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

A unique set of relaxation experiments with a forecast model initialized during December–January 1999–2019 is used to explore tropical influence on the Northern Hemisphere polar stratosphere and stratosphere–troposphere coupling, to quantify predictability benefits due to the perfect knowledge of the tropical variability. On average, predictability of the polar stratosphere, as represented by the 50 hPa geopotential height anomalies north of 50°N (Z50), increases from 17 days in freely running (control) forecasts to 21 days in the tropical relaxation experiments. At sub‐seasonal time‐scales, a statistically significant improvement in weekly mean skill scores can be demonstrated in 14%–20% of individual forecast ensembles, mostly in cases when the skill of the corresponding control forecast is worse than average. In these forecasts, root‐mean‐square errors and forecast spread of Z50 during forecast weeks 3–5 are decreased by 10%–15%. Stratospheric improvements are detected during periods of both vortex strengthening and vortex weakening, including most major Sudden Stratospheric Warmings that occurred during the study period, via modulation of the upward wave activity fluxes. An active Madden–Julian Oscillation (MJO) is found in most of these events with MJO phase 5–7 preceding vortex weakening and MJO phase 3–4 preceding vortex strengthening. Forecasts with improved stratospheric circulation also have improved tropospheric circulation during and after the periods when improvements are detected in the stratosphere. We attribute these improvements to both stratosphere–troposphere coupling and tropospheric tropical teleconnections. [ABSTRACT FROM AUTHOR]

Gravity waves (GWs) are among the key drivers of the meridional overturning circulation in the mesosphere and upper stratosphere. Their representation in climate models suffers from insufficient resolution and limited observational constraints on their parameterizations. This obscures assessments of middle atmospheric circulation changes in a changing climate. This study presents a comprehensive analysis of stratospheric GW activity above and downstream of the Andes from 1 to 15 August 2019, with special focus on GW representation ranging from an unprecedented kilometer-scale global forecast model (1.4 km ECMWF IFS), ground-based Rayleigh lidar (CORAL) observations, modern reanalysis (ERA5), to a coarse-resolution climate model (EMAC). Resolved vertical flux of zonal GW momentum (GWMF) is found to be stronger by a factor of at least 2–2.5 in IFS compared to ERA5. Compared to resolved GWMF in IFS, parameterizations in ERA5 and EMAC continue to inaccurately generate excessive GWMF poleward of 60°S, yielding prominent differences between resolved and parameterized GWMFs. A like-to-like validation of GW profiles in IFS and ERA5 reveals similar wave structures. Still, even at ∼1 km resolution, the resolved waves in IFS are weaker than those observed by lidar. Further, GWMF estimates across datasets reveal that temperature-based proxies, based on midfrequency approximations for linear GWs, overestimate GWMF due to simplifications and uncertainties in GW wavelength estimation from data. Overall, the analysis provides GWMF benchmarks for parameterization validation and calls for three-dimensional GW parameterizations, better upper-boundary treatment, and vertical resolution increases commensurate with increases in horizontal resolution in models, for a more realistic GW analysis. Significance Statement: Gravity wave–induced momentum forcing forms a key component of the middle atmospheric circulation. However, complete knowledge of gravity waves, their atmospheric effects, and their long-term trends are obscured due to limited global observations, and the inability of current climate models to fully resolve them. This study combines a kilometer-scale forecast model, modern reanalysis, and a coarse-resolution climate model to first compare the resolved and parameterized momentum fluxes by gravity waves generated over the Andes, and then evaluate the fluxes using a state-of-the-art ground-based Rayleigh lidar. Our analysis reveals shortcomings in current model parameterizations of gravity waves in the middle atmosphere and highlights the sensitivity of the estimated flux to the formulation used. [ABSTRACT FROM AUTHOR]

The weak stratospheric polar vortex (SPV) is usually linked to Northern Hemisphere cold spells. Based on the fifth generation of ECMWF atmospheric reanalysis and WACCM model experiments, we use K-means cluster analysis to extract the zonally asymmetric pattern of October-February stratospheric variability, which involves a stretched SPV and hence leads to cold surges in Northern Hemispheric mid-latitudes. There are contrasting effects and mechanisms between autumn (October–November) and late winter (February) SPV stretching events. In October, anomalies in the stratospheric circulation affect the near-surface 16–20 days after the weakening of the SPV. This contributes to a shift in the North Atlantic Oscillation (NAO) towards its negative phase, leading to cold anomalies over northern Eurasia. Together with the weakening of planetary wave-1 during days 31–40, the second stratosphere–troposphere coupling strengthens the East Asian trough and the Siberian high, resulting in Eurasian high-latitude cooling. For November, the suppressed upward propagation of wave-1 during days 11–15 is conducive to anomalous high pressure over northern Europe and thereby European cooling through a stratospheric pathway, while for days 21–30, the weakening of propagating wave-2 over Eastern Europe intensifies the mid-latitude wave train through a tropospheric pathway, favorable for cold temperatures in mid-latitude East Asia. In contrast, the late winter SPV stretching events and the attendant Eurasian coldness during 11–25 days are likely to have been simultaneously driven by the long-lived European high anomaly, which enhanced the upward-propagating tropospheric waves into the stratosphere and thus favored SPV stretching. It indicates that the tropospheric pathway, rather than the stratospheric pathway, plays a dominant role in cold Eurasia following February SPV stretching events. [ABSTRACT FROM AUTHOR]

• For the SH 2019 SSW, the wave driving observed was the highest recorded since 1979. • An enhanced upwelling led to cooling in the tropical stratosphere. • Tropical upper stratospheric water vapor increased while ozone decreased after SSW. The changes in the intensity of Brewer-Dobson circulation (BDC) in the record-breaking Southern Hemisphere sudden stratospheric warming (SH SSW) event in 2019 are investigated in the present study. Wave driving was observed to be at its highest in 2019 SH SSW, despite the event being classified as a minor warming event. The Eliassen-Palm flux divergence, indicative of wave driving, was observed to be increasing before the SSW, and subsiding afterwards. The stratospheric circulation followed the wave driving and intensified along the warming episode. The resulting stratospheric distribution from Aura Microwave Limb Sounder showed a negative ozone anomaly (∼20%) in the tropical lower stratosphere and a downward propagating positive anomaly in the Southern Hemisphere polar stratosphere around the central dates. The water vapor mixing ratio showed an increase of more than 25% in the polar lower stratosphere and 10 % in the tropical upper stratosphere. A cooling of about 5 K was observed in the tropical middle stratosphere. The results indicate that SH SSW can change the intensity of the stratospheric circulation which in turn affects the ozone and water vapor transport from the tropics to the pole. [ABSTRACT FROM AUTHOR]

Sulfur‐based stratospheric aerosol intervention (SAI) can cool the climate, but also heats the tropical lower stratosphere if done with injections at low latitudes. We explore the role of this heating in the climate response to SAI, by using mechanistic experiments that remove the effects of longwave absorption of sulfate aerosols above the tropopause. If longwave absorption by stratospheric aerosols is disabled, the heating of the tropical tropopause and most of the related side effects are strongly alleviated and the cooling per Tg‐S injected is 40% bigger. Such side‐effects include the poleward expansion of eddy‐driven jets, acceleration of the stratospheric residual circulation, and delay of Antarctic ozone recovery. Our results add to other recent findings on SAI side effects and demonstrate that SAI scenarios with low‐latitude injections of absorptive materials may result in atmospheric effects and regional climate changes that are comparable to those produced by the CO2 warming signal. Plain Language Summary: We explore the effects of the injection of sulfur into the tropical stratosphere to lower global average temperatures from those projected under a high‐emission greenhouse gas scenario to the levels of a mid‐emission scenario. We found that some detrimental effects of this injection are of a similar magnitude to those from climate change itself in some regions. This includes a strong warming 15 km above the tropics, which alters large‐scale weather patterns in the atmosphere. In comparison to the mid‐emission scenario, we find enhanced surface warming in the polar regions and modification in regional precipitation patterns over land, therefore not completely alleviating the warming of the high‐emission scenario in high northern latitudes. We show that the tropical stratospheric heating is responsible for a large portion of these side effects on tropospheric climate. Key Points: Many side effects of sulfur‐based stratospheric aerosol intervention are caused by heating of the tropical lower stratosphereSome regional patterns of change tied to atmospheric circulation can be of the same magnitude as those that are CO2‐drivenThe absorptivity of the aerosol particles increases their lifetime but decreases their cooling efficiency per Tg‐S per year by 40% [ABSTRACT FROM AUTHOR]

Antarctic Total Column Ozone (TCO) gradually began to recover around 2000, and a large number of studies have pointed out that the recovery of the Antarctic TCO is most significant in the austral early spring (September). Based on the Bodeker Scientific Filled Total Column Ozone and ERA5 reanalysis dataset covering 1979–2019, the variation characteristics of the Antarctic TCO and stratospheric circulation for the TCO 'depletion' period (1979–1999) and the 'recovery' period (2000–2019) are analyzed in September. Results show that: (1) Stratospheric elements significantly related to the TCO have corresponding changes during the two eras. (2) The interannual variability of the TCO and the above-mentioned stratospheric circulation elements in the recovery period are stronger than those in the depletion period. (3) Compared with the depletion period, due to the stronger amplitude of the planetary wave 1, stronger Eliassen–Palm (EP) flux corresponds to EP flux convergence, larger negative eddy heat flux, and positive eddy momentum flux in the stratosphere during the recovery period. The polar temperature rises in the lower and middle stratosphere and the polar vortex weakens in the middle and upper stratosphere, accompanied by the diminished area of PSC. This contributes to the understanding of Antarctic ozone recovery. [ABSTRACT FROM AUTHOR]

