东亚地区一次切断低压引起的平流层、对流层交换数值模拟研究

用中尺度模式(MM5)模拟研究东亚地区(网格中心45°N,120°E)一次强切断低压过程(2000年4月8日～12日)引起的平流层、对流层交换.用Wei公式计算此次过程由平流层到对流层的净交换量为5.0×1014 kg,平均交换通量为-0.72×10-3 kg m-2 s-1.地面锋与高空锋的相互作用,在急流入口处和高空锋区出现对流层顶折叠,并引起平流层、对流层物质交换,其中空气的水平运动起主要作用.利用历史天气图统计东亚地区(20～70°N,80～180°E)切断低压的年平均个数为60,占北半球极区类型切断低压的52%.由此,推断东亚地区的平流层、对流层交换对整个北半球的平流层、对流层交换贡献很大.     

不同季节内剩余环流时空结构转换及其演变特征的研究

利用ERA-Interim资料,采用改进的变形欧拉平均方法对1979—2011年剩余环流季节内时空结构转换及其演变特征进行了分析。结果表明:(1)150 hPa附近4—8月剩余环流上升中心发生了整体向北移动的趋势,而9月至次年2月则表现为整体向南的移动,并且6—8月和9—11月的移动较为明显,分别向北和向南移动了3.168°和2.277°。(2)对流层内的剩余质量输送显著增强,但是热带环流上升区以及穿越对流层顶进入平流层的输送存在着减弱的趋势。(3)两半球高纬度100 hPa附近从最低平流层向下输送的质量通量以及热带对流层顶附近向上输送的质量通量在各季节年代际变化中基本都是减弱的,仅在6—8月和9—11月北半球向下质量通量出现了增强。     

东亚地区平流层、对流层交换对臭氧分布影响的模拟研究

首先将区域酸沉降模式(RADM)进行改造,加入平流层化学模块以替代对流层模块.然后用MM5单向耦合改造后的化学模式M-RADM,对东亚地区一次切断低压(2000年4月8～12日)引起的对流层顶折叠对上对流层、下平流层臭氧分布形式的影响进行模拟.结果显示:(1)随着高空槽的发展与切断,高空槽的底部及切断低压四周臭氧有显著增加.对流层顶折叠使高空臭氧向下输送,这种向下的输送可以到达对流层中部,对上对流层的臭氧影响最大.(2)模式可以成功地模拟出在北京地区探测到的臭氧垂直廓线的双峰结构.250 hPa处的臭氧分压比背景值增加近5倍.模拟表明改造后的区域化学模式M-RADM可以用于研究天气过程引起的对流层顶附近臭氧的演变情况.(3)上对流层臭氧分布形势的变化主要是由动力过程中的水平平流作用引起的,但是对流层顶附近臭氧的化学过程是不可忽略的.     

全球穿越对流层顶质量通量的时空变化特征

利用1958—2001年44年的ECMWF再分析值资料,采用Euler算法的Wei诊断模型,计算估计了全球平流层与对流层之间穿越对流层顶的质量通量(cross-tropopause flux,即CTF),并对其时空变化特征进行了分析。结果表明:(1)全球CTF的空间分布呈纬向型,且基本与全球经向环流相配合,其中对流层顶断裂带中向下的通量形势较复杂,南半球通量交换的空间变化较均匀,北半球有多峰结构的空间变化,东亚在全球的CTF中有重要作用。(2)南北半球热带对流层顶纬向平均向上的净通量极值分别在5°S为0.873×10-4kg.m-2.s-1和10°N为0.155×10-3kg.m-2.s-1;极地对流层顶向上和向下的通量极值对应在62.5°S为0.510×10-3kg.m-2.s-1,75°S是-0.365×10-3kg.m-2.s-1,55°N为0.257×10-3kg.m-2.s-1,75°N是-0.234×10-3kg.m-2.s-1,两极向上的通量极值在87.5°S为0.355×10-3kg.m-2.s-1,90°N为0.300×10-3kg.m-2.s-1;对流层顶断裂带中向下的净通量极值在35°S是-0.416×10-3kg.m-2.s-1,35°N为-0.333×10-3kg.m-2.s-1。(3)全球的净通量变化出现非对称性的季节波动,南半球和北半球净通量的季节与年际变化趋势完全相反,全球极地对流层顶控制区域的半球年平均质量交换量为-3.55×108Tg.a-1。(4)全球平均的CTF有显著的QBO特征,南北半球的年代际变化明显,特别是20世纪70年代中期至80年代中期出现了质量通量振幅的异常突变现象。     

