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The fast decline of Arctic sea ice necessitates a stronger focus on understanding the Arctic sea ice predictability and developing advanced forecast methods for all seasons and for pan-Arctic and regional scales. In this study, the operational forecasting system combining an advanced eddy-permitting ocean–sea ice ensemble reanalysis ORAS5 and state-of-the-art seasonal model-based forecasting system SEAS5 is used to investigate effects of sea ice dynamics and thermodynamics on seasonal (growth-to-melt) Arctic sea ice predictability in 1993–2020. We demonstrate that thermodynamics (growth/melt) dominates the seasonal evolution of mean sea ice thickness at pan-Arctic and regional scales. The thermodynamics also dominates the seasonal predictability of sea ice thickness at pan-Arctic scale; however, at regional scales, the predictability is dominated by dynamics (advection), although the contribution from ice growth/melt remains perceptible. We show competing influences of sea ice dynamics and thermodynamics on the temporal change of ice thickness predictability from 1993–2006 to 2007–20. Over these decades, there was increasing predictability due to growth/melt, attributed to increased winter ocean heat flux in both Eurasian and Amerasian basins, and decreasing predictability due to advection. Our results demonstrate an increasing impact of advection on seasonal sea ice predictability as the region of interest becomes smaller, implying that correct modeling of sea ice drift is crucial for developing reliable regional sea ice predictions. This study delivers important information about sea ice predictability in the “new Arctic” conditions. It increases awareness regarding sea ice state and implementation of sea ice forecasts for various scientific and practical needs that depend on accurate seasonal sea ice forecasts.

The anticyclonic anomaly over the Ural Mountains, or the Ural high (UH), has recently received much attention as a factor related to weather anomalies across Eurasia. Here we studied how the UH affects the occurrence of cold wintertime episodes over eastern Europe and northern China. By employing three methods to identify the UH, we found that a method based on the sea level pressure anomaly captures a stronger cooling signal over eastern Europe and this method includes nonblocking cases associated with low-level anticyclones that do not affect the upper troposphere. However, with UH occurrence, a stronger cooling over northern China is detected by a method based on 500-hPa geopotential height fields. Cold events over eastern Europe typically occur when UH formation is associated with a Rossby wave breaking in the upper level. Our results show that the horizontal temperature advection plays an important role in formation of cold episodes in both eastern Europe and northern China. The advection is balanced by diabatic processes, which show an opposite sign to the temperature advection in both regions. Also adiabatic warming contributes to balancing the advection in northern China. We find that the exact location of the positive SLP anomaly during UH episodes is the most important factor controlling whether or not eastern Europe or northern China will experience a cold episode. If the positive SLP anomaly develops more northwest than usual, eastern Europe will experience a cold episode. When the anomaly moves eastward, northern China will be cold.

The number of sudden stratospheric warmings (SSWs) per year is affected by the phase of the El Niño–Southern Oscillation (ENSO), yet there are discrepancies between the observed and modelled relationship. We investigate how systematic model biases in atmospheric winds and temperatures may affect the ENSO–SSW connection. A two-step bias correction process is applied to the troposphere, stratosphere, or full atmosphere of an atmospheric general circulation model. ENSO-type sensitivity experiments are then performed by adding El Niño and La Niña sea surface temperature (SST) anomalies to the model's prescribed SSTs, to reveal the impact of differing climatologies on the ENSO–SSW teleconnection. The number of SSWs per year is overestimated in the control run, and this statistic is improved when biases are reduced in both the stratosphere and troposphere. The seasonal cycle of SSWs is also improved by the bias corrections. The composite SSW responses in the stratospheric zonal wind, geopotential height, and surface response are well represented in both the control and bias-corrected runs. The model response of SSWs to ENSO phase is more linear than in observations, in line with previous modelling studies, and this is not changed by the reduced biases. However, the ratio of wave 1 events to wave 2 events as well as the tendency to have more wave 1 events during El Niño years than La Niña years is improved in the bias-corrected runs.

Correctly capturing the teleconnection between the El Niño–Southern Oscillation (ENSO) and Europe is of importance for seasonal prediction. Here we investigate how systematic model biases may affect this teleconnection. A two-step bias correction process is applied to an atmospheric general circulation model to reduce errors in the climatology. The bias corrections are applied to the troposphere and stratosphere independently and jointly to produce a range of climates. ENSO-type sensitivity experiments are then performed to reveal the impact of differing climatologies on the ENSO–Europe teleconnections. The bias corrections do not affect the response of the tropical atmosphere or the Aleutian low to the strong ENSO anomalies imposed in our experiments. However, in El Niño experiments the anomalous upward wave flux and the response of the Northern Hemisphere polar vortex differ between the climatologies. We attribute this to a reduced sensitivity of the upward wave fluxes to the Aleutian low response in the bias correction experiments, where the reduced biases result in a deepened Aleutian low in the base state. Despite the differing responses of the polar vortex, the North Atlantic Oscillation (NAO) response is similar between the climatologies, implying that for strong ENSO events the stratospheric pathway may not be the dominant pathway for the ENSO–North Atlantic teleconnection.

