Poleward-moving recurrent auroral arcs associated with impulse-excited standing hydromagnetic waves

In Earth's high-latitude ionosphere, the poleward motion of east–west elongated auroral arcs has been attributed to standing hydromagnetic waves, especially when the auroral arcs appear quasi-periodically with a recurrence time of a few minutes. The validation of this scenario requires spacecraft observations of ultra-low-frequency hydromagnetic waves in the magnetosphere and simultaneous observations of poleward-moving auroral arcs near the spacecraft footprints. Here we present the first observational evidence from the multi-spacecraft THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission and the conjugated all-sky imager to support the scenario that standing hydromagnetic waves can generate the quasi-periodic appearance of poleward-moving auroral arcs. In this specific event, the observed waves were toroidal branches of the standing hydromagnetic waves, which were excited by a pulse in the solar wind dynamic pressure. Multi-spacecraft measurements from THEMIS also suggest higher wave frequencies at lower L shells (consistent with the distribution of magnetic field line eigenfrequencies), which indicates that the phase difference across latitudes would increase with time. As time proceeds, the enlarged phase difference corresponds to a lower propagation speed of the auroral arcs, which agrees very well with the ground-based optical data.

Contribution of Oceanic Circulation to the Poleward Heat Flux

Oceanic contribution to the poleward heat flux in the climate system includes two components: the sensible heat flux and the latent heat flux. Although the latent heat flux has been classified as atmospheric heat flux exclusively, it is argued that oceanic control over this component of poleward heat flux should play a critically important role. The so-called swamp ocean model practice is analyzed in detail, and the critical role of oceanic circulation in the establishment of the meridional moisture transport is emphasized.

Trend of Storm Surge Induced by Typical Landfall Super Typhoons During 1975–2021 in the Eastern China Sea

Climate change affects the activity of global and regional tropical cyclones(TCs).Among all TCs,typical super typhoons(STYs)are particularly devastating because they maintain their intensity when landing on the coast and thus cause casualties,economic losses,and environmental damage.Using a 3D tidal model,we reconstructed the typhoon(TY)wind field to simulate the storm surge induced by typical STYs.The TY activity was then analyzed using historical data.Results showed a downtrend of varying degrees in the annual frequency of STYs and TCs in the Western North Pacific(WNP)Basin,with a significant trend change observed for TCs from 1949 to 2021.A large difference in the interannual change in frequency was found between STYs and TCs in the WNP and Eastern China Sea(ECS).Along the coast of EC,the frequency of landfall TCs showed a weak downtrend,and the typical STYs showed reverse micro growth with peak activity in August.Zhejiang,Fujian,and Taiwan were highly vulnerable to the frontal hits of typical STYs.Affected by climate change,the average lifetime maximum intensity(LMI)locations and landfall locations of typical STYs in the ECS basin showed a significant poleward migration trend.In addition,the annual average LMI and accumulated cyclone energy showed an uptrend,indicating the increasing severity of the disaster risk.Affected by the typical STY activity in the ECS,the maximum storm surge area also showed poleward migration,and the coast of North China faced potential growth in high storm surge risks.

A Quasi-Linear Relationship between Planetary Outgoing Longwave Radiation and Surface Temperature in a Radiative-Convective-Transportive Climate Model of a Gray Atmosphere

In this study,we put forward a radiative-convective-transportive energy balance model of a gray atmosphere to examine individual roles of the greenhouse effect of water vapor,vertical convection,and atmospheric poleward energy transport as well as their combined effects for a quasi-linear relationship between the outgoing longwave radiation(OLR)and surface temperature(T_(S)).The greenhouse effect of water vapor enhances the meridional gradient of surface temperature,thereby directly contributing to a quasi-linear OLR-T_(S) relationship.The atmospheric poleward energy transport decreases the meridional gradient of surface temperature.As a result of the poleward energy transport,tropical(high-latitude)atmosphere-surface columns emit less(more)OLR than the solar energy input at their respective locations,causing a substantial reduction of the meridional gradient of the OLR.The combined effect of reducing the meridional gradients of both OLR and surface temperature by the poleward energy transport also contributes to the quasi-linear OLR-T_(S) relationship.Vertical convective energy transport reduces the meridional gradient of surface temperature without affecting the meridional gradient of OLR,thereby suppressing part of the reduction to the increasing rate of OLR with surface temperature by the greenhouse effect of water vapor and poleward energy transport.Because of the nature of the energy balance in the climate system,such a quasi-linear relationship is also a good approximation for the relationship between the annual-mean net downward solar energy flux at the top of the atmosphere and surface temperature.