Calculation of stratosphere–troposphere exchange in East Asia cut-off lows:cases from the Lagrangian perspective

本文使用三维非绝热朗格朗日轨迹模式OFFLINE3,对春末夏初东亚地区的切断低压所主导的双向平流层-对流层交换(STE)进行定量计算。通过对10个东亚地区切断低压的识别、计算、分析,发现切断低压附近发生平流层向对流层质量通量(STT)与对流层向平流层质量通量(TST)量级相当,但是分布范围不同:STT出现在低压中心西南部,最大通量位置出现在低压中心东南,TST最大值出现在槽前,并且从低压中心向外STT与TST交替分布。从本文所取的切断低压个例而言,切断低压产生的STT质量通量量级为10-4 kg m-2 s-1,促成的STE的净输送方向为从平流层向对流层,量级为10-4 kg m-2 s-1,比全球平均STE质量通量大1–2个量级。     

冬季北半球平流层季节内振荡与对流层季节内振荡的关系

通过平流层大气ISO与对流层大气ISO的比较分析,发现在中高纬地区平流层大气ISO与对流层大气ISO有着许多相同点.北半球冬半年平流层大气环流主要低频模态也可认为是北极涛动(AO),其空间分布的主要特征为; 高纬度地区与中低纬度地区为反位相变化,北极地区附近具有最大的变化值; 其季节内振荡的正位相对应于AO增强,负位相对应于AO减弱.北半球冬半年平流层100 hPa 和70 hPa 位势高度场的低频遥相关分析表明,北极地区(概指北纬60°N以北)和北半球其他大部分地区呈负相关,北极涛动扮演了非常重要的角色.同时,北半球冬半年平流层的主要低频波列是从欧亚大陆中部到西北太平洋,并且由纬向型低频波列(欧亚大陆-西伯利亚-太平洋)和经向型低频波列(欧亚大陆-北极-太平洋)共同构成.平流层30 hPa 和对流层500 hPa 上经过带通滤波(15～90 d)位势高度场的EOF第一主分量的形势有十分类似的特征,它们对应的时间系数序列有显著的延迟相关关系.因此可以认为,北半球平流层大气ISO的变化要先于对流层大气,在滞后35 d 左右其相关系数达到最大.大气环流模式(SAMIL)的数值模拟试验结果也表明,平流层的低频扰动可以在14 d 之后便在对流层500 hPa上激发出低频响应,其谱峰在30 d 左右.这进一步表明,通过大气季节内振荡,平流层的异常可以影响到对流层.     

远距离热带低压影响下山东半岛特大暴雨成因分析

用1948～2000年共53年的NCEP/NCAR再分析资料,分析了南北半球大气质量交换的周期变化特征和两半球大气质量交换通道的空间分布.结果表明:两半球的大气质量交换从长时间来看基本上是平衡的.两半球质量交换的季节变化表现为明显的半年周期,冬季和夏季的交换比较显著,且以夏季为主;具有显著的十年际变化;年际变化特征总体来看不显著,只在1980年代～1990年代存在较为显著的2～5年的年际振荡.NCEP资料存在较大变更的三个时段(以1968、1979年为分界)的周期变化特征确实存在一定的差异.全球平均异常向南的越赤道质量输送不存在很强的季节依赖性,主要由低空北半球太平洋-大西洋副热带高压南侧东北信风和亚洲季风区对流层高层副热带高压南侧的越赤道东北气流决定,亚洲季风区冬季南下的冷空气也起到很大的作用;而异常向北的越赤道质量净输送则主要发生在夏季,主要由亚洲季风区越赤道西南季风决定,特别是索马里越赤道急流从对流层低层到高层整层的向北输送,同时对流层低层赤道中东太平洋-大西洋信风区的东南信风也有较大的贡献.     

