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An international joint research project, entitled Interhemispheric Coupling Study by Observations and Modelling (ICSOM), is ongoing. In the late 2000s, an interesting form of interhemispheric coupling (IHC) was discovered: when warming occurs in the winter polar stratosphere, the upper mesosphere in the summer hemisphere also becomes warmer with a time lag of days. This IHC phenomenon is considered to be a coupling through processes in the middle atmosphere (i.e., stratosphere, mesosphere, and lower thermosphere). Several plausible mechanisms have been proposed so far, but they are still controversial. This is mainly because of the difficulty in observing and simulating gravity waves (GWs) at small scales, despite the important role they are known to play in middle atmosphere dynamics. In this project, by networking sparsely but globally distributed radars, mesospheric GWs have been simultaneously observed in seven boreal winters since 2015/16. We have succeeded in capturing five stratospheric sudden warming events and two polar vortex intensification events. This project also includes the development of a new data assimilation system to generate long‐term reanalysis data for the whole middle atmosphere, and simulations by a state‐of‐the‐art GW‐permitting general circulation model using the reanalysis data as initial values. By analyzing data from these observations, data assimilation, and model simulation, comprehensive studies to investigate the mechanism of IHC are planned. This paper provides an overview of ICSOM, but even initial results suggest that not only GWs but also large‐scale waves are important for the mechanism of the IHC. Plain Language Summary: In the late 2000s, an interesting form of the coupling between the Northern and Southern Hemispheres was discovered: when the winter polar stratosphere warms, the upper summer mesosphere also warms several days later. An international research project called Interhemispheric Coupling Study by Observations and Modelling (ICSOM) is ongoing to examine the mechanism of this interhemispheric coupling (IHC). This IHC phenomenon is thought to be the connection in the middle atmosphere (i.e., stratosphere, mesosphere, and lower thermosphere). Several promising mechanisms have been proposed, but they remain controversial. This is because gravity waves (GWs) having small scales, which are difficult to observe and simulate, are thought to play a crucial role in the coupling. So, we have performed observations of GWs by networking radars over seven Northern Hemisphere winters, and succeeded in capturing five stratospheric warming events and two opposite events. We also developed a new data assimilation system for the entire middle atmosphere and used the global data produced by the system to simulate GWs with a high‐resolution global model. By combining these research tools, we plan to elucidate the mechanism of IHC comprehensively. This paper presents an overview of ICSOM. Initial results show that not only GWs but also large‐scale waves are important for the IHC mechanism. Key Points: An international project is ongoing to elucidate the mechanism of interhemispheric coupling (IHC) in the middle atmosphereGravity waves (GWs), which are thought to play a key role in IHC, were observed by a radar network and simulated by high‐resolution global modelInitial results suggest that not only GWs but also large‐scale waves are important for the IHC mechanism [ABSTRACT FROM AUTHOR]

It is well known that stratospheric sudden warmings (SSWs) are a result of the interaction between planetary waves (PWs) and the stratospheric polar vortex. SSWs occur when breaking PWs slow down or even reverse this zonal wind jet and induce a sinking motion that adiabatically warms the stratosphere and lowers the stratopause. In this paper we characterize this downward progression of stratospheric temperature anomalies using 18 years (2003–2020) of Sounding of the Atmosphere using Broadband Radiometry (SABER) observations. SABER temperatures, derived zonal winds, PW activity and gravity wave (GW) activity during January and February of each year indicate a high‐degree of year‐to‐year variability. From 11 stratospheric warming events (9 major and 2 minor events), the descent rate of the stratopause altitude varies from 0.5 to 2 km/day and the lowest altitude the stratopause descends to varies from <20 to ∼50 km (i.e., no descent). A composite analysis of temperature and squared GW amplitude anomalies indicate that the downward descent of temperature anomalies from 50 to ∼25 km lags the downward progression of increased GW activity. This increased GW activity coincides with the weakening and reversal of the westward zonal winds in agreement with previous studies. Our study suggests that although PWs drive the onset of SSWs at 30 km, GWs also play a role in contributing to the descent of temperature anomalies from the stratopause to the middle and lower stratosphere. Key Points: Eighteen‐years of Sounding of the Atmosphere using Broadband Radiometry (SABER) temperatures indicate variable downward progression of stratospheric sudden warming (SSW) related temperature anomaliesDescent rate of temperature anomalies varies from ∼0.5 to 2 km/day with lower descent leading to warmer temperatures at 30 kmGravity waves contribute to the downward progression of temperature anomalies from the stratopause and the onset of SSWs at 30 km [ABSTRACT FROM AUTHOR]

Long‐term efforts have sought to extend global model resolution to smaller scales enabling more accurate descriptions of gravity wave (GW) sources and responses, given their major roles in coupling and variability throughout the atmosphere. Such studies reveal significant improvements accompanying increasing resolution, but no guidance on what is sufficient to approximate reality. We take the opposite approach, using a finite‐volume model solving the Navier‐Stokes equations exactly. The reference simulation addresses mountain wave (MW) generation and responses over the Southern Andes described using isotropic 500 m, central resolution by Fritts et al. (2021), https://doi.org/10.1175/JAS-D-20-0207.1 and Lund et al. (2020), https://doi.org/10.1175/JAS-D-19-0356.1. Reductions of horizontal resolution to 1 and 2 km result in (a) systematic increases in initial MW breaking altitudes, (b) weaker, larger‐scale generation of secondary GWs and acoustic waves accompanying these dynamics, and (c) significantly weaker and less extended responses in the mesosphere in latitude and longitude. Horizontal resolution of 4 km largely suppresses instabilities, but allows weak, sustained mean‐flow interactions. Responses for 8 km resolution are very weak and fail to capture any aspects of the high‐resolution responses. The chosen mean winds allow efficient MW penetration into the mesosphere and lower thermosphere, hence only exhibit strong pseudo‐momentum deposition and mean wind decelerations at higher altitudes. A companion paper by Fritts et al. (2022), https://doi.org/10.1029/2021JD036035 explores the impacts of decreasing resolution on responses in the thermosphere. Plain Language Summary: Mountain waves play major roles in the large‐scale circulation and structure of the atmosphere extending well into the thermosphere. Their primary influences at large scales depend on energy and momentum transports from sources at lower altitudes and deposition at higher altitudes accompanying wave‐breaking dynamics and instabilities that cannot currently be described by global models. Responses to these small‐scale dynamics in the stratosphere and lower mesosphere include local mean flow decelerations that extend large distances downstream, upstream, and laterally. The ability to describe these dynamics degrades rapidly with decreasing resolution, causing current global models to exhibit systematic circulation biases that significantly limit and degrade weather and climate prediction. We assess these impacts with a suite of simulations of wintertime flow over the S. Andes at horizontal resolutions from 0.5 to 8 km. Our results reveal that spatial resolution of ∼2 km or better is required to adequately address these deficiencies extending into the mesosphere. Key Points: Simulations of mountain wave (MW) dynamics reveal decreasing fidelity in MW and large‐scale fields at degraded resolutionTwo‐km resolution or better yields reasonable fidelity to MW fields, momentum fluxes, and large‐scale fields into the mesosphereFour‐km resolution fails to capture realistic MWs, fluxes, and responses; 8 km resolution yields no resemblance to well‐resolved MW fields [ABSTRACT FROM AUTHOR]

The circulation of the stratosphere transports important trace gases, including ozone, and can be thought of as having a fast horizontal mixing component and a slow meridional overturning component. Measuring the strength of the circulation directly is not possible, and so it must be inferred from tracer measurements. Long‐lived trace gases can be related to the idealized tracer age of air, which describes how long an air parcel has been in the stratosphere. In this paper, we derive a quantitative relationship between the vertical gradient of age and the horizontal mixing between the tropics and the extratropics using a "leaky pipe" framework in isentropic coordinates. Mixing rates of air into and out of the tropics are related to the vertical gradient of age in the tropics and in the extratropics, respectively. The derivation is repeated with the hemispheres separated so that the vertical structure of the mixing in the two hemispheres can be compared directly. These theories are applied to output from an idealized model of the stratosphere and from a realistic chemistry‐climate model to test our assumptions and calculate the mixing rates in the models. We then perform a quantitative comparison of the mixing rates in the Northern and Southern hemispheres along with an examination of where such a separation is valid. Finally, we perform a very preliminary calculation of mixing efficiency with satellite data to demonstrate the use of the mixing metric to compare mixing in models and data. Key Points: A new metric for adiabatic mixing in the stratosphere is developed from the age of air tracerWe examine hemispheric asymmetry and discuss the slow time evolution of the flowWe calculate this metric to perform a preliminary comparison between models and satellite data [ABSTRACT FROM AUTHOR]

It has recently been recognized that, in addition to greenhouse gases, anthropogenic emissions of ozone‐depleting substances (ODS) can induce long‐term trends in the Brewer‐Dobson circulation (BDC). Several studies have shown that a substantial fraction of the residual circulation acceleration over the last decades of the twentieth century can be attributed to increasing ODS. Here the mechanisms of this influence are examined, comparing model runs to reanalysis data and evaluating separately the residual circulation and mixing contributions to the mean age of air trends. The effects of ozone depletion in the Antarctic lower stratosphere are found to dominate the ODS impact on the BDC, while the direct radiative impact of these substances is negligible over the period of study. We find qualitative agreement in austral summer BDC trends between model and reanalysis data and show that ODS are the main driver of both residual circulation and isentropic mixing trends over the last decades of the twentieth century. Moreover, aging by isentropic mixing is shown to play a key role on ODS‐driven age of air trends. Plain Language Summary: Concentrations of human‐emitted ozone‐depleting substances in the stratosphere have increased substantially during the last decades of the twentieth century, which are found in the Southern Hemisphere. These substances have caused stratospheric ozone depletion, which not only has important direct impacts on human health but also leads to important changes in the atmospheric circulation and climate. Here the impact of these substances on the stratospheric mean transport circulation is examined using chemistry‐climate model simulations and reanalysis data. In addition to destroying ozone, these substances act as greenhouse gases. In this paper, we find that the radiative forcing impacts of human‐emitted ozone‐depleting substances on the stratospheric circulation over the last decades of the twentieth century are negligible. In contrast, the Antarctic ozone hole has contributed substantially to the acceleration of stratospheric tracer transport not only in the polar region but globally. The impacts, largest in the austral summer stratosphere, are examined in detail separating the overturning circulation and irreversible mixing. It is shown that trends in the model compare qualitatively well with estimates from two different reanalyses. This paper contributes to advance our understanding of the impact of human emissions on the stratospheric circulation and to untangle the forcings of recent trends. Key Points: Increasing ODS have played a key role in observed BDC trends over the last decades of the twentieth centuryOzone depletion dominates the ODS impact on BDC trends, while the radiative effect of these substances is negligibleIncluding changes in mixing in addition to the residual circulation is crucial to interpreting past mean age trends [ABSTRACT FROM AUTHOR]

This paper describes an updated method for automated detection of overshooting cloud tops (OT) using a combination of spatial infrared (IR) brightness temperature patterns and modeled tropopause temperature. IR temperatures are normalized to the tropopause, which serves as a stable reference that modulates how cold a convective cloud should become within a given region. Anvil clouds are identified using histogram analysis and cold spots embedded within anvils serve as OT candidate regions. OT candidates are then assigned an OT probability, which can be interpreted as a metric of storm intensity and an estimate of confidence in a detection for a particular pixel. It is produced using an original mathematical composition of four factors: Tropopause‐normalized temperature, prominence relative to the surrounding anvil, surrounding anvil area, and spatial uniformity of anvil temperature, which are calculated from empirically derived sensitivity curves. The shape of the curves is supported by independent analysis of a large sample of matched IR and radar‐derived OT regions. An optimal sensitivity for each factor was determined by maximizing correlation between the OT probability and a set of human‐identified OT regions. Coarser spatial resolution of GOES‐13 data cause OTs to be less prominent compared to GOES‐16, necessitating different sensitivities for each satellite. Detection performance is quantified for each satellite based on human OT identifications and as a function of how prominent the OT appeared in visible and IR imagery. Based on analyses of human‐identified OTs, OT detection accuracy, defined by the area under a receiver operating characteristic curve, is determined to be 0.94 for GOES‐16 and 0.78 for GOES‐13. Key Points: A new OT detection method combines tropopause‐relative infrared (IR) temperature, anvil‐relative prominence, anvil area and its spatial uniformityOvershooting cloud tops (OT) probability derived from spatial cloud analyses is validated with human‐identified OTs, revealing improvements over previous methodsDiffering imagery spatial resolution necessitates optimization of sensitivity curves to account for warmer observed OTs from coarser imagery [ABSTRACT FROM AUTHOR]

Daily radiosonde observations between 2003 and 2013 from Halley research station, Antarctica (75°S, 26°W), are used to determine climatologies of gravity wave properties in the lower stratosphere (between 15 km and 22 km altitude). Individual waves are extracted from the radiosonde profile using wavelet analysis and separated into upward and downward propagating waves. An increase in the percentage of downward propagating waves (~30% of the waves) is seen during the winter months. For the upward and downward propagating waves, their horizontal and vertical wavelength, intrinsic frequency, energy density, pseudomomentum flux, and direction of propagation are determined. The upward propagating wavefield is found to be dominated by waves with short vertical wavelength (~1 km) and low intrinsic frequency ( ω ~ f). The downward propagating wavefield is composed of a wider distribution of vertical wavelength waves and has a larger proportion of higher-frequency waves present. The upward propagating waves show an increase in total energy density in autumn and spring; the larger increase occurs during spring (up to 1.7 J kg−1 in September). The downward propagating waves increase in total energy density occurs during wintertime (up to 0.7 J kg−1 in June). During winter the contributions of the upward and downward propagating waves to the total energy density and pseudomomentum flux are almost equal. This paper details the first study of individual gravity wave properties combined into upward and downward propagating wave climatologies in the lower stratosphere above Halley. [ABSTRACT FROM AUTHOR]