极盖区等离子体云块触发的极向边界点亮事件的观测研究

极盖区等离子体云块是电子密度比背景高出2倍及以上的高密度不均匀体,极向边界点亮是夜侧极光卵极向边界亮度显著增强的极光结构。探究极盖区等离子体云块与极向边界点亮现象的形成和演化以及二者之间的关系对理解极区电离层-磁层耦合具有重要意义。本文基于北极黄河站全天空极光成像仪、欧洲非相干散射雷达(EISCAT)、SuperDARN雷达和GPS TEC等多手段观测数据,揭示极盖区等离子体云块运动到夜侧极光卵极向边界,进而触发极向边界点亮的完整过程。研究结果表明,(1)高密度等离子体云块从极盖区运动到夜侧极光卵极向边界时,极向边界附近的极光强度明显增加,即出现了极向边界点亮现象。(2)极光亚暴发生后,极光卵极向膨胀,极向边界到达EISCAT Svalbard 42 m雷达观测视野。该雷达观测到了高密度、高电子和离子温度的结构,并伴随着电子密度峰值高度下降和离子上行等现象,这与高密度等离子体云块的输运和当地的极光粒子沉降密切相关。(3)从日侧向夜侧输运的等离子体云块到达夜侧极光卵极向边界附近时可能触发极向边界点亮,并且影响等离子体云块和极光粒子沉降接触区域的等离子体特征,这有助于加深我们对夜侧极区电离层-磁层耦合过程的理解。

Surface and tropospheric ozone trends in the Southern Hemisphere since 1990: possible linkages to poleward expansion of the Hadley circulation

Increases in free tropospheric ozone over the past two decades are mainly in the Northern Hemisphere that have been widely documented,while ozone trends in the Southern Hemisphere(SH)remain largely unexplained.Here we first show that in-situ and satellite observations document increases of tropospheric ozone in the SH over 1990–2015.We then use a global chemical transport model to diagnose drivers of these trends.We find that increases of anthropogenic emissions(including methane)are not the most significant contributors.Instead,we explain the trend as due to changes in meteorology,and particularly in transport patterns.We propose a possible linkage of the ozone increases to meridional transport pattern shifts driven by poleward expansion of the SH Hadley circulation(SHHC).The SHHC poleward expansion allows more downward transport of ozone from the stratosphere to the troposphere at higher latitudes,and also enhances tropospheric ozone production through stronger lifting of tropical ozone precursors to the upper troposphere.These together may lead to increasing tropospheric ozone in the extratropical SH,particularly in the middle/upper troposphere and in austral autumn.Poleward expansion of the Hadley circulation is partly driven by greenhouse warming,and the associated increase in tropospheric ozone potentially provides a positive climate feedback amplifying the warming that merits further quantification.

Sensitivity Experiments on the Poleward Shift of Tropical Cyclones over the Western North Pacific under Warming Ocean Conditions

Recent studies found that in the context of global warming, the observed tropical cyclones （TCs） exhibit signific-ant poleward migration trend in terms of the mean latitude where TCs reach their lifetime-maximum intensity in the western North Pacific （WNP）. This poleward migration of TC tracks can be attributed to not only anthropogenic for-cing （e.g., continuous increase of sea surface temperature （SST））, but also impacts of other factors （e.g., natural vari- ability）. In the present study, to eliminate the impacts of other factors and thus focus on the impact of unvaried SST on climatological WNP TC tracks, the mesoscale Weather Research and Forecasting （WRF） model is used to con- duct a suite of idealized sensitivity experiments with increased SST. Comparisons among the results of these experi- ments show the possible changes in climatological TC track, TC track density, and types of TC track in the context of SST increase. The results demonstrate that under the warmer SST conditions, the climatological mean TC track sys-tematically shifts poleward significantly in the WNP, which is consistent with the previous studies. Meanwhile, the ocean warming also leads to the decreased （increased） destructive potential of TCs in low （middle） latitudes, and thus northward migration of the region where TCs have the largest impact. Further results imply the possibility that under the ocean warming, the percentage of TCs with westward/northwestward tracks decreases/increases distinctly.