西风角动量输送的气候特征及其与急流关系研究

利用1958～1997年NCEP/NCAR一日四次的风场再分析资料,系统地分析了季节平均西风角动量（即u角动量）经向、垂直输送通量及其三个分量（平均经圈环流、定常波、瞬变涡输送通量）的气候特征,特别是讨论了12～2月、6～8月它们与东、西风带、副热带西风急流、极夜急流之间的联系。结果表明:（1）包含纬度因子的角动量通量与动量通量在高纬地区存在显著差别,高纬对流层上部的强动量输送中心在角动量通量中不明显。而u角动量强经向输送主要在中低纬对流层顶附近和冬半球高纬平流层顶附近,副热带西风急流和极夜西风急流均位于u角动量强向极输送中心及其高纬一侧的辐合区中。（2）发现三个输送分量对急流维持的作用随纬度、季节不同。北半球冬季（夏季）的副热带西风急流主要由平均经圈环流（强度相当的定常波和瞬变涡）强经向输送及辐合维持;南半球西风急流全年均由平均经圈环流和瞬变涡旋输送及辐合维持;冬半球中平流层极夜急流主要由定常波、瞬变涡旋输送及其辐合共同维持。（3）热带东风区是牵连角动量（即Ω角动量）的高值区,它主要由平均经圈环流向对流层上部输送;冬半球副热带及中纬西风区存在u角动量垂直输送的切变区,它主要由平均经圈环流和瞬变涡旋完成;热带对流层顶附近有u角动量的定常波弱向下输送。     

平流层剩余环流及其时间演变特征

平流层剩余环流是由剩余速度经向分量和垂直分量构成的平流层经向－垂直环流，它对于对流层－平流层相互作用和物质交换起着十分重要的作用。作者利用1979～2003年NCEP II再分析资料计算了剩余速度经向分量和垂直分量， 并与数值模拟结果进行了比较，再用计算的剩余环流讨论了它的季节变化、年际变化和长期变化趋势。计算结果表明，剩余环流的上升气流从低纬度赤道地区对流层顶上升到平流层下部，然后向极向下运动，在中纬度地区下沉， 进入对流层，这也就是Brewer－Dobson环流。计算结果同数值模拟结果比较一致。由此可见，可以利用NCEP资料得到比较清晰的剩余环流和Brewer－Dobson环流。剩余环流有明显的季节变化，上升气流的中心随着季节的变化在赤道地区南北移动，春秋季节其中心基本上位于赤道附近，南北半球大致呈对称分布，只是北半球副热带地区的下沉气流要比南半球强。在冬夏季节，上升气流的中心分别位于南北纬10°附近。北半球夏季的上升气流要比南半球夏季的上升气流强，同时冬半球的下沉气流比夏半球的下沉气流强。剩余环流还有年际变化和准两年周期振荡特征，在纬向风为西风位相时，赤道地区的上升气流比较弱；而在东风位相时，上升气流和水平方向的输送相对比较强。剩余环流的十年际变化表现为，1979～1983年、1990～1995年、2000～2003年较强，其他年份则较弱。在过去25年，就总的变化趋势而言，剩余环流的上升气流有所增强，平流层下部向中纬度地区的输送也有所增强，环流整体形势是增强的。     

NCEP／NCAR再分析资料中南北半球大气质量交换的特征研究

用1948～2000年共53年的NCEP／NCAR再分析资料,分析了南北半球大气质量交换的周期变化特征和两半球大气质量交换通道的空间分布。结果表明：两半球的大气质量交换从长时间来看基本上是平衡的。两半球质量交换的季节变化表现为明显的半年周期,冬季和夏季的交换比较显著,且以夏季为主;具有显著的十年际变化;年际变化特征总体来看不显著,只在1980年代～1990年代存在较为显著的2-5年的年际振荡。NCEP资料存在较大变更的三个时段（以1968、1979年为分界）的周期变化特征确实存在一定的差异。全球平均异常向南的越赤道质量输送不存在很强的季节依赖性,主要由低空北半球太平洋．大西洋副热带高压南侧东北信风和亚洲季风区对流层高层副热带高压南侧的越赤道东北气流决定,亚洲季风区冬季南下的冷空气也起到很大的作用;而异常向北的越赤道质量净输送则主要发生在夏季,主要由亚洲季风区越赤道西南季风决定,特别是索马里越赤道急流从对流层低层到高层整层的向北输送,同时对流层低层赤道中东太平洋．大西洋信风区的东南信风也有较大的贡献。     

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.     

