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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]

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]

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]

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]

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 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]

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]

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]

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]

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]

Reliability of GCM (general circulation model) simulations is a significant predictor selection criteria and seems to be essential for outcomes of the statistical downscaling. This article investigates the reliability of a wide ensemble of CMIP5 climate models in reproducing seasonal atmospheric circulation patterns over Europe and the North Atlantic, namely sea level pressure (SLP) and 500 hPa geopotential height (G500). GCMs were evaluated with respect to 1971-2000 NCEP/NCAR reanalysis data by means of MAE, correlation coefficient and standard deviation. An attempt was made to determine the range of biases introduced into the statistical models by biases in the GCM simulations. For this purpose, canonical correlation analysis (CCA) models linking large-scale atmospheric circulation and air temperature in Poland were driven by historical GCM simulations expressed as anomalies with reference to the reanalysis data. It was shown that reliability of GCMs varies considerably, both seasonally and on an inter-model basis, thus becoming unfavourable in the context of statistical downscaling application. Overall, GCM ability to reproduce G500 values is better than for SLP. The most important finding from the statistical downscaling perspective is that SLP and G500 biases may introduce considerable air temperature biases (T-GCMbiases) into the downscaled reconstructions or projections of climate change. Seasonally, the highest T-GCMbiases were found in DJF, when a significant overestimation of air temperature results from a GCM tendency to overestimate the meridional pressure gradient. A similar situation appears to be a significant factor in MAM and SON. Multiyear average GCM biases do not, however, explain a significant part of the inter-model variability of T-GCMbias, thus highlighting the need for a more in-depth evaluation. Nevertheless, good GCM performance seems to ensure that T-GCMbias is not of a considerable value. Overall, the results of the article emphasise the necessity for undertaking an application-oriented assessment of GCM reliability prior to any statistical downscaling approach. [ABSTRACT FROM AUTHOR]

The Southern Appalachian Mountains (SAM) are a heavily forested mid-latitude mountain region that provide an ideal location for assessing the suitability of satellite-derived snow maps and explicitly linking atmospheric circulation to the spatial patterns of new snowfall. Although a variety of synoptic-scale circulation regimes contribute to mean annual snowfall, which ranges from roughly 25 cm in the lowest valleys to over 250 cm at the highest elevations, atmospheric circulation processes have largely been absent from efforts seeking to quantify the spatial patterns of new snowfall. In this paper, we examine the suitability of fractional snow cover (FSC) maps from the Moderate Resolution Imaging Spectroradiometer (MODIS) and determine the spatial extent of snowfall according to synoptic-scale circulation. FSC maps are analysed after 122 snow events from 2006 to 2012 to provide a suitability analysis of MODIS products for use in the SAM. For each event, we calculate FSC distribution and total snow-covered area and compare it with available in situ observations. Results indicate that the SAM presents unique meteorological, physical, and spectral characteristics that are ideal for evaluating the suitability of MODIS for measuring snow cover. Out of 122 observed snow events, 63 are considered suitable for analysis with the FSC maps. The highest FSC values are observed after Gulf/Atlantic lows and south-eastward tracking clipper systems. We conclude that MODIS data can be successfully used to link broader atmospheric circulation processes of snowfall with the spatial patterns of snow cover. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

ABSTRACT This paper contributes to knowledge of the impact of global warming on viticulture, using major vineyard regions of New Zealand as a case study to illustrate regional disparities in climate change impacts resulting from downscale effects of larger scale atmospheric circulation. Recent trends in air temperature in New Zealand vineyard areas are investigated. Trends since 1941 in Marlborough show an increase in temperature range, with both rising maximum temperatures and declining minimum temperatures, but no change in the annual mean. More hot days and frosts are of concern to viticulturalists, and this trend not only differs from other major vineyard areas, but also occurs at other sites in eastern parts of the country. The relationship between changes in atmospheric circulation over New Zealand and observed trends in temperature and frost occurrence at Blenheim is investigated using the Kidson weather type classification for the period 1958-2010 and the Trenberth M1 index. The increased temperature range is expected to be associated with more frequent clear skies along the east coast of the country, resulting from changes in weather patterns (more frequent anticyclones, fewer low-pressure systems, increased zonal and southerly flow). These changes in weather patterns are shown to be closely linked to larger scale changes in atmospheric circulation via the Southern Annular Mode (SAM) and Southern Oscillation. The results show that significant regional variations in the impact of global warming can occur over areas of complex terrain such as New Zealand. Observed differences in local temperature and frost trends can be caused by the interaction of changing weather systems with mountainous terrain. These changing weather systems themselves are seen to be the result of major shifts in the larger scale atmospheric circulation. These results are important for assessing possible impacts on viticulture and in developing adaptation strategies for agriculture in response to predicted future climates. [ABSTRACT FROM AUTHOR]