Dayside poleward moving auroral forms and ionospheric convection under stable interplanetary magnetic field (IMF) conditions

Using high temporal resolution optical data obtained from three-wavelength all-sky imagers over six winters continuously at Yellow River Station (78.92°N,11.93°E) in Arctic,we statistically investigated the dependence of location of poleward moving auroral forms (PMAFs) on the interplanetary magnetic field (IMF) B z and B y components as a function of MLT and MLAT under stable IMF conditions.It is found that more PMAFs occurred in lower latitude for Bz<0 and there was less evident IMF B y-related prenoon-postnoon asymmetry for Bz<0 than for Bz>0.We found that the PMAFs were distributed over a wide range of MLT when Bz<0,which indicates that the reconnection X-line might spread like an 'S' shape.However,during northward IMF,PMAFs were observed predominantly prenoon for IMF By>0 and postnoon for IMF By<0 associating with the effect of the high-latitude reconnection,which is largely consistent with the theoretical model of the convection flow.

“北极放大”现象驱动因素及其影响的研究进展综述

21世纪以来,受全球变暖的影响,北极近地表快速升温,升温速率可达全球平均水平的2倍,称为“北极放大”(Arctic Amplification,AA)现象。本文首先探讨了北极放大效应产生的原因:在局地气候反馈方面,海冰减少导致开阔水面增多,水体吸收太阳辐射增多使海-气相互作用增强,海冰进一步融化,形成正反馈;由于北极地区绝对温度较低,根据普朗克黑体辐射定律,普朗克反馈在北极为负反馈,促进北极放大效应;开阔水面和增温使北极地区大气湿度增加导致云量增多,从而使下行长波辐射增加,促使近地面层升温,这种水汽和云反馈对北极变暖的贡献有显著的季节性,主要发生在冬季;在北极以外地区水汽和热量的极向输送方面,温度的升高使大气柱增厚,高层气压梯度的变化引起纬向风变化,纬向分量减少,经向分量增加,形成槽脊,较低纬度地区可通过槽脊实现与北极的热量和水汽交换。此外,北极涛动(Arctic Oscillation,AO)与北大西洋涛动(North Atlantic Oscillation,NAO)也被证实与北极变暖有关。大西洋经向翻转环流(Atlantic Meridional Overturning Circulation,AMOC)异常曾被认为是北极变暖的驱动因素,而近期研究表明其更可能是北极变暖导致的结果。与此同时,“北极放大”现象会通过影响大气环流对中纬度地区甚至全球气候产生影响。北极变暖会导致东亚冬季严寒和北美东部冷事件的发生,形成“暖北极-冷欧亚”的格局并影响北美冬季气候波的形成。目前认为北极变暖与海冰减退可以通过两种机制影响中纬度地区:一是通过影响气压场的分布改变纬向风,使阻塞事件增加;二是通过对流层-平流层耦合作用影响北半球大气环流。但以上两种机制还存在争议。

2020年4月20日磁暴恢复相期间舌状中性结构的垂直变化

在发生于2020年4月20日的地磁暴恢复相阶段,GOLD(Global-scale Observations of the Limb and Disk)成像仪在第112天(day of year,DOY 112)中低纬地区观测到氧原子(O)和氮气分子(N_(2))的柱密度比(ΣO/N_(2))的舌状中性结构(TON).TON结构一般指发生于中高纬且形成于两个ΣO/N_(2)暴时衰减结构之间的ΣO/N_(2)增强结构.热层-电离层电动力学大气环流模式(Thermosphere-Ionosphere-Electrodynamics General Circulation Model,TIEGCM)定性地模拟再现了在本次磁暴恢复相期间观测到的ΣO/N_(2)增强结构,并且发现这个结构在前一天(DOY 111)当地下午形成,通过中性风的输运被逐渐耗散.模拟结果呈现了不同高度O/N_(2)的TON结构的垂直变化,其强度和纬度范围有明显的高度依赖性,并且随磁暴演化不断变化.诊断分析表明:下沉流(downwelling)驱动的垂直输运首先导致较低纬(约30°N—70°N)O/N_(2)的增强,然后通过极向风驱动的水平输运将其向更高纬地区输运.在中低热层(约120~300 km高度),主导O/N_(2)的TON结构演化的中低纬极向风主要是由气压梯度力导致的,同时科里奥利力对极向风也有一定的正贡献.而在约300 km高度以上的高热层,极向风主要由气压梯度力和与其作用相反的垂直黏性力两项控制.

