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During February 2023, the quasi‐4‐day wave (Q4DW) with westward zonal wavenumber 2 (W2) reached its largest amplitude of ∼400 m in the Southern Hemisphere (SH) geopotential height observations since 2004, which occurred simultaneously with an Arctic major sudden stratospheric warming (SSW) with an elevated stratopause (ES). However, the Q4DW‐W2 perturbations in the Northern Hemisphere (NH) were unexpectedly suppressed despite the unstable Arctic stratosphere and mesosphere during the 2023 ES‐SSW. Diagnostic analysis shows that the westward winds at ∼54°N–70°N in the upper stratosphere of ∼‐79 m/s during the 2023 ES‐SSW were the strongest during boreal winters over the past two decades, which benefited from the onset of a preceding minor SSW at the end of January. The strongest westward wind generated a wave geometry configuration of full reflection for Q4DW‐W2 in the NH, while the Q4DW‐W2 enhancement in the SH was induced by the in‐situ amplification of the surviving seeding perturbations. Plain Language Summary Among the planetary waves with a period of about 4 days, the eastward quasi‐4‐day wave (Q4DW) excited by the double‐jet structure in the Southern Hemisphere is the most famous one, while another Q4DW with westward zonal wavenumber 2 (W2) belonging to the normal modes of the Earth's atmosphere has attracted little attention. During the February 2023 sudden stratospheric warming (SSW) with an elevated stratopause (ES), the exceptional enhancement and suppression of Q4DW‐W2 were captured in the Southern Hemisphere and Northern Hemisphere, respectively. Such global structure is first observed and abnormal among traveling planetary waves during SSW since they usually have a peak region in the unstable winter hemisphere. We found that the rare sequence of stratospheric disturbances in the 44‐year historical record that a minor SSW followed by an ES‐SSW led to the excessively strong westward wind during the 2023 ES‐SSW, which resulted in a wave geometry configuration of full reflection for Q4DW‐W2 in the Northern Hemisphere. Our current results well establish the relationship between a new global structure of traveling planetary waves during SSW and the rare sequence of stratospheric disturbances. This will deepen the understanding of the less studied Q4DW‐W2 and the Earth's whole atmospheric coupling during SSW. Key Points The anomalous burst and suppression of quasi‐4‐day wave (Q4DW) with westward zonal wavenumber 2 (W2) were observed in the Southern Hemisphere and Northern Hemisphere (NH) respectively during the February 2023 major sudden stratospheric warming (SSW) with elevated stratopause (ES) The rare sequence of SSWs in January and February 2023 led to the strongest westward wind during boreal winters over the past two decades The strongest westward wind during the 2023 ES‐SSW created a wave geometry configuration of full reflection weakening the Q4DW‐W2 in the NH

During the 2023/2024 austral summer, the quasi‐two‐day (QTDW) with westward zonal wavenumber 3 (W3) abnormally reached its maximum amplitude at the December solstice (22 December 2023) for the first time in 20 years of Aura Microwave Limb Sounder observations, while the strongest event during austral summer usually occurs ∼2–6 weeks after the December solstice (on average January 21). Diagnostic analysis reveals that the westward winds in the Southern (summer) Hemisphere were anomalously strong (maximum of ∼90 m/s) during December 2023, which significantly shortened the e‐folding time of QTDW‐W3, and additionally generated the QTDW‐W3 critical layers at the tropical summer stratopause from December 7. These two factors contributed to the earliest amplification of QTDW‐W3. In essence, the cold equatorial stratosphere triggered the exceptionally strong westward winds in the Southern Hemisphere via thermal wind balance, which was related to the enhanced upward middle‐atmosphere Hadley circulation during a prolonged Arctic stratopause warming event. Plain Language Summary As one of the prominent dynamic features in the Earth's summer middle and upper atmosphere, the quasi‐two‐day wave (QTDW) with westward zonal wavenumber 3 (W3) has significant influences on the global zonal‐mean circulation and temperature. Long‐term satellite and ground‐based observations have shown that the QTDW‐W3 is stably amplified twice yearly after the solstice, namely January/February in the Southern Hemisphere and July/August in the Northern Hemisphere. Such climatological features are very sensitive to the spatial distribution of the zonal‐mean zonal wind in the summer stratosphere and mesosphere. This study focuses on the unusual amplification of QTDW‐W3 just at the December solstice during the recent austral summer of 2023/2024, which has been never observed in the past 20 years of Aura Microwave Limb Sounder observations. Further analysis indicates that this earliest amplification of QTDW‐W3 was induced by a persistent and dramatic stratopause warming event in the Arctic (winter) stratosphere during December 2023, which caused the westward winds in the Southern (summer) Hemisphere to be anomalously strong via secondary Rossby wave‐induced interhemispheric coupling process. The current results first reveal that stratopause warming also has great potential to alter the global middle and upper atmospheric dynamics like the famous sudden stratospheric warming. Key Points The quasi‐two‐day wave (QTDW) with s=3$s=3$(W3) was maximized at the December solstice for the first time during the 2023/2024 austral summer The burst window for QTDW‐W3 appeared 2 weeks earlier than usual due to the anomalously strong summer westward winds in December 2023 The prolonged winter stratopause warming in December 2023 greatly enhanced the summer westward winds by cooling the equatorial stratosphere