平流层异常下传对2009年12月北半球大范围降雪过程的影响

2009年12月北半球中纬度出现大范围持续低温、暴风雪等天气。采用NCEP／NCAR再分析资料研究了平流层AO（Arctic Oscillation,北极涛动）异常信号下传的特征及其对本次极端气候事件的影响,并讨论了与平流层异常信号下传相关的行星波活动。结果表明：1）与此次极端气候事件相联系的负位相A0异常11月首先发生在平流层,维持将近1个月后于12月初开始下传,并且迅速传至地面。12月整个对流层的位势高度及温度在极区附近出现强的正异常,而中纬度地区则为负异常。2）平流层AO异常信号下传后,地面出现有利于低温降雪过程的环流异常。12月上旬,亚洲大陆东部及北美大陆西部出现异常偏北风,造成了俄罗斯、北美西部大面积负的温度异常;12月中下旬,欧洲大陆盛行偏西北气流,同时蒙古高压增强,欧亚大陆北部包括中国北方出现大片负的温度异常。3）在此次极端气候事件之前,北半球高纬度地区有异常强的行星波上传至平流层,导致平流层出现负位相的AO异常,并维持了一个月;随后,上传到平流层的行星波减弱,同时平流层负位相的AO异常迅速传至地面,导致了有利于低温降雪的环流异常。     

北半球准定常行星波气候平均态的资料分析和数值模拟

利用NCEP/NCAR再分析资料和大气环流模式(CCSR/NIES AGCM Ver 5.6),对北半球准定常行星波的气候平均态分布进行分析和模拟.再分析资料分析的结果表明:北半球冬季,准定常行星波沿两支波导向上传播,其中一支在对流层上层转向中低纬度传播,另外一支折向高纬度,通过极地波导上传到平流层.其中,1波和2波可以上传到平流层,因而其振幅分布除在中低纬的对流层上层出现一个次大值外,在高纬度平流层中上层会出现一个最大值,3波则主要限制在对流层,其振幅分布除在副热带对流层上层出现一个次大值外,最大值出现在中纬度对流层上层.北半球夏季,整个平流层为东风环流,极地波导不存在,行星波不能上传到平流层,在对流层活动也较弱,1波、2波、3波的传播情况大致相似,表现为在对流层上层由中纬度向赤道地区的传播.相应的振幅分布是,对1波和2波而言,最大值出现在中低纬对流层顶附近,同时在中高纬对流层上层出现一个次大值,而3波的振幅分布正好相反,最大值出现在中高纬对流层上层,次大值则在中低纬对流层顶附近.利用大气环流模式进行的数值模拟表明,模式可以比较好地模拟冬夏季准定常行星波的传播路径,但模拟的北半球冬季沿极地波导向平流层的传播明显偏弱,其结果是对1波、2波而言,高纬度平流层中上层的振幅最大值明显小于再分析资料的数值.文中还讨论了数值模拟与资料分析中行星波的差异可能对大气环流模拟的影响.     

涡动在南北半球平流层极涡崩溃过程中作用的比较

比较了南北半球春季平流层极涡的崩溃过程以及涡动在此崩溃过程中的作用。极涡的崩溃时间以平流层极夜急流核区最后一次西风转换为东风的时间来确定。结果表明南北半球平流层极涡的崩溃过程有着共同的特点,涡动和非绝热加热过程都对极涡的崩溃起着重要的作用,在极涡崩溃前平流层行星尺度波动活动明显,极涡崩溃以后,这种波动活动便迅速减弱。其中从对流层上传的行星波决定着极涡的具体崩溃时间。两个半球的差别主要表现在南半球极涡崩溃过程一般始于平流层高层,然后逐渐下传,而北半球这种下传不是很明显。其次,北半球平流层极涡崩溃偏晚年,极涡的减弱有两次过程,第一次为快速变化过程,第二次变化比较缓慢,而南半球平流层极涡崩溃无论早晚年只有一次减弱过程。长期的变化趋势分析表明南北半球平流层极涡的崩溃时间逐渐推迟,特别是20世纪90年代中后期以来,这种推迟更加明显。进一步的研究还发现,伴随着平流层极涡的崩溃过程平流层和对流层存在强烈的动力耦合,南北半球极涡迅速减弱前,各自半球的环状模指数也由负指数增加为正指数,表明低层环流对于平流层极涡的崩溃起到重要的作用;同时极涡不同强度所对应的低层环状模指数也不同,这可能与不同强度平流层极涡对于上传的行星波的反射有关。     