多层海洋背景下的位涡均一化理论研究

位涡均一化理论是现代温跃层理论的基石,是引起层化而又弱耗散的大洋次表层产生运动的重要途径之一。本文通过对位涡均一化理论的基本构造方程扩充求解,将构造理论扩充到四层大洋模式、n层大洋模式中。结果表明,在四层大洋模式中,第二层及第三层都会出现位涡均一化的区域,并且其位涡恒定值相等,皆向沿流涡北边界处的值均一化,第三层位涡均一化区域的面积小于第二层位涡均一化区域的面积。在n层模式中,除表层和底层外的n-2层皆有位涡值均一的区域,所有发生位涡均一化的层均一化区域内的位涡恒定值皆等于流涡北边界处的位涡值,且随着深度的增加,发生位涡均一化的区域面积逐渐减小,均一化区域圆心发生极向运动。整个位涡均一化区域呈现出南缓北陡,东西对称的碗状结构。

2004年2月11日Cluster卫星和CUTLASS雷达同时观测的磁通量传输事件

本文分析了2004年2月11日11：00-11：40UT期间Cluster卫星簇的磁通门磁力计（FGM）、等离子体电子及电流试验仪（PEACE）和CUTLASS芬兰雷达对多个磁通量传输事件（FTEs）的同时观测.在此期间,Cluster卫星簇位于北半球外极隙区附近,并于11：18UT左右穿出磁层顶进入磁鞘,四颗卫星同时观测到了多个FTEs,其出现具有准周期性,周期约为130s.利用Cluster四颗卫星的多点同时观测数据,采用最小方向微分法和时空微分方法,我们推断这些FTEs是尺度大小约为（0.87-1.81）RE的准二维结构,其运动方向为东北方向,与Cooling模型预测方向基本一致.CUTLASS芬兰雷达在相应的电离层区域观测到了明显的＂极向运动雷达极光＂结构,这些结构与Cluster卫星簇观测的FTEs有着很好的对应关系,它们是FTEs的雷达观测特征.

Polar Vortex Oscillation Viewed in an Isentropic Potential Vorticity Coordinate

The stratospheric polar vortex oscillation （PVO） in the Northern Hemisphere is examined in a semiLagrangian θ-PVLAT coordinate constructed by using daily isentropic potential vorticity maps derived from NCEP/NCAR reanalysis Ⅱdataset covering the period from 1979 to 2003. In the semi-Lagrangian θ-PVLAT coordinate, the variability of the polar vortex is solely attributed to its intensity change because the changes in its location and shape would be naturally absent by following potential vorticity contours on isentropic surfaces. The EOF and regression analyses indicate that the PVO can be described by a pair of poleward and downward propagating modes. These two modes together account for about 82% variance of the daily potential vorticity anomalies over the entire Northern Hemisphere. The power spectral analysis reveals a dominant time scale of about 107 days in the time series of these two modes, representing a complete PVO cycle accompanied with poleward propagating heating anomalies of both positive and negative signs from the equator to the pole. The strong polar vortex corresponds to the arrival of cold anomalies over the polar circle and vice versa. Accompanied with the poleward propagation is a simultaneous downward propagation. The downward propagation time scale is about 20 days in high and low latitudes and about 30 days in mid-latitudes. The zonal wind anomalies lag the poleward and downward propagating temperature anomalies of the opposite sign by 10 days in low and high latitudes and by 20 days in mid-latitudes. The time series of the leading EOF modes also exhibit dominant time scales of 8.7, 16.9, and 33.8 months. They approximately follow a double-periodicity sequence and correspond to the 3-peak extratropical Quasi-Biennial Oscillation （QBO） signal.

