Gravity Wave Activity and Stratosphere-Troposphere Exchange During Typhoon Molave(2020)

To investigate the stratosphere-troposphere exchange(STE)process induced by the gravity waves(GWs)caused by Typhoon Molave(2020)in the upper troposphere and lower stratosphere,we analyzed the ERA5 reanalysis data provided by the European Centre for Medium-Range Weather Forecasts and the CMA Tropical Cyclone Best Track Dataset.We also adopted the mesoscale forecast model Weather Research and Forecasting model V4.3 for numerical simulation.Most of the previous studies were about typhoon-induced STE and typhoon-induced GWs,while our research focused on the STE caused by typhoon-induced gravity waves.Our analysis shows that most of the time,the gravity wave signal of Typhoon Molave appeared below the tropopause.It was stronger on the east side of the typhoon center(10°-20°N,110°-120°E)than on the west side,suggesting an eastward tilted structure with height increase.When the GWs in the upper troposphere and lower stratosphere region on the west side of the typhoon center broke up,it produced strong turbulence,resulting in stratosphere-troposphere exchange.At this time,the average potential vorticity vertical flux increased with the average ozone mass mixing ratio.The gravity wave events and STE process simulated by the WRF model were basically consistent with the results of ERA5 reanalysis data,but the time of gravity wave breaking was different.This study indicates that after the breaking of the GWs induced by typhoons,turbulent mixing will also be generated,and thus the STE.

The Role of Ozone Depletion in the Lack of Cooling in the Antarctic Upper Stratosphere during Austral Winter

Temperature trends in the upper stratosphere are investigated using satellite measurements from Stratospheric Sounding Unit(SSU)outputs and simulations from chemistry-climate models(CCMs)and the Coupled Model Intercomparison Project Phase 6(CMIP6).Observational evidence shows a lack of cooling in the Antarctic,in contrast to strong cooling at other latitudes,during austral winter over 1979-97.Analysis of CCM simulations for a longer period of1961-97 also shows a significant contrast in the upper stratospheric temperature trends between the Antarctic and lower latitudes.Results from two sets of model integrations with fixed ozone-depleting substances(ODSs)and fixed greenhouse gases(GHGs)at their 1960 levels suggest that the ODSs have made a major contribution to the lack of cooling in the Antarctic upper stratosphere.Results from CMIP6 simulations with prescribed GHGs and ozone confirm that changes in the dynamical processes associated with observed ozone depletion are largely responsible for the lack of cooling in the Antarctic upper stratosphere.The lack of cooling is found to be dynamically induced through increased upward wave activity into the upper stratosphere,which is attributed mainly to ODSs forcing.Specifically,the radiative cooling caused by the ozone depletion results in a stronger meridional temperature gradient between middle and high latitudes in the upper stratosphere,allowing more planetary waves propagating upward to warm the Antarctic upper stratosphere.These findings improve our understanding of the chemistry-climate coupling in the southern upper stratosphere.

Influence of topography on the fine structures of stratospheric gravity waves:An analysis using COSMIC-2 temperature data

We derive the potential energy of gravity waves(GWs)in the upper troposphere and stratosphere at 45°S-45°N from December 2019 to November 2022 by using temperature profiles retrieved from the Constellation Observing System for Meteorology,Ionosphere,and Climate-2(COSMIC-2)satellite.Owing to the dense sampling of COSMIC-2,in addition to the strong peaks of gravity wave potential energy(GWPE)above the Andes and Tibetan Plateau,we found weak peaks above the Rocky,Atlas,Caucasus,and Tianshan Mountains.The land-sea contrast is responsible for the longitudinal variations of the GWPE in the lower and upper stratosphere.At 40°N/S,the peaks were mainly above the topographic regions during the winter.At 20°N/S,the peaks were a slight distance away from the topographic regions and might be the combined effect of nontopographic GWs and mountain waves.Near the Equator,the peaks were mainly above the regions with the lowest sea level altitude and may have resulted from convection.Our results indicate that even above the local regions with lower sea level altitudes compared with the Andes and Tibetan Plateau,the GWPE also exhibits fine structures in geographic distributions.We found that dissipation layers above the tropopause jet provide the body force to generate secondary waves in the upper stratosphere,especially during the winter months of each hemisphere and at latitudes of greater than 20°N/S.

