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The study presents (a) a 44‐year wintertime climatology of resolved gravity wave (GW) fluxes and forcing in the extratropical stratosphere using ERA5, and (b) their composite evolution around gradual (final warming) and abrupt (sudden warming) transitions in the wintertime circulation, focusing on lateral fluxes. The transformed Eulerian mean equations are leveraged to provide a glimpse of the importance of GW lateral propagation (i.e., horizontal propagation) toward driving the wintertime stratospheric circulation by analyzing the relative contribution of the vertical versus meridional flux dissipation. The relative contribution from lateral propagation is found to be notable, especially in the Austral winter stratosphere where lateral (vertical) momentum flux convergence provides a peak climatological forcing of up to −0.5 (−3.5) m/s/day around 60°S at 40–45 km altitude. Prominent lateral propagation in the wintertime midlatitudes also contributes to the formation of belts of GW activity in both hemispheres. Plain Language Summary: Atmospheric Gravity Waves (GWs) are atmospheric disturbances created by processes like convection, thunderstorms, flow over topography, etc. These waves can have wavelengths as small as 1 km to as large as 1,000–2,000 km. Most atmospheric GWs are not resolved in coarse‐resolution climate models. As a result, they are represented in climate models using parameterizations, which are approximate models that can be subject to various idealizations. One such idealization is the assumption of pure vertical propagation of GWs. In this study, we use multidecadal records from ERA5 reanalysis—which combines a high‐resolution model with assimilated observations to produce a close‐to‐observed state of the atmosphere and resolves some of these GWs—to quantify the impact of this assumption on the mean state of the extratropical stratosphere. This is done by extracting GWs from ERA5 data, computing horizontal momentum fluxes carried by these waves, and comparing the net acceleration/deceleration provided by these fluxes on the peak winter stratospheric circulation and key episodes of abrupt changes in the circulation. Analysis using ERA5 reveals that horizontal propagation of GWs can be notable in the midlatitude stratosphere, highlighting the need to develop GW parameterizations that represent this essential property of atmospheric GWs. Key Points: Climatology of lateral fluxes from ERA5 shows substantial lateral propagation of gravity waves in both hemispheresContribution of both lateral and vertical GW fluxes toward zonal wind forcing is the same order of magnitudeAbrupt changes in GW forcing in the upper stratosphere around sudden stratospheric warmings persist even 20 days following the event [ABSTRACT FROM AUTHOR]

Abstract The study presents (a) a 44‐year wintertime climatology of resolved gravity wave (GW) fluxes and forcing in the extratropical stratosphere using ERA5, and (b) their composite evolution around gradual (final warming) and abrupt (sudden warming) transitions in the wintertime circulation, focusing on lateral fluxes. The transformed Eulerian mean equations are leveraged to provide a glimpse of the importance of GW lateral propagation (i.e., horizontal propagation) toward driving the wintertime stratospheric circulation by analyzing the relative contribution of the vertical versus meridional flux dissipation. The relative contribution from lateral propagation is found to be notable, especially in the Austral winter stratosphere where lateral (vertical) momentum flux convergence provides a peak climatological forcing of up to −0.5 (−3.5) m/s/day around 60°S at 40–45 km altitude. Prominent lateral propagation in the wintertime midlatitudes also contributes to the formation of belts of GW activity in both hemispheres.

Stratospheric dynamics are strongly affected by the absorption/emission of radiation in the Earth's atmosphere and Rossby waves that propagate upward from the troposphere, perturbing the zonal flow. Reduced order models of stratospheric wave–zonal interactions, which parameterize these effects, have been used to study interannual variability in stratospheric zonal winds and sudden stratospheric warming (SSW) events. These models are most sensitive to two main parameters: Λ , forcing the mean radiative zonal wind gradient, and h, a perturbation parameter representing the effect of Rossby waves. We take one such reduced order model with 20 years of ECMWF atmospheric reanalysis data and estimate Λ and h using both a particle filter and an ensemble smoother to investigate if the highly-simplified model can accurately reproduce the averaged reanalysis data and which parameter properties may be required to do so. We find that by allowing additional complexity via an unparameterized Λ (t) , the model output can closely match the reanalysis data while maintaining behavior consistent with the dynamical properties of the reduced-order model. Furthermore, our analysis shows physical signatures in the parameter estimates around known SSW events. This work provides a data-driven examination of these important parameters representing fundamental stratospheric processes through the lens and tractability of a reduced order model, shown to be physically representative of the relevant atmospheric dynamics. [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]

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]

Despite offsetting global mean surface temperature, various studies demonstrated that stratospheric aerosol injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far, most of the assessments of such an approach have come from climate model simulations in which SO 2 is injected only in a single location or a set of locations. Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections, namely an equatorial injection, an annual injection of equal amounts of SO 2 at 15 ∘ N and 15 ∘ S, an annual injection of equal amounts of SO 2 at 30 ∘ N and 30 ∘ S, and a polar strategy injecting SO 2 at 60 ∘ N and 60 ∘ S only in spring in each hemisphere. We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres, and changes in the speed of the residual circulation. These impacts tend to maximise under the equatorial injection strategy and become smaller as the aerosols are injected away from the Equator into the subtropics and higher latitudes. In conjunction with the differences in direct radiative impacts at the surface, these different stratospheric changes drive different impacts on the extratropical modes of variability (Northern and Southern Annular modes), including important consequences on the northern winter surface climate, and on the intensity of tropical tropospheric Walker and Hadley circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical, and chemical processes driving the atmospheric circulation and ozone response to SAI and lay the foundation for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons. [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]

Water vapor plays an important role in many aspects of the climate system, by affecting radiation, cloud formation, atmospheric chemistry and dynamics. Even the low stratospheric water vapor content provides an important climate feedback, but current climate models show a substantial moist bias in the lowermost stratosphere. Here we report crucial sensitivity of the atmospheric circulation in the stratosphere and troposphere to the abundance of water vapor in the lowermost stratosphere. We show from a mechanistic climate model experiment and inter-model variability that lowermost stratospheric water vapor decreases local temperatures, and thereby causes an upward and poleward shift of subtropical jets, a strengthening of the stratospheric circulation, a poleward shift of the tropospheric eddy-driven jet and regional climate impacts. The mechanistic model experiment in combination with atmospheric observations further shows that the prevailing moist bias in current models is likely caused by the transport scheme, and can be alleviated by employing a less diffusive Lagrangian scheme. The related effects on atmospheric circulation are of similar magnitude as climate change effects. Hence, lowermost stratospheric water vapor exerts a first order effect on atmospheric circulation and improving its representation in models offers promising prospects for future research. This study investigates the role of water in the lower-most stratosphere, affecting dynamics of the stratosphere and troposphere, and shows that common water vapor transport schemes can cause biases, present in nearly all modern climate models. [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]

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 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]

Despite offsetting global mean surface temperature, various studies demonstrated that Stratospheric Aerosol Injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far most of the assessments of such an approach have come from climate model simulations in which SO2 is injected only in a single location or a set of locations. Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections: an equatorial injection, an annual injection of equal amounts of SO2 at 15° N and 15° S, an annual injection of equal amounts of SO2 at 30° N and 30° S, and a polar strategy injecting SO2 at 60° N and 60° S only in spring in each hemisphere. We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres and changes in the speed of the residual circulation. In conjunction with the differences in direct radiative impacts at the surface, these drive different impacts on the extratropical modes of variability (Northern and Southern Annular Mode), including important consequences on the northern winter surface climate, as well as on the intensity of tropical tropospheric Walker and Hadley Circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical and chemical processes driving the atmospheric response to SAI, as well as lay the ground for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons. [ABSTRACT FROM AUTHOR]

