside the middle stratospheric polar vortex （24-36 km） during the 2002-2003 Arctic winter is studied by analyzing Michelson Interferometer for Passive Atmospheric Sounding （MIPAS） satellite data.A comprehensive global chemical transport model （Model for Ozone and Related Chemical Tracers,MOZART-3） is used to analyze the observed variation in polar vortex ozone during the stratospheric sudden warming （SSW） events.Both MIPAS measurement and MOZART-3 calculation show that a pronounced increase （26-28 DU） in the polar vortex ozone due to the SSW events.Due to the weakening of the polar vortex,the exchange of ozone mass across the edge of the polar vortex increases substantially and amounts to about 3.0 × 10 7 kg according to MOZART-3 calculation.The enhanced downward transport offsets about 80% of polar vortex ozone mass increase by horizontal transport.A ＂passive ozone＂ experiment shows that only ～55% of the vertical ozone mass flux in February and March can be attributed to the variation in vertical transport.It is also shown that the enhanced downward ozone above ～32 km should be attributed to the springtime photochemical ozone production.Due to the increase of air temperature,the NO x reaction rate increases by 40%-80% during the SSW events.As a result,NO x catalytic cycle causes another 44% decrease in polar vortex ozone compared to the net ozone changes due to dynamical transport.It is also shown that the largest change in polar vortex ozone is due to horizontal advection by planetary waves in January 2003.

平流层爆发性增温期间中高层大气行星波研究进展

极区平流层爆发性增温(SSW)是冬季半球最剧烈的大气扰动现象之一.SSW期间温度和风场的剧烈变化被认为是冬季半球中高层大气波动能量异常增强的主要原因.流星雷达是能够稳定连续探测中间层和低热层风场的重要地基探测设备.主要依托国家重大科技基础设施建设项目:“子午工程”,我国已建设了多个流星雷达观测台站,对中间层和低热层风场进行了长期稳定连续的监测,为揭示SSW期间中间层和低热层波动异常变化的物理机制提供了重要的观测资料.本文简述了近年来以我国“子午工程”流星雷达监测数据为核心,SSW期间中高层大气行星波的研究进展和成果;深入讨论了冬季半球中高层大气行星波发生异常变化的主要激发机制.随着“子午工程”二期十个流星雷达台站即将建成,本文对利用“子午工程”流星雷达监测台网进一步研究SSW期间中高层大气波动的变化特性进行了展望.

冬季SSW低频特性及平流层低层和对流层中层环流异常

研究了１９８３～１９８４年冬季及１９８５～１９８６年冬季平流层爆发性增温（简记为ＳＳＷ，下同）的活动特征，发现ＳＳＷ的活动具有３０～６０天的季节内低频振荡特性。通过对１９８５～１９８６年冬季ＳＳＷ和反ＳＳＷ时段温压场特征的对比分析，发现ＳＳＷ事件不仅引起平流层低层环流的异常，而且在对流层中层的环流也有明显的异常，甚至表现更显著。伴随着ＳＳＷ和反ＳＳｗ的低频循环，极涡流型、中高纬和中低纬环流的经向度以及中纬度的西风急流中心均显示出相应的变化。