Changes in concentrations of greenhouse gases lead to changes in radiative fluxes throughout the atmosphere. The value of this change, the instantaneous radiative forcing, varies across climate models, due partly to differences in the distribution of clouds, humidity, and temperature across models and partly due to errors introduced by approximate treatments of radiative transfer. This paper describes an experiment within the Radiative Forcing Model Intercomparision Project that uses benchmark calculations made with line‐by‐line models to identify parameterization error in the representation of absorption and emission by greenhouse gases. Clear‐sky instantaneous forcing by greenhouse gases is computed using a set of 100 profiles, selected from a reanalysis of present‐day conditions, that represent the global annual mean forcing from preindustrial times to the present day with sampling errors of less than 0.01 W m−2. Six contributing line‐by‐line models agree in their estimate of this forcing to within 0.025 W m−2 while even recently developed parameterizations have typical errors 4 or more times larger, suggesting both that the samples reveal true differences among line‐by‐line models and that parameterization error will be readily identifiable. Agreement among line‐by‐line models is better in the longwave than in the shortwave where differing treatments of the water vapor continuum affect estimates of forcing by carbon dioxide and methane. The impacts of clouds on instantaneous radiative forcing are estimated from climate model simulations, and the adjustment due to stratospheric temperature changes estimated by assuming fixed dynamical heating. Adjustments are large only for ozone and for carbon dioxide, for which stratospheric cooling introduces modest nonlinearity. Key Points: Mean clear‐sky instantaneous radiative forcing by greenhouse gases is computed with six benchmark models using 100 atmospheric profilesSampling error is several times smaller than the level of disagreement among models, which is itself smaller than parameterization errorThe impacts of clouds and stratospheric adjustment are roughly estimated; adjustments are large only for carbon dioxide and ozone [ABSTRACT FROM AUTHOR]

A rare disturbance of the stratospheric Antarctic polar vortex in September 2019 led to a significantly higher than usual polar total ozone column. We use assimilation of ozone, HCl, and N2O data from the Microwave Limb Sounder with the Global Earth Observing System (GEOS) Constituent Data Assimilation System driven by reanalysis meteorology to study the evolution of the 2019 Antarctic polar ozone. We find that the maximum 2019 ozone hole area was near 10 × 106 km2, and as little as 20% of that in 2018 in mid‐September. However, the magnitude of vortex‐averaged chemical ozone depletion was not significantly different between the 2 years despite earlier chlorine deactivation in 2019. The assimilation results show that most of the differences between 2018 and 2019 Antarctic ozone resulted from two factors: (1) the geometry of the 2019 vortex, with ozone‐rich middle‐stratospheric air masses overlying the lower portion of the vortex and leading to a significant reduction of the total column, and (2) significantly reduced vortex volume. The anomalously small ozone hole of 2019 was comparable in size to the record breaking 2002 case and the mechanisms responsible were similar in the two cases. While the 2019 sudden stratospheric warming is classified as minor, its impact on ozone was very significant. Plain Language Summary: A significant dynamical disturbance of the stratospheric polar vortex occurred over Antarctica in September 2019. Named sudden stratospheric warmings (SSWs), such events are relatively common in the Northern Hemisphere but exceedingly rare over the southern high latitudes. SSWs are known to disturb the chemical composition of the polar vortex, which slows down the reactions that lead to ozone depletion. In this paper we use observations of chemical composition of the 2019 Antarctic polar vortex from National Aeronautics and Space Administration (NASA)'s satellite sensor, the Microwave Limb Sounder, combined with a Global Earth Observing System model simulation to study the chemistry and dynamics of this unusual polar winter/spring season. We show that the 2019 SSW, although not classified as "major," led to more than a 50% reduction of the ozone hole area at its peak extent, compared to other years since 2004. Furthermore, we demonstrate that most of this reduction was caused by an unusual geometry and small size of the polar vortex, while the chemistry of the 2019 ozone hole was comparable to that in the undisturbed year 2018. The 2019 event can be regarded as an example of a minor sudden stratospheric warming with a major impact. Key Points: The 2019 ozone hole area was about 10 ×106 km2 or less, compared to over 20×106 km2 typical for Septembers 2005–2018The anomalously high Antarctic total ozone resulted from an unusual polar vortex size and geometry rather than from chemistryEven a minor sudden stratospheric warming in the Southern Hemisphere can have a big impact on polar total ozone [ABSTRACT FROM AUTHOR]

Stratospheric water vapor (SWV) is recognized as a potentially important positive feedback in global warming. The SWV change induces significant downward radiative flux perturbation at the tropopause and therefore is hypothesized to substantially amplify the surface warming. To test this hypothesis, we use a global climate model to quantify the surface warming contributed by the SWV change in the context of the quadrupled CO2. By prescribing the SWV increase as an external forcing, we find that SWV only accounts for 0.42 K surface warming, making up merely 5.4% of the total CO2‐caused surface warming (7.7 K). The efficacy of the stratosphere‐adjusted SWV forcing is small (38%), where the efficacy is defined as the ratio of the global temperature response per unit radiative forcing relative to that of the CO2 forcing. With the aid of a series of auxiliary experiments, we find that although the stratosphere‐adjusted SWV forcing at the top of atmosphere (TOA) is significant (1.13 W m−2), more than half of the forcing is offset by a high‐cloud decrease and an upper tropospheric warming in the tropospheric adjustment. The direct radiative impact of the SWV increase on surface temperature is negligible, and the SWV‐induced surface temperature change is a result of interactions between the radiative and nonradiative processes. Plain Language Summary: What role does the stratospheric water vapor play in global warming? Previous works demonstrated that stratospheric water vapor increase exerts significant downward radiative flux perturbation at the tropopause. This paper investigates whether this SWV radiative perturbation can substantially amplify the surface warming. Our simulation shows a negative answer to this question. The surface warming attributed to the stratospheric water vapor change is small, due to the effect of the tropospheric adjustment. The stratospheric water vapor increase causes a decrease in the high clouds and an increase in the upper tropospheric temperature, which offsets more than half of the stratospheric water vapor‐induced radiative perturbation. Key Points: The surface warming efficacy of the stratosphere‐adjusted SWV forcing is low (38%)SWV increase accounts for about 5% of total warming in the context of CO2‐driven global warmingMore than half of the stratosphere‐adjusted SWV forcing is offset by the high cloud decrease and the upper tropospheric warming in the tropospheric forcing adjustment [ABSTRACT FROM AUTHOR]

A carbonaceous aerosol plume associated with wildfires in British Columbia in August 2017 reached the stratosphere a few days following the fires onset. The Earth Polychromatic Imaging Camera (EPIC) sensor onboard the Deep Space Climate Observatory (DSCOVR) satellite observed the spatial and temporal evolution of the plume for about 6 weeks. EPIC's near‐hourly observations of the Earth's sunlit disk allowed for monitoring the smoke plume several times per day. High ultraviolet aerosol index values pointed to the presence of an aerosol layer in the upper troposphere and lower stratosphere, confirmed with Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar observations. EPIC aerosol optical depth retrievals and CALIOP information on layer height were used to estimate the stratospheric portion of the aerosol mass, reaching a maximum of 268 ± 86 Kt 3 days after the onset of the wildfires. About 10% of this aerosol mass reached the stratosphere on 13 August, whereas the remaining 90% did so over the following 2 days. Observed daytime and inferred nighttime rates of stratospheric aerosol mass increase showed a more rapid daytime increase, consistent with variability in diurnal insolation, suggesting that solar radiation driven processes may have contributed to the observed stratospheric aerosol mass increase. Global modeling results support the possible role of diabatic aerosol self‐lofting. Ozone Mapping and Profiler Suite Limb Profiler (OMPS‐LP) aerosol retrievals showed the spread of stratospheric aerosols in the Northern Hemisphere, reaching peak concentration between October 2017 and January 2018, when about half the Northern Hemisphere experienced a detectable enhancement of stratospheric aerosols with respect to preinjection conditions. The lifetime and aerosol mass of this smoke event were comparable to those of a moderate volcanic eruption. Plain Language Summary: The 2017 wildfire season in western North America was unusually intense. As documented by several media outlets (https://globalnews.ca/news/3921710/b‐c‐year‐in‐review‐2017‐wildfires/), over 1,300 fires and more than 1.2 million hectares burned were reported between April and November in the Canadian British Columbia province. In mid‐August, the combination of very intense boreal fires and typical summer meteorological conditions led to the rapid formation fire‐triggered convective clouds of considerable vertical extent. Because of the pyroconvective activity of these clouds, thousands of tons of carbon‐containing smoke particles, also known as aerosols, were injected above 10 km in a few hours, some of which may have been directly injected in the stratosphere. Heating of the aerosol layer resulting from absorption of solar radiation induced lofting of the smoke plume into the stratosphere (generally above 12 km in this area), a region of the atmosphere where aerosol removal processes are slow. Because of their long residence time, stratospheric aerosols are very important as they affect Earth's radiative budget and climate. In this paper, we use satellite observations and models to describe the extended three‐dimensional spatial distribution and long lifetime of the resulting aerosol plume that made this an unusual aerosol event. Key Points: Carbonaceous aerosols generated by the 2017 Canadian wildfires reached the upper troposphere and lower stratosphereDiabatic heating from aerosol absorption of solar radiation may have triggered the lofting of the aerosol layer from the upper troposphere to the stratosphereThe aerosol layer covered the northern hemisphere from about 30°N to the North Pole for a 10‐month period between August 2017 to June 2018 [ABSTRACT FROM AUTHOR]

Compatibility of the stratospheric ozone profile data from the Ozone Mapping and Profiler Suite Limb Profiler (OMPS‐LP) and the Microwave Limb Sounder (MLS) is assessed in the context of a continuity requirement for future reanalyses. A methodology for the assimilation of OMPS‐LP data into the Goddard Earth Observing System data assimilation system with a stratospheric chemistry module is developed. It is demonstrated that a simple homogenization technique significantly reduces the bias between OMPS‐LP and MLS analyses. With the homogenization applied, the mean difference between the two analyses is within 3% and the difference standard deviation within 10%, consistent with the estimated uncertainties of the satellite ozone data. Larger differences are seen in the tropical lower stratosphere. The MLS and OMPS‐LP assimilation experiments agree very well with independent data from ozonesondes and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer. Heterogeneous ozone depletion during polar spring in both hemispheres as well as ozone transport during the 2016 quasi‐biennial oscillation disruption event are realistically represented in both analyses. This work establishes a step toward achieving continuity of the post‐2004 ozone record in future reanalyses, necessary for trend and long‐term ozone variability studies, although further development is needed to address a drift in the OMPS‐LP ozone data. Plain Language Summary: Following a period of decline in the second half of the twentieth century, stratospheric ozone has begun its road to recovery owing to the successful implementation of the Montreal Protocol and its amendments. Monitoring of the evolution of stratospheric ozone continues to be of interest to the scientific community and the public. This paper presents a step toward a future multidecadal analysis of stratospheric ozone using data from two types of satellite instruments: The Microwave Limb Sounder (MLS, 2004 to present) and the Ozone Mapping and Profiler Suite Limb Profiler (OMPS‐LP, 2012 to present with projected future missions). We show that a simple homogenization methodology can be applied to remove the relative bias between the two data sets. We produce global records of stratospheric ozone for the year 2016 using a data assimilation methodology, which combines the satellite observations with an atmospheric chemistry model output. We show that MLS and OMPS‐LP assimilation experiments are in excellent agreement with independent data and with each other, providing a strong basis for a consistent future multidecadal ozone reanalysis, in which OMPS‐LP data will replace MLS once the latter becomes unavailable. We also emphasize that further work is needed to reduce a long‐term spurious drift in OMPS‐LP data. Key Points: OMPS Limb Profiler stratospheric ozone data are assimilated into GEOSA simple homogenization method significantly reduces the bias between MLS and OMSP‐LP ozone in assimilationCombined MLS and OMPS‐LP data can provide continuity of ozone record in future reanalysis once the OMPS‐LP drift problem is addressed [ABSTRACT FROM AUTHOR]

Abstract: To better understand the potential effects of climate change on atmospheric dynamics, this paper studies Rossby wave breaking and isentropic stratosphere‐troposphere exchange (STE) in the Northern Hemisphere between 320 and 380 K during 1981–2015 using the Modern‐Era Retrospective Analysis for Research and Application version 2 data. The isentropic STE is estimated using Contour Advection. Our results show that anticyclonic wave breaking events have become more frequent, especially in summer at higher isentropic surfaces, and cyclonic wave breaking events have become less frequent at 320 K. The anticyclonic wave breaking has shifted poleward in summer. The isentropic STE is found to be strongest over the regions where Rossby wave breaking activities are most frequent. Both isentropic STE and Rossby wave breaking are shown to be strongest in summer and weakest in winter. Our results do not show any discernable trends during isentropic STE during 1981–2015. [ABSTRACT FROM AUTHOR]