Ozone in the Arctic stratosphere is subject to large interannual variability, driven by both chemical ozone depletion and dynamical variability. Anomalies in Arctic stratospheric ozone become particularly important in spring, when returning sunlight allows them to alter stratospheric temperatures via shortwave heating, thus modifying atmospheric dynamics. At the same time, the stratospheric circulation undergoes a transition in spring with the final stratospheric warming (FSW), which marks the end of winter. A causal link between stratospheric ozone anomalies and FSWs is plausible and might increase the predictability of stratospheric and tropospheric responses on sub-seasonal to seasonal timescales. However, it remains to be fully understood how ozone influences the timing and evolution of the springtime vortex breakdown. Here, we contrast results from chemistry climate models with and without interactive ozone chemistry to quantify the impact of ozone anomalies on the timing of the FSW and its effects on surface climate. We find that ozone feedbacks increase the variability in the timing of the FSW, especially in the lower stratosphere. In ozone-deficient springs, a persistent strong polar vortex and a delayed FSW in the lower stratosphere are partly due to the lack of heating by ozone in that region. High-ozone anomalies, on the other hand, result in additional shortwave heating in the lower stratosphere, where the FSW therefore occurs earlier. We further show that FSWs in high-ozone springs are predominantly followed by a negative phase of the Arctic Oscillation (AO) with positive sea level pressure anomalies over the Arctic and cold anomalies over Eurasia and Europe. These conditions are to a significant extent (at least 50 %) driven by ozone. In contrast, FSWs in low-ozone springs are not associated with a discernible surface climate response. These results highlight the importance of ozone–circulation coupling in the climate system and the potential value of interactive ozone chemistry for sub-seasonal to seasonal predictability. [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]

Sudden stratospheric warmings (SSWs) can have a strong impact on the troposphere. Their fingerprint is often associated with the negative phase of the Northern Annular Mode (NAM) and the North Atlantic Oscillation (NAO), and extreme weather with high societal impact. However, the mechanisms behind this downward impact are not well understood. We investigate this surface impact through its associated predictability limits, by studying the 2018 SSW event. We search for predictability barriers that occurred after the onset of the SSW and before its surface impact. It is found that dynamical tropospheric events consisting of two cyclogenesis events were the main reasons for these predictability barriers in the prediction of negative NAM/NAO anomalies reaching the surface. This work corroborates that individual synoptic events might constitute predictability barriers during the downward impact of SSW events, and thereby sheds light on stratosphere‐troposphere coupling. Plain Language Summary: Sudden stratospheric warmings (SSWs) can have a strong impact on near‐surface weather. They have the potential to alter the atmospheric circulation and associated extreme events, thus affecting society. However, the way the stratosphere couples with surface weather is not fully understood. In this work, we investigate this relationship by analyzing the predictability of the surface impact of a stratospheric warming event that occurred in 2018, and which left widespread societal impacts over Europe due to a severe cold spell and extraordinary rainy and windy conditions. We find that the impact of this SSW event on the surface was regulated by two low‐pressure systems that developed over the North Atlantic. Even though the SSW event was well developed, alternative behavior of these systems could have led to drastically different weather conditions over Europe. Thus, this study highlights the role of the tropospheric circulation in the effectiveness of the stratospheric impact on surface weather. Key Points: The surface impact of the sudden stratospheric warming in 2018 was associated with a Greenland blocking in the troposphereMedium‐range uncertainties in the prediction of the Greenland blocking were linked to two cyclogenesis events over the North AtlanticIndividual synoptic events constitute "predictability barriers" for subsequent forecast lead times after sudden stratospheric warming events [ABSTRACT FROM AUTHOR]

The teleconnection between the Quasi‐Biennial Oscillation (QBO) and the Arctic polar vortex is investigated using Coupled Model Intercomparison Project 6 (CMIP6) models. Output from 14 CMIP6 models is compared with reanalysis, three experiments with prescribed QBOs, one of which has no free polar stratospheric variability, and transient experiments in which a QBO is prescribed in runs previously devoid of a QBO. Each CMIP6 model underestimates the Holton‐Tan effect (HTE), the weakening of the polar vortex expected with QBO easterlies in the tropical lower stratosphere. To establish why, potential vorticity maps are used to investigate longitudinal variations in the teleconnection. Prescribing easterly QBO in the transient experiments promotes more high‐latitude planetary wave breaking by influencing the mid‐latitude stratospheric circulation, particularly over Asia. CMIP6 models that better simulate this response over Asia better simulate the HTE. These models also have stronger 10 hPa QBO westerlies. Plain Language Summary: In the tropics at altitudes between two and three times as high as commercial airplanes fly (10–30 kilometers), successive sets of easterly and westerly winds propagate downward approximately every 28 months. This pattern of winds is called the Quasi‐Biennial Oscillation (QBO). Despite being in the middle to lower tropical stratosphere, the QBO affects the global circulation in ways that ultimately influence regional weather. One example of this occurs during winter, when the QBO changes the strength of the westerly flow that forms in the Arctic polar stratosphere in the absence of solar radiation, the polar vortex. Although scientists have known about this phenomenon for 40+ years, this long‐distance relationship is complicated. Models of the coupled land, ocean, atmosphere system have steadily improved at representing the QBO. These models also represent the QBO's relationship with the polar vortex, but each model does it differently. The difference between models that represent the QBO‐polar vortex relationship well and models that do not, teaches us more about the relationship. We learn here that the QBO communicates with the polar stratosphere partly by changing the atmospheric circulation 30 kilometers above Asia. This phenomenon is best represented by models that have stronger QBOs. Key Points: Coupled Model Intercomparison Project 6 models underestimate the weakening of the polar vortex expected with Quasi‐Biennial Oscillation (QBO) easterlies in the tropical lower stratosphereThe mid‐latitude stratospheric response to the QBO over Asia influences how the QBO affects the strength of the polar vortex in the modelsModels simulating more realistic extratropical responses to the QBO have stronger QBO westerlies in the tropical middle stratosphere [ABSTRACT FROM AUTHOR]

Rare events arising in nonlinear atmospheric dynamics remain hard to predict and attribute. We address the problem of forecasting rare events in a prototypical example, sudden stratospheric warmings (SSWs). Approximately once every other winter, the boreal stratospheric polar vortex rapidly breaks down, shifting midlatitude surface weather patterns for months. We focus on two key quantities of interest: the probability of an SSW occurring, and the expected lead time if it does occur, as functions of initial condition. These optimal forecasts concretely measure the event's progress. Direct numerical simulation can estimate them in principle but is prohibitively expensive in practice: each rare event requires a long integration to observe, and the cost of each integration grows with model complexity. We describe an alternative approach using integrations that are short compared to the time scale of the warming event. We compute the probability and lead time efficiently by solving equations involving the transition operator, which encodes all information about the dynamics. We relate these optimal forecasts to a small number of interpretable physical variables, suggesting optimal measurements for forecasting. We illustrate the methodology on a prototype SSW model developed by Holton and Mass and modified by stochastic forcing. While highly idealized, this model captures the essential nonlinear dynamics of SSWs and exhibits the key forecasting challenge: the dramatic separation in time scales between a single event and the return time between successive events. Our methodology is designed to fully exploit high-dimensional data from models and observations, and has the potential to identify detailed predictors of many complex rare events in meteorology. [ABSTRACT FROM AUTHOR]