This paper describes the recommended solar forcing dataset for CMIP6 and highlights changes with respect to CMIP5. The solar forcing is provided for radiative properties, namely total solar irradiance (TSI), solar spectral irradiance (SSI), and the F10.7 index as well as particle forcing, including geomagnetic indices Ap and Kp, and ionization rates to account for effects of solar protons, electrons, and galactic cosmic rays. This is the first time that a recommendation for solar-driven particle forcing has been provided for a CMIP exercise. The solar forcing datasets are provided at daily and monthly resolution separately for the CMIP6 preindustrial control, historical (1850–2014), and future (2015–2300) simulations. For the preindustrial control simulation, both constant and time-varying solar forcing components are provided, with the latter including variability on 11-year and shorter timescales but no long-term changes. For the future, we provide a realistic scenario of what solar behavior could be, as well as an additional extreme Maunder-minimum-like sensitivity scenario. This paper describes the forcing datasets and also provides detailed recommendations as to their implementation in current climate models.For the historical simulations, the TSI and SSI time series are defined as the average of two solar irradiance models that are adapted to CMIP6 needs: an empirical one (NRLTSI2–NRLSSI2) and a semi-empirical one (SATIRE). A new and lower TSI value is recommended: the contemporary solar-cycle average is now 1361.0 W m−2. The slight negative trend in TSI over the three most recent solar cycles in the CMIP6 dataset leads to only a small global radiative forcing of −0.04 W m−2. In the 200–400 nm wavelength range, which is important for ozone photochemistry, the CMIP6 solar forcing dataset shows a larger solar-cycle variability contribution to TSI than in CMIP5 (50 % compared to 35 %).We compare the climatic effects of the CMIP6 solar forcing dataset to its CMIP5 predecessor by using time-slice experiments of two chemistry–climate models and a reference radiative transfer model. The differences in the long-term mean SSI in the CMIP6 dataset, compared to CMIP5, impact on climatological stratospheric conditions (lower shortwave heating rates of −0.35 K day−1 at the stratopause), cooler stratospheric temperatures (−1.5 K in the upper stratosphere), lower ozone abundances in the lower stratosphere (−3 %), and higher ozone abundances (+1.5 % in the upper stratosphere and lower mesosphere). Between the maximum and minimum phases of the 11-year solar cycle, there is an increase in shortwave heating rates (+0.2 K day−1 at the stratopause), temperatures ( ∼  1 K at the stratopause), and ozone (+2.5 % in the upper stratosphere) in the tropical upper stratosphere using the CMIP6 forcing dataset. This solar-cycle response is slightly larger, but not statistically significantly different from that for the CMIP5 forcing dataset.CMIP6 models with a well-resolved shortwave radiation scheme are encouraged to prescribe SSI changes and include solar-induced stratospheric ozone variations, in order to better represent solar climate variability compared to models that only prescribe TSI and/or exclude the solar-ozone response. We show that monthly-mean solar-induced ozone variations are implicitly included in the SPARC/CCMI CMIP6 Ozone Database for historical simulations, which is derived from transient chemistry–climate model simulations and has been developed for climate models that do not calculate ozone interactively. CMIP6 models without chemistry that perform a preindustrial control simulation with time-varying solar forcing will need to use a modified version of the SPARC/CCMI Ozone Database that includes solar variability. CMIP6 models with interactive chemistry are also encouraged to use the particle forcing datasets, which will allow the potential long-term effects of particles to be addressed for the first time. The consideration of particle forcing has been shown to significantly improve the representation of reactive nitrogen and ozone variability in the polar middle atmosphere, eventually resulting in further improvements in the representation of solar climate variability in global models.