The climatologies of the stratopause height and temperature in the UA‐ICON model are examined by comparing them to 17‐years (2005–2021) of Microwave Limb Sounder (MLS) observations. In addition, the elevated stratopause (ES) event occurrence, their main characteristics, and driving mechanisms in the UA‐ICON model are examined using three 30‐year time‐slice experiments. While UA‐ICON reasonably simulates the large‐scale stratopause properties similar to MLS observations, at polar latitudes in the Southern Hemisphere the stratopause is ∼8 K warmer and ∼3 km higher than observed. A time lag of about two months also exists in the occurrence of the tropical semiannual oscillation of the stratopause compared to the observations. ES events occur in ∼20% of the boreal winters, after major sudden stratospheric warmings (SSWs). Compared to the SSWs not followed by ES events (SSW‐only), the ES events are associated with the persistent tropospheric forcing and prolonged anomalies of the stratospheric jet. Our modeling results suggest that the contributions of both gravity waves (GW)s and resolved waves are important in explaining the enhanced residual circulation following ES events compared to the SSW‐only events but their contributions vary through the lifetime of ES events. We emphasize the role of the resolved wave drag in the ES formation as in the sensitivity test when the non‐orographic GW drag is absent, the anomalously enhanced resolved wave forcing in the mesosphere gives rise to the formation of the elevated stratopause at about 85 km. Plain Language Summary: Using 17 years (2005–2021) of Microwave Limb Sounder (MLS) observations, we show negative (cooling stratopause temperatures and decreasing stratopause heights) trends in most regions and seasons. The largest negative trend in the stratopause temperature (by considering all regions and all seasons) is found in the Southern Hemisphere (SH)'s polar region during austral spring. The seasonal average of cooling rates is comparable in the mid‐latitudes of Northern Hemisphere and SH. In the UA‐ICON simulations, the elevated stratopause events (ESEs) occur after major sudden stratospheric warmings (SSWs). ESEs frequency is 2 events per decade in UA‐ICON simulations. Our results show that the wind reversal is stronger and long‐lasting in the ESEs compared to SSW‐only events. In addition, the easterlies extend to the mesosphere in the composites of ESEs, but the reversed winds are limited to below 60 km in the case of SSW‐only events. We show that the non‐orographic gravity wave drag induces anomalous residual circulation after SSW that causes the ESEs. We also show that the ESEs form even in the absence of non‐orographic gravity wave drag. In this case, the anomalous residual circulation is due to the anomalously enhanced resolved wave forcing in the mesosphere that gives rise to the formation of the ESEs at about 85 km. Key Points: The largest stratopause trend is found in the Southern Hemisphere polar region during austral springbased on Microwave Limb Sounder observationsThe suppression of gravity waves in UA‐ICON reveals the importance of resolvedwaves and their ability to compensate missing dragIn the polar regions, the simulated stratopause is too warm and the tropical semi‐annual oscillation is about two months out of phase [ABSTRACT FROM AUTHOR]

We conducted two 10‐day observational campaigns in 2019 targeting turbulence in the troposphere and lower stratosphere by adopting a frequency domain radar interferometric imaging technique using Program of the Antarctic Syowa radar (PANSY radar) and radiosonde observations obtained at Syowa Station in the Antarctic. Seventy three Kelvin‐Helmholtz (K‐H) billows were detected, and two characteristic cases likely excited by the same cyclone were examined in detail. In the first case with the longest observational duration of ∼6.5 hr, the K‐H billows had a thickness of ∼800 m and a horizontal wavelength of ∼2,500 m. According to a numerical simulation, continuously existing gravity waves associated with the cyclone maintained strong vertical wind shear sufficient to cause the K‐H instability. In the second case with the deepest thickness of ∼1,600 m, the K‐H billows had a horizontal wavelength of ∼4,320 m. Numerical simulation suggested that an enhanced upper‐tropospheric jet associated with a well‐developed synoptic‐scale cyclone caused the K‐H instability. Such background conditions, frequently observed in the Antarctic coastal region, are typical mechanisms for K‐H excitation. Linear stability analysis also indicated that the characteristics of the observed K‐H billows were consistent with the most unstable modes. Furthermore, statistical analysis was performed using data of all 73 observed cases. The characteristics of K‐H billows observed at Syowa Station are similar to those observed over Japan. However, the K‐H billows tend to have longer wave periods over Syowa Station than over Japan, likely due to the weaker tropospheric jet in the Antarctic. Plain Language Summary: Two 10‐day observational campaigns were conducted in 2019 for tropospheric and lower stratospheric turbulence with the Program of the Antarctic Syowa radar (PANSY radar) and radiosondes at Syowa Station in the Antarctic. Seventy three Kelvin‐Helmholtz (K‐H) billows were detected and two distinct cases were studied in detail by numerical simulations: the first case has the longest observational duration of the 73 cases and was most likely caused by instability due to continuous gravity waves associated with a cyclone that imparts strong vertical wind shear. The second case is the deepest and is most likely caused by instability due to a strong upper‐tropospheric jet associated with a well‐developed synoptic‐scale cyclone. Such background conditions are frequently observed in the Antarctic coastal region, suggesting typical mechanisms for K‐H excitation in this region. Theoretical calculations also show that the observed K‐H billow characteristics were consistent with the most unstable mode. Furthermore, statistical analysis using the 73 observed cases indicates that the characteristics of K‐H billows observed at Syowa Station are similar to those over Japan except for the long wave periods, which may be due to the weaker tropospheric jet in the Antarctic. Key Points: Seventy‐three Kelvin‐Helmholtz (K‐H) billows were detected in Program of the Antarctic Syowa radar observations using a frequency radar interferometric imaging techniqueGravity waves and well‐developed cyclones unique to the Antarctic coastal region cause strong vertical shearK‐H billows in the coastal Antarctic region have comparable spatial scale and strength to those in the mid‐latitudes [ABSTRACT FROM AUTHOR]

Deep convective overshooting has been shown to transport water vapor into the midlatitude lower stratosphere. However, it has not been demonstrated how the convective water vapor plumes evolve after the overshoots collapse. Furthermore, there is a lack of characterization of the convective water vapor plumes, nor is it clear whether satellite instruments can observe the characteristics. We use a high‐resolution numerical weather prediction model to study a convective system over North America. Multiple overshoots transport water vapor in the overworld stratosphere, forming a moist layer between 16.2 and 16.8 km (389.8–399.7 K), with horizontal diameters of about 300–400 km, and a maximum water vapor mixing ratio of 10.0 ppmv (4.3 ppmv anomaly). Lagrangian trajectories and mass integrations show the overworld water vapor plumes are maintained after the convective system weakens. In the lowermost stratosphere (LMS), water vapor plumes are less stable and ice is present, because there is perturbation by ongoing convection. Lagrangian trajectories and mass integrations show the LMS parcels partly return to the troposphere, and that the LMS water vapor mass is reduced by half after the convection weakens. On average, the LMS moistening is between 15.0 and 15.8 km (362.6–382.0 K), with horizontal diameters of about 150 km, and a maximum water vapor mixing ratio of 31.1 ppmv (18.4 ppmv anomaly). Although current satellites have difficulty observing the fine structure of the convective water vapor plumes, a new satellite instrument under development (SHOW) with 1‐km vertical and 100‐km horizontal resolution will be able to verify the plume characteristics. Plain Language Summary: We use a high‐resolution numerical model to study convective moistening in the midlatitude lower stratosphere, including their evolution after the convection weakens, their observable characteristics, and whether their characteristics can be detected by satellite instruments. Our results show that the convective water vapor plumes above 380‐K potential temperature are stable in the stratosphere. They form a thin moist layer of 0.6 km with a horizontal size of about 300–400 km. The convective water vapor plumes closer to the tropopause and cloud tops contain ice, with vertical sizes of 0.8 km and horizontal sizes of about 150 km. They are less stable in the stratosphere, and their mass is reduced by half after the convection weakens. Current satellite instruments are limited by their resolutions in observing the characteristics of convective water vapor plumes. We show that new satellite instruments with a vertical resolution of 1 km will be able to observe the fine structures of convective water vapor plumes. Key Points: Water vapor plumes in the overworld stratosphere are more stable and maintain the mass in the stratosphere after the convection weakensWater vapor plumes in the lowermost stratosphere partly return to troposphere due to ongoing convective perturbationNew satellites with ≤ $\le $1‐km vertical resolution are required for observing the fine structure of the water vapor plumes [ABSTRACT FROM AUTHOR]

The Pyrocumulonimbus (pyroCb) events over British Columbia in 2017 were observed in the lower stratosphere for about 8–10 months after the smoke injections. Several previous studies used global climate models to investigate the physical parameters for the 2017 pyroCb events, but the conclusions show strong model dependency. In this study, we use the Energy Exascale Earth System Model (E3SM) and complete an ensemble of runs exploring three injection parameters: smoke aerosol mass, the percentage of black carbon within the smoke aerosols, and plume injection height. Additionally, we consider the heterogeneous reaction of ozone and primary organic matter. According to the satellite daily observed aerosol optical depth (AOD), we find that the best ensemble member is the simulation with 0.4 Tg of smoke, 3% of which is black carbon, a 13.5 km smoke injection height, and a 10−5 probability factor of the heterogeneous reaction. Besides AOD, we examine the ensemble score based on the metrics used in previous studies: the metrics of the extinction coefficient at 18 km altitude and the maximum plume height. The conclusion of the best estimate of the injection parameters for 2017 pyroCb events shows strong not only model but also evaluation metric dependency. We use the Random Forest machine learning technique to quantify the relative importance of each parameter in accurately simulating the 2017 pyroCb events and find that the injection height is the most critical feature, no matter which metric is used to score the ensemble members. Plain Language Summary: Pyrocumulonimbus (pyroCb), or clouds caused by fires, can transport a large amount of smoke particles into the middle atmosphere, which then affects the climate. Several previous studies used global climate models, along with satellite observations of smoke, to investigate the key parameters for the 2017 pyroCb over British Columbia, but the conclusions show strong model dependency. This study uses the Energy Exascale Earth System Model (E3SM) atmosphere model version 1 (EAMv1) and completes an ensemble of simulations targeting three loosely constrained smoke injection parameters. The conclusion of the best estimate of the injection parameters for 2017 pyroCb events shows strong not only model but also evaluation metric dependency. We also use a machine learning technique to quantify the importance of each parameter in accurately simulating the 2017 pyroCb event. Finally, we find the E3SM model shows the most sensitivity to the smoke injection height. The estimated smoke lifetime for the 2017 pyroCb events is about 6 months, and a global‐averaged shortwave surface cooling of ∼‐0.3 W m−2 was observed for about 10 months. Key Points: An ensemble study of E3SM simulations determines the best combination of smoke parameters to match observations from the 2017 Pyrocumulonimbus (pyroCb) eventsThe random forest technique is used to quantify the relative importance of parameters and finds the greatest sensitivity to injection heightThe lifetime of stratospheric smoke from the 2017 pyroCb events is similar to the observations ranging from 137 to 188 days [ABSTRACT FROM AUTHOR]

The Arctic stratospheric polar vortex is an important driver of winter weather and climate variability and predictability in North America and Eurasia, with a downward influence that on average projects onto the North Atlantic Oscillation (NAO). While tropospheric circulation anomalies accompanying anomalous vortex states display substantial case‐by‐case variability, understanding the full diversity of the surface signatures requires larger sample sizes than those available from reanalyses. Here, we first show that a large ensemble of seasonal hindcasts realistically reproduces the observed average surface signatures for weak and strong vortex winters and produces sufficient spread for single ensemble members to be considered as alternative realizations. We then use the ensemble to analyze the diversity of surface signatures during weak and strong vortex winters. Over Eurasia, relatively few weak vortex winters are associated with large‐scale cold conditions, suggesting that the strength of the observed cold signature could be inflated due to insufficient sampling. For both weak and strong vortex winters, the canonical temperature pattern in Eurasia only clearly arises when North Atlantic sea surface temperatures are in phase with the NAO. Over North America, while the main driver of interannual winter temperature variability is the El Niño–Southern Oscillation (ENSO), the stratosphere can modulate ENSO teleconnections, affecting temperature and circulation anomalies over North America and downstream. These findings confirm that anomalous vortex states are associated with a broad spectrum of surface climate anomalies on the seasonal scale, which may not be fully captured by the small observational sample size. Plain Language Summary: The strength of the winds in the stratosphere over the Arctic provides useful information for seasonal forecasts of wintertime weather over Europe and North America. When we study these linkages, it is a challenge that we have few winters—only about 40—with reliable observations from the stratosphere. Here, we use data from a seasonal forecast model to generate a large collection of 3,000 possible winters, and we use these to examine different patterns of surface temperature and sea level pressure for winters with the strongest and weakest winds in the polar stratosphere. Some real‐world episodes have attracted wide attention, including recent cold winters linked to weak stratospheric winds, and there seems to be an anticipation that weak winds in the stratosphere are synonymous with extremely cold weather in many regions. However, our results indicate that these expected surface signatures are in fact not particularly common. There are also scenarios when instead the opposite surface signature emerges. We find that it is not sufficient to know the state of the stratosphere; regional sea surface temperatures can either support or counteract the stratospheric influence on winter weather in any given year. Key Points: The broad spectrum of surface signatures of stratospheric polar vortex anomalies is obscured by the small observational sample sizeA large ensemble indicates the observed magnitude of negative North Atlantic Oscillation and related surface anomalies during weak vortex winters may be inflatedOver North America the main driver of winter temperature variability is El Niño–Southern Oscillation (ENSO), but the stratosphere can modulate ENSO teleconnections [ABSTRACT FROM AUTHOR]