In the beginning of February 2018 a rapid deceleration of the westerly circulation in the polar Northern Hemisphere stratosphere took place, and on 12 February the zonal-mean zonal wind at 60∘ N and 10 hPa reversed to easterly in a sudden stratospheric warming (SSW) event. We investigate the role of the tropospheric forcing in the occurrence of the SSW, its predictability and teleconnection with the Madden–Julian oscillation (MJO) by analysing the European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecast. The SSW was preceded by significant synoptic wave activity over the Pacific and Atlantic basins, which led to the upward propagation of wave packets and resulted in the amplification of a stratospheric wavenumber 2 planetary wave. The dynamical and statistical analyses indicate that the main tropospheric forcing resulted from an anticyclonic Rossby wave breaking, subsequent blocking and upward wave propagation in the Ural Mountains region, in agreement with some previous studies. The ensemble members which predicted the wind reversal also reasonably reproduced this chain of events, from the horizontal propagation of individual wave packets to upward wave-activity fluxes and the amplification of wavenumber 2. On the other hand, the ensemble members which failed to predict the wind reversal also failed to properly capture the blocking event in the key region of the Urals and the associated intensification of upward-propagating wave activity. Finally, a composite analysis suggests that teleconnections associated with the record-breaking MJO phase 6 observed in late January 2018 likely played a role in triggering this SSW event.

Over recent years there have been concomitant advances in the development of stratosphere-resolving numerical models, our understanding of stratosphere–troposphere interaction, and the extension of long-range forecasts to explicitly include the stratosphere. These advances are now allowing for new and improved capability in long-range prediction. We present an overview of this development and show how the inclusion of the stratosphere in forecast systems aids monthly, seasonal, and annual-to-decadal climate predictions and multidecadal projections. We end with an outlook towards the future and identify areas of improvement that could further benefit these rapidly evolving predictions., Atmospheric Chemistry and Physics, 22 (4), ISSN:1680-7375, ISSN:1680-7367

Projected changes in the Northern Hemisphere stratospheric polar vortex are analyzed using Climate Model Intercomparison Project Phase 6 experiments. Previous studies showed that projections of the wintertime zonally averaged polar vortex strength diverge widely between climate models with no agreement on the sign of change, and that this uncertainty contributes to the regional climate change uncertainty. Here, we show that there remains large uncertainty in the projected strength of the polar vortex in experiments with global warming levels ranging from moderate (SSP245 runs) to large (Abrupt-4xCO2 runs), and that the uncertainty maximizes in winter. Partitioning of the uncertainty in wintertime polar vortex strength projections reveals that, by the end of the 21st century, model uncertainty contributes half of the total uncertainty, with scenario uncertainty contributing only 10%. Regression analysis shows that up to 20% of the intermodel spread in projected precipitation over the Iberian Peninsula and northwestern US, and 20%–30% in near-surface temperature over western US and northern Eurasian, can be associated with the spread in vortex strength projections after accounting for global warming. While changes in the magnitude and sign of the zonally averaged vortex strength are uncertain, most models (>95%) predict an eastward shift of the vortex by 8°–20° degrees in longitude relative to its historical location with the magnitude of the shift increasing for larger global warming levels. There is less agreement across models on a latitudinal shift, whose direction and magnitude correlate with changes in the zonally averaged vortex strength so that vortex weakening/strengthening corresponds to a southward/poleward shift., Journal of Geophysical Research: Atmospheres, 127 (18), ISSN:0148-0227, ISSN:2169-897X

Many recent studies have confirmed that variability in the stratosphere is a significant source of surface sub-seasonal prediction skill during Northern Hemisphere winter. It may be beneficial, therefore, to think about times in which there might be windows-of-opportunity for skillful sub-seasonal predictions based on the initial or predicted state of the stratosphere. In this study, we propose a simple, minimal model that can be used to understand the impact of the stratosphere on tropospheric predictability. Our model purposefully excludes state dependent predictability in either the stratosphere or troposphere or in the coupling between the two. Model parameters are set up to broadly represent current sub-seasonal prediction systems by comparison with four dynamical models from the Sub-Seasonal to Seasonal Prediction Project database. The model can reproduce the increases in correlation skill in sub-sets of forecasts for weak and strong lower stratospheric polar vortex states over neutral states despite the lack of dependence of coupling or predictability on the stratospheric state. We demonstrate why different forecast skill diagnostics can give a very different impression of the relative skill in the three sub-sets. Forecasts with large stratospheric signals and low amounts of noise are demonstrated to also be windows-of-opportunity for skillful tropospheric forecasts, but we show that these windows can be obscured by the presence of unrelated tropospheric signals.