This study investigates the stratospheric response to Arctic sea ice loss and subsequent near-surface impacts by analyzing 200-member coupled experiments using the Whole Atmosphere Community Climate Model version 6 (WACCM6) with preindustrial, present-day, and future sea ice conditions specified following the protocol of the Polar Amplification Model Intercomparison Project. The stratospheric polar vortex weakens significantly in response to the prescribed sea ice loss, with a larger response to greater ice loss (i.e., future minus preindustrial) than to smaller ice loss (i.e., future minus present-day). Following the weakening of the stratospheric circulation in early boreal winter, the coupled stratosphere–troposphere response to ice loss strengthens in late winter and early spring, projecting onto a negative North Atlantic Oscillation–like pattern in the lower troposphere. To investigate whether the stratospheric response to sea ice loss and subsequent surface impacts depend on the background oceanic state, ensemble members are initialized by a combination of varying phases of Atlantic multidecadal variability (AMV) and interdecadal Pacific variability (IPV). Different AMV and IPV states combined, indeed, can modulate the stratosphere–troposphere responses to sea ice loss, particularly in the North Atlantic sector. Similar experiments with another climate model show that, although strong sea ice forcing also leads to tighter stratosphere–troposphere coupling than weak sea ice forcing, the timing of the response differs from that in WACCM6. Our findings suggest that Arctic sea ice loss can affect the stratospheric circulation and subsequent tropospheric variability on seasonal time scales, but modulation by the background oceanic state and model dependence need to be taken into account. Significance Statement: This study uses new-generation climate models to better understand the impacts of Arctic sea ice loss on the surface climate in the midlatitudes, including North America, Europe, and Siberia. We focus on the stratosphere–troposphere pathway, which involves the weakening of stratospheric winds and its downward coupling into the troposphere. Our results show that Arctic sea ice loss can affect the surface climate in the midlatitudes via the stratosphere–troposphere pathway, and highlight the modulations from background mean oceanic states as well as model dependence. [ABSTRACT FROM AUTHOR]

Cold winters over Eurasia often coincide with warm winters in the Arctic, which has become known as the "warm Arctic–cold Eurasia" pattern. The extent to which this observed correlation is indicative of a causal response to sea ice loss is debated. Here, using large multimodel ensembles of coordinated experiments, we find that the Eurasian temperature response to Arctic sea ice loss is weak compared to internal variability and is not robust across climate models. We show that Eurasian cooling is driven by tropospheric and stratospheric circulation changes in response to sea ice loss but is counteracted by tropospheric thermodynamical warming, as the local warming induced by sea ice loss spreads into the midlatitudes by eddy advection. Although opposing effects of thermodynamical warming and dynamical cooling are found robustly across different models or different sea ice perturbations, their net effect varies in sign and magnitude across the models, resulting in diverse model temperature responses over Eurasia. The contributions from both tropospheric dynamics and thermodynamics show substantial intermodel spread. Although some of this spread in the Eurasian winter temperature response to sea ice loss may stem from model uncertainty, even with several hundred ensemble members, it is challenging to isolate model differences in the forced response from internal variability. [ABSTRACT FROM AUTHOR]

Based on the data of ERA5 reanalysis, the dates of spring and autumn changes in the stratospheric circulation on isobaric surfaces of 30, 20, and 10 hPa in the latitude zone of 30°–90° N in the period 1979–2020 have been obtained. Of the 42 cases of spring changes, 10 are early, 15 are middle, and 17 are late. The spread in the dates of spring changes on the surface of 10 hPa is 69 days. Most often, the spring changes of the circulation occurs from top to bottom; in some years, the delay of spring changes on the surface of 30 hPa relative to the surface of 10 hPa reaches 22–25 days. Autumn change takes place from the bottom up, and their terms at the 3 levels under consideration are close to each other. The relationship between the timing of the spring changes of the stratospheric circulation with solar activity and large sudden winter stratospheric warming is shown. An analysis of the fields of anomalies of daily temperature values and zonal wind velocity in the 1000-1 hPa layer in the period of January–May showed their significant spatiotemporal difference in the case of early and late spring changes. Thus, foci of positive anomalies of temperature and wind speed are formed initially in the upper stratosphere and then shifted from top to bottom. The interrelations between the layers of the atmosphere in different seasons are considered. [ABSTRACT FROM AUTHOR]

This study conducts a thorough investigation into the behaviors of analysis ensemble spreads linked to stratospheric sudden warming (SSW) events. A stratosphere-resolving ensemble data assimilation system is used here to document the evolution of analysis spread leading up to a pair of warming events. Precursory signals of the increased ensemble spreads were found a few days prior to two SSW events that occurred during December 2018 and August–September 2019 in the Northern and Southern Hemispheres, respectively. The signals appeared in the upper and middle stratosphere and did not appear at lower heights. When the signals appeared, it was found that both tendency by forecast and analysis increment in a forecast-analysis (data assimilation) cycle simultaneously became large. An empirical orthogonal function analysis showed that the dominant structures of the precursory signals were equivalent barotropic and were 90° out-of-phase with the analysis ensemble-mean field. Over the same period, the upper and middle stratosphere became more susceptible to barotropic instability than in their previous states. We conclude that the differing growth of barotropically unstable modes across ensemble members can amplify spread during the lead-up to SSW events. Significance Statement: Winds in the winter stratospheric polar vortex are typically westerly. Occasionally, however, warming over the pole leads to a reversal of the flow through a process known as stratospheric sudden warming. These events are difficult to predict even in state-of-the-art analysis and forecasting systems. In this study, we identify a precursor signal in the form of increased ensemble spread that appears to originate from differing realizations of growing barotropic modes across the ensemble. This signal could serve as a useful forecasting tool by enhancing situational awareness in the lead-up to potential stratospheric sudden warming events. [ABSTRACT FROM AUTHOR]

This study examines the dependence of Arctic stratospheric polar vortex (SPV) variations on the meridional positions of the sea surface temperature (SST) anomalies associated with the first leading mode of North Pacific SST. The principal component 1 (PC1) of the first leading mode is obtained by empirical orthogonal function decomposition. Reanalysis data, numerical experiments, and CMIP5 model outputs all suggest that the PC1 events (positive-minus-negative PC1 events), located relatively northward (i.e., North PC1 events), more easily weaken the Arctic SPV compared to the PC1 events located relatively southward (i.e., South PC1 events). The analysis indicates that the North PC1-related Aleutian low anomaly is located over the northern North Pacific and thus enhances the climatological trough, which strengthens the planetary-scale wave 1 at mid-to-high latitudes and thereby weakens the SPV. The weakened stratospheric circulation further extends into the troposphere and favors negative surface temperature anomalies over Eurasia. By contrast, the South PC1-related Aleutian low anomaly is located relatively southward, and its constructive interference with the climatological trough is less efficient at high latitudes. Thus, the South PC1 events could not induce an evident enhancement of the planetary-scale waves at high latitudes and thereby a weakening of the SPV on average. The Eurasian cooling associated with South PC1 events (positive-minus-negative PC1 events) is also not prominent. The results of this study suggest that the meridional positions of the PC1 events may be useful for predicting the Arctic SPV and Eurasian surface temperature variations. [ABSTRACT FROM AUTHOR]

The impacts of Stratospheric Aerosol Injection (SAI) on the atmosphere and surface climate depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. We use a set of CESM2(WACCM6) SAI simulations achieving three different levels of global mean surface cooling and demonstrate that unlike some direct surface climate impacts driven by the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in the high latitude circulation and ozone are more complex and could be non‐linear. This manifests in our simulations by disproportionally larger Antarctic springtime ozone loss, significantly larger intra‐ensemble spread of the Arctic stratospheric jet and ozone responses, and non‐linear impacts on the extratropical modes of surface climate variability under the strongest‐cooling SAI scenario compared to the weakest one. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI. Plain Language Summary: The injection of reflective aerosols, or their precursors, into the lower stratosphere (Stratospheric Aerosol Injection, SAI) has been proposed as a temporary measure to offset some of the adverse impacts of climate change whilst atmospheric concentrations of greenhouses are being stabilized and, ultimately, reduced. The impacts of SAI on the atmosphere and surface climate would depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. Here we analyze SAI impacts on stratospheric climate and ozone in a set of Earth system model simulations under varying magnitudes of the SAI‐induced global mean cooling. We demonstrate that unlike some of the direct surface climate impacts from the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in stratospheric circulation, chemistry and climate are more complex, with the model simulations pointing toward more non‐linear behavior of the high latitude circulation and ozone under higher SAI scenarios. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI. Key Points: Impacts of Stratospheric Aerosol Injection (SAI) depend on how much surface cooling is to be achievedHigh latitude circulation, ozone and modes of extratropical variability can vary non‐linearly with the SAI‐induced global surface coolingThese potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI [ABSTRACT FROM AUTHOR]