On the Differences and Climate Impacts of Early and Late Stratospheric Polar Vortex Breakup

The stratospheric polar vortex breakup (SPVB) is an important phenomenon closely related to the seasonal transition of stratospheric circulation. In this paper, 62-year NCEP/NCAR reanalysis data were employed to investigate the distinction between early and late SPVB. The results showed that the anomalous circulation signals extending from the stratosphere to the troposphere were reversed before and after early SPVB, while the stratospheric signals were consistent before and after the onset of late SPVB. Arctic Oscillation (AO) evolution during the life cycle of SPVB also demonstrated that the negative AO signal can propagate downward after early SPVB. Such downward AO signals could be identified in both geopotential height and temperature anomalies. After the AO signal reached the lower troposphere, it influenced the Aleutian Low and Siberian High in the troposphere, leading to a weak winter monsoon and large-scale warming at mid latitudes in Asia. Compared to early SPVB, downward propagation was not evident in late SPVB. The high-latitude tropospheric circulation in the Northern Hemisphere was affected by early SPVB, causing it to enter a summer circulation pattern earlier than in late SPVB years.     

北半球平流层中低层纬向风季节转换及转换层特征的分析研究

使用1979～2005年NCEP/NCAR 再分析数据,分析了北半球平流层中低层（300 hPa至10 hPa）纬向风的季节转换规律,并采用二维空间场相似性方法确定了平流层的季节过渡日期。分析表明,平流层大气环流基本为冬夏二元状态,冬夏转换具有突变性;其季节过渡在纬向是接近同步进行的,而在经向则有时间差异,无论是冬夏转换还是夏冬转换高纬都要早于低纬。在平流层中部（10～70 hPa）季节过渡是自上而向下进行的;而在平流层下部（100～200 hPa）季节过渡的上下传递关系则比较复杂,在不同的纬度带有不同的表现。在北半球热带外地区,平流层中部东风期的起止日期与相似性方法计算得到的平流层季节过渡日期之间具有较好的对应关系,在东风期之前和之后往往各存在持续10天左右的零风—弱风期。     

Stratospheric climate and variability from a general circulation model and observations

The climate and natural variability of the large-scale stratospheric circulation simulated by a newly developed general circulation model are evaluated against available global observations. The simulation consisted of a 30-year annual cycle integration performed with a comprehensive model of the troposphere and stratosphere. The observations consisted of a 15-year dataset from global operational analyses of the troposphere and stratosphere. The model evaluation concentrates on the simulation of the evolution of the extratropical stratospheric circulation in both hemispheres. The December–February climatology of the observed zonal mean winter circulation is found to be reasonably well captured by the model, although in the Northern Hemisphere upper stratosphere the simulated westerly winds are systematically stronger and a cold bias is apparent in the polar stratosphere. This Northern Hemisphere stratospheric cold bias virtually disappears during spring (March–May), consistent with a realistic simulation of the spring weakening of the mean westerly winds in the model. A considerable amount of monthly interannual variability is also found in the simulation in the Northern Hemisphere in late winter and early spring. The simulated interannual variability is predominantly caused by polar warmings of the stratosphere, in agreement with observations. The breakdown of the Northern Hemisphere stratospheric polar vortex appears therefore to occur in a realistic way in the model. However, in early winter the model severely underestimates the interannual variability, especially in the upper troposphere. The Southern Hemisphere winter (June–August) zonal mean temperature is systematically colder in the model, and the simulated winds are somewhat too strong in the upper stratosphere. Contrary to the results for the Northern Hemisphere spring, this model cold bias worsens during the Southern Hemisphere spring (September–November). Significant discrepancies between the model results and the observations are therefore found during the breakdown of the Southern Hemisphere polar vortex. For instance, the simulated Southern Hemisphere stratosphere westerly jet continuously decreases in intensity more or less in situ from June to November, while the observed stratospheric jet moves downward and poleward.This paper was presented at the Third International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 4–8 Sept. 1995 under the auspice of the Max Planck Institute for Meteorology, Hamburg. Editor for these papers is L. Dümenil.     