ABSTRACT The ability of numerical models in reproducing the observed statistical links between the sea surface temperature of central and eastern tropical Pacific Ocean is analysed in this paper. Significant links in the whole spectrum of frequency were studied for models of the World Climate Research Program (WCRP-CMIP3/IPCC-AR4) using the methodology of wavelet coherence. During the second half of the twentieth century, in-phase or quasi-in-phase links in oscillations of 2-6 years, a lack of relation in oscillations of 8-12 years and time lagged links in the longest periodicities are detected in the observed dataset. Some of the analysed models are able to reproduce the observed relationships in oscillations shorter than 6 years and few of them also reproduce the lacks of significant relation in the band of 8-12 years. On the other hand, all models have serious problems representing the observed time lagged links in the longest waves. The analysis shows that climate models are still unable to reproduce specific features of tropical Pacific interactions with potential effect on the Southern Hemisphere atmospheric circulation and, in particular, the South American climatic variability. [ABSTRACT FROM AUTHOR]

Changes of the upper-tropospheric temperature teleconnection and the associated atmospheric circulations in spring (Mar-April-May, MAM) during the mid-Holocene are preliminarily examined by using the CCSM3 simulation data archived in the Paleoclimate Modeling Intercomparison Project (PMIP) database. The simulation result shows that in the mid-Holocene the atmospheric circulation over the Asian-North Pacific region showed the variability of the teleconnection pattern resembling the present Asian-Pacific oscillation (APO), which is characterized by a warming (cooling) upper troposphere over Asia corresponding to a cooling (warming) upper troposphere over the North Pacific and indicates a thermal contrast between Asia and the North Pacific. In comparison with the present climate, the mid-Holocene upper-tropospheric temperature in spring was modelled to be lower over Asia and higher over the North Pacific, indicative of a weaker spring APO. Associated with such a condition, both the upper-tropospheric high (low) and the lower-tropospheric low (high) over Asia (central-eastern Pacific) were weakened in the mid-Holocene. Accordingly, anomalous descending (ascending) motion appeared over East Asia (central-eastern Pacific). This circulation background is unfavourable (favourable) for the occurrence of precipitation, and thus the mid-Holocene precipitation decreases (increases) in this region as compared with the present. Hence, the change of the spring thermal contrast between Asia and the North Pacific played a crucial role in the anomalies of the atmospheric circulations and precipitation over the Asian-Pacific region during the mid-Holocene. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

This paper traces the history of the use of ships' logbooks in climatological studies from the earliest days of the 17th century to the present day. Although early theories concerning global air circulations by Halley and Hadley were based on information gathered by mariners and recorded in their logbooks, it has only been in the last two decades that interest has returned to this important, but long-overlooked, source of climatic information. Attention is drawn to the many advantages offered by logbooks, in particular the long period of time that they collectively cover, their near-global geographic range, the large number of such documents that have survived, and the degree of detailed and reliable record that they provide. Before the mid-19th century, much of the recorded information is noninstrumental in character but its scientific potential is reflected in the variety of approaches and methods that have been used in its analysis and in the equally wide range of outcomes, from databases to synoptic charts and long-time series indices that have emerged from the most recent research. Attention is also drawn to the benefits to be derived from old and more recent instrumental logbook data. As only a small percentage of the tens of thousands of logbooks that have been preserved have thus far been examined, the potential for yet further development is enormous. [ABSTRACT FROM AUTHOR]