Based on the Modern‐Era Retrospective analysis for Research Applications Version 2 reanalysis, an upper stratosphere/lower mesosphere (USLM) disturbance in early January and a sudden stratospheric warming (SSW) in early February are both coincidentally captured preceding the anomalously strong polar vortex during the late boreal winters of 2010/2011 and 2019/2020. Our analysis indicates that planetary wave breaking during the USLM disturbance and SSW decelerates the westerly polar night jet (PNJ) in the upper stratosphere, which leads to a continuous downward and poleward propagation of the PNJ during the early and middle winter. As the peak height of the PNJ descends, the region with negative vertical wind shear and negative waveguides also extends downward and reaches an altitude of ∼40 km during the late winter, which significantly limits the upward propagation of planetary waves, inducing a stable polar vortex in the lower stratosphere. In addition, the transformed Eulerian mean diagnostic results based on the specified dynamics simulation of the Whole Atmosphere Community Climate Model eXtended version confirm that the planetary wave‐induced downwelling is the main cause of the stratospheric warming during the two perturbations. More importantly, the simulation results indicate that the strength of the two upper stratospheric perturbations is appropriate to maintain the reflective configuration during the late winter, which suggests that such a specific sequence of perturbations during the early and middle winter, namely a USLM disturbance followed by a minor SSW, is favorable for the development of a late‐winter unusually strong Arctic polar vortex in the lower stratosphere. Plain Language Summary: The ozone depletion in the Arctic is rarely observed, which is usually related to exceptionally strong westerly winds and polar vortices in the Arctic lower stratosphere since they will block the tropics‐to‐pole mass transport. Since the 21st century, only two strong Arctic polar vortex events in the lower stratosphere during late winter have occurred; one in 2010/2011 and the other in 2019/2020. Before the late‐winter strong polar vortex in the two winters, the planetary waves disturb the polar vortex in the upper stratosphere/lower mesosphere twice so that further wave activity becomes reflected away. Such early and middle winter evolution is unique in the 43‐year historical record. The strength of these perturbations is appropriate for reconfiguring the circulation to reflect planetary waves and help maintain a strong polar vortex, which finally results in a less disturbed late winter. This phenomenon to some extent can be regarded as a negative feedback in the middle atmosphere. Our results show the high similarity and uniqueness in the winter evolution preceding the two strong Arctic polar vortex events, and highlight the role of the vertical coupling process, which may improve the predictability of the late‐winter strong polar vortex and ozone depletion in the Arctic. Key Points: A USLM disturbance and an SSW are both captured preceding the late‐winter unusually strong Arctic polar vortex in 2010/2011 and 2019/2020The continuous descent of the peak height of the polar night jet due to the two perturbations is the main cause of the strong polar vortexThe roles of planetary and gravity waves are distinguished in the vertical coupling based on model simulations in the two winters [ABSTRACT FROM AUTHOR]

Parameterizations of convective gravity‐wave (CGW) drag (CGWD) require cloud information as input parameters. As cloud information provided from reanalyses includes some uncertainties, observed cloud information is required for better representation of CGWs. For this, characteristics of the latent heating rate (LHR) based on the Global Precipitation Measurement (GPM) satellite over 6 yr (June 2014 to May 2020) are investigated, and the CGW momentum flux and CGWD based on an offline CGWD parameterization are calculated using the GPM‐LHR and the Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2) background variables. Additionally, they are compared with those using LHR afforded by MERRA‐2. The averaged cloud‐bottom height is lower than that from MERRA but the cloud top height is similar for the both data, yielding deeper clouds from GPM that can generate more high phase‐speed components of CGWs. The column‐maximum heating rate, which is an input of the CGW momentum flux, of GPM‐LHR is maximal near the equator and the secondary maximum locates in the winter hemisphere storm tracks. The maximum of the cloud top momentum flux (CTMF) of CGWs locates in the winter hemisphere storm tracks, with the GPM‐CTMF being much larger than MERRA‐CTMF, as extreme convective events occur more frequently in GPM. In the equatorial region above z = 40 km, the GPM‐CGWD is significantly larger because high phase‐speed components of CGWs that survive up to the upper stratosphere are abundant for GPM‐CTMF, and this will contribute to drive more realistic semi‐annual oscillation. Key Points: Convective gravity wave momentum flux (CGWMF) and drag (CGWD) are calculated using the Global Precipitation Measurement (GPM) and Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2) heating rateThe CGWMF at the cloud top calculated using GPM is greater at high‐phase speeds than that using MERRA‐2 due to deeper clouds from GPMUsing two different heating rates does not change significantly CGWD below z = 40 km in the tropics [ABSTRACT FROM AUTHOR]

We report a hitherto unknown latitudinal double‐peak structure in the amplitude of stationary planetary wave 1 (SPW1) geopotential height in the stratosphere and lower mesosphere during austral winter. The primary peak is located at 60–70°S and 30–40 km, and the secondary peak is at 30–50°S and 40–60 km. According to 36 years (1981–2016) of the Modern‐Era Retrospective analysis for Research and Applications, version 2, reanalysis data, the double‐peak structure occurs with frequencies of ~61%, 97%, 53%, and 25% in May, June, July and August, respectively, while it rarely exists in other seasons. Significant downward Eliassen‐Palm fluxes suggestive of downward‐propagating waves are often found above the secondary peak, and phase progressions show opposite directions on its two sides. From the free‐running Whole Atmosphere Community Climate Model and linear mechanistic model simulations, the secondary peak is likely generated by the interference of primary upward‐ and secondary downward‐propagating SPW1 excited in situ by gravity wave forcing in the upper mesosphere. The strengths of the primary and secondary waves need to be comparable to form an effective interference pattern, which may explain the missing double‐peak feature in boreal winter as the primary waves are too dominant. Summer‐to‐winter interhemispheric wave coupling is identified in the austral midwinter and appears to originate from the secondary SPW1 generated in the summer hemisphere. Since the double‐peak structure of SPW1 is sensitive to the mean wind, wave‐mean flow, and wave‐wave interactions, this study provides a reference for general circulation and mechanistic models to simulate the middle atmosphere wave dynamics in austral winter. Plain Language Summary: The winter‐hemisphere stationary planetary waves are the most dominant perturbations in the lower and middle atmosphere of the Earth. They make significant contribution to the dynamics, chemistry, transport, and transition of the stratosphere and lower mesosphere. It is therefore important to understand their characteristics, forming mechanisms, and impacts. In this paper, we newly identify an intriguing latitudinal double‐peak feature of stationary planetary waves in austral winter months. Its statistical significance has been proved by using long‐term satellite observations and reanalysis data. The underlying mechanisms suggested by modeling work not only rely on the structure of the polar vortex but also involve the secondary planetary wave generation and wave‐wave interactions. This work uses a combination of satellite measurements, reanalysis product, whole atmosphere model, and mechanistic model, which provides a powerful tool to digest the important atmospheric phenomenon. Key Points: We report a novel finding of double‐peak structure in geopotential field of stationary planetary wave 1 in austral winter middle atmosphereDownward SPW1 generated by in situ GW forcing interferes with upward SPW1 which generates the secondary peak near midlatitude stratopauseSummer‐to‐winter interhemispheric wave coupling is identified at 70‐80 km and opposite in direction from winter‐to‐summer ducting above it [ABSTRACT FROM AUTHOR]

Trends in monthly average, zonal average temperature in the stratosphere as retrieved from highly accurate modern satellite data are intercompared and compared with climate reanalyses from 2003 through 2014. The data sets used are those of Atmospheric Infrared Sounder and Global Positioning System (GPS) radio occultation, and the reanalyses are those of MERRA and ERA‐Interim. Trends produced by all data sets agree to within 0.02 K/year in the lower stratosphere and 0.05 K/year in the middle stratosphere. A number of retrieval errors are found that introduce incorrect trends and seasonal anomalies. Adding microwave data to the infrared retrieval changes trends by approximately 0.01 K/year, thus improving agreement with the other data sets. The signature of the quasi‐biennial oscillation in temperature and the annual cycle of temperature over Antarctica as retrieved from infrared data contain null‐space errors of more than 3 K due to erroneous priors used in retrieval. Nonuniformity in GPS radio occultation gives rise to errors because changes in received GPS signal strength alter the upper boundary initialization in radio occultation retrieval. Finally, an incorrect specification of atmospheric water vapor introduces an erroneous seasonal cycle of temperature as retrieved from GPS radio occultation data in the upper troposphere. All of these time‐dependent retrieval errors can be corrected with future research and improvements to spectral infrared and GPS radio occultation retrieval systems. Plain Language Summary: Satellite instruments that have been observing the atmosphere since 2003 are highly calibrated and particularly rich in information. Two of them have been measuring temperature trends in the atmosphere between 7 and 30 km altitude, and their results differ by an uncomfortable amount. Because the measurement techniques have nothing in common, it is possible to trace the causes of the differences to fairly well‐known problems in their formulations that translate basic measurements to air temperature. The results of this paper will direct those responsible for retrieving temperature from modern satellite data to fix these problems and thereby bring multiple, independent observations of temperature trends into better agreement with each other. Key Points: Temperature trends in the lower and middle stratosphere as retrieved from modern satellite data differ by 0.02 and 0.05 K/yearNull‐space error affects temperature anomalies in infrared data, and receiver performance affects trends in GPS RO dataTime‐dependent biases inherent to infrared and GPS RO retrieval systems can be removed by modification of the retrieval systems [ABSTRACT FROM AUTHOR]

We use our forward domain filling trajectory model coupled with a new convective parcel injection system to explore stratosphere‐troposphere exchange (STE). We use Modern‐Era Retrospective Analysis for Research and Applications, Version 2 reanalysis and study the December‐February (DJF) and June‐August (JJA) seasons averaged for the years 2006–2015. Maps of age‐of‐air and STE regions show coincidence with stationary forced tropical Rossby waves that straddle the equator. The anticyclonic circulation generated by these waves isolates air over the Asian monsoon, the Tropical West Pacific, and North and South America. We map the preferred locations of troposphere to stratosphere (T2S) and stratosphere to troposphere (S2T) exchange. During DJF, the highest concentration of T2S locations is spread over the tropical west Pacific and in a broad region over Central/South America and Africa. During JJA, the highest concentration T2S locations is in the vicinity of the Asian monsoon. The highest concentration T2S crossing points is nearly always positioned on the eastern side of the Rossby waves where the tropopause is higher. Along the jets that bracket the tropics, the STE locations are consistent with earlier studies of Rossby wave breaking and two‐way exchange. S2T exchange above 12 km occurs at the edges of the tropics along the jet boundaries for both seasons. Plain Language Summary: Air exchange between the troposphere and the stratosphere determines the water vapor concentration in the stratosphere and provides a pathway for pollutants to reach the ozone layer. In this study, we use a Lagrangian model to map regions where the exchange is more frequent. Exchange regions are associated with the presence of stationary Rossby waves in the upper tropical troposphere in boreal winter, and the Asian monsoon during boreal summer. At the border between the tropical troposphere and the extra‐tropical stratosphere exchange occurs due to wave breaking along the jet. Key Points: Rossby wave pairs dominate the seasonal climatology of the upper tropical tropospheric circulationStratosphere‐troposphere exchange in the core tropics is modulated by the Rossby wavesEnhanced exchange also occurs at the edge of the tropics due to wave‐breaking [ABSTRACT FROM AUTHOR]

Cosmogenic isotopes are produced by cosmic rays mostly in the middle‐low atmosphere and then take part in the complicated processes of atmospheric transport and deposition that are different for the stratosphere and troposphere. Cosmogenic isotopes are continuously produced by galactic cosmic rays (GCRs) with a hard energy spectrum and sporadically by solar energetic particles (SEPs) with much softer energy spectra. The partition of the isotope production between the stratosphere and troposphere in different latitudinal zones depends on the spectrum of cosmic rays, geomagnetic shielding, and the tropopause shape. The exact zonal distribution of the isotope production is not accurately known and needs to be precisely modeled as well as the isotopes' transport and deposition in climatic reconstruction. Here, we present the results of the computations of 14C, 36Cl, 10Be, and 7Be cosmogenic isotope production in the Earth's atmosphere using the CRAC model. We provide zonal mean production rates separately for the stratosphere and troposphere in three latitudinal zones: tropical (0°–30°), midlatitude (30°–60°), and polar (60°–90°). The computations were performed for four scenarios: (a) production by GCR during a solar‐cycle minimum; (b) production by GCR during a solar‐cycle maximum; (c) SEP event with the hardest known spectrum (GLE#5); and (d) the strongest known soft‐spectrum SEP event (GLE#24). The results confirm that, while all the latitudinal zones are relatively important for the isotopes produced by GCR, isotope production by SEPs in the tropical zone is small and can be neglected. The results are coupled to the SOCOL‐AER2‐BE chemistry‐climate model and can be used for a simplified parametric modeling of the isotopes' atmospheric transport, for the conditions typical for the Holocene. Key Points: Latitudinal zonal partition of cosmogenic isotope production in the Earth's atmosphere is precisely modeled for different conditionsProduction by galactic cosmic rays and solar particles is important in all zones and only polar stratosphere, respectivelyThe results are coupled to a climate‐chemistry model and can be used for the Holocene conditions [ABSTRACT FROM AUTHOR]