Solar cycle 24 from December 2008 to December 2019 had an unusual progression and was the weakest cycle in a century, providing an ideal scenario to study perceivable effect of wave forcing from lower atmosphere. This study examines the ionospheric response to meteorological phenomenon of sudden stratospheric warming (SSW) during solar cycle 24 (Arctic winter 2008/2009 to 2018/2019) using Global Positioning System‐derived total electron content (VTEC) and its deviation from monthly median (ΔVTEC) for four longitudinal chains. Each chain comprises eight stations, covering varied latitudes in both hemispheres. A longitude‐ and latitude‐dependent response of VTEC is observed within ∼3 weeks of peak polar stratospheric temperature anomaly (ΔTmax), with maximum variations seen for low‐latitude stations at American and Australian‐East Asian sectors. It is seen that variations during minor warming events are comparable or at times even larger than during major warming events. These responses increase with an increase in solar activity, with major SSW events primarily seen during moderate and low solar activity periods and minor SSW during high solar activity period. Under similar ionizing conditions, a quite similar ionospheric response is observed irrespective of ΔTmax and of SSW type (major or minor); however, under similar SSW strength, no prominent pattern in ionospheric response is observed. Latitude‐dependent semidiurnal ionospheric behavior is observed for only a few events. This vertical coupling between lower and upper atmosphere during SSW is predominantly influenced by 13–16 days periodicities in VTEC, along with other periodicities of 9, 7, 5, and 3 days. Plain Language Summary: Sun is the primary source of ionization for the ionized layer of the atmosphere, the ionosphere. However, solar cycle 24, extending from December 2008 to December 2019, was the weakest cycle in the last 100 years. With such low levels of solar forcing, the ionosphere is expected to be strongly influenced by the wave forcing from below. Sudden stratospheric warming (SSW) is one of the important meteorological phenomena which impacts the ionosphere and will have perceivable effects during such ideal background scenario. This study focuses on the characteristics of ionospheric variations, that is, ionospheric response to SSW as the solar cycle progresses, thereby characterizing these variations as a function of solar activity, type of SSW, longitude/latitude, and dominant periodicities observed in these variations during 11 SSW events from Arctic winter 2008/2009 to 2018/2019. It is seen that variations during minor warming events are comparable or at times even larger than during major warming events. These responses increase with an increase in solar activity, with major SSW events primarily seen during moderate and low solar activity periods and minor SSW during high solar activity periods. These factors influencing the ionospheric variations during SSW are discussed. Key Points: Ionospheric variation (ΔVTEC) during minor warming events is comparable or at times even larger than during major warming eventsSemidiurnal behavior in ΔVTEC is observed for only a few sudden stratospheric warming (SSW) events and is found to depend on latitudeIonospheric variations during SSW show dominant 13–16 days periodicity along with periodicities of 9, 7, 5, and 3 days [ABSTRACT FROM AUTHOR]

The connection between the polar stratospheric vortex and the vertical component of the Eliassen–Palm flux in the lower stratosphere and upper troposphere is examined in model level data from ERA5. The particular focus of this work is on the conditions that lead to upward wave propagation between the tropopause and the bottom of the vortex near 100 hPa. The ability of four different versions of the index of refraction to capture this wave propagation is evaluated. The original Charney and Drazin index of refraction includes terms ignored by Matsuno that are shown to be critical for understanding upward wave propagation just above the tropopause both in the climatology and during extreme heat flux events. By adding these terms to the Matsuno index of refraction, it is possible to construct a useful tool that describes wave flux immediately above the tropopause and at the same time also describes the role of meridional variations within the stratosphere. It is shown that a stronger tropopause inversion layer tends to restrict upward wave propagation. It is also shown that while only 38% of extreme wave-1 Eliassen–Palm flux vertical component (Fz) at 100 hPa events are preceded by extreme Fz at 300 hPa, there are almost no extreme events at 100 hPa in which the anomaly at 300 hPa is of opposite sign or very weak. Overall, wave propagation near the tropopause is sensitive to vertical gradients in buoyancy frequency, and these vertical gradients may not be accurately captured in models or reanalysis products with lower vertical resolutions. [ABSTRACT FROM AUTHOR]

The impact of global orography on Northern Hemisphere wintertime climate is revisited using the Whole Atmosphere Community Climate Model, version 6 (WACCM6). A suite of experiments explores the roles of both resolved orography and the parameterized effects of unresolved orographic drag (hereafter parameterized orography), including gravity waves and boundary layer turbulence. Including orography reduces the extratropical tropospheric and stratospheric zonal mean zonal wind U ¯ by up to 80%; this is substantially greater than previous estimates. Ultimately, parameterized orography accounts for 60%–80% of this reduction; however, away from the surface most of the forcing of U ¯ by parameterized orography is accomplished by resolved planetary waves. We propose that a catalytic wave–mean-flow positive feedback in the stratosphere makes the stratospheric flow particularly sensitive to parameterized orography. Orography and land–sea contrast contribute approximately equally to the strength of the midlatitude stationary waves in the free troposphere, although orography is the dominant cause of the strength of the Siberian high and Aleutian low at the surface and of the position of the Icelandic low. We argue that precisely quantifying the role of orography on the observed stationary waves is an almost intractable problem, and in particular should not be approached with linear stationary wave models in which U ¯ is prescribed. We show that orography has less impact on stationary waves, and therefore on U ¯ , on a backward-rotating Earth. Last, we show that atmospheric meridional heat transport shows remarkable constancy across our simulations, despite vastly different climates and stationary wave strengths. [ABSTRACT FROM AUTHOR]

The Northern Hemisphere wintertime circulation response to volcanic eruptions has been explored extensively using general circulation models. In observations and some models, the response is characterized by an enhanced stratospheric polar vortex (SPV), a positive mode of the North Atlantic Oscillation (NAO), and warm surface temperatures during the winter over North America and Eurasia. A weak surface air temperature signal in previous studies has led to conflicting conclusions on the robustness of the response. Here, we use simulations with the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM) of six nuclear war scenarios to present a new perspective on the connection between stratospheric aerosol heating, the SPV, and the surface temperature response. We show that stratospheric aerosol heating by soot is the primary contributor to the SPV response in nuclear war simulations, which is coupled to the troposphere and projects as a positive mode of the NAO at the surface. Winter warming is observed across northern Eurasia, albeit poleward of the response after volcanic eruptions. We compare the results to simulations of volcanic eruptions and find that observed Eurasian warming in the first winter after the 1963 Agung, 1982 El Chichón, and 1991 Pinatubo volcanic eruptions is simulated with the NCAR CAM5 climate model only when tropical sea surface temperatures, including the observed El Niño, are specified along with the volcanic aerosols. This suggests an undiagnosed tropospheric mechanism connecting the tropics and high latitudes, as without specifying sea surface temperatures, internal variability dominates the simulated winter warming response after historical volcanic eruptions. Plain Language Summary: Explosive volcanic eruptions can create sulfur aerosols in the upper atmosphere, which both cool Earth's surface and heat the lower stratosphere. While large heating is well known to affect the atmospheric circulation, favoring a pattern that produces winter surface warming over Eurasia, whether the observed winter warming from eruptions like the 1991 Pinatubo was caused by those eruptions has been questioned. We use nuclear war simulations to show that large heating of the upper atmosphere by smoke would produce this response. Finally, we find that when there is an El Niño combined with a large volcanic eruption, there is a greater likelihood of winter warming over Eurasia. Key Points: Acceleration of the stratospheric polar vortex (SPV) occurs due to heated stratospheric aerosols in nuclear war and volcanic eruption simulationsA tropospheric mechanism for acceleration of the SPV is detected in a 150 Tg soot nuclear war scenarioWinter warming is simulated after the 1963 Agung, 1982 El Chichón, and 1991 Pinatubo volcanic eruptions, when the observed sea surface temperatures, which include an El Niño in each case, are specified [ABSTRACT FROM AUTHOR]