The number of sudden stratospheric warmings (SSWs) per year is affected by the phase of the El Niño–Southern Oscillation (ENSO), yet there are discrepancies between the observed and modeled relationship. We investigate how systematic model biases may affect the ENSO-SSW connection. A two-step bias-correction process is applied to the troposphere, stratosphere or full atmosphere of an atmospheric general circulation model. ENSO type sensitivity experiments are then performed to reveal the impact of differing climatologies on the ENSO–SSW teleconnection. The number of SSWs per year is overestimated in the control run, and this statistic is improved when stratospheric biases are reduced. The seasonal cycle of SSWs is also improved by the bias corrections. The composite SSW responses in the stratospheric zonal wind, geopotential height and surface response are well represented in both the control and bias corrected runs. The model response of SSWs to ENSO phase is more linear than in observations, in line with previous modelling studies, and this is not changed by the reduced biases. However, the trend of more wave-1 events during El Niño years than La Niña years is improved in the bias corrected runs.

Over recent years there have been parallel advances in the development of stratosphere resolving numerical models, our understanding of stratosphere-troposphere interaction and the extension of long-range forecasts to explicitly include the stratosphere. These advances are now allowing new and improved capability in long range prediction. We present an overview of this development and show how the inclusion of the stratosphere in forecast systems aids monthly, seasonal and decadal climate predictions. We end with an outlook towards the future of climate forecasts and identify areas for improvement that could further benefit these rapidly evolving predictions.

Variability in the circumpolar westerly winds of the Northern Hemisphere winter polar stratosphere-- the stratospheric polar vortex-- has a known downward influence on the extratropical surface climate on sub-seasonal timescales. On longer timescales, observed trends towards a weakening stratospheric polar vortex have been linked to cooling surface temperatures over Eurasia from 1990-2009. Here, we show that 10-40 year polar vortex weakening trends occur as often as strengthening trends in large-ensemble historical climate simulations, and that decadal variability in polar vortex trends is significantly linked to decadal variability in regional surface temperature trends across the Northern Hemisphere even as the climate warms. We find that 74% of ensemble members with cooling trends over Eurasia during an 1850-2099 climate simulation also exhibit a weakening polar vortex, while 70% of members with accelerated warming over Eurasia exhibit a strengthening polar vortex. Decadal variability in the polar vortex thus modulates extratropical anthropogenically-forced warming trends.

In recent decades, the warming in the Arctic has been much faster than in the rest of the world, a phenomenon known as Arctic amplification (AA). Numerous studies report that Arctic is warming either twice, more than twice, or even three times as fast as the globe on average. However, the lack of consensus of AA definition precludes its precise quantification. Here we show, by using several observational datasets which cover the Arctic region and adopting a simple definition of AA, that during the last 40 years the Arctic has been warming almost four times faster than the globe as a whole, which is a higher ratio than generally reported in literature. Furthermore, we compared the observed AA ratio to the ratio simulated by state-of-the-art climate models, and show that the models largely underestimate the present AA, a finding that is not very sensitive to the exact definition of AA. The underestimation of AA by climate models most likely results from their inability to realistically simulate feedback mechanisms between sea ice melt and atmospheric temperatures. Our results imply that the underestimated AA leads to biased projections of climate change both in the Arctic and mid-latitudes.

The weather-dependent planning and decision-making benefit greatly from subseasonal to seasonal (S2S) weather predictions made for up to six weeks ahead. At this timescale anomalies in the extratropical stratospheric circulation, which can last for several weeks in the Northern Hemisphere during winter, are known to affect climate at the surface and can extend the predictability of tropospheric weather conditions. The downward influence of the stratospheric circulation anomalies on the troposphere is projected by the Northern Annular Mode (NAM). Because of the long persistence of stratospheric anomalies beyond typical weather timescale, the increase in forecast skill is possible for the regions influenced by the atmospheric circulation variability associated with NAM based on the stratospheric predictor.In this study, we investigate the predictability of the Eurasian severe and persistent cold spells during winter and its dependence on the state of the stratosphere. We first detected the below-normal surface temperature events over northern Eurasia (cold spells) in the ERA5 re-analysis. Then, to assess the predictability of these cold spells and to evaluate the skill in the probabilistic re-forecasts we divided them into groups associated with different stratospheric circulation anomalies which took place prior to the below-normal temperature events. When the stratospheric vortex is strong it is not expected to favor cold air outbreaks in this region. Therefore, in these cases, the cold air outbreaks result from internal tropospheric dynamics and their predictability is limited by the chaotic behavior of the weather systems. On the other hand, the weakening of the vortex is characterized by a more negative NAM index. This weakening is often followed by an equatorward shift of the tropospheric jets, an increase in the frequency of occurrence of tropospheric blocking, and cold air outbreaks over northern Eurasia. In these cases, the stratospheric vortex weakening can lead to the statistically significant improvement of the skill of cold air outbreak forecasts in case if the forecast model is capable of properly representing the coupling between the stratosphere and the troposphere. We show that the predictability of cold spells in the European Centre for Medium-range Weather Forecasts (ECMWF) model is enhanced under weak vortex conditions starting from week 3 before the event. We also evaluate how the surface predictability is affected by model imperfections by comparing the predictability across different S2S models.