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.

Representation of the Stratospheric Circulation in CRA-40 Reanalysis:The Arctic Polar Vortex and the Quasi-Biennial Oscillation

The representation of the Arctic stratospheric circulation and the quasi-biennial oscillation(QBO)during the period 1981–2019 in a 40-yr Chinese global reanalysis dataset(CRA-40)is evaluated by comparing two widely used reanalysis datasets,ERA-5 and MERRA-2.CRA-40 demonstrates a comparable performance with ERA-5 and MERRA-2 in characterizing the winter and spring circulation in the lower and middle Arctic stratosphere.Specifically,differences in the climatological polar-mean temperature and polar night jet among the three reanalyses are within±0.5 K and±0.5 m s^(–1),respectively.The onset dates of the stratospheric sudden warming and stratospheric final warming events at 10 hPa in CRA-40,together with the dynamics and circulation anomalies during the onset process of warming events,are nearly identical to the other two reanalyses with slight differences.By contrast,the CRA-40 dataset demonstrates a deteriorated performance in describing the QBO below 10 hPa compared to the other two reanalysis products,manifested by the larger easterly biases of the QBO index,the remarkably weaker amplitude of the QBO,and the weaker wavelet power of the QBO period.Such pronounced biases are mainly concentrated in the period 1981–98 and largely reduced by at least 39%in 1999–2019.Thus,particular caution is needed in studying the QBO based on CRA-40.All three reanalyses exhibit greater disagreement in the upper stratosphere compared to the lower and middle stratosphere for both the polar region and the tropics.

Himalayas as a global hot spot of springtime stratospheric intrusions:Insight from isotopic signatures in sulfate aerosols

Downward transport of stratospheric air into the troposphere(identified as stratospheric intrusions)could potentially modify the radiation budget and chemical of the Earth's surface atmosphere.As the highest and largest plateau on earth,the Tibetan Plateau including the Himalayas couples to global climate,and has attracted widespread attention due to rapid warming and cryospheric shrinking.Previous studies recognized strong stratospheric intrusions in the Himalayas but are poorly understood due to limited direct evidences and the complexity of the meteorological dynamics of the third pole.Cosmogenic^(35)S is a radioactive isotope predominately produced in the lower stratosphere and has been demonstrated as a sensitive chemical tracer to detect stratospherically sourced air mass in the planetary boundary layer.Here,we report 6-month(April–September 2018)observation of^(35)S in atmospheric sulfate aerosols(^(35)SO_(4)^(2-))collected from a remote site in the Himalayas to reveal the stratospheric intrusion phenomenon as well as its potential impacts in this region.Throughout the sampling campaign,the^(35)SO_(4)^(2-)concentrations show an average of 1,070±980 atoms/m^(3).In springtime,the average is 1,620±730 atoms/m^(3),significantly higher than the global existing data measured so far.The significant enrichments of^(35)SO_(4)^(2-)measured in this study verified the hypothesis that the Himalayas is a global hot spot of stratospheric intrusions,especially during the springtime as a consequence of its unique geology and atmospheric couplings.In combined with the ancillary evidences,e.g.,oxygen-17 anomaly in sulfate and modeling results,we found that the stratospheric intrusions have a profound impact on the surface ozone concentrations over the study region,and potentially have the ability to constrain how the mechanisms of sulfate oxidation are affected by a change in plateau atmospheric properties and conditions.This study provides new observational constraints on stratospheric intrusions in the Himalayas,which would further provide additional information for a deeper understanding on the environment and climatic changes over the Tibetan Plateau.

2-D Modeling and Calculations of Stratospheric Ozone and Influences of Convection, Diffusion, and Time

An engineering system approach of 2-D cylindrical model of transient mass balance calculations of ozone and other concerned chemicals along with fourteen photolysis, ozone-generating and ozone-depleting chemical reaction equations was developed, validated, and used for studying the ozone concentrations, distribution and peak of the layer, ozone depletion and total ozone abundance in the stratosphere. The calculated ozone concentrations and profile at both the Equator and a 60˚N location were found to follow closely with the measured data. The calculated average ozone concentration was within 1% of the measured average, and the deviation of ozone profiles was within 14%. The monthly evolution of stratospheric ozone concentrations and distribution above the Equator was studied with results discussed in details. The influences of slow air movement in both altitudinal and radial directions on ozone concentrations and profile in the stratosphere were explored and discussed. Parametric studies of the influences of gas diffusivities of ozone DO3 and active atomic oxygen DO on ozone concentrations and distributions were also studied and delineated. Having both influences through physical diffusion and chemical reactions, the diffusivity (and diffusion) of atomic oxygen DO was found to be more sensitive and important than that of ozone DO3 on ozone concentrations and distribution. The 2-D ozone model present in this paper for stratospheric ozone and its layer and depletion is shown to be robust, convenient, efficient, and executable for analyzing the complex ozone phenomena in the stratosphere. .