Measurements of long-lived trace gases (SF 6 , CFC-11, CFC-12, HCFC-22, CCl 4 , N 2 O, CH 4 , H 2 O, and CO) performed with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) have been used to infer the stratospheric and mesospheric meridional circulation. The MIPAS data set covers the time period from July 2002 to April 2012. The method used for this purpose was the direct inversion of the two-dimensional continuity equation for the concentrations of trace gases and air density. This inversion predicts an "effective velocity" that gives the best fit for the evolution of the concentrations on the assumption that an explicit treatment of Fickian diffusion can be neglected. These effective velocity fields are used to characterize the mean meridional circulation. Multiannual monthly mean effective velocity fields are presented, along with their variabilities. According to this measure, the stratospheric circulation is found to be highly variable over the year, with a quite robust annual cycle. The new method allows us to track the evolution of various circulation patterns over the year in more detail than before. According to the effective velocity characterization of the circulation, the deep branch of the Brewer–Dobson circulation and the mesospheric overturning pole-to-pole circulation are not separate but intertwined phenomena. The latitude of stratospheric uplift in the middle and upper stratosphere is found to be quite variable and is not always found at equatorial latitudes. The usual schematic of stratospheric circulation with the deep and the shallow branch of the Brewer–Dobson circulation and the mesospheric overturning circulation is an idealization which best describes the observed atmosphere around equinox. Sudden stratospheric warmings and the quasi-biennial oscillation cause a pronounced year-to-year variability of the meridional circulation. [ABSTRACT FROM AUTHOR]

This paper reports a comprehensive instability analysis of a 3D Charney-like model with an observationally compatible generic stratosphere. It is found that the values of a single nondimensional parameter Γ =   λ f o 2 / (β D N 1 2 ) (detailed definition in text), in conjunction with representative values of four other nondimensional parameters, would dictate the existence of multiple branches of unstable modes as a function of the zonal wavenumber. Prototype Charney mode, Green mode, and two additional structurally distinct modes (Charney+ mode and tropopause mode) are identified. The latter result from the additional strong influence of the tropopause. The dynamical nature of all modes is delineated in terms of their meridional fluxes of heat and potential vorticity. The three-dimensional structure of the ageostrophic velocity field in each mode is presented to identify its potential of inducing frontogenesis. Optimal-mode analyses are also performed to ascertain how a disturbance with a predisposed structure would explosively develop toward each type of normal mode. In view of the pivotal role of Γ in baroclinic instability and for historical reason, we name it the Charney number. It is most instructive to think of Γ as a ratio of the meridional gradient of PV associated with the basic flow in the troposphere,   λ f o 2 / (D N 1 2) , to that associated with Earth's rotation, β. [ABSTRACT FROM AUTHOR]

Total columns of the trace gases nitric acid (HNO3) and hydrogen chloride (HCl) are sensitive to variations in the lower stratospheric age of air, a quantity that describes transport time scales in the stratosphere. Analyses of HNO3 and HCl columns from the Network for the Detection of Atmospheric Composition Change panning 77°S to 79°N have detected changes in the extratropical stratospheric transport circulation from 1994 to 2018. The HNO3 and HCl analyses combined with the age of air from a simulation using the MERRA2 reanalysis show that the Southern Hemisphere lower stratosphere has become 1 month/decade younger relative to the Northern Hemisphere, largely driven by the Southern Hemisphere transport circulation. The analyses reveal multiyear anomalies with a 5‐ to 7‐year period driven by interactions between the circulation and the quasi‐biennial oscillation in tropical winds. This hitherto unrecognized variability is large relative to hemispheric transport trends and may bias ozone trend regressions. Plain Language Summary: Our analyses of the 25‐year Network for the Detection of Atmospheric Composition Change column HNO3 and HCl data records from nine stations provide observational evidence that air in the Southern Hemisphere lower stratosphere has been getting younger relative to the Northern Hemisphere at a rate of 1 month/decade since 1994. This stands in contrast to several model studies that predict that Antarctic ozone hole recovery in this century will increase the Southern Hemisphere age of air relative to the Northern Hemisphere. The analyses also reveal extratropical variability with a 5‐ to 7‐year period driven by interactions between the circulation and tropical winds. This previously unrecognized, low‐frequency variability is much larger than hemispheric transport trends and is likely to cause bias in trends calculated using data records shorter than about two decades. Understanding and quantifying changes in the transport circulation matters to our ability to model how our protective O3 layer will evolve in the future. Key Points: Trace gas data show southern stratospheric air is getting younger relative to the Northern Hemisphere, in contrast to model predictionsBoth extratropical hemispheres have dynamically driven 5‐ to 7‐year periodicity with large amplitude relative to the age trendMultidecadal data sets are required to quantify transport trends that are not confounded by large extratropical variability [ABSTRACT FROM AUTHOR]

Mountain ranges are regional features on Earth, as are the regions of mountain-wave drag (MWD) exerted by dissipating atmospheric gravity waves generated by flow over them. However, these regional drags have significant global- or zonal-mean impacts on Earth's atmospheric general circulation (e.g., slowing of the polar night jet). The objective of this work is to understand the regional to global evolution of these impacts. The approach is to track the evolution of MWD-generated potential vorticity (PV) over the winter using the Whole Atmosphere Community Climate Model (WACCM). Within an ensemble of winter-long runs with and without MWD, lower-stratospheric PV is generated over mountains and advected downstream, generating large-scale PV banners. These PV banners are diffused but survive this diffusion and are reinforced over downstream mountain ranges, accumulating into zonal-mean or global features within WACCM. A simple 2D model representing sources, advection, and diffusion of "passive PV" recreates the salient features in the WACCM results, suggesting the winter-long evolution of MWD-generated PV can be crudely understood in terms of horizontal advection and diffusion within a global vortex. In the Northern Hemisphere, cyclonic, equatorward PV banners accumulate zonally into a single zonally symmetric positive PV anomaly. The anticyclonic, poleward PV banners also accumulate into a zonally symmetric feature, but then diffuse over the North Pole into a negative PV polar cap. In the Southern Hemisphere, the same processes are at work, though the different geographic configuration of mountain ranges leads to different patterns of impacts. [ABSTRACT FROM AUTHOR]

Understanding and predicting midlatitude cold spells is of scientific and public interest, given often associated severe impacts. However, large-scale atmospheric dynamics related to these events are not fully understood. The winter of 2017/18 was characterized by several cold spells affecting large parts of North America and Eurasia. Here, the role of stratosphere–troposphere coupling for the occurrence of cold spells in this winter is investigated using different wave propagation diagnostics. While the European cold spell in late February 2018 was influenced by a major sudden stratospheric warming (SSW) associated with wave absorption, the cold spells over North America at the end of December 2017 and early February 2018 were related to downward reflected waves over the North Pacific. Previously proposed wave reflection indices, however, either miss these reflection events or are not able to distinguish them from the major SSW related to wave absorption. To overcome this, a novel simple index based on eddy heat flux is proposed here, capturing regional wave reflection over the North Pacific. Reflection events detected with this index are shown to be followed by North Pacific blocking and negative temperature anomalies over North America. An improved understanding of the contribution of wave reflection for cold spells is crucial to better predict such events in the future. [ABSTRACT FROM AUTHOR]