The precursory atmospheric gravity wave (AGW) activity in the stratosphere has been investigated in our previous paper by studying an inland Kumamoto earthquake (EQ). We are interested in whether the same phenomenon occurs or not before another major EQ, especially an oceanic EQ. In this study, we have examined the stratospheric AGW activity before the oceanic 2011 Tohoku EQ (Mw 9.0), while using the temperature profiles that were retrieved from ERA5. The potential energy (EP) of AGW has enhanced from 3 to 7 March, 4–8 days before the EQ. The active region of the precursory AGW first appeared around the EQ epicenter, and then expanded omnidirectionally, but mainly toward the east, covering a wide area of 2500 km (in longitude) by 1500 km (in latitude). We also found the influence of the present AGW activity on some stratospheric parameters. The stratopause was heated and descended; the ozone concentration was also reduced and the zonal wind was reversed at the stratopause altitude before the EQ. These abnormalities of the stratospheric AGW and physical/chemical parameters are most significant on 5–6 March, which are found to be consistent in time and spatial distribution with the lower ionospheric perturbation, as detected by our VLF network observations. We have excluded the other probabilities by the processes of elimination and finally concluded that the abnormal phenomena observed in the present study are EQ precursors, although several potential sources can generate AGW activities and chemical variations in the stratosphere. The present paper shows that the abnormal stratospheric AGW activity has also been detected even before an oceanic EQ, and the AGW activity has obliquely propagated upward and further disturbed the lower ionosphere. This case study has provided further support to the AGW hypothesis of the lithosphere-atmosphere-ionosphere coupling process.

Based on the zonal winds from the Modern‐Era Retrospective Analysis for Research and Applications, version 2 reanalysis data, we investigate the enhancement of an eastward propagating 4‐day wave (E4DW) during the 2019 Antarctic sudden stratospheric warming (SSW). The amplitudes of E4DW are centered at 50°S and 50 km with a maximum amplitude of ∼20 m/s. The enhanced E4DW significantly exceeds the climatological level and has the strongest amplitude in the Southern Hemisphere since 1980. Our results show that the E4DW is generated by the atmospheric instabilities via a double‐jet configuration of the zonal mean zonal winds. Our analysis further indicates that the special structure of the zonal mean zonal winds caused by the 2019 Antarctic SSW at the stratopause is responsible for the excitation of the record‐strong E4DW in 2019 in the Southern Hemisphere. Plain Language Summary An extraordinary Antarctic sudden stratospheric warming (SSW) occurred in September 2019. Responses of westward propagating quasi‐6‐day and quasi‐10‐day planetary waves to the 2019 Antarctic SSW have been widely reported in recent studies. However, the enhancement of an eastward propagating 4‐day wave (E4DW) is also observed during this SSW. The amplitude of the E4DW is significantly larger than the climatological level and is the strongest in the Southern Hemisphere since 1980. We further explore the trigger mechanism of the record‐strong E4DW that occurred during the 2019 Antarctic SSW for the first time. The special configuration of zonal mean zonal winds with the abnormal instabilities at the stratopause during the 2019 Antarctic SSW is the main reason for the enhancement of E4DW. Our results could help in understanding the activity of eastward propagating planetary waves in the middle atmosphere during SSWs. Key Points The eastward propagating 4‐day wave (E4DW) in September 2019 has the strongest amplitude in the Southern Hemisphere since 1980 The exceptional E4DW is triggered by atmospheric instabilities under a double‐jet configuration of the zonal mean zonal winds The 2019 Antarctic sudden stratospheric warming results in a special vertical wind shear at the stratopause for the generation of E4DW