A statistical learning approach to produce seasonal temperature forecasts in western Europe and Scandinavia was implemented and tested. The leading principal components (PCs) of sea surface temperature (SST) and the geopotential at the 150-hPa level (GPT) were derived from reanalysis datasets and used at different lags (from one to five seasons) as predictors. Random sampling of both the fitting years and the potential predictors together with the Least Absolute Shrinkage and Selection Operator regression (LASSO) was used to create a large ensemble of statistical models. Applying the models to independent test years shows that the ensemble performs well over the target areas and that the ensemble mean is more accurate than the best individual ensemble member on average. Skillful results were especially found for summer and fall, with the anomaly correlation coefficient values ranging between 0.41 and 0.68 for these seasons. The correct simulation of decadal trends, using sufficiently long time series for fitting (70 years), and the use of lagged predictors increased the prediction skill. The decadal-scale variability of SST, most importantly the Atlantic multidecadal oscillation (AMO), and different PCs of GPT are the most important individual predictors among all predictors. Both SST and GPT bring equally much predictive power, although their importance is different in different seasons.

Correctly capturing the teleconnection between the ENSO and Europe is of importance for seasonal prediction. Here we investigate how systematic model biases may affect this teleconnection. A two–step bias–correction process is applied to an atmospheric general circulation model to reduce errors in the climatology. The bias–corrections are applied to the troposphere and stratosphere separately and together to produce a range of climates. ENSO type sensitivity experiments are then performed to reveal the impact of differing climatologies on ENSO–Europe teleconnections.The bias–corrections do not affect the response of the tropical atmosphere, nor the Aleutian Low, to strong ENSO anomalies. However, the anomalous upward wave flux and the response of the northern hemisphere polar vortex differs between the climatologies. We attribute this to a reduced sensitivity of waves to the strength of the Aleutian Low. Despite the differing responses of the polar vortex, the NAO response is similar between the climatologies, implying that for strong ENSO events the stratospheric response may not be the primary driver for the ENSO–North Atlantic teleconnection.

We investigate the effect of systematic model biases on teleconnections influencing the Northern Hemisphere wintertime circulation. We perform a two-step nudging and bias-correcting scheme for the dynamic variables of the ECHAM6 atmospheric model to reduce errors in the model climatology relative to ERA-Interim. One result is a significant increase in the strength of the Northern Hemisphere wintertime stratospheric polar vortex, reducing errors in the December–February mean zonal stratospheric winds by up to 75%. The bias corrections are applied to the full atmosphere or the stratosphere only. We compare the response of the bias-corrected and control runs to an increase in Siberian snow cover in October—a surface forcing that, in our experiments, weakens the stratospheric polar vortex from October to December. We find that despite large differences in the vortex strength the magnitude of the stratospheric weakening is similar among the different climatologies, with some differences in the timing and length of the response. Differences are more pronounced in the stratosphere–troposphere coupling, and the subsequent surface response. The snow forcing with the stratosphere-only bias corrections results in a stratospheric response that is comparable to control, yet with an enhanced surface response that extends into early January. The full-atmosphere bias correction’s snow response also has a comparable stratospheric response but a somewhat suppressed surface response. Despite these differences, our results show an overall small sensitivity of the Eurasian snow teleconnection to the background climatology.

Correctly capturing the teleconnection between the El Niño–Southern Oscillation (ENSO) and Europe is of importance for seasonal prediction. Here we investigate how systematic model biases may affect this teleconnection. A two–step bias–correction process is applied to an atmospheric general circulation model to reduce errors in the climatology. The bias–corrections are applied to the troposphere and stratosphere separately and together to produce a range of climates. ENSO type sensitivity experiments are then performed to reveal the impact of differing climatologies on ENSO–Europe teleconnections. The bias–corrections do not affect the response of the tropical atmosphere, nor the Aleutian Low, to strong ENSO anomalies. However, the anomalous upward wave flux and the response of the northern hemisphere polar vortex differs between the climatologies. We attribute this to a reduced sensitivity of waves to the strength of the Aleutian Low. Despite the differing responses of the polar vortex, the NAO response is similar between the climatologies, implying that for strong ENSO events a stratospheric response may not be necessary for the ENSO–North Atlantic teleconnection.

Public attention has recently focused on high-impact extreme weather events in midlatitudes that originate in the sub-Arctic. We investigate movements of the stratospheric polar vortex (SPV) and related changes in lower atmospheric circulation during the February-March 2018 &ldquo, Beast from the East&rdquo, cold winter event that dramatically affected much of Europe and north-central North America. This study demonstrates that the movement of the SPV is a key linkage in late winter subarctic and northern midlatitude extreme weather events. February&ndash, March 2018 saw two types of subarctic-midlatitude weather connections. In the first type, the SPV was displaced from the pole to lower latitudes over North America in February and then was found over northern Siberia in March. Mid-February and mid-March are examples of persistent near vertically aligned geopotential height structures of the atmospheric circulation. These structures over North America and Eurasia advected cold Arctic air southward. The second type of cold surface event was associated with a weak regional SPV and a sudden stratospheric warming event over Europe during the second half of February. These late winter linkage events that arise through dynamic instabilities of the SPV are more common in the last decade, but the potential role of enhanced Arctic amplification is uncertain.