Recent Arctic sea ice loss in fall has been posited to drive midlatitude circulation changes into winter and even spring. Past work has shown that sea ice loss can indeed trigger a weakening of the stratospheric polar vortex, which can lead to delayed surface weather changes. But the mechanisms of such changes and their relevant time scales have remained unclear. This study uses large ensembles of idealized GCM simulations to identify how and over what time scales the atmospheric circulation responds to short-term surface heat flux changes in high latitudes. The ensemble-mean response of the atmospheric circulation is approximately linear in the amplitude of the surface forcing. It is also insensitive to whether the forcing is zonally asymmetric or symmetric, that is, whether stationary waves are generated or not. The circulation response can be decomposed into a rapid thermal response and a slower dynamic adjustment. The adjustment arises through weakening of vertical wave activity fluxes from the troposphere into the stratosphere in response to polar warming, a mechanism that differs from sudden stratospheric warmings yet still results in a weakened stratospheric circulation. The stratospheric response is delayed and persists for about 2 months because the thermal response of the stratosphere is slow compared with that of the troposphere. The delayed stratospheric response feeds back onto the troposphere, but the tropospheric effects are weak compared with natural variability. The general pathway for the delayed response appears to be relatively independent of the atmospheric background state at the time of the anomalous surface forcing. [ABSTRACT FROM AUTHOR]

The atmospheric composition is strongly influenced by a change in atmospheric dynamics, which is potentially related to climate change. A prominent example is the doubling of the stratospheric ozone component at the Zugspitze summit station (2962 m a.s.l., Garmisch-Partenkirchen, Germany) between the mid-seventies and 2005, roughly from 11 to 23 ppb (43 %). Systematic efforts for identifying and quantifying this influence have been made since the late 1990s. Meanwhile, routine lidar measurements of ozone and water vapour carried out at Garmisch-Partenkirchen (German Alps) since 2007, combined with in situ and radiosonde data and trajectory calculations, have revealed that stratospheric intrusion layers are present on 84 % of the yearly measurement days. At Alpine summit stations the frequency of intrusions exhibits a seasonal cycle with a pronounced summer minimum that is reproduced by the lidar measurements. The summer minimum disappears if one looks at the free troposphere as a whole. The mid- and upper-tropospheric intrusion layers seem to be dominated by very long descent on up to hemispheric scale in an altitude range starting at about 4.5 km a.s.l. Without interfering air flows, these layers remain very dry, typically with RH ≤5 % at the centre of the intrusion. Pronounced ozone maxima observed above Garmisch-Partenkirchen have been mostly related to a stratospheric origin rather than to long-range transport from remote boundary layers. Our findings and results for other latitudes seem to support the idea of a rather high contribution of ozone import from the stratosphere to tropospheric ozone. [ABSTRACT FROM AUTHOR]

This study employs 40 years of ERA-Interim data to quantify the periodic behavior of the integral amount of potential energy stored in the tropical upper UTLS (upper troposphere and lower stratosphere) originally placed there by high-entropy outflow of deep convection and tropical cyclones (TCs). The upper UTLS is defined to be isentropic layers above the level of zero net radiation (LZNR) in the tropics. Once there, the trapped high-entropy air mass must flow into the extratropics where radiative loss will allow it to subside back to lower-entropy levels. Mean poleward fluxes of isentropic mass are prevented by an inertial wall associated with Earth's rotation. This causes the mass of these isentropic layers to build up, creating a bubble of high potential energy. Periodic releases of this mass into the extratropics in the form of tropical plumes (TPs) help drain the bubble of its mass buildup over time. We present a metric to quantify the energy trapped and stored in this bubble to be the jet available potential energy (JAPE), in reference to the availability of this energy to fuel the kinetic energy of the subtropical jet stream (STJ), which bounds the tropical JAPE bubble (TJB) on its poleward extremities. We also calculate the isentropic mass M, which measures the isentropic thickness. Results of a 40-yr-time-series analysis of upper-UTLS JAPE and M show that the TJB features periodicity in the buildup and release of JAPE and M. [ABSTRACT FROM AUTHOR]

Both large-scale atmospheric circulation and moisture content in the atmosphere govern regional precipitation. We partition recent changes in mean, heavy, and extreme precipitation for all seasons over Canada to changes in synoptic circulation patterns (dynamic changes) and in atmospheric moisture conditions (thermodynamic changes) using 500-hPa geopotential height and precipitation data over 1979–2014. Using the self-organizing map (SOM) cluster analysis, we identify statistically significant trends in occurrences of certain synoptic circulation patterns over the Canadian landmass, which have dynamically contributed to observed changes in precipitation totals and occurrence of heavy and extreme precipitation events over Canada. Occurrences of circulation patterns such as westerlies and ridges over western North America and the North Pacific have considerably affected regional precipitation over Canada. Precipitation intensity and occurrences of precipitation extremes associated with each SOM circulation pattern also showed statistically significant trends resulting from thermodynamic changes in the atmospheric moisture supply for precipitation events. A partition analysis based on the thermodynamic–dynamic partition method indicates that most (~90%) changes in mean and extreme precipitation over Canada resulted from changes in precipitation regimes occurring under each synoptic circulation pattern (thermodynamic changes). Other regional precipitation changes resulted from changes in occurrences of synoptic circulation patterns (dynamic changes). Because of the high spatial variability of precipitation response to changes in thermodynamic and dynamic conditions, dynamic contributions could offset thermodynamic contributions to precipitation changes over some regions if thermodynamic and dynamic contributions are in opposition to each other (negative or positive), which would result in minimal changes in precipitation intensity and occurrences of heavy and extreme precipitation events. [ABSTRACT FROM AUTHOR]

Previous studies have reported that the tropospheric climate shifted around the 2000s, but the recent trend in the stratospheric wave intensity (SWI) is still unclear. The results in this study show that the SWI in December during 2001–2015 weakens, which is opposite to that during 1979–2000, implying a shift around the 2000s. The weakened SWI is dominated by its wave number‐1 component, which is related to the wave propagation impeded by the decreased refractive index at high latitudes and the weakened wave activity in the troposphere. Changes in refractive index are mainly contributed by changes in the meridional potential vorticity gradient via the barotropic term. This shift in the wave number‐1 waves leads to a shift in the stratospheric Arctic temperature, that is, warming (cooling) during 1979–2000 (2001–2015), implying that similar shifting phenomena may appear regardless of the continued greenhouse gas emissions. Plain Language Summary: Recent studies have reported that some tropospheric climate (e.g., global mean surface temperature, the Pacific trade wind, and the Walker circulation) and stratospheric circulations (e.g., Arctic vortex and Brewer‐Dobson circulation) have shifted around the 2000s. The planetary waves, which play a key role in the stratosphere‐troposphere dynamical coupling, can not only impact the stratospheric temperature and circulation via wave‐mean interactions but also modify the tropospheric weather and climate via downward wave coupling. Therefore, it is important to examine whether the planetary waves have shifted around the 2000s. In this study, we investigated the trends in stratospheric wave intensity in December during 1979–2000 and 2001–2015 and found that the stratospheric wave intensity during 2001–2015 weakens, which is opposite to that during 1979–2000, implying a shift around the 2000s. This shift in planetary waves suggests that temperature, circulation, and tracers, associated with planetary waves may have also shifted around the 2000s, even though global warming continues. Key Points: The stratospheric wave intensity after the 2000s has a weakening trend, different from the strengthening trend during 1979–2000The weakened stratospheric wave intensity is dominated by its wave number‐1 component that is related to the impeded wave propagationThis shift in the wave number‐1 may lead to a shift in the stratospheric Arctic temperature [ABSTRACT FROM AUTHOR]