基于局部运动向量时空统计的极光事件检测

极光动态过程的分析与理解对极光发生机制研究具有重要意义。本文提出了一种基于动态过程的极光事件检测方法。首先利用多尺度流体光流的方法提取出极光的局部运动场信息,然后基于局部运动场时空统计特性表征极光视频序列,最后实现对特殊极光事件的检测。实验结果表明,本文方法能够高效、准确地检索到特殊极光事件,并且检测结果不依赖于目标事件的选择。这一成果为开展基于大量连续观测的极光视频对极光动态过程进行统计分析的研究奠定了基础。

Nighttime meridional neutral wind responses to SAPS simulated by the TIEGCM:A universal time effect

The present work uses the Thermosphere Ionosphere Electrodynamics General Circulation Model(TIEGCM),under geomagnetically disturbed conditions that are closely related to the southward interplanetary magnetic field(IMF),to investigate how the nighttime poleward wind(30°–50°magnetic latitude and 19–22 magnetic local time)responds to subauroral polarization streams(SAPS)that commence at different universal times(UTs).The SAPS effects on the poleward winds show a remarkable UT variation,with weaker magnitudes at 00 and 12 UT than at 06 and 18 UT.The strongest poleward wind emerges when SAPS commence at 06 UT,and the weakest poleward wind develops when SAPS occur at 00 UT.A diagnostic analysis of model results shows that the pressure gradient is more prominent for the developing of the poleward wind at 00 and 12 UT.Meanwhile,the effect of ion drag is important in the modulation of the poleward wind velocity at 06 and 18 UT.This is caused by the misalignment of the geomagnetic and geographic coordinate systems,resulting in a large component of ion drag in the geographically northward(southward)direction due to channel orientation of the SAPS at 06 and 18 UT(00 and 12 UT).The Coriolis force effect induced by westward winds maximizes(minimizes)when SAPS commence at 12 UT(00 UT).The centrifugal force due to the accelerated westward winds shows similar UT variations as the Coriolis force,but with an opposite effect.

Case study of an Equatorial Plasma Bubble Event investigated by multiple ground-based instruments at low latitudes over China

Observational evidence is insufficient to understand how equatorial plasma bubbles(EPBs)form over low latitudes.The mechanism of plasma-density enhancement(formation of"plasma blobs")at low latitudes is in dispute.In this paper,we use data from multiple ground-based instruments(one all-sky airglow imager,five digisondes,and one Fabry–Perot interferometer)to investigate the evolution of an EPB event that occurred at low latitudes over China on the night of 06 December 2015(06-Dec-2015).We provide observational evidence that an enhanced equatorward wind most likely induced by a substorm could have initiated the Rayleigh–Taylor instability(RTI)that destabilized several EPB depletions in an upwelling region of a large-scale wave-like structure(LSWS)in the bottomside ionosphere.Those EPB depletions were forced to surge poleward,from nearly 10°to 19°magnetic latitude,two hours before midnight.Smaller-scale bifurcations evolved rapidly from tips of airglow depletions by a secondary E×B instability when the aforementioned substorm-induced southwestward wind blew through.During the growth phase of the EPB depletions,a westward polarization electric field inside the LSWS is likely to have compressed plasma downward,inducing the two airglow-type blobs observed in the bottomside ionosphere,by a mechanism of LSWS-blob connection that we propose.We also provide observational evidence of brightness airglow depletions.We find that an enhanced poleward wind associated with a passing-by brightness wave(BW)is likely to have transported plasma to fill the airglow depletions,which finally evolved into brightness airglow structures.This study investigates the physical processes accompanied by the EPB event and those two-airglow blobs observed at low-latitudes over China.

What Controls Recent Changes in the Circulation of the Southern Hemisphere: Polar Stratospheric or Equatorial Surface Temperatures?