The quasibiennial oscillation (QBO) dominates the variability of the equatorial stratosphere and also affects the circulation and temperature of the extratropical region. In this paper we review previous work showing that the mid- to high-latitude circulation is weaker (stronger) when QBO easterlies (westerlies) dominate in the low equatorial stratosphere. The accepted explanation for the extratropical QBO signature is based on the QBO modulation of upward propagating planetary Rossby waves. This mechanism is consistent with the strong seasonality observed in the extratropical QBO. The largest QBO signature in the northern extratropical stratosphere occurs during winter when the dominating westerly wind allows the penetration of planetary waves in the northern stratosphere. However, during the southern winter, planetary waves do not disrupt the southern stratospheric vortex and the largest QBO signature is found during the late spring (November). To further illustrate these mechanisms, we analyze the QBO signature on the mid- to high-latitude circulation of the stratosphere using data from the ERA-40 reanalysis. The extratropical signature in both hemispheres is evaluated as a function of the latitude–height structure of the zonal wind in the tropical region in order to determine how the extratropical response depends on the vertical phase structure of the tropical QBO. We also analyze the QBO impact on planetary wave activity in order to determine how this modulation can explain the observed extratropical QBO signal. [ABSTRACT FROM AUTHOR]

Investigates the temporal and spatial variability of the seasonal mean of maximum air temperature in Romania and its links with the large?scale atmospheric circulation. Significant increasing trends were found for winter and summer mean maximum temperature in Romania, with upward shifts around 1947 and 1985 respectively. During autumn, a decreasing trend with a downward shift around 1969 was detected. These changes seem to be real, since they are connected to similar changes in the large-scale circulation. The intensification of the southwesterly circulation over Europe since 1933, which overlapped with the enhancement of westerly circulation after the 1940s, could be the reason for the change in winter mean maximum temperature.

Analyzes the decadal variability of the Danube river flow in the lower basin and its connection with the North Atlantic Oscillation (NAO) for the period 1931?1995. Associated linkages with precipitation (PP) in the European sector, global sea surface temperature (SST) and atmospheric circulation for the period 1931?81, and the 500 hPa geopotential heights (G500) over the Northern Hemisphere for the period 1948?95 are also investigated. The results show that there is an out-of-phase relationship between the time series of the Danube river flow anomalies and the NAO index. The time series of a PP index presents a time evolution similar to that of the river flow anomalies. Correlation maps between the river flow anomalies and global SST show coherent large-scale patterns. Physically consistent sea level pressure and 500 hPa geopotential height are obtained.

Sudden stratospheric warmings (SSWs) are significant source of enhanced subseasonal predictability, but whether this source is untapped in operational models remains an open question. Here we report on the prediction of the SSW on 12 February 2018, its dynamical precursors, and surface climate impacts by an ensemble of dynamical forecast models. The ensemble forecast from 1 February predicted 3 times increased odds of an SSW compared to climatology, although the lead time for SSW prediction varied among individual models. Errors in the forecast location of a Ural high and underestimated magnitude of upward wave activity flux reduced SSW forecast skill. Although the SSW's downward influence was not well forecasted, the observed northern Eurasia cold anomaly following SSW was predicted, albeit with a weaker magnitude, due to persistent tropospheric anomalies. The ensemble forecast from 8 February predicted the SSW, its subsequent downward influence, and a long‐lasting cold anomaly at the surface. Plain Language Summary: Predicting climate anomalies more than 2 weeks ahead remains a challenging task. However, there are periods when climate is more predictable. In particular, it is known that in the wintertime Northern Hemisphere midlatitudes, climate may be more predictable during several weeks following sudden stratospheric warmings (SSWs), when a cold stratospheric polar vortex suddenly breaks and the stratosphere warms. In this article we evaluate quality of monthly forecasts by several up‐to‐date forecast systems during a sudden stratospheric warming observed in February 2018 focusing both on predictability of the stratospheric anomalies and on predictability of their surface impacts. We find that models predicted enhanced probability of sudden stratospheric warming at least 11 days before the observed event. Four days before the event it was forecast by all models with a very high certainty. We further show that successful prediction of the SSW required correct forecasting of anticyclonic anomaly over Ural and associated upward propagation of planetary‐scale atmospheric waves. The observed cold anomaly across Eurasia in February 2018 was predicted more than 2 weeks ahead although at this lead time the predictability was more likely associated with persistence of tropospheric anomalies in the forecast, with smaller downward influence of the stratospheric anomalies. Key Points: Subseasonal multimodel ensemble predicted increased odds of a large sudden stratospheric warming in February 2018 at a lead time of 11 daysExtended cold anomaly over Eurasia in February 2018 was predicted more than 2 weeks ahead, but its intensity varied among individual modelsAccurate forecasting of a Ural high and associated upward wave activity flux are essential for improved prediction of stratospheric warming [ABSTRACT FROM AUTHOR]