THE EXCHANGE OF MASS BETWEEN STRATOSPHERE AND TROPOSPHERE OVER THE TIBETAN PLATEAU AND ITS SURROUNDINGS IN 1998

n this paper,using NCEP dataset and the Wei's method,we calculate the exchange of massacross the thermal tropopause during 1998 over the Tibetan Plateau and its surroundings.Theresults indicate that:(1)There is strong air transport from troposphere to stratosphere in summerover the Tibetan Plateau and its surroundings.The air transport reaches the summit in midsummerwith three large value centers among which the Bay of Bengal is the largest and the other two largecenters lie in the east and northwest of the Tibetan Plateau,respectively.In May and October thecross-tropopause mass exchange reaches balance.In other months the mass transport is fromstratosphere to troposphere.(2)As far as the cross-tropopause mass exchange from June toSeptember in 1998 is concerned,the net mass transport is 13.7×10~(18) kg from troposphere tostratosphere,So the area from Tibetan Plateau to the Bay of Bengal is a channel through which airmass gets into stratosphere from troposphere.     

Decadal variation of the Northern Hemisphere Annular Mode and its influence on the East Asian trough

We analyze the decadal variation of the stratosphere troposphere coupled system around the year 2000 by using the NCEP reanalysis-2 data.Specifically,the relationship between the Northern Hemisphere Annular Mode(NAM) and the tropospheric East Asian trough is investigated in order to find the effective stratospheric signals during cold air outbreaks in China.Statistical analyses and dynamic diagnoses both indicate that after 2000,increased stratospheric polar vortex disturbances occur and the NAM is mainly in negative phase.The tropospheric polar areas are directly affected by the polar vortex,and in the midlatitudes,the Ural blocking high and East Asian trough are more active,which lead to enhanced cold air activities in eastern and northern China.Further investigation reveals that under this circulation pattern,downward propagations of negative NAM index are closely related to the intensity variation of the East Asian trough.When negative NAM anomalies propagate down to the upper troposphere and reach a certain intensity(standardized NAM index less than 1),they result in apparent reinforcement of the East Asian trough,which reaches its maximum intensity about one week later.The northerly wind behind the trough transports cold air southward and eastward,and the range of influence and the intensity are closely associated with the trough location.Therefore,the NAM index can be used as a measure of the signals from the disturbed stratosphere to give some indication of cold air activities in China.     

A two-way stratosphere-troposphere coupling of submonthly zonal-mean circulations in the Arctic

This paper examines the dominant submonthly variability of zonally symmetrical atmospheric circula- tion in the Northern Hemisphere （NH） winter within the context of the Northern Annular Mode （NAM）, with particular emphasis on interactive stratosphere-troposphere processes. The submonthly variability is identified and measured using a daily NAM index, which concentrates primarily on zonally symmetrical circulation. A schematic lifecycle of submonthly variability is developed that reveals a two-way coupling pro- cess between the stratosphere and troposphere in the NH polar region. Specifically, anomalous tropospheric zonal winds in the Atlantic and Pacific sectors of the Arctic propagate upwards to the low stratosphere, disturbing the polar vortex, and resulting in an anomalous stratospheric geopotential height （HGT） that subsequently propagates down into the troposphere and changes the sign of the surface circulations. From the standpoint of planetary-scale wave activities, a feedback loop is also evident when the anoma- lous planetary-scale waves （with wavenumbers 2 and 3） propagate upwards, which disturbs the anomalous zonally symmetrical flow in the low stratosphere, and induces the anomalous HGT to move poleward in the low stratosphere, and then propagates down into the troposphere. This increases the energy of waves at wavenumbers 2 and 3 in the low troposphere in middle latitudes by enhancing the land-sea contrast of the anomalous HGT field. Thus, this study supports the viewpoint that the downward propagation of stratospheric NAM signals may not originate in the stratosphere.