Determining the habitability and interpreting future atmospheric observations of exoplanets requires understanding the atmospheric dynamics and chemistry from a 3D perspective. Previous studies have shown significant spatial variability in the ozone layer of synchronously rotating M-dwarf planets, assuming an Earth-like initial atmospheric composition. We simulate Proxima Centauri b in an 11.2-d orbit around its M-type host star using a 3D coupled climate-chemistry model to understand the spatial variability of ozone and identify the mechanism responsible for it. We document a previously unreported connection between the ozone production regions on the photochemically active dayside hemisphere and the nightside devoid of stellar radiation and thus photochemistry. We find that stratospheric dayside-to-nightside overturning circulation can advect ozone-rich air to the nightside. On the nightside, ozone-rich air subsides at the locations of two quasi-stationary Rossby gyres, resulting in an exchange between the stratosphere and troposphere and the accumulation of ozone at the gyre locations. The mechanism drives the ozone distribution for both the present atmospheric level (PAL) and a 0.01 PAL O2 atmosphere. We identify the hemispheric contrast in radiative heating and cooling as the main driver of the stratospheric dayside-to-nightside circulation. An age-of-air experiment shows that the mechanism also impacts other tracer species in the atmosphere (gaseous and non-gaseous phase) as long as chemical lifetimes exceed dynamical lifetimes. These findings, applicable to exoplanets in similar orbital configurations, illustrate the 3D nature of planetary atmospheres and the spatial and temporal variability that we can expect to impact spectroscopic observations of exoplanet atmospheres. [ABSTRACT FROM AUTHOR]

The Canadian Earth System Model version 5.0 (CanESM5.0), the most recent major version of the global climate model developed at the Canadian Centre for Climate Modelling and Analysis (CCCma) at Environment and Climate Change Canada (ECCC), has been used extensively in climate research and for providing future climate projections in the context of climate services. Previous studies have shown that CanESM5.0 performs well compared to other models and have revealed several model biases. To address these biases, the CCCma has recently initiated the "Analysis for Development" (A4D) activity, a coordinated analysis activity in support of CanESM development. Here we describe the goals and organization of this effort and introduce two variants ("p1" and "p2") of a new CanESM version, CanESM5.1, which features important improvements as a result of the A4D activity. These improvements include the elimination of spurious stratospheric temperature spikes and an improved simulation of tropospheric dust. Other climate aspects of the p1 variant of CanESM5.1 are similar to those of CanESM5.0, while the p2 variant of CanESM5.1 features reduced equilibrium climate sensitivity and improved El Niño–Southern Oscillation (ENSO) variability as a result of intentional tuning of the atmospheric component. The A4D activity has also led to the improved understanding of other notable CanESM5.0 and CanESM5.1 biases, including the overestimation of North Atlantic sea ice, a cold bias over sea ice, biases in the stratospheric circulation and a cold bias over the Himalayas. It provides a potential framework for the broader climate community to contribute to CanESM development, which will facilitate further model improvements and ultimately lead to improved climate change information. [ABSTRACT FROM AUTHOR]

The peculiarities of the troposphere and stratosphere dynamic processes that accompanied cooling events (cold waves) with a duration of more than a week in the northwest of European Russia (on the example of St. Petersburg) and in its north and center (on the example of Moscow) in early and late December 2021, respectively, are analyzed using reanalysis data, station observations, and trajectory modeling. In both cases the daily temperatures dropped below −10°C, and cold waves were accompanied by the air mass transport from the northeast [ABSTRACT FROM AUTHOR]

The large-scale extratropical circulation in the Southern Hemisphere is much more zonally symmetric than that of the Northern Hemisphere, but its zonal departures, albeit highly relevant for regional impacts, have been less studied. In this study we analyse the joint variability of the zonally asymmetric springtime stratospheric and tropospheric circulation using Complex Empirical Orthogonal Functions (cEOF) to characterise planetary waves of varying amplitude and phase. The leading cEOF represents variability of a zonal wave 1 in the stratosphere that correlates slightly with the Symmetric Southern Annular Mode (S-SAM). The second cEOF (cEOF2) is an alternative representation of the Pacific-South American modes. One phase of this cEOF is also very highly correlated with the Asymmetric SAM (A-SAM) in the troposphere. Springs with an active ENSO tend to lock the cEOF2 to a specific phase, but have no consistent impact on its magnitude. Furthermore, we find indications that the location of Pacific Sea Surface Temperature anomalies affect the phase of the cEOF2. As a result, the methodology proposed in this study provides a deeper understanding of the zonally asymmetric springtime extratropical SH circulation. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) are extreme disruptions of the wintertime polar vortex that can alter the tropospheric weather for over 2 months. However, the reasons why only some SSWs have a tropospheric impact are not yet clear. This study analyses the tropospheric impact of SSWs over the Atlantic region as measured by the latitudinal displacement of the North Atlantic eddy-driven jet following SSWs. We use reanalysis data for the period 1950–2020 to examine differences in the stratospheric and tropospheric circulation for SSWs with an equatorward (EQ) or a poleward (POLE) shift. Our results show a stronger and more persistent Northern Annular Mode (NAM) signal in the lower stratosphere for EQ than for POLE, beginning 2 weeks before the onset date. In the troposphere, we find precursory signals of the Atlantic jet behavior over Siberia, consistent with previous studies, and also over the central North Pacific and central Europe. In particular, our results suggest that the noncanonical poleward jet shift response to SSWs is in part modulated by circulation anomalies over the central North Pacific, and that these are in turn connected to the cold phase of El Niño–Southern Oscillation. Further analysis of the enhanced predictability given by these precursors suggests that the sign of the lower-stratospheric NAM and the geopotential anomalies over the central North Pacific significantly affect the probability of having an EQ or POLE response of the Atlantic jet. [ABSTRACT FROM AUTHOR]

The dominant pattern of Northern Hemisphere extratropical climate variability is the Northern Annular Mode (NAM), which in wintertime represents the coupling of the stratospheric and tropospheric circulations. The internal variability associated with the NAM has been shown to give rise to substantial uncertainty in climate projections of regional surface air temperatures (SATs), but it's unclear how much of this variability arises from stratosphere‐troposphere coupling processes. Here, using three large‐ensemble coupled climate model simulations from 1850 to 2100 with varying fidelity of stratosphere‐troposphere coupling processes, we demonstrate that regional SAT variability across timescales is amplified when the tropospheric NAM is coupled to the stratospheric NAM. Moreover, models that lack adequate simulation of stratosphere‐troposphere coupling processes may not capture the magnitude of uncertainty in near‐term SAT projections associated with this coupling. Plain Language Summary: Large‐scale atmospheric circulation variability drives substantial variation in regional surface air temperatures (SATs), so that even on 30–40 year timescales local cooling trends are possible even as global‐mean temperatures increase. We provide evidence that when the stratospheric mean flow is coupled to the tropospheric mean flow, the decadal variability of SATs is amplified in some regions. Some models do not represent well the observed coupling of the stratosphere to the troposphere, and these models may underestimate decadal variability associated with this coupling. Key Points: The Northern Annular Mode (NAM) exhibits decadal variability that can accelerate or slow the rate of anthropogenic regional surface warmingAmplified decadal variability of extratropical surface temperatures occurs when the NAM is coupled from the stratosphere to the troposphereModels that fail to adequately capture vertical coupling may underestimate the magnitude of decadal variations in surface air temperatures [ABSTRACT FROM AUTHOR]

Using 20 quasi-biennial oscillation (QBO)-resolving models from phases 5/6 of the Coupled Model Intercomparison Project (CMIP5/6), this study examines the projected Southern Hemisphere (SH) extratropical response to the QBO. Nine of the 22 models simulate decelerated circumpolar westerlies during easterly QBO (EQBO) in the historical climate as is observed, though only ∼30% of the observed change is reproduced in the multimodel ensemble mean (MME) of these high-skill models. These high-skill models project an enhanced stratospheric QBO teleconnection for both emissions scenarios. Further, the stratospheric wind anomaly in high latitudes is projected to move to midlatitudes in future scenarios. The climatological subtropical jet is projected to strengthen, while tropical easterlies are projected to weaken. As a consequence, upward wave activity in the future appears to become more sensitive to the QBO phase. Enhanced upward propagation of waves in mid- to high latitudes during EQBO are much stronger in future scenarios than in historical simulations. The anomalous downwelling over the Antarctic, as part of the Brewer–Dobson deep branch response to EQBO, is also projected to strengthen, corresponding to increased warm anomalies. In future scenarios, the areal extent of the deep convective response to EQBO over the Maritime Continent widens and includes a sharper reduction in outgoing longwave radiation, albeit with southward expansion from Indonesia to Australia. The enhancement and spatial shift in the stratospheric polar vortex pathway and tropical convection pathway subsequently lead to changes in the tropospheric QBO signal. An annular mode–like response forms in the troposphere and near-surface in the present climate, whereas this pattern shifts farther equatorward in future projections with circulation anomalies in the tropical Indian Ocean amplifying. Significance Statement: The influence of the equatorial stratospheric quasi-biennial oscillation (QBO) has been reported to intensify in the Northern Hemisphere under climate change, even as the QBO wind magnitude weakens. It is still unknown whether the influence of the QBO on the Southern Hemisphere intensifies or weakens in a warm climate. Using simulations from multiple models, this study finds that the influence of the QBO on the Southern Hemisphere is projected to strengthen in most models even as the QBO weakens. Two QBO pathways are explored, including the stratospheric polar vortex route and the tropical convection pathway, both of which are projected to amplify in both the moderate- and high-emissions scenarios. In addition to the amplification of the QBO signals in the Southern Hemisphere extratropics, an equatorward shift of the maximum anomalies is also projected. That is, the maximum easterly anomalies in the Antarctic stratosphere during easterly QBO and the annular mode–like pattern in the troposphere or the near-surface shifts farther equatorward. [ABSTRACT FROM AUTHOR]