Differential pressure difference based altitude control of a stratospheric satellite

An autonomous altitude adjustment system for a stratospheric satellite(StratoSat)platform is proposed.This platform consists of a helium balloon,a ballonet,and a two-way blower.The helium balloon generates lift to balance the platform gravity.The two-way blower inflates and deflates the ballonet to regulate the buoyancy.Altitude adjustment is achieved by tracking the differential pressure difference(DPD),and a threshold switching strategy is used to achieve blower flow control.The vertical acceleration regulation ability is decided not only by the blower flow rate,but also by the designed margin of pressure difference(MPD).Pressure difference is a slow-varying variable compared with altitude,and it is adopted as the control variable.The response speed of the actuator to disturbance can be delayed,and the overshoot caused by the large inertia of the platform is inhibited.This method can maintain a high tracking accuracy and reduce the complexity of model calculation,thus improving the robustness of controller design.

HCl Quasi-Biennial Oscillation in the Stratosphere and a Comparison with Ozone QBO

HALOE data from 1992 to 2003 are used to analyze the interannual variation of the HCl volume mixing ratio and its quasi-biennial oscillation （QBO） in the stratosphere, and the results are compared with the ozone QBO. Then, the NCAR two-dimensional interactive chemical, dynamical and radiative model is used to study the effects of the wind QBO on the distribution and variation of HCl in the stratosphere. The results show that the QBO signals in the HCl mixing ratio are mainly at altitudes from 50 hPa to 5 hPa; the larger amplitudes are located between 30 hPa and 10 hPa; a higher HCl mixing ratio usually corresponds to the westerly phase of the wind QBO and a lower HCl mixing ratio usually corresponds to the easterly phase of the wind QBO in a level near 20 hPa and below. In the layer near 10 hPa-5 hPa, the phase of the HCl QBO reverses earlier than the phase of the wind QBO; the QBO signals for HCl in the extratropics are also clear, but with reversed phase compared with those over the Tropics. The HCl QBO signals at 30°N are clearer than those at 30°S; the QBOs for HCl and ozone have a similar phase at the 50 hPa-20 hPa level while they are out of phase near 10 hPa; the simulated structures of the HCl QBO agree well with observations. The mechanism for the formation of the HCl QBO and the reason for differences in the vertical structure of the HCl and ozone QBO are attributed to the transport of HCl and ozone by the wind QBO-induced meridional circulation.

Parallel Comparison of the 1982/83,1997/98 and 2015/16 Super El Nios and Their Effects on the Extratropical Stratosphere

This study uses multiple sea surface temperature （SST） datasets to perform a parallel comparison of three super El Nifios and their effects on the stratosphere. The results show that, different from ordinary El Nifios, warm SST anomalies appear earliest in the western tropical Pacific and precede the super El Nifio peak by more than 18 months. In the previous winter, relative to the mature phase of El Nifio, as a precursor, North Pacific Oscillation-like circulation anomalies are observed. A Pacific-North America （PNA） teleconnection appears in the extratropical troposphere during the mature phase, in spite of the subtle differences between the intensities, as well as the zonal position, of the PNA lobes. Related to the negative rainfall response over the tropical Indian Ocean, the PNA teleconnection in the winter of 1997/98 is the strongest among the three super El Nifios. The northern winter stratosphere shows large anomalies in the polar cap temperature and the circumpolar westerly, if the interferences from other factors are linearly filtered from the circulation data. Associated with the positive PNA response in a super El Nino winter, positive polar cap temperature anomalies and circumpolar easterly anomalies, though different in timing, are also observed in the mature winters of the three super El Nifios. The stratospheric polar vortex in the next winter relative to the 1982/83 and 1997/98 events is also anomalously weaker and warmer, and the stratospheric circulation conditions remain to be seen in the coming winter following the mature phase of the 2015/16 event.