This study documents the contribution of equatorial waves and mesoscale gravity waves to the momentum budget of the quasi-biennial oscillation (QBO) in a 110-level version of the Whole Atmosphere Community Climate Model. The model has high vertical resolution, 500 m, above the boundary layer and through the lower and middle stratosphere, decreasing gradually to about 1.5 km near the stratopause. Parameterized mesoscale gravity waves and resolved equatorial waves contribute comparable easterly and westerly accelerations near the equator. Westerly acceleration by resolved waves is due mainly to Kelvin waves of zonal wavenumber in the range k = 1–15 and is broadly distributed about the equator. Easterly acceleration near the equator is due mainly to Rossby–gravity (RG) waves with zonal wavenumbers in the range k = 4–12. These RG waves appear to be generated in situ during both the easterly and westerly phases of the QBO, wherever the meridional curvature of the equatorial westerly jet is large enough to produce reversals of the zonal-mean barotropic vorticity gradient, suggesting that they are excited by the instability of the jet. The RG waves produce a characteristic pattern of Eliassen–Palm flux divergence that includes strong easterly acceleration close to the equator and westerly acceleration farther from the equator, suggesting that the role of the RG waves is to redistribute zonal-mean vorticity such as to neutralize the instability of the westerly jet. Insofar as unstable RG waves might be present in the real atmosphere, mixing due to these waves could have important implications for transport in the tropical stratosphere.

A data assimilation system (DAS) is described for global atmospheric reanalysis from 0- to 100-km altitude. We apply it to the 2014 austral winter of the Deep Propagating Gravity Wave Experiment (DEEPWAVE), an international field campaign focused on gravity wave dynamics from 0 to 100 km, where an absence of reanalysis above 60 km inhibits research. Four experiments were performed from April to September 2014 and assessed for reanalysis skill above 50 km. A four-dimensional variational (4DVAR) run specified initial background error covariances statically. A hybrid-4DVAR (HYBRID) run formed background error covariances from an 80-member forecast ensemble blended with a static estimate. Each configuration was run at low and high horizontal resolution. In addition to operational observations below 50 km, each experiment assimilated 105 observations of the mesosphere and lower thermosphere (MLT) every 6 h. While all MLT reanalyses show skill relative to independent wind and temperature measurements, HYBRID outperforms 4DVAR. MLT fields at 1-h resolution (6-h analysis and 1–5-h forecasts) outperform 6-h analysis alone due to a migrating semidiurnal (SW2) tide that dominates MLT dynamics and is temporally aliased in 6-h time series. MLT reanalyses reproduce observed SW2 winds and temperatures, including phase structures and 10–15-day amplitude vacillations. The 0–100-km reanalyses reveal quasi-stationary planetary waves splitting the stratopause jet in July over New Zealand, decaying from 50 to 80 km then reintensifying above 80 km, most likely via MLT forcing due to zonal asymmetries in stratospheric gravity wave filtering.

Based on the specified dynamics simulation of Whole Atmosphere Community Climate Model with ionosphere/thermosphere extension (SD-WACCM-X), the composite response of polar hydroxyl radical (OH) layer in the mesosphere and lower thermosphere (MLT) to the Arctic sudden stratospheric warming (SSW) events with elevated stratopause (ES) during 2004–2023 is investigated. A total of ten ES-SSW events are systematically analyzed. Before the onset of ES-SSW events, the OH concentration climatologically peaks at ∼ 7.4 ppbv near 82.4 km. During the stratospheric warming phase, relative to the climatology, the peak height of OH layer undergoes a distinct upward displacement reaching ∼ 85.9 km accompanied by a reduction in the OH concentration to ∼ 2.9 ppbv. This shift is closely linked to an ∼ 11 % and ∼ 90.8 % reduction in mesospheric temperature and atomic oxygen, respectively, due to enhanced upward residual circulation. During the elevated stratopause phase, the peak height of OH layer experiences a pronounced downward shift to ∼ 80.6 km with a maximum in OH concentration to ∼ 6.8 ppbv. This phase is characterized by ∼ 3.7 % and ∼ 137.3 % enhancements in mesospheric temperature and atomic oxygen concentrations, respectively, which is driven by intensified downward residual circulation. Further analysis suggests that OH concentration variations are positively correlated to mesospheric temperature anomalies and atomic oxygen redistribution induced by vertical transport, which is attributed to the significant influence of ES-SSW on gravity wave drag (GWs) in the mesosphere.