During the recent four decades since 1980, a series of modern climate satellites were launched, allowing for the measurement and record-keeping of multiple climate parameters, especially over the polar regions where traditional observations are difficult to obtain. China has been actively engaging in polar expeditions. Many observations were conducted during this period, accompanied by improved Earth climate models, leading to a series of insightful understandings concerning Arctic and Antarctic climate changes. Here, we review the recent progress China has made concerning Arctic and Antarctic climate change research over the past decade. The Arctic temperature increase is much higher than the global-mean warming rate, associated with a rapid decline in sea ice, a phenomenon called the Arctic Amplification. The Antarctic climate changes showed a zonally asymmetric pattern over the past four decades, with most of the fastest changes occurring over West Antarctica and the Antarctic Peninsula. The Arctic and Antarctic climate changes were driven by anthropogenic greenhouse gas emissions and ozone loss, while tropical-polar teleconnections play important roles in driving the regional climate changes and extreme events over the polar regions. Polar climate changes may also feedback to the entire Earth climate system. The adjustment of the circulation in both the troposphere and the stratosphere contributed to the interactions between the polar climate changes and lower latitudes. Climate change has also driven rapid Arctic and Southern ocean acidification. Chinese researchers have made a series of advances in understanding these processes, as reviewed in this paper. [ABSTRACT FROM AUTHOR]

The representation of the stratosphere and stratosphere–troposphere coupling processes is evaluated in the subseasonal Global Ensemble Forecast System, version 12 (GEFSv12), hindcasts. The GEFSv12 hindcasts develop systematic stratospheric biases with increasing lead time, including a too strong boreal wintertime stratospheric polar vortex. In the tropical stratosphere, the GEFSv12 winds and temperatures associated with the quasi-biennial oscillation (QBO) tend to decay with lead time such that they underestimate the observed amplitudes; consistently, the QBO-associated mean meridional circulation is too weak. The hindcasts predict extreme polar vortex events (including sudden stratospheric warmings and vortex intensifications) about 13–14 days in advance, and extreme lower-stratospheric eddy heat flux events about 6–10 days in advance. However, GEFSv12’s ability to predict these events is likely affected by its zonal-mean circulation biases, which increases the rates of false alarms and missed detections. Nevertheless, GEFSv12 shows stratosphere–troposphere coupling relationships that agree well with reanalysis and other subseasonal forecast systems. For instance, GEFSv12 reproduces reanalysis relationships between polar vortex strength and the Northern Annular Mode in the troposphere. It also exhibits enhanced weeks 3–5 prediction skill of the North Atlantic Oscillation index when initialized during strong and weak polar vortex states compared to neutral states. Furthermore, GEFSv12 shows significant differences in Madden–Julian oscillation (MJO) amplitudes and enhanced MJO predictive skill in week 4 during easterly versus westerly QBO phases, though these results are sensitive to the level used to define the QBO. Our results provide a baseline from which future GEFS updates may be measured. [ABSTRACT FROM AUTHOR]

The sensitivity of atmospheric ozone to geomagnetic storms and reduced cosmic rays' (CRs) intensity (called Forbush decrease [FD]) has been noticed for many years. However, it is still unclear what the factors affecting ozone density are—whether these are the complex changes induced by geomagnetic storms in the upper atmosphere and ionosphere, or it is the severe reduction of CRs accessing Earth's lower atmosphere. Analyzing two strong geomagnetic storms, accompanied by FDs, and another FD occurred in geomagnetically quiet conditions, we conclude that observed ozone changes support the idea about the existence of an additional source of ozone in the lower stratosphere. The ion‐molecular reactions of catalytic ozone production are initiated by secondary ionization, produced by cosmic radiation at these levels. This conclusion is supported by the fact that the strongest ozone depletion beneath its maximum is found in regions with the severest reduction of CR flux. Moreover, the time delay of ozone response (∼2 days), is short enough to be attributed to changes in circulation. In addition, we have examined the changes in the sea level pressure during the analyzed events and found out that the surface pressure follows dynamically the spatial pattern of the ozone changes, forced by the reduced amount of precipitating energetic particles in the atmosphere. Plain Language Summary: Theoretical estimates of the efficiency of the lower stratospheric ion‐molecular reactions suggest an existence of another source of ozone (different from the well‐known photo‐dissociation mechanism). The conditions necessary for activation of autocatalytic ozone production require: (a) increased density of the low energy electrons (naturally produced at these levels by cosmic rays) and (b) reduced amount of water molecules above the tropopause. The short‐lasting periods of a significantly decreased cosmic radiation—known as Forbush decreases (FDs)—provide a good opportunity for testing the above hypothesis. In this paper, we have compared the spatial distribution of three different FDs with changes in the lower stratospheric ozone density. We show that the strongest ozone depletion (beneath the ozone layer maximum) coincides very well with the largest magnitude of the FD. The time delay of ozone response to the applied forcing is too short, so the ozone depletion could not be attributed to the stratospheric circulations (as usually done). This result could be interpreted as an indirect confirmation of the importance of energetic particles, precipitating in the Earth's atmosphere, for the spatial‐temporal variability of the lower stratospheric ozone density. Key Points: The response of ozone profile to the reduced cosmic ray intensity (known as Forbush decrease [FD]) is investigatedWe found that atmospheric ozone beneath its maximum is reduced in periods of FDsThis result is an indirect confirmation of the existence of secondary source of ozone in the lower stratosphere [ABSTRACT FROM AUTHOR]

Tropospheric methane (CH4) shows large inter‐annual variability in its growth rates on top of decadal trends, which is usually interpreted as changes in sources or sinks. The contribution of the stratosphere‐troposphere exchange (STE) is often omitted in such analyses. Here, we quantify annual anomalies of stratosphere‐troposphere CH4 fluxes using varied model‐ or observation‐based methods. Globally, the inter‐annual variability (standard deviation) of this flux during 2000–2020 is 2.0 Tg a−1, producing a variability of ∼0.6 ppb a−1 at the surface, which is only ∼20% of observed surface CH4 growth rate variability. Compared to global averages, high latitudes experience a larger STE‐induced surface variability, which is 80% of the observed surface anomalies in the Antarctic and 44% in the Arctic. Our results suggest that although the STE process is a minor contributor globally, it has a considerable impact over polar regions on the inter‐annual variability of surface CH4 growth rates. Plain Language Summary: In addition to surface emissions and tropospheric removal, tropospheric CH4 concentrations are also affected by exchange with the stratosphere. However, the changes in the stratosphere‐troposphere flux are often unaccounted for in the analysis of surface CH4 observations. Here we evaluate this stratospheric influence by quantifying stratosphere‐troposphere CH4 fluxes. We find that the surface CH4 variability induced by the stratosphere‐troposphere exchange is relatively small globally but can be substantial in polar regions at anomalies of the stratospheric circulation. Key Points: The inter‐annual variability of the CH4 flux between the stratosphere and the troposphere is 2.0 Tg a−1Globally the stratosphere‐troposphere exchange is a minor contributor to observed inter‐annual variation of tropospheric CH4 growthThe surface impact can be substantial at high latitudes during the maximum or minimum of the Brewer‐Dobson Circulation [ABSTRACT FROM AUTHOR]

There are myriad ways atmospheric circulation responds to increased CO2. In the troposphere, the region of the tropical upwelling narrows, the Hadley cells expand, and the upper-level subtropical zonal winds that comprise the subtropical jet strengthen. In the stratosphere, the tropical upwelling narrows and strengthens, enhancing the Brewer–Dobson circulation. Despite the robustness of these projections, dynamical coupling between the features remains unclear. In this study, we analyze output from the NASA Goddard Institute for Space Studies (GISS) ModelE coupled climate model to examine any connection between the upper tropospheric and lower stratospheric circulation by considering the features' seasonality, hemispheric asymmetry, scaling, and transient response to a broad range of CO2 forcings. We find that a narrowing and strengthening of upper tropospheric upwelling occurs with a strengthening of the subtropical jet. There is also a narrowing and strengthening of lower stratospheric upwelling that is related to an equatorward shift in critical latitude for wave breaking and the associated strengthening of the subtropical lower stratosphere's zonal winds. However, the stratospheric responses display different seasonal, hemispheric, and transient patterns than those in the troposphere, indicating independent circulation changes between the two domains. [ABSTRACT FROM AUTHOR]