Recent research suggests that both tropical ocean warming and stratospheric temperature anomalies due to ozone depletion have led to a poleward displacement of the midand high-latitude circulation of the Southern Hemisphere over the past century. In this study, we attempt to distinguish the influences of ocean warming and stratospheric cooling trends on seasonal changes of both the zonally symmetric and asymmetric components of the southern hemisphere circulation. Our analysis makes use of three data sets-the ERA40 reanalysis and results from two different runs of the GFDL global atmosphere and land model (AM2.1) for the period 1870 to 2004. A regression analysis was applied to two variables in each of the three data sets-the zonal component of the surface wind U(10 m) and the height at 300 hPa—to determine their correlation with zonally averaged polar stratospheric temperatures (T_polar—at 150 hPa, averaged over a band from 70S - 80S) and low-level equatorial temperatures (T_equator—at 850 hPa averaged over a band at 5S - 5N). Our analysis shows that the zonally symmetric surface winds have a considerably enhanced intensity in high latitudes of the southern hemisphere over the summer period, and that the stratospheric temperature trend, and thus ozone depletion, is the dominant contributor to that change. However, the climatic change of the asymmetric component of zonal wind component at z = 10 m (U10) as well as of 300hPa heights has been found to be large for both summer and winter periods. Our regression results show that correlation with T_equator (our proxy for global warming) explains most of the climatic changes for the asymmetric component of U10 and 300 hPa heights for summer and winter periods, suggesting the influence of warming of the global oceans on anticyclones south of the Indian Ocean and south-eastern Pacific Ocean.

南半球风暴路径的加强与南北扩张

基于NCEP/NCAR再分析资料,分析了近60年来南半球300hPa风暴路径的变化趋势。研究发现,1956—2015年期间,南半球风暴路径出现了较为明显的增强趋势,尤其是在中高纬地区最为显著。通过构造的风暴路径边界指数,发现南半球风暴路径的北界和南界分别存在向赤道和向极的扩张趋势,其中以南界向极偏移趋势最为明显,整个南半球风暴活动带变得更加宽阔和靠近极地。作为风暴路径获取能量的主要来源,背景场西风急流以及下垫面海表面温度的变化对其强弱和位置变化有显著影响,南半球西风急流南强北弱和海表面温度南冷北暖的变化趋势,是南半球风暴路径增强和扩张的重要原因。

夏季进入北极温带气旋的活动特征及其影响因素

进入北极地区的气旋在移动过程中往往伴有大风、强降水等特征,对北极气候变化有深刻的影响。基于NCEP2再分析资料,识别并跟踪了北半球夏季(6—8月)从中纬度进入北极的温带气旋,考察了其年际变化特征和影响因素。结果表明:1979—2019年夏季进入北极的温带气旋共867个,其中消失在北极边缘区域和中心区域的数量分别为688个和161个,且后者平均强度更大、平均持续时间更长。分区域研究发现,夏季从陆地进入极区的气旋个数较多,而从海洋进入极区的气旋强度更大,活动更为剧烈。对进入北极的气旋年际时间序列进行分析发现,夏季进入北极的气旋个数和强度均存在年际变率,其中气旋个数的年际变率尤为显著。气旋个数年际变率主要周期为5 a,强度的主要周期约为2.7 a。进一步分析发现,引导气流是影响气旋向北移动的重要因素。此外,夏季北大西洋气旋强度与同期北大西洋涛动(North Atlantic Oscillation,NAO)指数存在较好相关。研究还表明,进入极区气旋活动的年际变化受大气斜压不稳定性的影响,在北太平洋地区区域平均的Eady增长率与气旋个数和强度的相关性均最强,相关系数分别为0.4和0.5。

The extreme Arctic warm anomaly in November 2020

2020年11月北极东部显著偏暖,表面气温暖异常为1979年以来第二强,且北极表层偏暖可以延伸至对流层上层.本文进一步研究了此次北极极端偏暖的可能原因.2020年秋季北极海冰大幅减少,11月从北大西洋向北极的水汽输送显著增加,且二者的变化幅度均超过了1979年以来的最高纪录,进而导致北极出现极端暖异常.此外,从中纬度向北极的Rossby波传播有利于向极水汽输送增加,且此过程可能与北大西洋中纬度海温异常有关.