Gravity waves play a significant role in driving the semiannual oscillation (SAO) of the zonal wind in the tropics. However, detailed knowledge of this forcing is missing, and direct estimates from global observations of gravity waves are sparse. For the period 2002–2018, we investigate the SAO in four different reanalyses: ERA-Interim, JRA-55, ERA-5, and MERRA-2. Comparison with the SPARC zonal wind climatology and quasi-geostrophic winds derived from Microwave Limb Sounder (MLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite observations show that the reanalyses reproduce some basic features of the SAO. However, there are also large differences, depending on the model setup. Particularly, MERRA-2 seems to benefit from dedicated tuning of the gravity wave drag parameterization and assimilation of MLS observations. To study the interaction of gravity waves with the background wind, absolute values of gravity wave momentum fluxes and a proxy for absolute gravity wave drag derived from SABER satellite observations are compared with different wind data sets: the SPARC wind climatology; data sets combining ERA-Interim at low altitudes and MLS or SABER quasi-geostrophic winds at high altitudes; and data sets that combine ERA-Interim, SABER quasi-geostrophic winds, and direct wind observations by the TIMED Doppler Interferometer (TIDI). In the lower and middle mesosphere the SABER absolute gravity wave drag proxy correlates well with positive vertical gradients of the background wind, indicating that gravity waves contribute mainly to the driving of the SAO eastward wind phases and their downward propagation with time. At altitudes 75–85 km, the SABER absolute gravity wave drag proxy correlates better with absolute values of the background wind, suggesting a more direct forcing of the SAO winds by gravity wave amplitude saturation. Above about 80 km SABER gravity wave drag is mainly governed by tides rather than by the SAO. The reanalyses reproduce some basic features of the SAO gravity wave driving: all reanalyses show stronger gravity wave driving of the SAO eastward phase in the stratopause region. For the higher-top models ERA-5 and MERRA-2, this is also the case in the lower mesosphere. However, all reanalyses are limited by model-inherent damping in the upper model levels, leading to unrealistic features near the model top. Our analysis of the SABER and reanalysis gravity wave drag suggests that the magnitude of SAO gravity wave forcing is often too weak in the free-running general circulation models; therefore, a more realistic representation is needed.

Rising emissions of anthropogenic greenhouse gases (GHG) have led to tropospheric warming and stratospheric cooling over recent decades. As a thermodynamic consequence, the troposphere has expanded and the rise of the tropopause, the boundary between the troposphere and stratosphere, has been suggested as one of the most robust fingerprints of anthropogenic climate change. Conversely, at altitudes above ∼55 km (in the mesosphere and thermosphere) observational and modeling evidence indicates a downward shift of the height of pressure levels or decreasing density at fixed altitudes. The layer in between, the stratosphere, has not been studied extensively with respect to changes of its global structure. Here we show that this atmospheric layer has contracted substantially over the last decades, and that the main driver for this are increasing concentrations of GHG. Using data from coupled chemistry-climate models we show that this trend will continue and the mean climatological thickness of the stratosphere will decrease by 1.3 km following representative concentration pathway 6.0 by 2080. We also demonstrate that the stratospheric contraction is not only a response to cooling, as changes in both tropopause and stratopause pressure contribute. Moreover, its short emergence time (less than 15 years) makes it a novel and independent indicator of GHG induced climate change.