We investigate the effect of systematic model biases on teleconnections influencing the Northern Hemisphere wintertime circulation. We perform a two-step nudging and bias-correcting scheme for the dynamic variables of the ECHAM6 atmospheric model to reduce errors in the model climatology relative to ERA-Interim. The developed scheme is efficient in removing errors in model’s climatology. In particular, large negative bias in December-February mean zonal stratospheric winds is reduced by up to 75%, significantly increasing the strength of the Northern Hemisphere wintertime stratospheric polar vortex. The bias-corrections are applied to the full atmosphere or stratosphere only.We compare the response of bias-corrected and control runs to internal stratospheric variability and surface forcings that are important on seasonal timescales: Siberian snow cover in October; the Quasi-Biennial Oscillation (QBO); and ENSO. We find the bias-corrected model has the potential for a strengthened and more realistic response to the teleconnections, either in the stratospheric or surface response. In particular, the bias-corrected model has a strong QBO teleconnection which modulates the extratropical polar vortex and sea level pressure variability in a manner similar to that seen in observations. The Siberian snow forcing with the stratosphere-only bias-corrections also leads to an enhanced surface response relative to the control. The mechanism behind the sensitivity of the teleconnections to model biases is discussed.

The stratosphere can have a significant impact on winter surface weather on subseasonal to seasonal (S2S) timescales. This study evaluates the ability of current operational S2S prediction systems to capture two important links between the stratosphere and troposphere: (1) changes in probabilistic prediction skill in the extratropical stratosphere by precursors in the tropics and the extratropical troposphere and (2) changes in surface predictability in the extratropics after stratospheric weak and strong vortex events. Probabilistic skill exists for stratospheric events when including extratropical tropospheric precursors over the North Pacific and Eurasia, though only a limited set of models captures the Eurasian precursors. Tropical teleconnections such as the Madden‐Julian Oscillation, the Quasi‐Biennial Oscillation, and El Niño–Southern Oscillation increase the probabilistic skill of the polar vortex strength, though these are only captured by a limited set of models. At the surface, predictability is increased over the United States, Russia, and the Middle East for weak vortex events, but not for Europe, and the change in predictability is smaller for strong vortex events for all prediction systems. Prediction systems with poorly resolved stratospheric processes represent this skill to a lesser degree. Altogether, the analyses indicate that correctly simulating stratospheric variability and stratosphere‐troposphere dynamical coupling are critical elements for skillful S2S wintertime predictions. ISSN:0148-0227 ISSN:2169-897X

Observations taken over the last few decades indicate that dramatic changes are occurring in the ArcticBoreal Zone (ABZ), which are having significant impacts on ABZ inhabitants, infrastructure, flora and fauna, and economies. While suitable for detecting overall change, the current capability is inadequate for systematic monitoring and for improving processbased and largescale understanding of the integrated components of the ABZ, which includes the cryosphere, biosphere, hydrosphere, and atmosphere. Such knowledge will lead to improvements in Earth system models, enabling more accurate prediction of future changes and development of informed adaptation and mitigation strategies. In Duncan et al. (2020), we review the strengths and limitations of current spacebased observational capabilities for several important ABZ components and make recommendations for improving upon these current capabilities. We recommend an interdisciplinary and stepwise approach to develop a comprehensive ABZ Observing Network (ABZON), beginning with an initial focus on observing networks designed to gain processbased understanding for individual ABZ components and systems that can then serve as the building blocks for a comprehensive ABZON.

The scales of dynamical processes in the Arctic atmosphere range from turbulence in the atmospheric boundary layer (ABL) via interactive mesoscale processes, such as orographic flows and Polar lows, to synoptic-scale cyclones, and further to hemispherical-scale circulations characterized by the Polar front jet stream and planetary waves. Specific boundary conditions for tropospheric dynamics in the Arctic include (a) sea ice and snow, which strongly affect the surface energy budget, (b) large transports of heat and moisture from lower-latitudes, and (c) the wintertime stratospheric Polar vortex, which has a large impact on tropospheric large-scale circulation and synoptic-scale cyclones. Knowledge on dynamics of the Arctic atmosphere is advancing but, compared to mid- and low-latitudes, still limited due to lack of process-level observations from the Arctic. The dynamics of the Arctic atmosphere poses a challenge for numerical weather prediction (NWP) and climate models, in particular in the case of ABL, orographic flows, Polar lows, and troposphere-stratosphere coupling. More research is also needed to better understand how the atmospheric dynamics affects and is affected by climate warming.

