北极平流层冬季早期变暖的GCM模拟研究

观测表明北极平流层自20世纪70年代末以来在冬季早期 (11～12月) 存在变暖的趋势。为了验证该趋势是否是由于海面温度 (SST) 升高造成的, 我们使用观测的全球SST强迫一个全球大气环流模式 (AGCM)。集合模拟的结果表明, 在SST强迫下, 北极平流层呈现统计显著的变暖趋势, 极地对流层也有相对较弱的变暖趋势, 但统计显著性较低。通过对模拟的位势高度进行经验正交函数 (EOF) 分析, 我们发现北半球位势高度第一模态 (EOF1) 的空间结构非常类似于北极涛动 (AO) 或北半球环状模 (NAM), 其平流层主分量时间序列在冬季早期呈现统计显著的负趋势。与负的AO趋势相对应的是, 对流层高纬度和平流层中高纬度波动增强, 说明极区变暖是由于波动增强产生的极区绝热加热增强造成的。另外, 模拟的结果还表明北极平流层不仅在冬季早期出现变暖的趋势, 在冬季晚期 (2～3月) 北极平流层低层也出现弱的变暖趋势。SST强迫导致北极平流层冬季变暖不利于异相臭氧化学反应的发生, 这对极地平流层臭氧恢复有着重要意义。     

平流层爆发性增温过程中臭氧的垂直分布特征

利用ECMWF资料分析了平流层爆发性增温 (SSW) 过程中臭氧体积混合比的垂直分布的变化, 结果表明: 平流层爆发性增温过程中臭氧体积混合比增大, 而其极大值大多数形成在增温盛期。同时臭氧体积混合比的高值区在爆发性增温过程中随高度发生一定的变化, 据此对其变化分为两类: (1) 下传型: 在增温初期臭氧体积混合的高值区随高度下传至一定高度, 在增温盛期形成极大值然后随高度抬升到大致增温前的高度。 (2) 增厚型: 在增温过程中臭氧体积混合比的高值区厚度增加, 同时附近区域的臭氧体积混合比也增大, 而且在增温前臭氧体积混合比高值区在高度上没有多大变化, 增温开始后有所抬升。平流层爆发性增温过程中臭氧高值区随高度变化的这两种类型, 是由于平流层爆发性增温期间剩余环流对臭氧输送的结果。臭氧变化的下传型是由于在爆发性增温前剩余环流存在着中纬度向极地的明显输送, 并且伴随着极地强烈的下沉运动, 这就使得中纬度输送来的臭氧向下输送, 因此出现了臭氧高值区的下传; 而臭氧变化的增厚型是由于在爆发性增温期间剩余环流不但有中纬度向极地的输送, 而且在极地附近5 hPa高度处出现了上下两支输送气流, 向上的输送气流使中纬度输送来的臭氧向上输送, 而向下的输送气流使中纬度输送来的臭氧向下输送, 进而使增温期间极地附近的臭氧的高值区增厚。同时分析还表明: 平流层爆发性增温过程中中纬度臭氧体积混合比减少。     

2002/2003年与2003/2004年冬季爆发性增温期间的动力特征

利用ECMWF提供的60层气象场资料诊断分析了2002/2003和2003/2004年两个冬季的爆发性增温(stratospheric sudden warming,SSW)过程,比较了两次SSW期间高纬温度和纬向风的差异,计算了SSW期间的EP通量和剩余环流.结果表明:2003/2004年增温持续时间长、强度大,而2002/2003年则发生了波动;增温都是从平流层上层开始向下传播,但是2003/2004年高层极涡崩溃后迅速恢复,低层极涡恢复得慢,2002/2003年极涡在高层和低层都是缓慢恢复;SSW期间行星渡活动较多,2003/2004年极地EP通量的辐合引起东风长时间持续从而阻止了行星渡再次上传,而2002/2003年行星波则发生多次上传;2002/2003年SSW发生时高纬地区为下沉气流,没有形成环流圈,增温后形成逆时针的环流圈比2003/2004年偏低.     

Northern Hemisphere Stratospheric Polar Vortex Extremes in February under the Control of Downward Wave Flux in the Lower Stratosphere

Using ECWMF ERA-40 and Interim reanalysis data, the planetary wave fluxes associated with the February extreme stratospheric polar vortex were studied. Using the three-dimensional Eliassen-Palm (EP) flux as a measure of the wave activity propagation, the authors show that the unusual warm years in the Arctic feature an anomalous weak stratosphere-troposphere coupling and weak downward wave flux at the lower stratosphere, especially over the North America and North Atlantic (NANA) region. The extremely cold years are characterized by strong stratosphere-troposphere coupling and strong downward wave flux in this region. The refractive index is used to examine the conception of planetary wave reflection, which shows a large refractive index (low reflection) for the extremely warm years and a small refractive index (high reflection) for the extremely cold years. This study reveals the importance of the downward planetary wave propagation from the stratosphere to the troposphere for explaining the unusual state of the stratospheric polar vortex in February.     