Sudden stratospheric warming (SSW) events can exhibit long‐lasting surface impacts that promise improvements in medium‐range to seasonal predictability. Their surface impact is dominated by the negative phase of the North Atlantic Oscillation (NAO). Hence, the question arises if stratospheric variability, and in particular the frequency of SSW events, can in turn be estimated from surface NAO conditions. This is especially relevant for the period before frequent upper air observations became available, while daily surface observations of the NAO date back to 1850. The surface impact is here quantified by NAO characteristics that are commonly observed after SSW events: a switch from a positive to a negative NAO and an extended persistence of the negative NAO, termed NAO events. Two thirds of SSW events are found to be followed by either a persistence or switch NAO event, and a quarter of SSW events are followed by both. On the other hand, less than 25% of winter surface NAO events are preceded by a SSW event. Based on these findings, an index purely based on surface NAO observations is derived that estimates SSW frequency for the satellite era and extends it back to 1850, indicating that decadal stratospheric variability was present for the entire time series, with no significant trend. The minimum in SSW frequency in the 1990s is found to be coincident with the longest absence of NAO events since 1850, indicating that the early 1990s may constitute the longest absence of SSW events for the 150‐year record. Plain Language Summary: Extreme events in the stratosphere, so‐called sudden stratospheric warming (SSW) events, have long‐lasting impacts on the weather over the North Atlantic that can be characterized by variability in the North Atlantic Oscillation (NAO). While daily observations of the NAO date back to 1850, estimates of SSW event frequency only go back to the 1950s. This study explores to what extent the frequency of SSW events can be estimated from surface NAO variability as a first step toward reconstructing SSW frequency before the 1950s. It is found that characteristic effects in the surface NAO that typically occur after SSW events are only associated with SSW events in about a quarter of all cases. Nevertheless, an index can be generated entirely based on surface information that can to a good extent reproduce the SSW frequency for the period for which information for the stratosphere is available. Extending the index back to 1850 indicates that the frequency of SSW events exhibited decadal variability, as observed in the second half of the twentieth century. It is also found that the early 1990s, a period where very few SSW events were observed, was likely the longest absence of SSW events since 1850. Key Points: Two thirds of SSW events are followed by a persistent negative NAO and/or a switch to a negative NAO at the surfaceWinter surface NAO anomalies commonly observed after SSW events are preceded by a SSW in less than 25% of all casesThe "SSW drought" in the 1990s coincides with the longest absence of NAO events since 1850 [ABSTRACT FROM AUTHOR]

An unprecedented disruption of the quasi-biennial oscillation (QBO) started to develop from late 2015. The early development of this event is analyzed using the space–time spectra of eddies from reanalysis data. While the extratropical waves propagating horizontally into the tropics were assumed to be the main driver for the disruption, it was not clear why these waves dissipated near the jet core instead of near the jet edge as linear theory predicts. This study shows that the drastic deceleration of the equatorial jet was largely brought about by a single strong wave packet, and the local winds experienced by the wave packet served as a better indicator of the wave breaking latitude than the zonal mean winds. Surprisingly, tropical mixed Rossby–gravity waves also made an appreciable contribution to the deceleration of the equatorial westerly jet by the horizontal eddy momentum fluxes, especially before January 2016. The horizontal eddy momentum fluxes associated with the tropical waves arise from the deformation of the wave structure when background westerlies increase with height. These horizontal eddy momentum anomalies from the tropical waves are commonly observed in the reanalysis data but are typically much weaker than those in the 2015/16 winter. The possibility exists that exceptionally strong equatorially trapped waves precondition the flow to disruption by an extratropical disturbance. [ABSTRACT FROM AUTHOR]

The article describes the development of the version of the SL-AV global semi-Lagrangian atmospheric model with high spatial resolution in the stratosphere. The new model version uses the vertical grid of 100 levels, grid spacing of 500 m in the layer between 100 and 10 hPa, and the upper lid at 0.04 hPa. The parameterization of the non-orographic gravity wave drag is implemented. Numerous modifications are introduced in the block for the numerical solution of dynamical equations to enhance model stability. The experiment on atmospheric dynamics modeling for 28 years is carried out. It is shown that the SL-AV model reproduces the main features of stratospheric circulation, such as the polar night stratospheric jet formation and sudden stratospheric warming. The quasi-biennial oscillation of equatorial wind is reproduced with realistic period and amplitude. [ABSTRACT FROM AUTHOR]

A momentum budget is examined in the stratosphere, mesosphere, and lower thermosphere using simulation data over ~11 years from a whole-atmosphere model in terms of the respective contributions of gravity waves (GWs), Rossby waves (RWs), and tides. The GW forcing is dominant in the mesosphere and lower thermosphere (MLT), as indicated in previous studies. However, RWs also cause strong westward forcing, described by Eliassen–Palm flux divergence (EPFD), in all seasons in the MLT and in the winter stratosphere. Despite the relatively coarse model resolution, resolved GWs with large amplitudes appear in the MLT. The EPFD associated with the resolved GWs is eastward (westward) in the summer (winter) hemisphere, similar to the parameterized GW forcing. A pair of positive and negative EPFDs are associated with the RWs and GWs in the MLT. These results suggest that the RWs and resolved GWs are generated in situ in the MLT. Previous studies suggested that a possible mechanism of RW generation in the MLT is the barotropic/baroclinic instability. This study revisits this possibility and examines causes of the instability from a potential vorticity (PV) viewpoint. The instability condition is characterized as the PV maximum at middle latitudes on an isentropic surface. Positive EPFD for RWs is distributed slightly poleward of the PV maximum. Because the EPFD equals the PV flux, this feature indicates that the RW radiation acts to reduce the PV maximum. The PV maximum is climatologically maintained in both the winter and summer mesospheres, which is caused by parameterized GW forcing. [ABSTRACT FROM AUTHOR]

Abstract: The interannual variability of tropical lower stratosphere ozone and its connections to sea surface temperatures in the equatorial Pacific are examined using a combination of chemistry climate model simulations, satellite observations, and reanalyses. The model simulations and observations show large differences in the magnitude of interannual variability in ozone between northern tropic (NT; EQ‐18° N) and southern tropic (EQ‐18° S) during boreal summer but small differences in winter. The interannual variability during boreal summer is highly correlated with summer sea surface temperatures in the eastern and central Pacific Ocean and El Niño–Southern Oscillation (ENSO) events. Larger variability in NT ozone is primarily due to meridional advection, connected to the changes in the onset date and strength of the Asian summer monsoon anticyclone. The Asian summer monsoon anticyclone forms earlier in a season and tends to be stronger during cold (La Niña) events leading to more isentropic transport of ozone from the extratropics into the NT, with the reverse for warm (El Niño) events. [ABSTRACT FROM AUTHOR]

The polar night jet (PNJ) is a strong stratospheric westerly circumpolar wind at around 65°N in winter, and the strength of the climatological PNJ is widely recognized to increase from October through late December. Remarkably, the climatological PNJ temporarily stops increasing during late November. We examined this "short break" in terms of the atmospheric dynamical balance and the climatological seasonal march. We found that it results from an increase in the upward propagation of climatological planetary waves from the troposphere to the stratosphere in late November, which coincides with a maximum of the climatological Eliassen--Palm (EP) flux convergence in the lower stratosphere. The upward propagation of planetary waves at 100 hPa, which is strongest over Siberia, is related to the climatological strengthening of the tropospheric trough over Siberia. We suggest that longitudinally asymmetric forcing by land--sea heating contrasts caused by their different heat capacities can account for the strengthening of the trough. [ABSTRACT FROM AUTHOR]