Stratospheric water vapour influences the chemical ozone loss in the polar stratosphere via control of the polar stratospheric cloud formation. The amount of water vapour entering the stratosphere through the tropical tropopause differs substantially between simulations from chemistry–climate models (CCMs). This is because the present-day models, e.g. CCMs, have difficulties in capturing the whole complexity of processes that control the water transport across the tropopause. As a result there are large differences in the stratospheric water vapour between the models. In this study we investigate the sensitivity of simulated Arctic ozone loss to the simulated amount of water vapour that enters the stratosphere through the tropical tropopause. We used a chemical transport model, FinROSE-CTM, forced by ERA-Interim meteorology. The water vapour concentration in the tropical tropopause was varied between 0.5 and 1.6 times the concentration in ERA-Interim, which is similar to the range seen in chemistry–climate models. The water vapour changes in the tropical tropopause led to about 1.5 ppmv less and 2 ppmv more water vapour in the Arctic polar vortex compared to the ERA-Interim, respectively. The change induced in the water vapour concentration in the tropical tropopause region was seen as a nearly one-to-one change in the Arctic polar vortex. We found that the impact of water vapour changes on ozone loss in the Arctic polar vortex depends on the meteorological conditions. The strongest effect was in intermediately cold stratospheric winters, such as the winter of 2013/2014, when added water vapour resulted in 2 %–7 % more ozone loss due to the additional formation of polar stratospheric clouds (PSCs) and associated chlorine activation on their surface, leading to ozone loss. The effect was less pronounced in cold winters such as the 2010/2011 winter because cold conditions persisted long enough for a nearly complete chlorine activation, even in simulations with prescribed stratospheric water vapour amount corresponding to the observed values. In this case addition of water vapour to the stratosphere led to increased areas of ICE PSCs but it did not increase the chlorine activation and ozone destruction significantly. In the warm winter of 2012/2013 the impact of water vapour concentration on ozone loss was small because the ozone loss was mainly NOx-induced. The results show that the simulated water vapour concentration in the tropical tropopause has a significant impact on the Arctic ozone loss and therefore needs to be well simulated in order to improve future projections of the recovery of the ozone layer.

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.

Major sudden stratospheric warmings (SSWs) are striking phenomena of wintertime stratospheric circulation usually defined as a reversal of zonal mean circulation from westerlies to easterlies. SSWs often have significant impact on tropospheric circulation and cause anomalies in surface climate lasting for up to 2 months. For this reason, dynamics and predictability of SSW receive considerable attention. It is however well-known that not all SSWs cause significant, long-lasting impact on the troposphere. In order to explain differences in tropospheric impacts following SSWs, several reasons have been previously proposed, including differences in type of SSW (split or displacement), persistence of stratospheric anomalies, preconditioning of the tropospheric circulation, and whether or not SSW was accompanied by a planetary wave reflection in the stratosphere. Here we address the predictability of tropospheric impacts by SSWs by seeking early precursors of the impacts. We separate midwinter SSWs into two groups: those which are followed by significant, long-lasting impacts on the tropospheric circulation (defined in terms of anomalous Northern Annular Mode) and those not followed by significant anomalies in the annular mode. We show that SSWs characterised by a more negative Northern Annular Mode index in the lower stratosphere around 150 hPa and enhanced wave activity propagation to the stratosphere during the first few days following the central date have a larger probability of being followed by tropospheric impact, both in reanalyses and in climate model runs. These anomalies play a more important role in the subsequent downward propagation of the signal to the troposphere than the type of SSW: whether it is a split or a displacement, or absorptive or reflective SSW. We propose that using these anomalies as precursors of tropospheric impacts of SSW can enhance climate predictability.

We present a case study of long range forecasts for Northern Hemisphere winter 2015/2016. This winter produced the strongest El Nino event since 1997/1998 and equatorial Pacific sea surface temperature anomalies exceeded 3 °C. Other factors relevant to the Northern Hemisphere extratropical atmosphere circulation included a strong westerly phase of the Quasi-Biennial Oscillation (QBO) and very strong winds in the stratospheric polar night jet in early winter. At the surface, intense cyclonic extratropical circulation anomalies occurred in early winter in both the North Pacific and North Atlantic, consistent with known teleconnections to these phases of El Nino–Southern Oscillation, the QBO and the polar night jet. The midlatitude flow was very westerly in early winter and less westerly and sometimes northerly in late winter, when sudden stratospheric warming events also occurred. We show that initialised climate predictions were able to capture the winter mean flow pattern at seasonal lead times from well before the start of winter. In this special case, not only the winter mean flow pattern, but also some aspects of the sub-seasonal evolution were skilfully predicted. We show that the winter of 1982/1983 was closely analogous to winter 2015/2016 in both the predictable driving factors and the forecast winter circulation. This case study adds to the evidence that the north Atlantic circulation can be predictable on seasonal timescales and advance warning of the increased risk of intense rainfall and storminess which caused extreme flooding in the UK in December was possible in this case.

The Antarctic ozone hole shifted the jet stream in the Southern Hemisphere poleward, leading to hemisphere-wide climatic changes. But the Montreal Protocol, which banned ozone-depleting substances, has halted the shift. The Antarctic ozone hole shifted the jet stream in the Southern Hemisphere poleward, leading to hemisphere-wide climatic changes. But the Montreal Protocol, which banned ozone-depleting substances, has halted the shift.