Twenty six years of medium frequency (MF) radar wind measurements made from 1994 to 2019 at Davis Station (68.6°S, 77.9°E) are used to study the mean response of the mesosphere‐lower thermosphere to stratospheric warmings in the Southern Hemisphere. Warming events were detected using Modern‐Era Retrospective Analysis for Research and Applications (MERRA2) data with a systematic search for reductions in the zonal‐mean circulation at 60°S and corresponding increases in polar temperatures. Some 37 events were identified, including the 2002 major warming and the large event of 2019, with an average of 1–2 warmings per year. At the 10 hPa level, the polar cap temperature increases ranged from 3 to 28 K, with a mean value of 12 K, while the zonal wind speed reductions varied between −6 and −43 ms−1, with a mean value of −15 ms−1. Peak values occurred near 40 km. Warmings occurred mainly between August and October, with a small peak in occurrence in April/May. The MF radar data showed an average reduction in the mesospheric eastward winds of about 5–7 ms−1 at heights near 75 km that occurred 3–4 days prior to the changes in the stratosphere. Warming events were driven by episodic intensifications in planetary wave amplitudes, with quasi‐stationary planetary scale waves (PW) 1 being especially important. PW Eliassen‐Palm flux divergences show a systematic behavior with time and height that is consistent with a poleward residual circulation and downwelling over the pole prior to the warming events and an equatorward flow and upwelling after the peak of the events. Key Points: Medium frequency radar wind data acquired at Davis Station between 1994 and 2019 are used to study the response of the mesosphere‐lower thermosphere to Southern Hemisphere stratospheric warmingsWarmings have an average duration of about 10 days and are driven by episodic penetration of planetary waves from the lower atmosphereThe mean mesospheric response to warmings is a decrease in the eastward flow of 5–7 ms−1 [ABSTRACT FROM AUTHOR]

Among the 70 northern hemisphere winters from 1950 to 2020, 61 have been classified into four independent scenarios with a new technique based on analyzing empirical orthogonal functions of stratospheric zonal wind fluctuation patterns at the edge of the polar vortex. These four scenarios represent a typical zonal wind evolution in the stratosphere modulated by timings of stratospheric warmings occurring in winter. First, there are three scenarios (January mode, February mode, and Double mode) for which the polar vortex generally breaks down with a reversing of zonal winds in midwinter, known as sudden stratospheric warmings (SSWs) events, at different times. The last one is a non‐perturbed scenario with two under‐modes for which only the timing of the polar vortex transition to its summer state differs, either radiative or dynamical. Consistently with these wind patterns, the wave‐1 and wave‐2 anomaly evolutions confirm that these scenarios are associated with independent dynamics behaviors and significant differences between wave‐1 and wave‐2 activities. We found that the wave‐1 anomaly evolutions drop systematically for each scenario when the stratospheric winds weaken, while this correlation with the wave‐2 anomaly evolutions is not observed for all scenarios. These different dynamical behaviors confirm that the scenarios are either mainly dynamically driven by wave‐1 or both driven by wave‐1 and wave‐2. Until early December, the modes possess similar increasing evolutions of the wave‐1 activity due to the seasonal cycle. After, the scenarios separate, suggesting that the mechanisms responsible for the winter unfolding act in the previous months. Key Points: The winter stratosphere in the northern hemisphere tends to follow four typical scenariosThe different scenarios show how the mid‐winter is connected with the winter endThe modes contain the information of important sudden stratospheric warmings timings [ABSTRACT FROM AUTHOR]

Tropopause‐overshooting convection in the midlatitudes rapidly transports lower troposphere air and cloud material to the upper troposphere and lower stratosphere (UTLS), with notable events resulting in the formation of above‐anvil cirrus plumes (AACPs). However, there is limited understanding of how transport driven by overshooting convection and AACP properties are modified by variations in UTLS environments, especially the extent of stratosphere‐to‐troposphere (i.e., downward) transport. Here, AACP development and UTLS transport sensitivities to lower stratosphere stability and the UTLS wind environment are evaluated. The Bryan Cloud Model is used to simulate midlatitude supercell convection and evaluate resulting UTLS composition changes within two common midlatitude thermodynamic environments: a single tropopause and a double tropopause, and two UTLS wind environments: one with a stronger stratospheric wind profile supportive of frequent gravity wave breaking and AACP development and the other with a weaker stratospheric wind profile to suppress gravity wave breaking and discourage AACP formation. Multiple passive tracers and water vapor concentrations are used to evaluate the exchange of air between the troposphere and stratosphere and transport confined to each layer. It is found that greater troposphere‐to‐stratosphere transport (TST) and stratospheric hydration occurs in storms with AACPs, while stratosphere‐to‐troposphere transport (STT) is greater in storms without AACPs. This STT is facilitated by a mechanical oscillation induced by the overshoot. AACP‐producing storms have increased downward transport of stratospheric overworld air to the lowermost stratosphere, which is enhanced within a double tropopause environment. More expansive AACPs, deeper overshoots, and greater TST also occurs in double tropopause environments. Key Points: Stratosphere stability has a minimal impact on stratosphere‐to‐troposphere transport in overshooting convectionStratosphere‐to‐troposphere transport is greater in storms without above‐anvil cirrus plumesA mechanical oscillation induced by the overshoot facilitates stratosphere‐to‐troposphere transport [ABSTRACT FROM AUTHOR]

This study investigates the tropospheric and lower stratospheric 3D wind frequency spectra at Andøya, Norway (69.30°N and 16.04°E) during the years 2017–2020 obtained with the Middle Atmosphere Alomar Radar System. We produce power spectral density S(f) in a broad frequency range (3.5 days−1 > f > 1h−1). We categorize the spectra into different ranges: two frequency ranges (lower and higher than (13 hr)−1 according to the local inertial period of 13 hr), four altitude ranges (lower troposphere, middle troposphere, tropopause region, and lower stratosphere), and four seasons (spring, summer, autumn, and winter). We approximate the power law S(f) ∝ fβ through a least‐square regression. Our results indicate that (a) the spectra of the horizontal winds follow a power law with slopes of about β = −5/3 and −2 at the high and low frequency, respectively, (b) the slopes get steeper around the tropopause and seasonally during the summer (which we report for the first time), and (c) the slope β in the vertical wind is shallow 0 > β > −1, which flattens with altitude. Key Points: Frequency spectra of the horizontal winds in the troposphere and lower stratosphere exhibit power laws with slopes of about −2 and −5/3The horizontal wind slopes steepen in altitude around the tropopause and during summer (particularly at the frequency higher than (13 hr)−1)The power law spectral slopes in the vertical wind are shallower than −1 and flatten with altitude [ABSTRACT FROM AUTHOR]

The stratospheric polar vortex (SPV) is a key part of the stratospheric system. A better understanding of its variability and its interaction with the troposphere might help us to improve tropospheric predictability. In winter 1987/88, the SPV intensity exhibited the most significant positive transition (enhanced from weak to strong) between the early winter (November and December) and late winter (January and February) during 1979–2019. Therefore, we selected winter 1987/88 as an extreme case to analyze its stratosphere–troposphere interaction processes. Results indicate that the positive transition of SPV intensity was probably caused by the negative transition of upward planetary wave intensity entering the stratosphere, especially planetary wave‐1. We further found that the anomalous strong (weak) upward planetary wave‐1 in the stratosphere might originate from the increased (reduced) generation of planetary wave‐1 in the lower troposphere. Accompanied by anomalous SPV intensity, the positive (negative) stratospheric zonal wind anomalies were able to propagate downwards into the troposphere during strong (weak) SPV stages. And these out‐of‐phase downward propagating anomalies might have contributed to the out‐of‐phase tropospheric zonal wind anomalies between these two stages, which appeared as North Atlantic Oscillation‐like zonal wind anomalies in the North Atlantic region and similar dipole‐like anomaly mode but in the Asia‐Arctic region at 500 hPa. Moreover, we found that this downward influence of SPV anomalies can also manifest in its statistical relationship with the 500‐hPa zonal winds on simultaneous monthly scales. Key Points: A significant positive transition of stratospheric polar vortex intensity existed between early and later winter of 1987/88This polar vortex transition was possibly caused by a negative transition of upward planetary waves entering the stratosphereThis polar vortex transition might have contributed to the out‐of‐phase tropospheric zonal wind anomalies [ABSTRACT FROM AUTHOR]

Aerosol emissions from spaceflight activities play a small but increasing role in the background stratospheric aerosol population. Rockets used by the global launch industry emit black carbon (BC) particles directly into the stratosphere where they accumulate, absorb solar radiation, and warm the surrounding air. We model the chemical and dynamical response of the atmosphere to northern mid‐latitude rocket BC emissions. We initially examine emissions at a rate of 10 Gg per year, which is an order of magnitude larger than current emissions, but consistent with extrapolations of space traffic growth several decades into the future. We also perform runs at 30 and 100 Gg per year in order to better delineate the atmosphere's response to rocket BC emissions. We show that a 10 Gg/yr rocket BC emission increases stratospheric temperatures by as much as 1.5 K in the stratosphere. Changes in global circulation also occur. For example, the annual subtropical jet wind speeds slow down by as much as 5 m/s, while a 10%–20% weakening of the overturning circulation occurs in the northern hemisphere during multiple seasons. Warming temperatures lead to a ozone reduction in the northern hemisphere by as much as 16 DU in some months. The climate response increases in a near linear fashion when looking at larger 30 and 100 Gg emission scenarios. Comparing the amplitude of the atmospheric response using different emission rates provides insight into stratospheric adjustment and feedback mechanisms. Our results show that the stratosphere is sensitive to relatively modest BC injections. Plain Language Summary: Emissions from spaceflight activities play an increasing role in the background stratospheric aerosol population. Rockets used by the global launch industry emit black carbon particles directly into the stratosphere where they accumulate, absorb solar radiation, and warm the surrounding air. We model the climate response of the stratosphere to an annual, black carbon emission source from rocket launches. We initially examine an emission rate of 10 Gg per year, an order of magnitude larger than current emissions but plausible within the next two decades based upon recent trends in space traffic growth. We also perform runs at 30 and 100 Gg per year in order to better understand the atmosphere's response to rocket black carbon emissions. We show that the rocket black carbon increases stratospheric temperatures and changes the global circulation, both of which cause a reduction in the total ozone column, mainly in the northern high latitudes. Comparing the amplitude of the atmospheric response using different emission rates provides insight into stratospheric adjustment and feedback mechanisms. Our results show that the stratosphere is sensitive to relatively modest black carbon injections. Key Points: The increased stratospheric BC burden from rocket launches warms the stratosphereStratospheric BC‐induced heating causes shifts in stratospheric dynamics, year‐round NH ozone loss, and a stronger Antarctic ozone holeThe climate response scales in a near linear fashion with increasing rocket launch emissions [ABSTRACT FROM AUTHOR]

There are few reports on quasi 6‐day wave (Q6DW) during stratospheric sudden warming (SSW) events, especially the eastward propagating one. By using observations from three meteor radars and three radiosondes combined with reanalysis data from January to March 2016, we study the eastward propagating Q6DWs during the SSWs from the troposphere to the mesosphere and lower thermosphere (MLT). There are SSW and stratospheric final warming (SFW) events around February and March in the northern hemisphere, respectively. The polar mean temperature rises by about 45 and 36 K at 10‐hPa level and the mean zonal wind decreases by about 38 and 49 ms−1 during the SSW and SFW, respectively. Large‐scale zonal wind reversal happens during the SSWs. Radar observations and reanalysis data show that two Q6DWs corresponding to the SSWs are robust from the troposphere to the MLT. They are eastward traveling waves with zonal wavenumbers 1–2. The Q6DWs propagate downward in the stratosphere and upward in the MLT, implying that the source of the Q6DWs is in the mesosphere. The meridional gradient of quasi‐geostrophic potential vorticity and Eliassen‐Palm flux divergence show that the instabilities possibly occur in the mesosphere, which likely in situ excite and strengthen the Q6DWs. The instabilities are also found in the stratosphere and may amplify the Q6DWs in the stratosphere at the three stations especially during the SSW. Key Points: The quasi 6‐day waves (Q6DWs) are robust from the troposphere to the mesosphere and lower thermosphere (MLT) and propagate eastward at mid latitudes during the stratospheric sudden warmings (SSWs)The Q6DWs are in situ excited in the mesosphere and propagate upward in the MLT and downward in the stratosphereLasting atmospheric instabilities can explain the excitation and amplification of the Q6DWs in the atmosphere during the SSWs [ABSTRACT FROM AUTHOR]