General circulation models display a wide range of future predicted changes in the Northern Hemisphere winter stratospheric polar vortex. The downward influence of this stratospheric uncertainty on the troposphere has previously been inferred from regression analyses across models and is thought to contribute to model spread in tropospheric circulation change. Here we complement such regression analyses with idealized experiments using one model where different changes in the zonal-mean stratospheric polar vortex are artificially imposed to mimic the extreme ends of polar vortex change simulated by models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The influence of the stratospheric vortex change on the tropospheric circulation in these experiments is quantitatively in agreement with the inferred downward influence from across-model regressions, indicating that such regressions depict a true downward influence of stratospheric vortex change on the troposphere below. With a relative weakening of the polar vortex comes a relative increase in Arctic sea level pressure (SLP), a decrease in zonal wind over the North Atlantic, drying over northern Europe, and wetting over southern Europe. The contribution of stratospheric vortex change to intermodel spread in these quantities is assessed in the CMIP5 models. The spread, as given by 4 times the across-model standard deviation, is reduced by roughly 10% on regressing out the contribution from stratospheric vortex change, while the difference between models on extreme ends of the distribution in terms of their stratospheric vortex change can reach up to 50% of the overall model spread for Arctic SLP and 20% of the overall spread in European precipitation. [ABSTRACT FROM AUTHOR]

Downward wave coupling (DWC) is an important process that characterizes the dynamical coupling between the stratosphere and troposphere via planetary wave reflection. A recent modeling study has indicated that natural forcing factors, including sea surface temperature (SST) variability and the quasi-biennial oscillation (QBO), influenceDWCand the associated surface impact in the Northern Hemisphere (NH). In light of this, the authors further investigate how DWC in the NH is affected by anthropogenic forcings, using a fully coupled chemistry--climate model CESM1(WACCM). The results indicate that the occurrence of DWC is significantly suppressed in the future, starting later in the seasonal cycle, with more events concentrated in late winter (February and March). The future decrease in DWC events is associated with enhanced wave absorption in the stratosphere due to increased greenhouse gases (GHGs), which is manifest as more absorbing types of stratospheric sudden warmings (SSWs) in early winter. This early winter condition leads to a delay in the development of the upper-stratospheric reflecting surface, resulting in a shift in the seasonal cycle of DWC toward late winter in the future. The tropospheric responses to DWC events in the future exhibit different spatial patterns, compared to those of the past. In the North Atlantic sector, DWC-induced circulation changes are characterized by a poleward shift and an eastward extension of the tropospheric jet, while in the North Pacific sector, the circulation changes are characterized by a weakening of the tropospheric jet. These responses are consistentwith a change in the pattern ofDWC-induced synoptic-scale eddy--mean flow interaction in the future. [ABSTRACT FROM AUTHOR]

Using reanalysis datasets and numerical simulations, the relationship between the stratospheric Arctic vortex (SAV) and the Pacific decadal oscillation (PDO) on decadal time scales was investigated. A significant in-phase relationship between the PDO and SAV on decadal time scales during 1950-2014 is found, that is, the North Pacific sea surface temperature (SST) cooling (warming) associated with the positive (negative) PDO phases is closely related to the strengthening (weakening) of the SAV. This decadal relationship between the North Pacific SST and SAV is different from their relationship on subdecadal time scales. Observational and modeling results both demonstrate that the decadal variation in the SAV is strongly affected by the North Pacific SSTs related to the PDO via modifying the upward propagation of planetary wavenumber-1 waves from the troposphere to the stratosphere. The decreased SSTs over the North Pacific tend to result in a deepened Aleutian low along with a strengthened jet stream over the North Pacific, which excites a weakened western Pacific pattern and a strengthened Pacific-North American pattern. These tropospheric circulation anomalies are in accordance with the decreased refractive index (RI) at middle and high latitudes in the northern stratosphere during the positive PDO phase. The increased RI at high latitudes in the upper troposphere impedes the planetary wavenumber-1 wave from propagating into the stratosphere, and in turn strengthens the SAV. The responses of the RI to the PDO are mainly contributions of the changes in the meridional gradient of the zonal-mean potential vorticity via alteration of the baroclinic term - f²2/ρ0(ρ0...z/N²)z. [ABSTRACT FROM AUTHOR]

The role of parameterized nonorographic gravity wave drag (NOGWD) and its seasonal interaction with the resolved wave drag in the stratosphere has been extensively studied in low-resolution (coarser than 1.9° × 2.5°) climate models but is comparatively unexplored in higher-resolution models. Using the European Centre for Medium-Range Weather Forecasts Integrated Forecast System at 0.7° × 0.7° resolution, the wave drivers of the Brewer–Dobson circulation are diagnosed and the circulation sensitivity to the NOGW launch flux is explored. NOGWs are found to account for nearly 20% of the lower-stratospheric Southern Hemisphere (SH) polar cap downwelling and for less than 10% of the lower-stratospheric tropical upwelling and Northern Hemisphere (NH) polar cap downwelling. Despite these relatively small numbers, there are complex interactions between NOGWD and resolved wave drag, in both polar regions. Seasonal cycle analysis reveals a temporal offset in the resolved and parameterized wave interaction: the NOGWD response to altered source fluxes is largest in midwinter, while the resolved wave response is largest in the late winter and spring. This temporal offset is especially prominent in the SH. The impact of NOGWD on sudden stratospheric warming (SSW) life cycles and the final warming date in the SH is also investigated. An increase in NOGWD leads to an increase in SSW frequency, reduction in amplitude and persistence, and an earlier recovery of the stratopause following an SSW event. The SH final warming date is also brought forward when NOGWD is increased. Thus, NOGWD is still found to be a very important parameterization for stratospheric dynamics even in a high-resolution atmospheric model. [ABSTRACT FROM AUTHOR]

Climate indicators related to Long Island Sound (LIS) water and air temperature variability were investigated. The Pacific decadal oscillation (PDO) and east Pacific/North Pacific (EP/NP) patterns are found to be strongly correlated with LIS air temperature anomalies during most seasons, especially during the winter. Additionally, the winter EP/NP index is strongly correlated with subsequent spring and summer LIS water temperature anomalies, potentially rendering the EP/NP index useful in extended LIS water temperature outlooks. Such lagged relationships are found to be related largely to the decorrelation time scale of seasonal water temperature anomalies. The atmospheric circulation pattern associated with anomalous LIS water temperature conditions is consistent with atmospheric Rossby wave trains emanating from the western equatorial Pacific. The EP/NP index has a characteristic time scale of approximately 5 to 10 years and such fluctuations are termed the quasi-decadal mode, the mode identified as varying coherently with LIS air and water temperature anomalies. Apparent PDO and EP/NP regime shifts in 1997 are found to coincide with a LIS water temperature regime shift. This result suggests that not all LIS warming experienced during recent decades is solely due to anthropogenic causes but rather is to some extent a result of natural variability. The results from this study provide a useful framework for both seasonal and decadal prediction of LIS water temperature variability. [ABSTRACT FROM AUTHOR]