The stratosphere has been identified as an important source of predictability for a range of processes on subseasonal to seasonal (S2S) time scales. Knowledge about S2S predictability within the stratosphere is however still limited. This study evaluates to what extent predictability in the extratropical stratosphere exists in hindcasts of operational prediction systems in the S2S database. The stratosphere is found to exhibit extended predictability as compared to the troposphere. Prediction systems with higher stratospheric skill tend to also exhibit higher skill in the troposphere. The analysis also includes an assessment of the predictability for stratospheric events, including early and midwinter sudden stratospheric warming events, strong vortex events, and extreme heat flux events for the Northern Hemisphere and final warming events for both hemispheres. Strong vortex events and final warming events exhibit higher levels of predictability as compared to sudden stratospheric warming events. In general, skill is limited to the deterministic range of 1 to 2 weeks. High‐top prediction systems overall exhibit higher stratospheric prediction skill as compared to their low‐top counterparts, pointing to the important role of stratospheric representation in S2S prediction models. ISSN:0148-0227 ISSN:2169-897X

We investigate factors influencing European winter (DJFM) air temperatures for the period 1979–2015 with the focus on changes during the recent period of rapid Arctic warming (1998–2015). We employ meteorological reanalyses analysed with a combination of correlation analysis, two pattern clustering techniques, and back‐trajectory airmass identification. In all five selected European regions, severe cold winter events lasting at least 4 days are significantly correlated with warm Arctic episodes. Relationships during opposite conditions of warm Europe/cold Arctic are also significant. Correlations have become consistently stronger since 1998. Large‐scale pattern analysis reveals that cold spells are associated with the negative phase of the North Atlantic Oscillation (NAO‐) and the positive phase of the Scandinavian (SCA+) pattern, which in turn are correlated with the divergence of dry‐static energy transport. Warm European extremes are associated with opposite phases of these patterns and the convergence of latent heat transport. Airmass trajectory analysis is consistent with these findings, as airmasses associated with extreme cold events typically originate over continents, while warm events tend to occur with prevailing maritime airmasses. Despite Arctic‐wide warming, significant cooling has occurred in northeastern Europe owing to a decrease in adiabatic subsidence heating in airmasses arriving from the southeast, along with increased occurrence of circulation patterns favouring low temperature advection. These dynamic effects dominated over the increased mean temperature of most circulation patterns. Lagged correlation analysis reveals that SCA‐ and NAO+ are typically preceded by cold Arctic anomalies during the previous 2–3 months, which may aid seasonal forecasting., Wintertime associations of warm weather in the Arctic and cold events in Europe have become stronger during the Arctic amplification of climate warming. Although winters have warmed in Europe, significant winter cooling has occurred in northeastern Europe in cases of airmasses originating from southeast. The positive phase of the North Atlantic Oscillation is typically preceded by cold Arctic anomalies during the previous 2–3 months, which may aid seasonal forecasting.

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.

This study evaluates the stratospheric water vapour distribution and variability in the Arctic. A FinROSE chemistry climate model simulation covering years 1990–2013 is compared to observations (satellite and frostpoint hygrometer soundings) and the sources of stratospheric water vapour are studied. According to observations and the simulations the water vapour concentration in the Arctic stratosphere started to increase after year 2006, but around 2011 the concentration started to decrease. Model calculations suggest that the increase in water vapour during 2006–2011 (at 56 hPa) is mostly explained by transport related processes, while the photochemically produced water vapour plays a relatively smaller role. The water vapour trend in the stratosphere may have contributed to increased ICE PSC occurrence. The increase of water vapour in the precense of the low winter temperatures in the Arctic stratosphere led to more frequent occurrence of ICE PSCs in the Arctic vortex. The polar vortex was unusually cold in early 2010 and allowed large scale formation of the polar stratospheric clouds. The cold pool in the stratosphere over the Northern polar latitudes was large and stable and a large scale persistent dehydration was observed. Polar stratospheric ice clouds and dehydration were observed at Sodankylä with accurate water vapour soundings in January and February 2010 during the LAPBIAT atmospheric sounding campaign. The observed changes in water vapour were reproduced by the model. Both the observed and simulated decrease of the water vapour in the dehydration layer was up to 1.5 ppm.

Using an international, multi-model suite of historical forecasts from the World Climate Research Programme (WCRP) Climate-system Historical Forecast Project (CHFP), we compare the seasonal prediction skill in boreal wintertime between models that resolve the stratosphere and its dynamics (“high-top”) and models that do not (“low-top”). We evaluate hindcasts that are initialized in November, and examine the model biases in the stratosphere and how they relate to boreal wintertime (Dec-Mar) seasonal forecast skill. We are unable to detect more skill in the high-top ensemble-mean than the low-top ensemble-mean in forecasting the wintertime North Atlantic Oscillation, but model performance varies widely. Increasing the ensemble size clearly increases the skill for a given model. We then examine two major processes involving stratosphere-troposphere interactions (the El Nino-Southern Oscillation/ENSO and the Quasi-biennial Oscillation/QBO) and how they relate to predictive skill on intra-seasonal to seasonal timescales, particularly over the North Atlantic and Eurasia regions. High-top models tend to have a more realistic stratospheric response to El Nino and the QBO compared to low-top models. Enhanced conditional wintertime skill over high-latitudes and the North Atlantic region during winters with El Nino conditions suggests a possible role for a stratospheric pathway.