Decadal Variation of the Impact of La Nina on the Winter Arctic Stratosphere

The impact of La Ni?a on the winter Arctic stratosphere has thus far been an ambiguous topic of research. Contradictory results have been reported depending on the La Ni?a events considered. This study shows that this is mainly due to the decadal variation of La Ni?a’s impact on the winter Arctic stratosphere since the late 1970 s. Specifically,during the period1951–78,the tropospheric La Ni?a teleconnection exhibits a typical negative Pacific–North America pattern,which strongly inhibits the propagation of the planetary waves from the extratropical troposphere to the stratosphere,and leads to a significantly strengthened stratospheric polar vortex. In contrast,during 1979–2015,the La Ni?a teleconnection shifts eastwards,with an anomalous high concentrated in the northeastern Pacific. The destructive interference of the La Ni?a teleconnection with climatological stationary waves seen in the earlier period reduces greatly,which prevents the drastic reduction of planetary wave activities in the extratropical stratosphere. Correspondingly,the stratospheric response shows a less disturbed stratospheric polar vortex in winter.

The Impact of Cut-off Lows on Ozone in the Upper Troposphere and Lower Stratosphere over Changchun from Ozonesonde Observations

In situ measurements of the vertical structure of ozone were made in Changchun （43.53°N, 125.13°E）, China, by the Institute of Atmosphere Physics, in the summers of 2010-13. Analysis of the 89 validated ozone profiles shows the vari- ation of ozone concentration in the upper troposphere and lower stratosphere （UTLS） caused by cut-off lows （COLs） over Changchun. During the COL events, an increase of the ozone concentration and a lower height of the tropopause are observed. Backward simulations with a trajectory model show that the ozone-rich airmass brought by the COL is from Siberia. A case study proves that stratosphere-troposphere exchange （STE） occurs in the COL. The ozone-rich air mass transported from the stratosphere to the troposphere first becomes unstable, then loses its high ozone concentration. This process usually happens during the decay stage of COLs. In order to understand the influence of COLs on the ozone in the UTLS, statistical analysis of the ozone profiles within COLs, and other profiles, are employed. The results indicate that the ozone concentrations of the in-COL profiles are significantly higher than those of the other profiles between ±4 km around the tropopause. The COLs induce an increase in UTLS column ozone by 32% on average. Meanwhile, the COLs depress the lapse-rate tropopause （LRT）/dynamical tropopause height by 1.4/1.7 km and cause the atmosphere above the tropopause to be less stable. The influence of COLs is durable because the increased ozone concentration lasts at least one day after the COL has passed over Changchun. Furthermore, the relative coefficient between LRT height and lower stratosphere （LS） column ozone is -0.62, which implies a positive correlation between COL strength and LS ozone concentration.

A multi-location joint field observation of the stratosphere and troposphere over the Tibetan Plateau

The unique geographical location and high altitude of the Tibetan Plateau can greatly influence regional weather and climate.In particular, the Asian summer monsoon(ASM) anticyclone circulation system over the Tibetan Plateau is recognized to be a significant transport pathway for water vapor and pollutants to enter the stratosphere. To improve understanding of these physical processes, a multi-location joint atmospheric experiment was performed over the Tibetan Plateau from late July to August in 2018, funded by the fiveyear(2018–2022) STEAM(stratosphere and troposphere exchange experiment during ASM) project, during which multiple platforms/instruments—including long-duration stratospheric balloons, dropsondes, unmanned aerial vehicles, special sounding systems, and ground-based and satellite-borne instruments—will be deployed. These complementary methods of data acquisition are expected to provide comprehensive atmospheric parameters(aerosol, ozone, water vapor, CO_2, CH_4, CO, temperature, pressure,turbulence, radiation, lightning and wind); the richness of this approach is expected to advance our comprehension of key mechanisms associated with thermal, dynamical, radiative, and chemical transports over the Tibetan Plateau during ASM activity.