平流层爆发性增温期间行星波的活动

利用逐日的NCEP再分析资料分析了1978～2004年期间发生的18次强爆发性增温时平流层中下层行星波1波和2波的异常以及极涡崩溃的形式.分析表明, 爆发性增温前期1波都异常增幅, 波振幅达到峰值之后的一段时间内发生爆发性增温, 然而在增温过程中行星波2波的变化却有明显不同, 可分为三种情况: (1) 在增温前期, 行星波2波很弱; 在增温期间, 行星波2波异常发展, 伴随着极涡崩溃的形式是由偏心型向偶极型过渡. (2) 在增温前期, 行星波2波较强; 在增温期间, 行星波2波明显减弱, 极涡的变化形式是远离极地, 在极地外围活动, 但不发生分裂.(3) 在增温前期, 行星波2波不太强; 在增温期间, 行星波2波有所发展, 但始终强不过1波, 极涡的变化是先偏离极地, 然后发生不对称的变形.作者还计算和分析了EP通量及其散度, 利用波流相互作用理论对这三种行星波的异常变化及其与爆发性增温的关系进行解释.     

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.     

The Role of Stationary and Transient Planetary Waves in the Maintenance of Stratospheric Polar Vortex Regimes in Northern Hemisphere Winter

Using 1958-2002 NCEPNCAR reanalysis data, we investigate stationary and transient planetary wave propagation and its role in wave-mean flow interaction which influences the state of the polar vortex (PV) in the stratosphere in Northern Hemisphere (NH) winter. This is done by analyzing the Eliassen-Palm (E-P) flux and its divergence. We find that the stationary and transient waves propagate upward and equatorward in NH winter, with stronger upward propagation of stationary waves from the troposphere to the stratosphere, and stronger equatorward propagation of transient waves from mid-latitudes to the subtropics in the troposphere. Stationary waves exhibit more upward propagation in the polar stratosphere during the weak polar vortex regime (WVR) than during the strong polar vortex regime (SVR). On the other hand, transient waves have more upward propagation during SVR than during WVR in the subpolar stratosphere, with a domain of low frequency waves. With different paths of upward propagation, both stationary and transient waves contribute to the maintenance of the observed stratospheric PV regimes in NH winter.     

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.     

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.     

北半球冬季平流层强、弱极涡事件演变过程的对比分析

利用1958～2017年逐日的NCEP/NCAR再分析资料对北半球冬季平流层强、弱极涡事件的演变过程进行了对比分析,同时比较了有平流层爆发性增温（SSW）和无SSW发生的两类弱极涡事件的环流演变和动力学特征。结果表明,强极涡的形成存在着缓慢发展和快速增强的过程,而弱极涡事件的建立非常迅速;和强极涡事件相比,弱极涡事件的峰值强度更强,异常中心的位置更高。此外,强、弱极涡事件的产生与波流相互作用的正反馈过程密切相关。对于强极涡事件,发展阶段的太平洋—北美（PNA）型异常削弱了行星波一波;当平流层西风达到一定强度,上传的行星波受到强烈抑制,使得极涡迅速增强达到峰值。而对于弱极涡事件,发展阶段一波型的异常增强了行星波上传,通过对纬向流的拖曳作用使得平流层很快处于弱西风状态,更多行星波进入平流层导致极涡急剧减弱甚至崩溃。针对有、无SSW发生的两类弱极涡事件的对比分析表明,有SSW发生的弱极涡事件发展阶段,平流层出现强的向上的一波Eliassen-Palm（EP）通量异常,通过正反馈过程使得一波和二波上传同时增强而导致极涡崩溃;无SSW发生的弱极涡事件发展阶段,平流层缺乏向上的一波通量,二波活动起到重要作用,其总的行星波上传远弱于有SSW发生的弱极涡事件。对于无SSW发生的弱极涡事件,其发展和成熟阶段对流层上部出现类似欧亚（EU）型的高度异常,伴随着强的向极的EP通量异常,导致对流层有极强的负北极涛动（AO）型异常。而有SSW发生的弱极涡事件发展阶段对流层上部主要表现为北太平洋上空来自低纬的波列异常,其后期的对流层效应更加滞后也不连续,对流层AO异常的强度明显弱于无SSW发生的弱极涡事件。     

Lagrangian Estimations of Ozone Loss in the Core and Edge Region of the Arctic Polar Vortex 1995/1996: Model Results and Observations

Ozone evolution and diabatic descent in the Arctic polar vortex in winter 1995/1996 was studied with a newly developed diabatic trajectory–chemistry model (DTCM). To study the chemical and dynamic evolution of the species in the polar vortex, 400 diabatic trajectories were calculated in the vortex core and edge region by using three-dimensional (3-D) wind data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). The averaged diabatic descending motion and ozone behavior were obtained for particles started from the core and from the edge region of the vortex. The difference in ozone-loss rates as well as the difference in descending rates between the vortex core and the vortex-edge region was not statistically significant. The average cumulative ozone loss of 65 ± 16% in the vortex core obtained from the model calculations was consistent with the estimates obtained with a different method (Match experiment). The model results for the vortex core were compared with those obtained using trajectories with the vertical winds calculated on the basis of radiative cooling rates as used by the SLIMCAT 3-D chemical transport model. Although the trajectories based on cooling rates exhibited lower descending rates than those based on 3-D analyzed wind data, the ozone behavior was similar for both types of trajectory. Ozonesonde data from two stations (Ny-Alesund in the vortex core and Yakutsk in the vortex edge) were compared with the model results. For Lagrangian estimation of the ozone loss at these stations, the descending rates obtained by the diabatic trajectory calculations were used. Good agreements were obtained between the model results and observations for both the vortex core and edge region. These results suggest that strong ozone depletion occurred not only in the core, but also in the edge region of the vortex, and that air masses from the mid-latitudes did not appreciably affect the degree of ozone depletion in this winter–spring period. The sensitivity of the model to different descending rates and to the presence of large nitric acid trihydrate (NAT) particles was also examined.     