Our previous study revealed the link between Barents–Kara sea ice and rainfall in eastern China. This study continues evaluating the performance of multiple models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating this linkage. Most CMIP6 models can simulate Arctic sea ice coverage in the present climate system, although the sea ice extent in the edge areas show some biases. Only a few models can roughly reproduce the observed rainfall dipole pattern associated with Arctic sea ice variability. The linkage between Arctic sea ice variability in winter and eastern China rainfall in early summer is performed through a long memory of the sea ice, the stratospheric variability as the mediator, and downward propagation of stratospheric signals. Very few CMIP6 models can exhibit a realistic interannual relationship between the Arctic sea ice and China rainfall. The selected high-skill models with a more realistic linkage between sea ice and China rainfall present a clear downward impact of the stratospheric circulation anomalies associated with sea ice variability. The reversal of the Northern Hemisphere Annular Mode (NAM) from the negative phase in early winter to the positive phase in spring in the high-skill models and observations denotes the important role of the stratosphere as a mediator to bridge the Arctic sea ice and China rainfall. The long memory of the Arctic sea ice with the stratosphere as the mediator has a deep implication on the seasonal forecasts of East Asian countries. [ABSTRACT FROM AUTHOR]

This work assesses the contribution of assimilating AMSU-A satellite-based radiance measurements to a global data assimilation system based on an atmospheric general circulation model and the local ensemble transform Kalman filter (LETKF). The radiance measurements were from three channels that are sensitive to the upper troposphere and lower stratosphere. The contribution of these measurements, or AMSU-A observation impact, was estimated both through ensemble-based forecast sensitivity to observations (EFSO) and observing system experiments (OSEs). Two streams of data-denial experiments for the AMSU-A observations were performed for about one month during winter in each hemisphere. The OSEs quantified the accumulated observation impact by cycling (repeating) data denials: including AMSU-A observations reduced the total observation impact for all observations of each data assimilation cycle. In contrast, EFSO estimated AMSU-A to increase the total observation impact. The opposing effects were attributed to the accumulated observation impact in the OSEs; the accumulation could stabilize the data assimilation cycles. In both experiments, the accumulated observation impact of AMSU-A was strongest in the upper troposphere, particularly in the austral midlatitudes where westerly jets exist and observations of other types are sparse. EFSO also assessed AMSU-A to have the most beneficial observation impact in similar locations. The AMSU-A observation impact tended to accumulate just downstream of where EFSO estimated the beneficial observation impact signals. The accumulated AMSU-A observation impact was tied to dynamic processes in the upper-tropospheric and general stratospheric circulation. Therefore, EFSO helps estimate the beneficial distributions of AMSU-A accumulated observation impact by considering their dynamical propagation. [ABSTRACT FROM AUTHOR]

The radiative balance of the upper atmosphere is dependent on the magnitude and distribution of greenhouse gases and aerosols in that region. Climate models predict that with increasing surface temperature, the primary mechanism for transporting tropospheric air into the stratosphere (known as the Brewer–Dobson circulation) will strengthen, leading to changes in the distribution of atmospheric water vapor, other greenhouse gases, and aerosols. Stratospheric relationships between greenhouse gases and other long-lived trace gases with various photochemical properties (such as N 2 O, SF 6 , and chlorofluorocarbons) provide a strong constraint for tracking changes in the stratospheric circulation. Therefore, a cost-effective approach is needed to monitor these trace gases in the stratosphere. In the past decade, the balloon-borne AirCore sampler developed at NOAA's Global Monitoring Laboratory has been routinely used to monitor the mole fractions of CO 2 , CH 4 , and CO from the ground to approximately 25 km above mean sea level. Our recent development work adapted a gas chromatograph coupled with an electron capture detector (GC-ECD) to measure a suite of trace gases (N 2 O, SF 6 , CFC-11, CFC-12, H-1211, and CFC-113) in the stratospheric portion of AirCores. This instrument, called the StratoCore-GC-ECD, allows us to retrieve vertical profiles of these molecules at high resolution (5–7 hPa per measurement). We launched four AirCore flights and analyzed the stratospheric air samples for these trace gases. The results showed consistent and expected tracer–tracer relationships and good agreement with recent aircraft campaign measurements. Our work demonstrates that the StratoCore-GC-ECD system provides a low-cost and robust approach to measuring key stratospheric trace gases in AirCore samples and for evaluating changes in the stratospheric circulation. [ABSTRACT FROM AUTHOR]

The increase of atmospheric CO2 concentrations changes the atmospheric temperature distribution, which in turn affects the circulation. A robust circulation response to CO2 forcing is the strengthening of the stratospheric Brewer-Dobson circulation (BDC), with associated consequences for transport of trace gases such as ozone. Ozone is further affected by the CO2-induced stratospheric cooling via the temperature dependency of ozone chemistry. These ozone changes in turn influence stratospheric temperatures and thereby modify the CO2-induced circulation changes. In this study, we perform dedicated model simulations to quantify the modification of the circulation response to CO2 forcing by stratospheric ozone. Specifically, we compare simulations of the atmosphere with preindustrial and with quadrupled CO2 climate conditions, in which stratospheric ozone is held fixed or is adapted to the new climate state. The results of the residual circulation and mean age of air show that ozone changes damp the CO2-induced BDC increase by up to 20%. This damping of the BDC strengthening is linked to an ozone-induced relative enhancement of the meridional temperature gradient in the lower stratosphere in summer, thereby leading to stronger stratospheric easterlies that suppress wave propagation. Additionally, we find a systematic weakening of the polar vortices in winter and spring. In the Southern Hemisphere, ozone reduces the CO2-induced delay of the final warming date by 50%. [ABSTRACT FROM AUTHOR]

Climate models underestimate the magnitude of Arctic warming in past warm climates, like the early Cretaceous and Paleogene periods, implying that certain physical processes might be missing or poorly represented. Previous studies suggest that a large increase in wintertime Arctic polar stratospheric clouds (PSCs) might have promoted Arctic amplification through additional greenhouse warming. High methane concentrations in warm climates might have increased stratospheric water vapor providing favorable conditions for PSCs. However, methane concentrations in past warm climates are extremely uncertain. Here, we revisit the PSC hypothesis by exploring PSC changes under very high methane levels, 4× preindustrial carbon dioxide, and strong polar-amplified surface warming, using a whole-atmosphere model with fully interactive chemistry. We find that with polar-amplified warming there is a large increase in Arctic outgoing longwave radiation (OLR) that reduces as the methane concentration is increased. PSCs increase monotonically with methane concentration. A large radiative cooling and an increase in water vapor in the stratosphere increases Arctic PSCs, which follow a power law with respect to relative humidity. Using a two-way partial radiative perturbation technique, we show that the OLR reduction due to PSCs is similar to the direct radiative forcing of methane for high methane levels. Thus, we find that PSCs could play an important role in Arctic warming in a warmer-than-present-day climate, but only if methane levels were higher than suggested by previous modeling studies for past warm climates. [ABSTRACT FROM AUTHOR]

The Canadian Earth System Model version 5.0 (CanESM5.0), the most recent major version of the global climate model developed at the Canadian Centre for Climate Modelling and Analysis (CCCma) at Environment and Climate Change Canada (ECCC), has been used extensively in climate research and for providing future climate projections in the context of climate services. Previous studies have shown that CanESM5.0 performs well compared to other models and have revealed several model biases. To address these biases, CCCma has recently initiated the 'Analysis for Development' (A4D) activity, a coordinated analysis activity in support of CanESM development. Here we describe the goals and organization of this effort and introduce two variants ("p1" and "p2") of a new CanESM version, CanESM5.1, which features substantial improvements as a result of the A4D activity. These improvements include the elimination of spurious stratospheric temperature spikes and an improved simulation of tropospheric dust. Other climate aspects of the p1 variant of CanESM5.1 are similar to those of CanESM5.0, while the p2 variant of CanESM5.1 features reduced equilibrium climate sensitivity and improved ENSO variability as a result of intentional tuning of the atmospheric component. The A4D activity has also led to the improved understanding of other notable CanESM5.0/5.1 biases, including the overestimation of North Atlantic sea ice, a cold bias over sea ice, biases in the stratospheric circulation and a cold bias over the Himalayas. It provides a potential framework for the broader climate community to contribute to CanESM development, which will facilitate further model improvements and ultimately lead to improved climate change information. [ABSTRACT FROM AUTHOR]