The skill of the Arctic stratospheric retrospective ensemble forecasts (hindcasts) of the European Centre for Medium-Range Weather Forecasts extended-range system is analyzed with a focus on the predictability of the major sudden stratospheric warmings (SSWs) during the period 1993-2016. Thirteen SSWs took place during this period. It is found that forecasts initialized 10-15 days before the SSWs show worse skill in the stratosphere than forecasts initialized during normal conditions in terms of root-mean-square errors but not in terms of anomaly correlation. Using the spread of ensemble members to estimate forecasted SSW probability, it is shown that some SSWs can be predicted with high (.0.9) probability at lead times of 12-13 days if a difference of 3 days between actual and forecasted SSW is allowed. Focusing on SSWs with significant impacts on the tropospheric circulation, on average, the forecasted SSW probability is found to increase from nearly 0 at 1-month lead time to 0.3 at day 13 before SSW, and then rapidly increases to nearly 1 at day 7. The period between days 8 and 12 is when most of the SSWs are predicted, with a probability of 0.5-0.9, which is considerably larger than the observed SSW occurrence frequency. Therefore, this period can be thought of as an estimate of the SSW predictability limit in this system. Indications that the predictability limit for some SSWs may be longer than 2 weeks are also found; however, this result is inconclusive and more studies are needed to understand when and why such long predictability is possible. [ABSTRACT FROM AUTHOR]

A configuration of an idealized general circulation model has been obtained in which a deep, stratospheric, equatorial, westerly jet is established that is spontaneously and quasi-periodically disrupted by shallow easterly jets. Similar to the disruption of the quasi-biennial oscillation (QBO) observed in early 2016, meridional fluxes of wave activity are found to play a central role. The possible relevance of two feedback mechanisms to these disruptions is considered. The first involves the secondary circulation produced in the shear zones on the upper and lower flanks of the easterly jet. This is found to play a role in maintaining the aspect ratio of the emerging easterly jet. The second involves the organization of the eddy fluxes by the mean flow: the presence of a weak easterly anomaly within a tall, tropical, westerly jet is demonstrated to produce enhanced and highly focused wave activity fluxes that reinforce and strengthen the easterly anomalies. The eddies appear to be organized by the formation of strong potential vorticity gradients on the subtropical flanks of the easterly anomaly. Similar wave activity and potential vorticity structures are found in the ERA-Interim for the observed QBO disruption, indicating this second feedback was active then. [ABSTRACT FROM AUTHOR]

The impact of snow cover in western and central China during late autumn on wintertime blocking occurrence is investigated using reanalysis data. The study results show that wintertime atmospheric circulations affected by late autumn snow cover anomalies form favorable conditions for increased blocking frequency (BF), especially in the North Pacific and North Atlantic. Evidence is also presented that the stratosphere-troposphere interactions are the key mechanism of the lag response of wintertime North Pacific and North Atlantic BFs to the late autumn snow cover. That is, positive anomalous snow cover can induce a dipole anomaly in the geopotential height field over the lower stratosphere, due to the decrease of the 300-1000-hPa thickness and the concurrent variation between the East Asian plateau jet and the polar front jet. The associated positive geopotential height anomalies are located over northwestern Eurasia. Meanwhile, western and central China shows remarkably negative geopotential height anomalies. Also, the corresponding atmospheric circulation in the lower stratosphere increases the Eliassen-Palm flux that propagates into the stratosphere through the constructive interference between the forced and climatological waves. The upward wave activity fluxes collapse the polar vortex in the stratosphere, resulting in the downward propagation of the geopotential and wind anomalies from the stratosphere. Consequently, the decreased zonal wind speed in the upper layer of the blocking region forms conditions favorable for wintertime blocking. [ABSTRACT FROM AUTHOR]

The Whole Atmosphere Community Climate Model, version 4 (WACCM4), is used to investigate the influence of stratospheric conditions on the development of sudden stratospheric warmings (SSWs). To this end, targeted experiments are performed on selected modeled SSW events. Specifically, the model is reinitialized three weeks before a given SSW, relaxing the surface fluxes, winds, and temperature below 10 km to the corresponding fields from the free-running simulation. Hence, the tropospheric wave evolution is unaltered across the targeted experiments, but the stratosphere itself can evolve freely. The stratospheric zonal-mean state is then altered 21 days prior to the selected SSWs and rerun with an ensemble of different initial conditions. It is found that a given tropospheric evolution concomitant with the development of an SSW does not uniquely determine the occurrence of an event and that the stratospheric conditions are relevant to the subsequent evolution of the stratospheric flow toward an SSW, even for a fixed tropospheric evolution. It is also shown that interpreting the meridional heat flux at 100 hPa as a proxy of the tropospheric injection of wave activity into the stratosphere should be regarded with caution and that stratospheric dynamics critically influence the heat flux at that altitude. [ABSTRACT FROM AUTHOR]

This study examines the influence of the Caspian Sea–Hindu Kush Index (CasHKI) on local and synoptic meteorology as well as on dust emissions over southwest (SW) Asia by means of National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP-NCAR) re-analysed mean sea-level pressure (MSLP), geopotential height at 700 hPa and surface meridional wind, along with meteorological data at Zabol, east Iran and Meteosat/Infrared Difference Dust Index (IDDI) retrievals. The analysis focuses on the summer period (June to September) of 2000-2014 and the winter period (November to March) of 1963-2014. The CasHKI values are mostly controlled by the MSLP anomalies over the Caspian Sea (CS) domain, varying from approximately -25 to +35 hPa in winter and from approximately-10 to +14 hPa in summer, but without a clear annual pattern. The CasHKI values are classified into four modes for each month depending on their intensity. In the summer months, the high CasHKI mode is associated with enhanced MSLP over central Asia and deepening of the Indo-Pakistan thermal low associated with the Indian summer monsoon. At 700-hPa level, the high CasHKI mode shows an enhancement of the Arabian ridge, expanding it to the north over Iran and the CS, with a concurrent strengthening of the Indian trough, leading to intensification of northerly winds along east Iran. This results in significant increase in dust activity over SWAsia, which is also apparent in the winter months. Furthermore, the intensification of the northerly flow associated with the high CasHKI modes drops the temperature and increases the relative humidity over Zabol, especially during winter. The SPRINTARS-model simulations also show increased dust emissions and concentrations for the high CasHKI values, confirming that the CasHKI variations modulate the dust activity over SW Asia throughout the year. [ABSTRACT FROM AUTHOR]

Previous studies have explored the influence of atmospheric cloud radiative effects (ACRE) on the tropospheric circulation. Here the authors explore the influence of ACRE on the stratospheric circulation. The response of the stratospheric circulation to ACRE is assessed by comparing simulations run with and without ACRE. The stratospheric circulation response to ACRE is reproducible in a range of different GCMs and can be interpreted in the context of both a dynamically driven and a radiatively driven component. The dynamic component is linked to ACRE-induced changes in the vertical and meridional fluxes of wave activity. The ACRE-induced changes in the vertical flux of wave activity into the stratosphere are consistent with the ACRE-induced changes in tropospheric baroclinicity and thus the amplitude of midlatitude baroclinic eddies. They account for a strengthening of the Brewer-Dobson circulation, a cooling of the tropical lower stratosphere, a weakening and warming of the polar vortex, a reduction of static stability near the tropical tropopause transition layer, and a shortening of the time scale of extratropical stratospheric variability. The ACRE-induced changes in the equatorward flux of wave activity in the low-latitude stratosphere account for a strengthening of the zonal wind in the subtropical lower to midstratosphere. The radiative component is linked to ACRE-induced changes in the flux of longwave radiation into the lower stratosphere. The changes in radiative fluxes lead to a cooling of the extratropical lower stratosphere, changes in the static stability and cloud fraction near the extratropical tropopause, and a shortening of the time scales of extratropical stratospheric variability. The results highlight a previously overlooked pathway through which tropospheric climate influences the stratosphere. [ABSTRACT FROM AUTHOR]