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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]

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]