We use the idealized tracer experiments and investigate the summertime transport from the surface region of northern India and Tibetan Plateau to the lower stratosphere. It is found that the transport, compared to other surrounding regions, has an overall younger modal age in the northern lower stratosphere away from the tropopause. Analysis of the tracer budget reveals that the tracer is transported to the tropical lower stratosphere rapidly in the first 5 days due to vertical eddy transport and afterward in a month or two advection associated with the Brewer‐Dobson circulation. Meanwhile the tracer is also transported to the northern extratropical lower stratosphere in the first 3 months due to horizontal eddy mixing. The results highlight the uniqueness of the northern India region in the summertime transport to the lower stratosphere and implications for the transport of short‐lived chemical species in the destruction of stratospheric ozone. Plain Language Summary: The atmospheric transport plays a critical role in determining the concentration and distribution of chemical species in the stratosphere, in addition to emission and chemistry. In this study, we have shown the sensitivity of the transport and its associated timescale to the surface emission region. Specifically, tracers emitted from the surface region of northern India and Tibetan Plateau travel to the Northern Hemisphere lower stratosphere in an overall shorter timescale, compared to tracers that are emitted from the surrounding regions. This suggests that short‐lived chemical species with lifetimes generally under 6 months, when emitted from the surface region of northern India and Tibetan Plateau, could be transported efficiently to the Northern Hemisphere middle and high latitude stratosphere and destruct the stratospheric ozone there. Key Points: Transport from northern India has an overall younger age in the northern lower stratosphereTransport pathway to tropical lower stratosphere due to vertical eddy transport first and Brewer‐Dobson circulation advection afterwardTransport pathway to northern extratropical lower stratosphere mostly due to horizontal eddy mixing [ABSTRACT FROM AUTHOR]

Using ensemble forecasts from the Global Ensemble Forecast System (GEFS) and controlled integrations with the Weather Research and Forecasting (WRF) model, we explore the stratospheric influence on the development of the 2018 European severe cold air outbreak (CAO). The 2018 CAO set in on 22 February, and its evolution was closely related to a growing tropospheric blocking ridge over the North Atlantic in late February that brought cold air mass into Europe. The event was also preceded by a split of the stratospheric polar vortex on 12 February, after which one child vortex persisted over northeastern Canada. GEFS forecasts initialized 1 week prior to this CAO substantially underestimated its severity, exhibiting large spread correlated with the strength and location of the Atlantic ridge, while forecasts initialized closer to the event captured its severity with higher fidelity. Based on a set of nudged simulations, we show that capturing the evolution of the stratospheric child vortex over northeastern Canada, particularly in the lowermost stratosphere, is of great importance for predicting this CAO, since it substantially affects the strength and location of the Atlantic ridge that in turn determines the evolution and severity of this CAO. When the stratosphere is nudged toward the observed evolution, the northward flow between the child vortex and the Atlantic ridge advects more low potential vorticity air into the Atlantic ridge, and therefore amplifies it. Further experiments demonstrate that the impact of the stratosphere on the development of the ridge is particularly strong over the North Atlantic. Key Points: Global Ensemble Forecast System forecasts initialized one week prior to the 2018 European severe cold air outbreak (CAO) underestimated its severityControlled Weather Research and Forecasting simulations demonstrate that the stratospheric polar vortex evolution is essential to the synoptic development of the CAOThe impact of the stratospheric polar vortex is particularly strong over the North Atlantic [ABSTRACT FROM AUTHOR]

To estimate reaction of the atmospheric circulation in the middle and upper atmosphere to changes in phases of equatorial stratospheric quasi‐biennial oscillation (QBO), the three‐dimensional nonlinear middle and upper atmosphere model (MUAM) is used. This model allows continuous simulations of atmospheric wave propagation from the ground to the thermosphere (300 km and above). The main atmospheric hydrodynamic fields (wind and temperature), components of residual meridional circulation (RMC), and fluxes of mass are calculated based on ensembles containing 16 pairs of model runs for initial conditions corresponding to easterly and westerly QBO phases. To minimize uncertainties in determination of the QBO phases, an approach based on the usage of empirical orthogonal functions (EOFs) is applied. Statistically significant results are obtained illustrating how changes in the planetary waves (PWs) structures promote the spread of QBO effects to polar latitudes and to the thermosphere, through changes in the Eliassen‐Palm (EP) flux and its divergence, or through the formation of an eddy meridional circulation. The main contribution to the cooling of the polar winter stratosphere during the westerly QBO is made by the weakening of wave activity, in particular, the weakening of the vertical EP flux, which leads to a weakening of the poleward heat flux. The sensitivity of the wave‐induced eddy circulation to changes in the QBO phase is higher than that of the RMC, demonstrating that PWs propagating from the lower troposphere are the most important mechanism for the transfer of global circulation disturbances from the equatorial QBO region to polar latitudes. Key Points: In the thermosphere, weakening in residual circulation occurs during the westerly QBO, accompanying by an increase in the temperatureThe sensitivity of the wave‐induced eddy circulation to changes in the QBO phase is higher than that of the residual circulationCooling of the polar winter stratosphere during westerly QBO is made by the weakening of planetary wave activity [ABSTRACT FROM AUTHOR]

In the follow‐up of the Australian "Black Summer" event that persisted from August 2019 to March 2020, we present the optical properties of the stratospheric aerosols injected into the atmosphere by these wildfires. The outbreak of pyrocumulonimbus (PyroCb) activity triggered between 2019/12/29 and 2020/01/04 has raised the stratospheric aerosol load of the Southern Hemisphere to unprecedented levels. Long‐range transport brought some of the plumes down to the Antarctic region, where general circulation patterns kept them circling around the continent. The 532 nm Rayleigh/Mie/Raman ground‐based lidar of the French Antarctic station Dumont d'Urville (66.6°S–140°E) acquired unprecedented time series of these carbonaceous aerosols starting approximately 20 days after the injection and up to the most recent measurements in October 2019 where local radiosonde reported anomalous ozone depletion as compared to the decadal average. The lidar provides a first and unique time series at high vertical and temporal resolution, complemented by satellite measurements from Ozone Monitoring Instrument, Ozone Mapping and Profiler Suite, and Microwave Limb Sounder. Aerosol backscatter ratio decreases from 1.9 to 1.2 between January and June 2020. Aerosol origin and persistence are characterized, as well as their optical properties and vertical distribution over several months. Plain Language Summary: Major wildfires are now known to significantly contribute to the stratospheric aerosol load, which in turn may have consequences at the global scale. The deep convection and radiative uplift processes that they can spark, now colloquially referred to as pyroconvection, can inject significant quantities of aerosols into the stratosphere. The major wildfires that raged in Australia at the end of 2019 are a case in point: they injected an unprecedented amount of material into the stratosphere. After some weeks and due to the general circulation patterns, a part of this material ended up trapped above the Antarctic region. The lidar located at the French Antarctic station Dumont d'Urville had a unique opportunity to probe these stratospheric aerosols for an extended time period, allowing to observe the evolution in time of their optical properties over months. This study presents these valuable measurements which contribute to our knowledge of biomass‐originated stratospheric aerosols and highlights the importance of operational ground‐based instrumental capabilities and coverage at high latitudes. Key Points: 2019–2020 Australian bushfires injected an unprecedented amount of aerosol into the stratosphere with impact at polar latitudesLongest ground‐based lidar smoke aerosol record acquired at Dumont d'Urville with persistent 532 nm scattering ratio throughout the year 2020Simultaneous presence of polar stratospheric clouds and smoke aerosols were observed during the wintertime [ABSTRACT FROM AUTHOR]

Carbonyl sulfide (OCS) is a non‐hygroscopic trace species in the free troposphere and a large sulfur reservoir maintained by both direct oceanic, geologic, biogenic, and anthropogenic emissions and the oxidation of other sulfur‐containing source species. It is the largest source of sulfur transported to the stratosphere during volcanically quiescent periods. Data from 22 ground‐based globally dispersed stations are used to derive trends in total and partial column OCS. Middle infrared spectral data are recorded by solar‐viewing Fourier transform interferometers that are operated as part of the Network for the Detection of Atmospheric Composition Change between 1986 and 2020. Vertical information in the retrieved profiles provides analysis of discreet altitudinal regions. Trends are found to have well‐defined inflection points. In two linear trend time periods ∼2002 to 2008 and ∼2008 to 2016 tropospheric trends range from ∼0.0 to (1.55 ± 0.30%/yr) in contrast to the prior period where all tropospheric trends are negative. Regression analyses show strongest correlation in the free troposphere with anthropogenic emissions. Stratospheric trends in the period ∼2008 to 2016 are positive up to (1.93 ± 0.26%/yr) except notably low latitude stations that have negative stratospheric trends. Since ∼2016, all stations show a free tropospheric decrease to 2020. Stratospheric OCS is regressed with simultaneously measured N2O to derive a trend accounting for dynamical variability. Stratospheric lifetimes are derived and range from (54.1 ± 9.7)yr in the sub‐tropics to (103.4 ± 18.3)yr in Antarctica. These unique long‐term measurements provide new and critical constraints on the global OCS budget. Plain Language Summary: Carbonyl sulfide (OCS) is the most abundant sulfur containing gas in the atmosphere. There are many sources and sinks of OCS and other sulfur species in the atmosphere but most other short lived sulfur species eventually are converted to OCS. It is important to quantify and understand OCS as it can be used to understand CO2 and the carbon cycle and also since it eventually is transported into the stratosphere where it maintains the sulfate aerosol layer at about 20 km into the atmosphere. This layer is very important for earth's energy balance and climate change. In contrast with earlier and less comprehensive reports, this global study from 22 observation stations worldwide, shows stratospheric OCS to be increasing north and south of the equator but decreasing near the equator and to be increasing in the troposphere to 2016 and decreasing since. The main drivers of OCS in the troposphere are the cumulative anthropogenic sources. Key Points: Global distribution of carbonyl sulfide (OCS) measured by NDACC solar absorption FTIR remote sensingTropospheric trends in OCS are non‐monotonic globally, driven by anthropogenic emissionsLongest term stratospheric trends are increasing outside of sub‐tropics [ABSTRACT FROM AUTHOR]

February‐March 2020 was marked by highly anomalous large‐scale circulations in the Northern extratropical troposphere and stratosphere. The Atlantic jet reached extreme strength, linked to some of the strongest and most persistent positive values of the Arctic Oscillation index on record, which provided conditions for extreme windstorms hitting Europe. Likewise, the stratospheric polar vortex reached extreme strength that persisted for an unusually long period. Past research indicated that such circulation extremes occurring throughout the troposphere‐stratosphere system are dynamically coupled, although the nature of this coupling is still not fully understood and generally difficult to quantify. We employ sets of numerical ensemble simulations to statistically characterize the mutual coupling of the early 2020 extremes. We find the extreme vortex strength to be linked to the reflection of upward propagating planetary waves and the occurrence of this reflection to be sensitive to the details of the vortex structure. Our results show an overall robust coupling between tropospheric and stratospheric anomalies: ensemble members with polar vortex exceeding a certain strength tend to exhibit a stronger tropospheric jet and vice versa. Moreover, members exhibiting a breakdown of the stratospheric circulation (e.g., sudden stratospheric warming) tend to lack periods of persistently enhanced tropospheric circulation. Despite indications for vertical coupling, our simulations underline the role of internal variability within each atmospheric layer. The circulation extremes during early 2020 may be viewed as resulting from a fortuitous alignment of dynamical evolutions within the troposphere and stratosphere, aided by each layer's modification of the other layer's boundary condition. Key Points: Large‐ensemble simulations are needed to fully characterize coupled extremes in the polar vortex and tropospheric jet in early 2020Details of the vortex structure play an important role in promoting either reflection or dissipation of upward propagating waves 1 and/or 2Modulation of lowermost stratospheric circulation from above and below facilitates co‐evolution of tropospheric and stratospheric extremes [ABSTRACT FROM AUTHOR]

We present a new version of the HIgh Altitude Mechanistic general Circulation Model (HIAMCM) with specified dynamics. We utilize a spectral method that nudges only the large‐scale flow to MERRA‐2 reanalysis. The nudged HIAMCM simulates gravity waves (GWs) down to horizontal wavelengths of about 200 km from the troposphere to the thermosphere like the free‐running model, including the generation of secondary and tertiary GWs. Case studies show that the simulated large‐scale GWs are consistent with those in the reanalysis, while the medium‐scale GWs compare well with observations in the northern winter 2016 stratosphere from the Atmospheric InfraRed Sounder (AIRS). GWs having wavelengths larger than about 1,350 km can be described with the nonlinear balance equation. The GWs relevant in the stratosphere, however, have smaller scales and require a different approach. We propose that the GW amplification due to kinetic energy transfer from the large‐scale flow combined with GW potential energy flux convergence helps to identify the mesoscale GW sources due to spontaneous emission. The GW amplification is strongest in the region of maximum large‐scale vertical wind shear in the mid‐stratosphere. Maps of the time‐averaged stratospheric GW activity simulated by the HIAMCM and computed from AIRS satellite data show a persistent hot spot over Europe during January 2016. At about 40 km, the average GW amplitudes are maximum in the region of fastest large‐scale flow. We argue that refraction of GWs originating in the troposphere, as well as GWs from spontaneous emission in the stratosphere contribute to this effect. Plain Language Summary: We present a new version of the HIgh Altitude Mechanistic general Circulation Model (HIAMCM) with specified dynamics in the troposphere, stratosphere, and lower mesosphere. The HIAMCM is a spectral model with high spatial resolution (the shortest horizontal wavelength is about 156 km) and a model top at about 450 km. The nudging of the model to reanalysis is formulated in spectral space and restricted to the large‐scale flow. This ensures that gravity waves (GWs) are self‐consistently simulated over the whole altitude range like in the free‐running model. The simulated large‐scale GWs are quantitatively consistent with those in the reanalysis, while the medium‐scale GWs compare well with satellite observations in the northern winter 2016 stratosphere. We analyze a case of GW generation in the stratosphere from imbalance of the polar vortex. We propose that this process can be identified by the transfer of kinetic energy transfer from the large‐scale flow to the GWs combined with the GW potential energy flux convergence. The in‐situ generation of GWs in the stratosphere likely contributes to the averaged distribution of GW activity in the upper stratosphere, which is maximum around the edge of the polar vortex. Key Points: Nudging in spectral space allows for self‐consistent simulation of gravity waves up to the thermosphereSimulated gravity waves in the winter stratosphere agree well with satellite observationsSpontaneous emission of GWs in the winter stratosphere depends critically on vertical wind shear [ABSTRACT FROM AUTHOR]