It has been demonstrated that there is a globally unified linear relationship between the interannual variations of the fall-to-spring polar ozone accumulation and the winter-mean poleward eddy heat flux on the 100 hPa pressure surface. The foundation of this relationship is investigated using time-slice experiments on a chemistry–climate model with two levels of ozone-depleting substances (ODSs). The features of the transport field are interpreted by decomposing the horizontal ozone flux caused by the residual circulation into contributing processes including the eddy heat flux with the aid of the transformed Eulerian-mean momentum equation followed by rearrangement of terms. The linear relationship between the interannual variations of the fall-to-spring ozone buildup integrated poleward and above a reference point Pref on a meridional plane and the poleward eddy heat flux during the corresponding period at Pref is realized for each hemisphere implying that the interhemispheric unification should be treated with caution. This relationship is interpreted using the fact that the interannual variation of poleward ozone transport in the upper stratosphere is captured well by the vertical convergence of the constituent-based Eliassen–Palm (EP) flux ( F ˜ ), which is defined as the product of the constituent (ozone) mixing ratio and EP flux. The eddy momentum flux contributes to the meridional ozone transport in combination with the eddy heat flux in the form of the divergence of F ˜ , although it is not responsible for realizing the linear relationship. The dependence of the linearity on the location of Pref and ODS levels is discussed. [ABSTRACT FROM AUTHOR]

The anomalous stratospheric state favoring the occurrence of sudden stratospheric warmings (SSWs) is usually referred to as vortex preconditioning. This study investigates the role of vortex preconditioning in triggering strong and weak SSWs by using ERA5 reanalysis data. Strong and weak SSWs are distinguished by the amplitude of zonal wind deceleration of sudden stratospheric deceleration events. The robust stratospheric anomalies before strong SSWs last longer compared to weak SSWs, accompanied by stronger amplification of stratospheric wave activity near the warming. The stratospheric anomalies before weak SSWs have two significant enhancement stages, with two processes of stratospheric wave amplification. Robust stratospheric anomalies generally appear before tropospheric wave events (TWEs) followed by the strong and weak SSWs, which are absent before TWEs without SSWs. Stratospheric meridional potential vorticity gradient events (SPVEs) are defined to represent the anomalous stratospheric state during vortex preconditioning. The SPVEs can effectively modulate the stratospheric upward wave activity. No strong lower-tropospheric wave forcing is seen for the composites of both strong and weak SSWs preceded by SPVEs. These SSWs account for about 59% of the total SSWs. Furthermore, about 23% of strong SSWs and 36% of weak SSWs are only preceded by SPVEs without TWEs, indicating the major role of vortex preconditioning in triggering these SSWs. The SPVEs can be caused by wave breaking in the surfzone or the enhanced polar vortex, while the SPVEs preceded by clear wave breaking may be more favorable to the occurrence of SSWs. [ABSTRACT FROM AUTHOR]

The radiative balance of the upper atmosphere is dependent on the magnitude and distribution of greenhouse gases and aerosols in that region of the atmosphere. Climate models predict that with increasing surface temperature, the primary mechanism for transporting tropospheric air into the stratosphere (known as the Brewer-Dobson Circulation) will strengthen, leading to changes in the distribution of atmospheric water vapor, other greenhouse gases, and aerosols in this region. Stratospheric relationships between greenhouse gases and other long-lived trace gases with various photochemical properties (such as N2O, SF6, and chlorofluorocarbons) provide a strong constraint for tracking changes in the stratospheric circulation. Therefore, a cost-effective approach is needed to monitor these trace gases in the stratosphere. In the past decade, the balloon-borne AirCore sampler developed at NOAA/GML has been routinely used to monitor the mole fractions of CO2, CH4, and CO from ground to approximately 25 km above mean sea level. Our recent development work adapted a gas chromatograph coupled with an electron capture detector (GC-ECD) to measure a suite of trace gases (N2O, SF6, CFC-11, CFC-12, H-1211, and CFC-113) in the stratospheric portion of AirCores. This instrument, called the StratoCore-GC-ECD, allows us to retrieve vertical profiles of these molecules at high resolution (5-7 hPa per measurement). We then launched four AirCore flights and analyzed the stratospheric air samples for these trace gases. The results showed consistent and expected tracer-tracer relationships and good agreement with recent aircraft campaign measurements. Our work demonstrates that the StratoCore-GC-ECD system provides a low-cost and robust approach to measuring key stratospheric trace gases in AirCore samples and for evaluating changes in the stratospheric circulation. [ABSTRACT FROM AUTHOR]

Global ECMWF IFS simulations with horizontal grid spacings of 1, 4, and 9 km are used to assess gravity wave forcing (GWF) in the extratropical stratosphere. Results with important implications for GWF parameterizations at high and intermediate resolutions are presented. A doubling in the zonal-mean resolved GWF is observed when the horizontal resolution is increased from 9 to 1 km. Small-scale gravity waves with horizontal wavelengths < 100 km dominate this increase. Over most regions, excluding the polar night jet in the Antarctic spring, the total (resolved + parameterized) GWF at 9 km (4 km) is underestimated by up to 30% (15%). This implies that the parameterization of GWF is still required at 9 and 4 km horizontal resolutions. Despite the small land area in the Southern Hemisphere (SH), the resolved orographic and nonorographic GWF contribute equally to the total GWF in the SH at 1 km resolution. This is not reflected in the partitioning of the parameterized GWF, which has a significantly larger nonorographic contribution at 9 km. As a result, a zonal-mean momentum budget analysis reveals that the total GWF contributes one-third of SH springtime polar vortex deceleration at 1 km, whereas the contribution is as much as 50% at 9 km. This suggests that a rebalancing of the parameterized nonorographic and orographic GWF is required. [ABSTRACT FROM AUTHOR]

The performance of the European Consortium Earth-system model (EC-EARTH) in the boreal winter stratosphere is comprehensively assessed for the first time, in particular its version 3.3 that contributes to CMIP6. A 100-year long simulation with prescribed climatological boundary conditions and fixed radiative forcing, representative of present-day climate, is used to evaluate the simulation of the climatological stratospheric circulation and to identify model biases. Results show that EC-EARTH has a large issue with the vertical distribution of stratospheric temperature from the tropics to mid-latitudes, seemingly linked to radiative processes of ozone, leading to a biased warm middle-upper stratosphere. Associated with this model bias, EC-EARTH simulates a stronger polar vortex at upper-stratospheric levels while the Brewer-Dobson circulation at middle/lower levels is weaker than reanalysis. The amplitude of the climatological planetary waves is overall underestimated, but the magnitude of the background wave injection from the troposphere into the stratosphere is overestimated, related to a weaker polar vortex at lower-stratospheric levels and, thus, a less effective wave filtering. This bias in the westerly flow could have a contribution from parameterized waves. The overestimation of background wave driving is maximum in early-winter, and may explain the overestimated frequency of sudden stratospheric warmings at this time, as compared to reanalysis. The spatial distribution of wave injection climatology has revealed a distinctive role of the climatological planetary waves: while large-scale waves (wavenumbers 1–2) dominate the eddy heat flux over the North Pacific, small-scale waves (wavenumbers 3–4) are responsible for the doubled-lobe structure of the eddy heat flux over Eurasia. EC-EARTH properly simulates this climatological feature, although overestimates its amplitude over central Eurasia. [ABSTRACT FROM AUTHOR]

Historically, observational information about atmospheric temperature has been limited due to a lack of suitable measurements. Recent advances in satellite observations provide new insight into the fine structure of the free atmosphere, with the upper troposphere and lower stratosphere comprising essential components of the climate system. This is a prerequisite for understanding the complex processes of this part of the atmosphere, which is also known to have a large impact on surface climate. With unprecedented resolution, latest climate observations reveal a dramatic warming of the atmosphere. The tropical upper troposphere has already warmed about 1 K during the first two decades of the 21st century. The tropospheric warming extends into the lower stratosphere in the tropics and southern hemisphere mid-latitudes, forming a prominent hemispheric asymmetry in the temperature trend structure. Together with seasonal trend patterns in the stratosphere, this indicates a possible change in stratospheric circulation. [ABSTRACT FROM AUTHOR]

Extreme stratospheric vortex states are often associated with extreme heat flux and upward wave propagation in the troposphere and lower stratosphere; however, the factors that dictate whether an upward-directed wave in the troposphere will reach the bottom of the vortex versus being reflected back to the troposphere are not fully understood. Following Charney and Drazin, an analytical quasigeostrophic planetary-scale model is used to examine the role of the tropopause inversion layer (TIL) in wave propagation and reflection. The model consists of three different layers: troposphere, TIL, and stratosphere. It is shown that a larger buoyancy frequency in the TIL leads to weaker upward transmission to the stratosphere and enhanced reflection back to the troposphere, and thus reflection of wave packets is sensitive not just to the zonal wind but also to the TIL's buoyancy frequency. The vertical–zonal cross section of a wave packet for a more prominent TIL in the analytical model is similar to the corresponding wave packet for observational events in which the wave amplitude decays rapidly just above the tropopause. Similarly, a less prominent TIL both in the model and in reanalysis data is associated with enhanced wave transmission and a weak change in wave phase above the tropopause. These results imply that models with a poor representation of the TIL will suffer from a bias in both the strength and phase of waves that transit the tropopause region. [ABSTRACT FROM AUTHOR]