Arctic oscillation and the antarcic oscillation modes in the atmospheric stratosphere

On the basis of the EOF analysis of global geopotential height anomaly(GHA)field at 10 hPa level,the arctic oscillation(AO)and the means antarctic oscillation(AAO)can bedetected more obviously at the upper level of atmosphere than the AO or the AAO in surface layer.Unlike the hemisphere pattern of the AO and the AAO in the surface lager given by previous authors,the AO or the AAO in the stratosphere has its global features.The zonal oscillations—the SouthernOscillation(SO)and the north oscillation(NO)in atmospheric surface layer become less clear inthe upper air.The first mode(AO mode,abbreviated to AOM hereafter)and the second mode(AAO mode,abbreviated to AAOM hereafter)respectively have 41.47%and 27.04%of the total variancecontribution.The cumulative variance contribution of the first two modes reaches 68.51%.These twomodes are the main components for the interdecadal or decadal oscillation in the stratosphere.Inaddition,there still exist two kinds of oscillation patterns with less probability,namely,thesymmetric pattern at mid-high latitudes in the Southern Hemisphere and the asymmetric pattern.Spectral analysis shows that the AOM and the AAOM all have a spectral peak for 22 a period,beingconsistent with the periodic variations of the solar magnetic field,and a peak for 11 a period,being consistent with the period of the numbers of sunspots.Step filter analysis shows that theinfluencing factor for the upper atmospheric oscillation is the solar activity.The fluctuation ofthe solar magnetic field is the more influencing factor than the variation of the sunspot number.

Study on the Trace Species in the Stratosphere and Their Impact on Climate

The trace gases （O3, HCl, CH4, H2O, NO, NO2） in the stratosphere play an important role, not only in the photochemical processes in which the ozone layer destroyed, but also in the radiative processes. In this paper, we review the works on the distribution and variation of the trace gases in the stratosphere and their impact on climate, which have been carried out at the University of Science and Technology of China in the recent 20 years. The Halogen Occultation Experiment （HALOE） data were used to analyse the distribution and variation of the mixing ratio of these trace gases and the temperature trends in the stratosphere in the most recent decade. And the reanalyzed National Centers of Environmental Prediction （NCEP）/NCAR data were also used to give the temperature trends and compared with the results from HALOE data. Numerical simulations were also carried out to study the impact of ozone depletion on the global climate. In this review, the distributions of the trace gases, especially those over the Qinghai-Xizang Plateau, are discussed, and the variations and trends for the trace gases in various levels in the stratosphere have been given for the most recent decade. The temperature variation and the cooling trend obtained from HALOE data in the middle and lower stratosphere for the last 13 years are significant, which agree well with the results from NCEP/NCAR data. While the temperature trend in the upper stratosphere in this period do not seem to have much cooling. The numerical simulations show that either the Antarctic ozone hole or the ozone valley over Qinghai-Xizang Plateau affect not only the temperature and circulation in the stratosphere, but also the temperature, pressure and wind fields in the troposphere, then lead to the global climate change.

On the Temporal and Spatial Structure of Troposphere-toStratosphere Transport in the Lowermost Stratosphere over the Asian Monsoon Region during Boreal Summer

This study produced a novel characterization of the troposphere-to-stratosphere transport （TST） over the Asian monsoon region during boreal summer, using a comprehensive analysis of 60-day backward trajectories initialized in the stratosphere. The trajectory datasets were derived from the high-resolution Lagrangian particle dispersion model （FLEXPART） simulation driven by the wind fields acquired from the National Center for Environmental Prediction （NCEP）. The results indicate that the distribution of residence time （tTST） of tropopause-crossing trajectories in the lowermost stratosphere represents a horizontal signature of the Asian summer monsoon. Vertically, the distribution of tTST can be roughly separated into two layers： a consistent lower layer with tTST 〈5 days forming a narrow band, corresponding to a layer-3 km thick following the location of the tropopause, and an upper layer at a larger distance from the local tropopause. The maximum residence time was -20 days, especially within the Asian high anticyclone consistent with its confinement effects. In general, the overall geographical distribution of dehydration points was not coincident with the location of tropopause crossing. TST trajectories, which were initialized in the stratosphere, underwent their Lagrangian cold points mostly in the tropics and subtropics 1 4 days after the TST event; they were characterized by a wide range of temperature differences, with a mean value of 3-12 K. The vertical extent of the influence of tropospheric intrusion on the Asian monsoon region in the stratosphere exhibited a peak at -16.5-18.5 km, and the uppermost height was -21 km.