We examine the influence of the solar cycle on the North Atlantic Oscillation (NAO) on its pathway from the upper stratosphere to the surface by applying lagged regression analyses to recent observations, historical observations covering 194 years, and an Earth system model simulation covering 165 years. The propagation of the solar signal can well be explained by a top‐down mechanism, but one that was strongly affected by air‐sea coupling. The solar signal first appears in the subtropical upper stratosphere as a temperature signal. The associated zonal wind signal then propagates downward to the surface in response to stratospheric variability known as the Polar‐night Jet Oscillation. The NAO signal tends to appear in February during years of peak solar activity. The solar signal is further modulated such that positive NAO signals tend to appear earlier in winter with increasing years after peak solar activity associated with the appearance of the NAO‐related SST signal. There are also 50‐year scale fluctuations and amplitude modulations of the solar‐NAO signal, but the tendency of increasing NAO amplitude with increasing NAO‐related SST amplitude suggests a strong influence of decadal ocean dynamics on these modulations. Key Points: Propagation of the solar signal can be explained by a top‐down mechanism modulated by air‐sea couplingSolar‐related North Atlantic Oscillation (NAO) signal tends to peak in February of solar maximum years but shows a 50‐year scale drift with lagged amplitudes of ∼2 yearsA positive NAO signal tended to appear earlier in winter as the years since peak solar activity increased [ABSTRACT FROM AUTHOR]

The inter‐annual variability in mid and lower stratospheric temperatures for the period 1984–2019 is decomposed into dynamical and radiative contributions using a radiative calculation perturbed with changes in dynamical heating, trace gases and aerosol optical depth. The temperature timeseries obtained is highly correlated with the de‐seasonalized ERA5 temperature (r2 > 0.6 in the region 15 to 70 hPa, 1992 to 2019–after the Pinatubo volcanic eruption). Ozone and dynamical heating contributions are found to be equally important, with water vapor, stratospheric aerosols, and carbon dioxide playing smaller roles. Prominent aspects of the temperature timeseries are closely reproduced, including the 1991 Pinatubo volcanic eruption, the year‐2000 water vapor drop, and the 2016 Quasi‐biennial oscillation (QBO) disruption. Below 20 hPa, ozone is primarily controlled by transport and is positively correlated to the upwelling. This ozone‐transport feedback acts to increase the temperature response to a change in upwelling by providing an additional ozone‐induced radiative temperature change. This can be quantified as an enhancement of the dynamical heating of about 20% at 70 hPa. A Principal Oscillation Pattern (POP) analysis is used to estimate the contribution of the ozone QBO (±1 K at 70 hPa). The non‐QBO ozone variability is also shown to be significant. Using the QBO leading POP timeseries as representative of the regular QBO signal, the QBO 2016 disruption is shown to have an anomalously large radiative impact on temperature due to the ozone change (>3K $ > 3\hspace*{.5em}\mathrm{K}$ at 70 hPa). Plain Language Summary: Temperatures in the lower stratosphere vary substantially from year to year and play a major role in determining the composition of the stratosphere, which in turn plays an important role in Earth's energy budget. Understanding this variability is thus key to understanding the properties of the stratosphere. We calculate the amount of heating due to changes in four main factors (circulation, ozone, water vapor, and aerosol). This allows us to decompose the observed temperature changes into these main factors with good agreement during the time period studied (1984–2019). Our method also allows us to analyze anomalously large temperature changes such as those during volcanic eruptions and the recent disruption in the Quasi‐biennial oscillation. We find that ozone changes play a key role throughout the time period analyzed. Our findings indicate that temperature changes related to composition (particularly ozone) are critical in determining the variability of temperatures in the lower tropical stratosphere. They highlight the need to validate these processes in global models. Key Points: Inter‐annual variability in lower tropical stratospheric temperatures has been reconstructedOzone and dynamical heating dominate. Aerosol and water wapor contributions are not negligibleOzone associated with the Quasi‐biennial oscillation contributes substantially, particularly during the 2016 disruption [ABSTRACT FROM AUTHOR]

Planetary wave (PW) associated dynamical variability in the equatorial and extratropical middle atmosphere during the September 2019 Southern hemisphere minor sudden stratospheric warming (SSW) is investigated utilizing meteor radar wind observations from São João do Cariri (7.4°S, 36.5°W) and Cachoeira Paulista (22.7°S, 45°W) and reanalysis data. Signature of the mesospheric warming in conjunction with the stratospheric cooling is found at low latitudes. The strong westerly wind at low latitudes decelerates notably near 65 km at the onset of the warming episode, although no wind reversal is observed. The wind spectra reveal a prevalent quasi‐16‐day wave (Q16DW) prior to the SSW and existence of a quasi‐6‐day wave (Q6DW) after the warming event. Possible existence of barotropic/baroclinic instability in the low and mid latitude middle atmosphere may be responsible for exciting the Q6DW. Both traveling and stationary waves exhibit notable activities during the warming event. Although involvement of both zonal wavenumbers 1 and 2 PWs are found in the event, PW with zonal wavenumber 1 seems to play a vital role in preconditioning the same. Furthermore, significant latitudinal mixing of airmass between the tropics and high latitudes is evident in the potential vorticity map. The Eliassen‐Palm flux diagnosis shows the propagation of the Q6DW and Q16DW from mid to low latitudes during the warming event. Plain Language Summary: A minor but impactful sudden stratospheric warming event occurred in the Southern hemisphere during September 2019. This event is characterized by a marked deceleration of the zonal mean westerlies at polar stratosphere although no wind reversal is observed. Simultaneously, stratospheric temperature at high latitudes increases sharply by more than 25 K within a few days. During the same warming event, stratospheric cooling and mesospheric warming is found at the present low latitude stations. The wind spectra reveal a long period wave of about 16 days prior to the warming event in the stratosphere followed by a relatively shorter period wave (∼6 days) in the mesosphere. The 6‐day wave component is possibly excited due to the instability in the middle atmosphere at low and middle latitudes during the warming episodes. Both traveling and stationary large‐scale waves are supposed to play a vital role in preconditioning the warming event. There is also significant meridional air mass mixing during the warming days owing to possible planetary wave (PW) driven weakening of the zonal mean westerlies. The PW fluxes for both the aforementioned components are observed to propagate from high and mid to low latitudes during the warming event. Key Points: Both traveling and stationary planetary waves with zonal wavenumber 1 seem to play a salient role in pre‐conditioning the warming eventExisting instability is believed to support the growth of the quasi‐6‐day wave (Q6DW) in post warming intervalThe Q6DW and Q16DW are found to propagate from high and mid latitudes to low latitudes during the warming [ABSTRACT FROM AUTHOR]

Using three‐dimensional wind observations from a Mesosphere‐Stratosphere‐Troposphere radar at Xianghe for two years from September 2011 to August 2013, we study inertia gravity wave (IGW) activity in the two height segments of 3–10 and 15–25 km to reveal the characteristics of waves in the source region at midlatitude. Case investigation indicates that three‐dimensional wind fluctuations are usually dominated by a strong IGW. Statistics on 15,617 IGWs extracted show that most waves have typical horizontal scales of tens to thousands of kilometers, the vertical scale of a few kilometers, intrinsic frequencies in 1–4 times the inertial frequency, and amplitudes of 1–4 ms−1 in horizontal winds but less than 0.3 ms−1 in the vertical wind. In the lower region, occurrence possibilities of upward and downward propagating waves are almost the same throughout the year. Whereas in the upper region, IGWs display a predominant upward propagation with a maximum of 85.3% in February but a minimum of 63.6% in July, and a preferential propagation in the zonal direction relative to the meridional direction. Meanwhile, wave energy has a prominent seasonal cycle with the maximum in winter and the minimum in summer. Subtropical jet, as wave source and propagation condition, plays an important role in these different features between the two regions. The annual average of momentum flux indicates that the IGWs in the lower stratosphere can apply a persistent eastward drag on the background flow at higher altitudes through momentum transported by these waves. Key Points: Upward propagating waves predominate obviously in the upper region but are nearly as many as downward propagating waves in the lower regionAs wave source and propagation condition, subtropical jet plays an important role in the different features of inertia gravity waves (IGWs) between the two regionsIGWs in the lower stratosphere can apply a persistent eastward drag on the background flow at higher levels through momentum transport [ABSTRACT FROM AUTHOR]

Quasi‐stationary planetary Rossby waves forced in the extratropical troposphere propagate upwards into the stratospheric vortex and can also be refracted toward the equator. To the extent that such waves penetrate the low latitudes they will be observable as quasi‐stationary zonal eddies in the equatorial middle stratosphere. These eddies should be strongly modulated by the phase of the Quasi‐Biennial Oscillation (QBO) of the mean zonal wind. In this project we used global reanalysis data to examine the quasi‐stationary waves in the monthly mean wind and geopotential fields near the equator and their relation to the QBO mean winds and the extratropical planetary waves. We discovered quasi‐stationary mid‐stratospheric eddy circulations near the equator that often take the form of equatorial planetary modes. Notably a quasi‐stationary equatorial Rossby mode structure can develop during westerly QBO phases, especially when the westerly mean jet is almost symmetric about the equator and deep in the vertical (often in November). More surprisingly, during the easterly QBO phase (EQBO), a modest amplitude quasi‐stationary equatorial Kelvin mode structure develops. Stationary equatorial waves forced in the troposphere will encounter critical surfaces at one or more levels below the mid‐stratosphere, and so presumably cannot account for our findings. A linear, steady model in an equatorial channel that was forced by observed fields specified at extratropical lateral boundaries also cannot fully reproduce the equatorial mode structures, especially for the Kelvin wave during EQBO. Other factors such as transient wave‐mean flow interaction or some dynamic instability might be responsible. Plain Language Summary: We studied the wind at high altitude (∼30 km) during 1979–2019 using recently available representations of the observed atmospheric flow on a fine‐resolution grid over the whole globe. Near the equator we confirmed the expected year‐to‐year variation of the dominant east‐west flow with prevailing easterly winds and prevailing westerly winds alternating in successive years. Contrary to expectations, however, we found that embedded in the prevailing easterly and westerly flow regimes are pressure patterns close to the equator with one high pressure and one low pressure center arranged on opposite sides of the earth. The pressure patterns are accompanied by swirling wind circulations. These patterns are fixed with respect to the ground and can persist for a month or more, but are quite different in the easterly and westerly prevailing wind periods. These patterns are fairly closely trapped near the equator which is somewhat counterintuitive as they may be forced by planetary scale waves originating at higher latitudes. Key Points: Equatorial zonal eddies sometimes take Rossby and Kelvin mode structures in the mid‐stratospheric QBO westerly and easterly, respectivelyThese modes show systematic seasonality and some correlation with the location and strength of planetary waves in the extratropicsA simple linear steady‐state model showed only limited success in explaining these modes [ABSTRACT FROM AUTHOR]

Many recent studies have confirmed that variability in the stratosphere is a significant source of surface sub‐seasonal prediction skill during Northern Hemisphere winter. It may be beneficial, therefore, to think about times in which there might be windows‐of‐opportunity for skillfull sub‐seasonal predictions based on the initial or predicted state of the stratosphere. In this study, we propose a simple, minimal model that can be used to understand the impact of the stratosphere on tropospheric predictability. Our model purposefully excludes state dependent predictability in either the stratosphere or troposphere or in the coupling between the two. Model parameters are set up to broadly represent current sub‐seasonal prediction systems by comparison with four dynamical models from the Sub‐Seasonal to Seasonal Prediction Project database. The model can reproduce the increases in correlation skill in sub‐sets of forecasts for weak and strong lower stratospheric polar vortex states over neutral states despite the lack of dependence of coupling or predictability on the stratospheric state. We demonstrate why different forecast skill diagnostics can give a very different impression of the relative skill in the three sub‐sets. Forecasts with large stratospheric signals and low amounts of noise are demonstrated to also be windows‐of‐opportunity for skillfull tropospheric forecasts, but we show that these windows can be obscured by the presence of unrelated tropospheric signals. Plain Language Summary: For successful forecasts of surface winter conditions between two weeks and one season ahead, the stratosphere has been shown to be a key source of information. Despite many studies examining how well the stratosphere can be predicted in computer‐based forecasting systems, there remains a lack of understanding of which surface forecasts the stratosphere is most important for. This study is an attempt to step back from examining the role of the stratosphere in any particular forecasting system and instead to determine a simple framework that can be used to understand when and how the stratosphere is important. Using our framework we can construct a series of simple experiments that help to understand how important the stratosphere is in the longer range forecasting problem. Our experiments show that forecasts made during periods in which the Arctic stratospheric winds are unusually strong or weak have greater skill, but this does not depend on how unusually weak or strong the stratospheric winds are. The results are particularly important for thinking about the times in which longer range forecasts might be more skillfull than on average, so called windows‐of‐opportunity, and how these depend on the stratosphere. Key Points: We propose a model that demonstrates how forecast skill present in the lowermost stratosphere contributes to tropospheric forecast skillThe model can explain the greater correlation skill in the troposphere for forecasts during weak or strong vortex eventsThe model shows how tropospheric skill arising from the stratosphere can sometimes be confounded by uncorrelated tropospheric signals [ABSTRACT FROM AUTHOR]