平流层强、弱极涡事件的演变过程及其对我国冬季天气的影响

本文利用1979～2010年的NCEP再分析资料,通过北半球环状模NAM指数挑选出的强、弱极涡个例,分析了北半球平流层异常变化过程中行星波的演变以及与之相联系的我国天气的变化特征。结果表明,在强极涡事件前,行星波1波会被反射回对流层,极地波导减弱,低纬波导增强,中高纬地区的E-P通量矢量有着从平流层传播到对流层的趋势;强极涡事件后,极地波导增强,低纬波导减弱。在弱极涡事件前,中、高纬度行星波1波沿着极地波导的传播明显增强;弱极涡事件后,极地波导明显减弱。与此对应的我国天气也有明显变化,在强极涡事件前,我国大部分地区温度偏低,南方地区偏湿而新疆西北部和云南西部地区偏干;在强极涡事件后,东亚冬季风进一步增强,冷空气加强南下,南方地区可降水量减少,新疆西北部仍然偏干,而云南大部分地区可将水量增加。在弱极涡事件前,东亚冬季风显著增强,使我国气温偏冷,降水减少,而弱极涡事件后,我国气温明显回升,中、东部地区和新疆西北部地区降水明显增加。     

Observational Subseasonal Variability of the PM2.5 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.     

A 1-D model of the formation and evolution of Polar Stratospheric Clouds

A 1-D model of the formation and seasonal evolution of Polar Stratospheric Clouds (PSCs) is described. The model considers PSCs of types 1 and 2 in the vertical range from 8 to 30 km and utilizes real temperature data. The micro-physical processes included in the model are the heterogeneous nucleation and condensation (or evaporation), while sedimentation, gas diffusion and vertical wind velocity are the processes responsible for transport. Model simulations have been compared with PSC data obtained by lidar at the South Pole: results for the winter 1990 are discussed. The different contribution of type 1 and type 2 PSCs to the measured backscattering coefficient has been evidenced. In the simulations, layers of NAT particles form when low values of the backscattering coefficient are measured; similarly, ice particles form when sharper and rapidly changeable structures with higher values of the backscattering coefficient are observed. Significant results on the condensation and depletion of HNO₃ and H₂O are presented. Water vapor profiles measured during winter 1990 are reproduced quite well.     

A Comparison of Match Ozonesonde-Derived and 3D Model Ozone Loss Rates in the Arctic Polar Vortex during the Winters of 1994/95 and 1995/96

Ozone loss rates from ozonesonde data reported in the Match experiments of winters 1994/95 and 1995/96 inside the Arctic polar vortex are compared with simulations of the same winters performed using the SLIMCAT 3D chemistry and transport model. For 1994/95 SLIMCAT reproduces the location and timing of the diagnosed ozone destruction, reaching 10 ppbv/sunlit hour in late January as observed. SLIMCAT underestimates the loss rates observed in February and March by 1–3 ppbv/sunlit hour. By the end of March, SLIMCAT ozone exceeds the observations by 25–35%. In January 1995 the ozonesonde-derived loss rates at levels above 525 K are not chemical in origin but due to poor conservation of air parcels. Correcting temperature biases in the model forcing data significantly improved the agreement between the model and observed ozone at the end of winter 1994/95, increasing ozone destruction in SLIMCAT in February and March. The SLIMCAT simulation of winter 1995/96 does not reproduce the maximum ozone loss rates diagnosed by Match of 13 ppbv/sunlit hour. Comparing the data for the two winters reveals that the SLIMCAT photochemistry is least able to reproduce observed losses at low temperatures or when low temperatures coincide with high solar zenith angles (SZA). When cold (T = 192 K), high SZA (90°)matches are excluded from the 1995/96 analysis, agreement between the diagnoses and SLIMCAT is better with ozone loss rates of up to 6 ppbv/sunlit hour. For the rest of the winter SLIMCAT consistently underestimates the Match rates of ozone loss by 1–3 ppbv/sunlit hour. In March 1996 the monthly mean SLIMCAT ozone is 50% greater than observations at 430–540 K. In both winters, ozone destruction rates peaked more rapidly and declined more slowly in the Match observations than in the SLIMCAT simulations. The differences between the observed and modelled cumulative ozone losses demonstrate that the total ozone destruction by the end of the winter is sensitive to errors in the instantaneous ozone loss rates of 1–3 ppbv/sunlit hour.