Simulation of the Effect of Water-vapor Increase on Temperature in the Stratosphere

To analyze the mechanism by which water vapor increase leads to cooling in the stratosphere, the effects of water-vapor increases on temperature in the stratosphere were simulated using the two-dimensional, interactive chemical dynamical radiative model （SOCRATES） of NCAR. The results indicate that increases in stratospheric water vapor lead to stratospheric cooling, with the extent of cooling increasing with height, and that cooling in the middle stratosphere is stronger in Arctic regions. Analysis of the radiation process showed that infrared radiative cooling by water vapor is a pivotal factor in middle-lower stratospheric cooling. However, in the upper stratosphere （above 45 km）, infrared radiation is not a factor in cooling; there, cooling is caused by the decreased solar radiative heating rate resulting from ozone decrease due to increased stratospheric water vapor. Dynamical cooling is important in the middle-upper stratosphere, and dynamical feedback to temperature change is more distinct in the Northern Hemisphere middle-high latitudes than in other regions and signiffcantly affects temperature and ozone in winter over Arctic regions. Increasing stratospheric water vapor will strengthen ozone depletion through the chemical process. However, ozone will increase in the middle stratosphere. The change in ozone due to increasing water vapor has an important effect on the stratospheric temperature change.

Study on Horizontal Relative Diffusion in theTroposphere and Lower Stratosphere

The behaviour of relative diffusion theory and Gifford’s random-force theory for long-range atmospheric diffusion is examined. When a puff scale is smaller than the Lagrangian length scale, √2KTL, an accelerative relative diffusion region exists, i.e., σy∝ t 3/2. While the puff diffusion enters a two-dimensional turbulence region, in which the diffusion scale is larger than 500 km, or time scale is larger than 1 day, divergence and convergence are main cause of horizontal diffusion. Between the two above-mentioned regimes, diffusion deviation is given by σy = √2KT. The large-scale horizontal relative diffusion parameters were obtained by analyzing the data of radioactive cloud width collected in air nuclear tests. Key words Tropospheric and lower stratospheric diffusion - Relative diffusion - Large scale turbulence - Nuclear explosion clouds This work is sponsored by the National Natural Science Foundation of China under Grant No. 49505064.The author would like to thank Prof. Chen Jiayi Department of Geophysics of Peking University and Dr. Cai Xiaoming School of Geography and Environmental Sciences of Birmingham University for their helpful discussions.

Kinetic Analysis of Vectored Electric Propulsion System for Stratosphere Airship

To enhance the controllability of stratosphere airship,a vectored electric propulsion system is used.By using the Lagrangian method,a kinetic model of the vectored electric propulsion system is established and validated through ground tests.The fake gyroscopic torque is first proposed,which the vector mechanism should overcome besides the inertial torque and the gravitational torque.The fake gyroscopic torque is caused by the difference between inertial moments about two principal inertial axes of the propeller in the rotating plane,appears only when the propeller is rotating and is proportional with the rotation speed.It is a sinusoidal pulse,with a frequency that is twice of the rotation speed.Considering the fake gyroscope torque pulse and aerodynamic efficiency,three blade propeller is recommended for the vectored propulsion system used for stratosphere airship.

Observed Long-and Short-lived North Atlantic Oscillation Events:Role of the Stratosphere

Utilizing three different sets of reanalysis data,this study examines the long-and short-lived observed positive North Atlantic Oscillation(NAO)events(referred to as NAO+_LE and NAO+_SE)and long-and short-lived observed negative NAO events(referred to as NAO−_LE and NAO−_SE).Composite results indicate that the NAO-like circulation anomalies associated with the long-lived NAO events can reach the stratosphere,while they are primarily confined to the troposphere in the short-lived NAO events.Thus,the coupling/connection of stratospheric and tropospheric circulation anomalies is much better(worse)in the long-lived(short-lived)NAO events.A series of modified stratospheric initial-value experiments conducted with a simplified model indicate that a better(worse)connection between stratospheric and tropospheric circulation anomalies in the initial-value fields tend to gradually induce the NAO-like tropospheric circulation anomalies in the troposphere on the subsequent days,and thus naturally elongate(reduce)the lifetimes of the original NAO events by altering the tropospheric synoptic eddy vorticity flux over the North Atlantic region.