The 15 January 2022 eruption of the Hunga Tonga‐Hunga Ha'apai underwater volcano (20.5°S, 175.4°W) injected a record amount of water directly into the stratosphere. This study attempts to quantify this impact on the temperature, as well as the subsequent changes to the stratospheric circulation, during the months following the eruption based on reanalysis fields. The extreme nature of the temperature, wind, and circulation changes are tracked through comparisons of the first six months of 2022 with the previous 42 years. Examination of the data assimilation process shows that at 20 hPa the thermal observations are forcing significant cooling, compensating for the absence of the excess stratospheric moisture in the model used for the reanalysis. In response to this cooling the atmosphere adjusts by creating strong westerly winds above the temperature anomaly and large changes to the downward and poleward mean meridional circulation. Plain Language Summary: While the stratosphere (15–55 km) region of the atmosphere contains the ozone layer, it is typically very dry, especially when compared to the troposphere. However, the remarkable eruption of the Hunga Tonga‐Hunga Ha'apai underwater volcano on 15 January 2022 injected a record amount of water directly into the stratosphere. Winds in the stratospheric soon carried this excess water vapor around the globe to all longitudes and spread the water vapor in latitude as well. Since water vapor can cool to space more rapidly than the stratospheric air, enhanced cooling in the stratosphere is expected. This study quantifies this cooling, as well as the subsequent changes to the stratospheric circulation, during the months following the eruption. The extreme nature of the stratospheric temperature, wind, and circulation changes are tracked through comparisons of the first six months of 2022 with the previous 42 years. Details of the stratospheric perturbations in latitude and pressure are presented for June 2022, where anomalously low temperatures are found at near 20 km altitude from 60 to 30°S. In response to this cooling the atmosphere adjusts by creating strong westerly winds above the temperature anomaly and large changes to the downward and poleward mean meridional circulation. Key Points: Extreme perturbations in the stratospheric winds and temperatures are linked to a volcanic eruptionHunga Tonga‐Hunga Ha'apai eruption effects are unique in the stratospheric recordData assimilation can track temperature and wind perturbations, even when some physics is missing [ABSTRACT FROM AUTHOR]

Using reanalysis datasets and a coupled general circulation model, the relationship between springtime Arctic total column ozone (TCO) and surface (5 m) ocean currents in the North Pacific is investigated. We found that as March Arctic TCO decreases, a statistically significant northwestward ocean current anomaly occurs in the northern North Pacific in surface layer, but an anomalous southward ocean current appears in the central North Pacific in April, and vice versa. The decreased Arctic TCO favors an enhanced Arctic stratospheric circulation, which tends to induce the tropospheric positive Arctic Oscillation anomaly with easterly anomalies over the midlatitude eastern Asia in late March through stratosphere-troposphere dynamical coupling. The easterly anomaly over eastern Asia in late March further extends eastward and induces an easterly anomaly over the midlatitude North Pacific, which favors negative North Pacific Oscillation (–NPO)-like circulation anomaly via anomalous zonal wind shear and the interactions between synoptic scale eddies and the mean flow in early-middle April. The –NPO anomaly forces anomalous northwestward/southward surface ocean currents in the northern/central North Pacific through direct friction of wind and the Coriolis force. Our coupled numerical simulations with high- and low-ozone scenarios also support that the Arctic stratospheric ozone affects the North Pacific surface ocean currents by NPO anomalies. Moreover, the ozone-related ocean current anomalies contribute to Victoria mode-like sea surface temperature anomalies in the North Pacific by horizontal heat advection. These results imply that Arctic ozone signal could be a predictor for variations of the North Pacific surface ocean currents. [ABSTRACT FROM AUTHOR]

Tropospheric trends in long‐lived source gases N2O and the chlorofluorocarbons cause trends in O3 through changes in their reactive product gases. Transport affects the product gases because it controls the distribution of the long‐lived source gases. We find that large changes in tropical upwelling 10–5 hPa since 2012 have strengthened the northern branch of the upper stratospheric (UpS) transport circulation, dramatically altering the abundances of N2O and its odd nitrogen product gases, NOx and HNO3. Increased upwelling is connected to stronger and more frequent Quasi‐Biennial Oscillation easterly winds at 10 hPa and above. We use simulations with and without time varying MERRA2 meteorology to quantify the impact of dynamical changes on O3 loss via the NOx and ClOx cycles. We find that dynamical impacts on these cycles explain the mid‐stratospheric tropical O3 increase and Arctic UpS O3 decrease since 2005. Plain Language Summary: Changes in the upper stratospheric circulation over the past decade have altered composition between 30 and 40 km altitude. In the tropics, faster ascent carried greater amounts of nitrous oxide (N2O), a gas produced at the surface, to the upper stratosphere. This increased the amount of the ozone‐destroying nitrogen radicals produced from N2O. Most of the extra nitrogen radicals traveled to the Arctic above ∼32 km, where they increased ozone destruction. Below the altitude where the radicals are produced (about 35 km), the faster tropical ascent carries reduced amounts of nitrogen radicals to the middle stratosphere (∼26 to 35 km), leading to an ozone increase. Observations by satellite instruments revealed changes in the composition. Model simulations showed that circulation change caused the composition changes that affected ozone. The circulation change was connected to changes in the tropical winds that circulate around the globe and change from easterly to westerly on a fairly regular basis. During the last 10 years, this long‐standing pattern changed. This could be variability or the start of a trend; it's too soon to tell. Either way, it complicates the identification and attribution of the ozone increases we expect due to international treaties that banned the use of manmade chlorocarbons. Key Points: Tropical upwelling 10–5 hPa increased up to 30% from 2005 to 2021, increasing upper stratospheric N2O and reactive nitrogen productionIncreased upwelling is connected to stronger and more frequent Quasi‐Biennial Oscillation easterlies at 10 hPa and aboveIncreased upwelling changed NOx‐driven O3 losses, increasing tropical O3 from 15 to 7 hPa while reducing Arctic O3 from 15 to 2 hPa [ABSTRACT FROM AUTHOR]

Even though winter land–sea thermal contrast (LSC) is expected to undergo a strong weakening in the future warmer climate, its effects have been poorly investigated. Here we run a set of idealized winter simulations featuring reduced LSC in the Northern Hemisphere (NH) extratropics, or in individual extratropical sectors of the NH (Atlantic and Pacific), using an intermediate-complexity atmospheric general circulation model. Reduced LSC is obtained by imposing a warming of surface land temperatures in East Asia and North America. For similar warming intensities over the two regions, the response of the model to East Asia forcing is significantly stronger and dominates the response to the sum of the two forcing patterns. We find that the LSC reduction causes a weakening and poleward shift of the midlatitude jet streams, and a strong interference with zonal wavenumbers 1 and 2. In particular, East Asian warming reduces the amplitude of waves 1 and 2, producing a strengthening of the stratospheric vortex, while a weaker vortex due to a moderate amplification of wave 1 is detected when warming North America. Eventually, stratospheric signals propagate downward in the troposphere affecting the midlatitude winter NH even remotely from the forcing. In this work we pinpoint some mechanisms by which weakened winter LSC influences the NH extratropical circulation: the results may become useful to interpret the response to long-term projections displaying reduced LSC along with other climate change forcing patterns. [ABSTRACT FROM AUTHOR]

Impacts of stratospheric polar vortex shift on the wintertime East Asian trough (EAT) on intraseasonal time scales are investigated using a reanalysis dataset and a climate model. The result based on composite analysis shows that the shift of the stratospheric polar vortex toward eastern Siberia (ES-shift event) is associated with higher geopotential height at 500 hPa than normal over East Asia, corresponding to the weakened EAT. Furthermore, the simulated EAT is also weakened when nudging the stratospheric state toward that during the ES-shift events. This study further found that there is no significant difference in the stratospheric polar vortex intensity between the ES-shift events and nonshift events, implying that the polar vortex strength change may have little influence on the possible connection between the polar vortex shift and the EAT change. The underlying mechanisms are listed as follows: First, the positive potential vorticity (PV) anomalies in the lower stratosphere associated with ES-shift events could explain approximately 40% of the local westerly anomalies in the upper troposphere to the north of East Asia via PV conservation, leading to the rise of the geopotential height over East Asia and the weakening of the EAT. Second, the shift of stratospheric polar vortex could modulate the synoptic-scale Rossby wave activity in the upper troposphere, favorable for the southward propagation of synoptic-scale waves and divergence of extended Eliassen-Palm flux in the upper troposphere. Finally, the transient wave feedback could enhance the tropospheric westerly anomalies in the north of East Asia and induce positive height anomalies to its south, further weakening the EAT. Our results revealed that the stratospheric polar vortex shift leads the EAT intensity variation by around 2-5 days, implying that the stratospheric polar vortex shift could be applicable to the prediction of the EAT intensity. [ABSTRACT FROM AUTHOR]

An idealized model is used to examine the tropospheric response to sudden stratospheric warmings (SSWs), by imposing transient stratospheric momentum torques tailored to mimic the wave-forcing impulse associated with spontaneously occurring SSWs. Such an approach enables us to examine both the ∼2–3-week forcing stage of an SSW during which there is anomalous stratospheric wave-activity convergence, as well as the recovery stage during which the wave forcing abates and the stratosphere radiatively recovers over 2–3 months. It is argued that applying a torque is better suited than a heating perturbation for examining the response to SSWs, due to the meridional circulation that is induced to maintain thermal-wind balance (i.e., the "Eliassen adjustment"); an easterly torque yields downwelling at high latitudes and equatorward flow below, similar to the wave-induced circulation that occurs during spontaneously occurring SSWs, whereas a heating perturbation yields qualitatively opposite behavior and thus cannot capture the initial SSW evolution. During the forcing stage, the meridional circulation in response to an impulse comparable to the model's internal variability is able to penetrate down to the surface and drive easterly-wind anomalies via Coriolis torques acting on the anomalous equatorward flow. During the recovery stage, after which the tropospheric flow has already responded, the meridional circulation associated with the stratosphere's radiative recovery appears to provide the persistent stratospheric forcing that drives the high-latitude easterly anomalies, whereas planetary waves are found to play a smaller role. This is then augmented by synoptic-wave feedbacks that drive and amplify the annular-mode response. [ABSTRACT FROM AUTHOR]