Characteristics of the gravity waves (GWs) in the lower stratosphere, produced by typhoon "Lekima" (2019) after its landfall, are studied using simulations with the high‐resolution WRF‐ARW4.2 model. In particular, the Characteristics and sources of the GWs are investigated. The results show that the GWs have a significant wavelength of 200–400 km and a significant period of 2.5–10 hr. The characteristics of stratospheric gravity wave induced by the typhoon after landing are affected by a westerly trough at mid‐latitude. The GWs are reconstructed by Fourier inversion transform in the significant spectral region in order to reveal the source of waves inducing the stratospheric GWs. The frequency of reconstruction gravity waves varies with time, and the meridional wavenumber varies with latitude. The GW intensity (GWI) has two peak values during the interested period from 10 to 11 August 2019. The first peak in GWI is the result of the in‐phase superposition of the GWs induced by the jet stream and the typhoon, when the two waves propagate almost in the same direction. The second intensity peak occurred when the trough moved away. The correlation between GWI and the typhoon and the trough are analyzed. It is found that the main factor affecting the changes of GWI is the northward movement of the trough. The location and strength of the typhoon and trough and their interaction are used to fit variations in GWI. Key Points: The characteristics of stratospheric gravity wave induced by the typhoon after landing are affected by a westerly trough at mid‐latitudeThe frequency of gravity waves varies with time, and the meridional wavenumber varies with latitudeThe intensity of gravity waves in this case is mainly affected by the northward movement of the trough [ABSTRACT FROM AUTHOR]

We report fallout radionuclide (FRN) and major/trace element (MTE) contributions to bulk atmospheric deposition in Hanover, NH, USA (43.7022°N, 72.2896°W). Deposition of 7Be, 210Pb, and SO4 covary [R2 > 0.4, n = 461] but are discriminated by production sources, depositional mechanisms, meteorological controls, MTE associations, and seasonal biases. 7Be is dominated by rainout (78% of total deposition), recharged by long‐range transport (+23% over mean, o.m.), influenced by stratosphere‐troposphere exchange (+9% o.m.), and solar activity (−2% per doubling of sunspot count). Correlation with particulate nitrogen (+9% per doubling of N) indicates 7Be affinity for biogenic aerosols. 210Pb is dominated by dry + washout deposition (54% of total) and convective storms (+107% o.m), depleted in marine moisture sources (−133% o.m.), correlated with S (+9% per doubling of S), and biased to autumn with Mn, Hg, and V (+7% o.m.). Coincident long‐term declines in S and 210Pb (−14%, −4% per year) suggest co‐scavenging by PbSO4. 7Be:210Pb ratios increase asymptotically with precipitation through the dry‐washout‐rainout transition and recharge of 7Be. At the global scale, 7Be:210Pb increases with precipitation for North American/European sites due to recharge of 7Be in midlatitude storm belts [R2 = 0.64, n = 31]. Conversely, 7Be:210Pb is independent of precipitation for Southeast/East Asian sites where 7Be recharge is low [R2 = 0.01, n = 40]. Globally, 7Be:210Pb ratios in dry deposition reflect resuspended aerosols with mean age of ca. 200 days, contributing <5% of 210Pb deposition. Different aerosol populations contributing to FRN deposition across spatial and temporal scales should emerge as a focus in terrestrial 7Be, 210Pb, and 10Be tracer applications. Plain Language Summary: Beryllium‐7 (7Be) and lead‐210 (210Pb) are natural radioactive elements that are produced in the atmosphere. They are powerful tools in earth and atmospheric sciences because they allow us to trace the movement of other atmospheric elements and particles such as sulphate (SO4), toxins such as mercury (Hg), or particulate matter small enough to damage human lungs (PM2.5). After production, 7Be, 210Pb, and associated particles reach Earth's surface in rainfall, pass through soils and sediments, and may be resuspended as dust that can pose ongoing risks of particulate exposure. To better understand how they trace atmospheric particles, we measured 7Be, 210Pb, SO4, and other elements and particles in rainwater each week for 9 years. By making many measurements we were able to see important differences in how 7Be, 210Pb, and SO4 are delivered to the Earth's surface, and differences among other elements that they associate with. These differences are likely to have implications for how 7Be and 210Pb are used to date geological deposits or to measure erosion or contaminant transport, but more work will be needed to fully understand them. Key Points: 7Be, 210Pb, and SO4 in bulk deposition represent unique mixtures of distinct aerosol populationsPrimary controls on 7Be:210Pb ratios include airmass altitude and origin, meteorology, precipitation depth, and resuspended aerosolsMultiple aerosol populations controlling FRN deposition should be investigated for their implications to terrestrial tracer studies [ABSTRACT FROM AUTHOR]

Tropopause‐overshooting convection transports air from the lower troposphere to the upper troposphere and lower stratosphere (UTLS) where the resulting chemistry and mixing of trace gases can modify the radiation budget. While recent work has examined output from model simulations as well as aircraft and satellite observations of the impacts of tropopause‐overshooting convection on UTLS composition, the range of potential impacts and their dependence on characteristics of storms and their environments is not known. Here, two 10‐day periods, one representative of springtime convection and one representative of summertime convection, were simulated with the Weather Research and Forecasting (WRF) model with Chemistry to examine the range of UTLS composition impacts from tropopause‐overshooting convection. Overall, springtime convection has a larger impact on UTLS composition than summertime convection, with a net effect of increasing water vapor (H2O) in the lower stratosphere and increasing ozone (O3) in the upper troposphere. Springtime convection frequently increases the domain average H2O mixing ratio in the lowermost stratosphere by over 20% while changes in stratospheric H2O from summertime convection are much lower (∼7%–11% increase), reflecting a dependence of the maximum possible H2O increase on UTLS temperature. Increases in upper troposphere O3 mixing ratios span the range 8%–19% from springtime convection and are minimal from summertime convection. Changes in the composition of the UTLS from tropopause‐overshooting convection are largely dependent on the height and temperature of the tropopause, with the largest changes being in environments with relatively low tropopause heights between 11 and 13 km (typical of springtime environments in the United States). Key Points: Moist convection frequently reaches the tropopause in the United States and mixes air between the troposphere and stratosphereTropopause‐overshooting convection occurring during May drastically increases lower stratospheric water vapor mixing ratiosThe impact that convection has on upper troposphere and lower stratosphere (UTLS) composition appears to largely depend on the height of the tropopause [ABSTRACT FROM AUTHOR]

This study investigated the influence of the interdecadal changes in the Pacific‐North America (PNA) and Western Pacific (WP) teleconnections on the planetary waves and stratospheric sudden warmings (SSWs), based on the National Center for Environmental Prediction reanalysis and Japanese 55‐year reanalysis. Before the 1980s, there were more negative PNA & negative WP combinations (−PNA&−WP), and after the 1980s, there were more +PNA&+WP. +PNA&−WP and −PNA&+WP had no significant interdecadal changes before and after the 1980s. The interdecadal changes in the teleconnection combinations may be related to the Pacific Decadal Oscillation (PDO). During +PNA&+WP (−PNA&−WP), the planetary waves with wavenumber 1 have the maximum (minimum) wave energy and propagating more (less) to the stratosphere, causing a greater (lesser) frequency of SSWs. Thus, the interdecadal changes in the teleconnection combinations are one of the important factors which could contribute to the possible increase in SSWs after the 1980s. Since there are no satellite observations on the stratosphere before 1979, there are still many uncertainties in the interdecadal changes in SSWs. Plain Language Summary: Stratospheric sudden warmings (SSWs) are rapid warming processes occurring in the stratosphere in winter. In the northern hemisphere, SSWs mitigate springtime ozone depletion and have potential effects on tropospheric weather. Nevertheless, the number of SSWs recorded in each decade is variable, and these interdecadal changes give rise to changes in SSW impacts. At the same time, the Pacific‐North America (PNA) and Western Pacific (WP) patterns are the most prominent modes of low frequency variability in the Northern Hemisphere. Before the 1980s, there were more −PNA&−WP (negative PNA and negative WP combinations), and after the 1980s, there were more +PNA&+WP. The interdecadal changes in the teleconnection combinations may be related to the Pacific Decadal Oscillation, which is an important decadal signal in the climate system. +PNA&+WP and −PNA&−WP have a significant influence on SSWs, since more (less) planetary waves enter the stratosphere during +PNA&+WP (−PNA&−WP). The interdecadal changes in the teleconnection combinations may have an important contribution to the possible increase in SSWs after the 1980s. Key Points: The teleconnection combinations of +PNA&+WP and −PNA&−WP had significant interdecadal changes around the 1980sThe interdecadal changes in +PNA&+WP and −PNA&−WP may be related to the Pacific Decadal OscillationThe interdecadal changes in +PNA&+WP and −PNA&−WP could contribute to the possible increase in SSWs after the 1980s [ABSTRACT FROM AUTHOR]

Using the geopotential height measurements from the Aura Microwave Limb Sounder and the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite, we found eight westward quasi‐8‐day wave (Q8DW) events with zonal wavenumber 1 (W1) during the Arctic sudden stratospheric warmings (SSWs) in 2005–2020. The W1 Q8DW perturbations are mainly confined to the middle and high latitudes in the Northern Hemisphere, and peak at ∼50–80 km. Specified Dynamics Whole Atmosphere Community Climate Model eXtended version simulations are utilized to reproduce the Q8DW activities. The diagnostic results indicate that the W1 Q8DWs originate from the high latitudes in the Northern Hemisphere stratosphere. Their excitation, propagation, and amplification are primarily influenced by the critical layers and baroclinic/barotropic instability, which are dependent on the background conditions during SSWs. Our statistical results show that the W1 planetary waves with periods shorter than 12 days during the Arctic SSWs are most frequently observed at the period of ∼8 days, which is significantly different from the climatological periods of the Rossby (1, 1) and (1, 2) normal modes (6.14 and 9.81 days, respectively). These results indicate that the W1 Q8DW may be a new wave mode. Finally, we found that the reversal (or deceleration) and the recovery of the zonal mean flow during SSWs can result in a zonally symmetric Q8DW, and the W1 Q8DW is likely the child wave generated by the nonlinear interaction between the stationary planetary wave with zonal wavenumber 1 and zonally symmetric Q8DW. Key Points: The westward quasi‐8‐day wave (Q8DW) with zonal wavenumber 1 (W1) is observed eight times during the Arctic sudden stratospheric warmings (SSWs) from 2005 to 2020The SD‐WACCM‐X simulation results show that the Q8DW activities during SSWs are highly influenced by the critical layers and instabilitiesThe W1 Q8DWs during the Arctic SSWs are most likely generated from the zonally symmetric wave‐stationary wave nonlinear interactions [ABSTRACT FROM AUTHOR]

The connections between the stratosphere and troposphere, for example, the stratospheric Arctic vortex (SAV) and Arctic Oscillation (AO), have been paid attention by scientists and the public in recent years. Previous studies suggested that the coupling between the stratosphere and troposphere is the strongest in winter. However, whether the link between SAV and AO during boreal winter on interannual timescales is steady is still unknown. Our results show that the interannual relationship between the SAV in early winter and AO in late winter during 1958–2018 is unsteady and has experienced a remarkable interdecadal change around the 2000s, with large and statistically significant positive correlations before the 2000s but small and insignificant correlations after the 2000s. The weakened linkage between the SAV and AO after the 2000s is possibly caused by (a) the weakened downward effect of SAV on the North Atlantic and polar sectors of AO, and (b) the opposite signs of sea level pressure (SLP) in the North Pacific and North Atlantic. The links of SAV‐AO are enforced (weakened) when AO‐related SLP anomalies over the North Pacific are the same (opposite) signs of those over the North Atlantic before (after) the 2000s. Differences in the tropospheric state over the North Pacific during two periods might be jointly contributed by the SAV and the SAV‐related significant (insignificant) sea surface temperature anomalies over central North Pacific in the earlier (latter) period. Our results may provide evidences to improve the predictability in seasonal or interannual timescales forecasts of weather and climate at mid‐latitudes. Key Points: The link of stratospheric Arctic vortex (SAV)‐Arctic Oscillation (AO) has experienced a remarkable interdecadal change around the 2000sThe opposite signs of the SAV‐related sea level pressure anomalies over the North Pacific and North Atlantic after the 2000s weaken the links of SAV‐AODifferences in AO's Pacific sector around the 2000s are jointly contributed by the SAV and its related sea surface temperature anomalies over central North Pacific [ABSTRACT FROM AUTHOR]