Progress and persistent biases in the simulation of the stratospheric polar vortex from three generations of Coupled Model Intercomparison Projects (CMIPs) are assessed. On average, the stratospheric cold bias is largest in CMIP3, but is improved in CMIP5 and CMIP6. The climatological ridge in the stratosphere over the North Pacific is underestimated persistently across CMIPs. Four parameters of the stratospheric polar vortex are evaluated among models and among CMIPs, and some common biases are identified for most models, including the too-large size of the vortex periphery, overstrong strength, too-small aspect ratio of the vortex shape, and too-westward displaced a vortex centroid. Intermodel spread in some parameters is highly correlated with the sea surface temperature (SST) bias in northern tropical oceans. Namely, a cold SST bias in the northern tropical Pacific is associated with a too-strong and too-large polar vortex, and a cold SST bias in the northern tropical Atlantic seems to be related to the westward bias of the vortex centroid. The implications of biases in the mean state of the vortex for stratospheric events such as sudden stratospheric warmings (SSWs) are also identified in this study. Models with a climatological vortex that is too strong, has too low an aspect ratio, or has a westward-biased centroid usually produce fewer SSWs, whereas ones with weaker strength, high aspect ratio, and eastward-biased centroid produce more. Based on the multimodel mean, the biases of all parameters have improved across three generations of CMIPs, although persistent biases in some individual models exist across CMIPs. Intermodel relationships between the polar vortex and SST highlights the important role of the SST simulation for the stratosphere. [ABSTRACT FROM AUTHOR]

Polar lows (PLs) are intense maritime mesoscale cyclones that can pose hazards to coastal communities and marine operation in the Arctic. This study examines the impacts of sudden stratospheric warmings (SSWs) on PL activity in the subarctic North Atlantic. The 20 days following SSWs are characterized by tropospheric circulation anomalies resembling the negative phase of the North Atlantic Oscillation. PL activity decreases significantly over the Labrador Sea, which can be attributed to the infrequent occurrence of low static stability and strong environmental baroclinicity, as well as reduced surface turbulent heat fluxes. These results suggest that a skillful prediction of SSWs can improve the extended-range forecast of PL activity over the Labrador Sea. For the Nordic seas, the results imply that the spatial structure of an SSW event is important for the PL modulation through different tropospheric circulation patterns. Situations with increased PL frequency in the Nordic seas are characterized by SSWs centered close to northern Greenland occurring over a smaller area, and a tropospheric response featuring enhanced cold-air outbreaks over the Norwegian Sea. Conversely, PL activity is suppressed over the Nordic seas when the SSW favors the formation of a tropospheric anticyclone above Greenland and Scandinavia. Significance Statement: This study investigates the relationships between polar lows (PLs) and sudden stratospheric warmings (SSWs) over the subarctic North Atlantic. A better understanding of the effect of SSWs on PL development has the potential to improve extended-range forecasts of PLs. It is shown that SSWs are responsible for the significantly suppressed regional PL activity over the Labrador Sea, suggesting that SSWs can serve as a predictor for the extended-range forecast of PLs over this region. Following SSW events, the thermodynamic state of atmosphere becomes more stable over the Labrador Sea and hinders the convective development of PLs. For the northern Nordic seas, the impacts of SSWs on PL activity are sensitive to the spatial structure of stratospheric warming. [ABSTRACT FROM AUTHOR]

The stratospheric polar vortices play a key role in springtime polar ozone depletion and can influence the stratospheric circulation. In this work, we use the method of vortex delineation based on geopotential values determined from the maximum temperature gradient and maximum wind speed, thus characterizing the edge of the Antarctic polar vortex. Using the vortex delineation method based on the ERA5 reanalysis data for 1979–2019, we characterized the dynamics of the Antarctic polar vortex in the lower stratosphere depending on the prevailing wind speed along the vortex edge. We filtered out cases of weakening of the dynamic barrier, which prevents the penetration of air masses into the polar vortex. We show that in the lower stratosphere the area of the Antarctic polar vortex usually exceeds 10 million km2, the mean wind speed along the vortex edge typically exceeds 30 m/s, and the mean temperature inside the vortex is in most cases less than −50 °C. [ABSTRACT FROM AUTHOR]

Modeling of the Antarctic stratospheric polar vortex from the Coupled Model Intercomparison Project phase 3 (CMIP3) to phase 6 (CMIP6) is evaluated in this study. On average, a wide coverage of warm biases appears in the Antarctic stratosphere, which is greatest in the early CMIP and is gradually diminished in the two later CMIPs with the number of models reproducing QBO increasing (0, 5, and 19). Four metrics of the Antarctic stratospheric polar vortex are assessed for three generations of CMIPs. Biases such as the overly weak strength, the overly large aspect ratio and the westward drifted vortex centroid are commonly shared across the CMIPs. While with improvements of the model resolution, model top, interactive chemistry and physical process, the intermodel spread narrows generation by generation, especially for high-top models than low-top models in the simulation of vortex area. Further, intermodel spread of Antarctic stratospheric vortex is obviously associated with the bias of austral winter sea surface temperature (SST). Specifically, as cold SST bias in the southern tropical Pacific Ocean and warm SST bias in the Southern Hemisphere Niño 1 + 2 region (Nino) improve, biases in the modelled vortex metrics also decrease, which can be partly attributed to the modifying of the upward propagations of planetary waves over tropical and extratropical oceans. The strengthened relationships in the focused regions further confirms the importance of the SST simulation for the stratosphere vortex simulation. In general, despite biases of the polar vortex existing across CMIPs, marked progresses have been achieved for most models. [ABSTRACT FROM AUTHOR]

Satellite observations of middle-atmosphere temperature are used to investigate the short-term global response to planetary wave activity in the winter stratosphere. The focus is on the relation between variations in the winter and summer hemispheres. The analysis uses observations from Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) for 2002–21 and Aura Microwave Limb Sounder (MLS) for 2004–21, and reanalysis temperatures and winds from MERRA-2 for 2002–21. We calculate temporal correlations of the Eliassen–Palm flux divergence in the winter stratosphere with global temperature. Results show a robust perturbation extending to midlatitudes of the Southern Hemisphere (SH) stratosphere during Northern Hemisphere (NH) winter. An increase in wave forcing is followed by a decrease in temperatures over the depth of the stratosphere in the SH, peaking at a lag of 3 days. Summer mesospheric temperature perturbations of the opposite sign are seen in many winters. Comparable signals in the NH summer middle-atmosphere are present during some SH winters but are weaker and less consistent than those in the SH during NH winter. A diagnostic evaluation of the patterns of correlation, the mesospheric zonal winds, and the stability criteria suggests that the temperature perturbations in the midlatitude summer mesosphere are more closely associated with the summer stratosphere directly below than with the wave activity in the winter stratosphere. This suggests that the interhemispheric coupling in the stratosphere is driving or contributing to the coupling between the winter stratosphere and the summer mesosphere that has been reported in several investigations. Significance Statement: There are many instances in which one part of the atmosphere is found to regularly respond to perturbations occurring in a distant region. In this study, we use observations to investigate one such pattern: temperature changes at high altitude (60–100 km) in the summer that follow dynamical changes near the winter pole at 40–60 km. Such analysis is useful to understand which physical processes contribute to the global connectivity and variability of the atmosphere. [ABSTRACT FROM AUTHOR]

A dry-core idealized general circulation model with a stratospheric polar vortex in the Northern Hemisphere is run with a combination of simplified topography and imposed tropospheric temperature perturbations, each located in the Northern Hemisphere with a zonal wavenumber of 1. The phase difference between the imposed temperature wave and the topography is varied to understand what effect this has on the occurrence of polar vortex displacements. Geometric moments are used to identify the centroid of the polar vortex for the purposes of classifying whether or not the polar vortex is displaced. Displacements of the polar vortex are a response to increased tropospheric wave activity. Compared to a model run with only topography, the likelihood of the polar vortex being displaced increases when the warm region is located west of the topography peak, and decreases when the cold region is west of the topography peak. This response from the polar vortex is due to the modulation of vertically propagating wave activity by the temperature forcing. When the southerly winds on the western side of the topographically forced anticyclone are collocated with warm- or cold-temperature forcing, the vertical wave activity flux in the troposphere becomes more positive or negative, respectively. This is in line with recent reanalysis studies that showed that anomalous warming west of the surface pressure high, in the climatological standing wave, precedes polar vortex disturbances. [ABSTRACT FROM AUTHOR]