Simulated stratospheric temperatures over the period 1979–2016 in models from the Chemistry-Climate Model Initiative are compared with recently updated and extended satellite data sets. The multimodel mean global temperature trends over 1979–2005 are -0.88 ± 0.23, -0.70 ± 0.16, and -0.50 ± 0.12 K/decade for the Stratospheric Sounding Unit (SSU) channels 3 (~40–50 km), 2 (~35–45 km), and 1 (~25–35 km), respectively (with 95% confidence intervals). These are within the uncertainty bounds of the observed temperature trends from two reprocessed SSU data sets. In the lower stratosphere, the multimodel mean trend in global temperature for the Microwave Sounding Unit channel 4 (~13–22 km) is -0.25 ± 0.12 K/decade over 1979–2005, consistent with observed estimates from three versions of this satellite record. The models and an extended satellite data set comprised of SSU with the Advanced Microwave Sounding Unit-A show weaker global stratospheric cooling over 1998–2016 compared to the period of intensive ozone depletion (1979–1997). This is due to the reduction in ozone-induced cooling from the slowdown of ozone trends and the onset of ozone recovery since the late 1990s. In summary, the results show much better consistency between simulated and satellite-observed stratospheric temperature trends than was reported by Thompson et al. (2012, https://doi.org/10.1038/nature11579) for the previous versions of the SSU record and chemistry-climate models. The improved agreement mainly comes from updates to the satellite records; the range of stratospheric temperature trends over 1979–2005 simulated in Chemistry-Climate Model Initiative models is comparable to the previous generation of chemistry-climate models.

Stratospheric water vapor plays a key role in radiative and chemical processes, it e.g. influences the chemical ozone loss via controlling the polar stratospheric cloud formation in the polar stratosphere. The amount of water entering the stratosphere through the tropical tropopause differs substantially between chemistry-climate models. This is because the present-day models have difficulties in capturing the whole complexity of processes that control the water transport across the tropopause. As a result there are large differences in the stratospheric water vapour between the models. In this study we investigate the sensitivity of simulated Arctic ozone loss to the amount of water, which enters the stratosphere through the tropical tropopause. We used a chemical transport model, FinROSE-CTM, forced by ERA-Interim meteorology. The water vapour concentration in the tropical tropopause was varied between 0.5 and 1.6 times the concentration in ERA-Interim, which is similar to the range seen in chemistry climate models. The water vapour changes in the tropical tropopause led to about 1.5 and 2 ppm more water vapour in the Arctic polar vortex compared to the ERA-Interim, respectively. We found that the impact of water vapour changes on ozone loss in the Arctic polar vortex depend on the meteorological conditions. Polar stratospheric clouds form in the cold conditions within the Arctic vortex, and chlorine activation on their surface lead to ozone loss. If the cold conditions persist long enough (e.g. in 2010/11), the chlorine activation is nearly complete. In this case addition of water vapour to the stratosphere increased the formation of ICE clouds, but did not increase the chlorine activation and ozone destruction significantly. In the warm winter 2012/13 the impact of water vapour concentration on ozone loss was small, because the ozone loss was mainly NOx induced. In intermediately cold conditions, e.g. 2013/14, the effect of added water vapour was more prominent than in the other studied winters. The results show that the simulated water vapour concentration in the tropical tropopause has a significant impact on the Arctic ozone loss and deserves attention in order to improve future projections of ozone layer recovery.

Anomalies in the extratropical stratospheric circulation are known to affect climate at the surface for up to 2 months during winter. Previously, several studies have proposed simple statistical models based on a stratospheric predictor to forecast surface climate; however, none of these studies considered a possibility, nor evaluated the skill, of categorical weather forecasts at individual sites. Here we evaluate the skill of the three-category (below-normal, near-normal and above-normal) monthly and two-monthly mean surface air temperature statistical forecasts, both deterministic and probabilistic, which use, as predictors, normalized mean polar cap geopotential height at 150 hPa and mean surface air temperature over the preceding 30-day period (persistence). We show that, during the period 1957–2013, at several sites influenced by the Northern Annular Mode variability, the monthly and two-monthly forecasts based on the stratospheric predictor possess a moderate skill in December–March at lead times of several days, but in many cases they only marginally improve forecasts based on persistence only. Nevertheless at some sites in northeastern Europe and northwestern Russia the improvements indicate that statistical climate forecasting at these sites can benefit from addition of stratospheric information. The results of this study further demonstrate usefulness of simple statistical models based on stratospheric predictors, both as a complementary source of forecasts, and as providing benchmarks for evaluating the skill of more complex dynamical systems.

The role of stationary planetary waves in the dynamical response of the Arctic winter stratosphere circulation to global warming is investigated here by analyzing simulations performed with atmosphere-only models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) driven by prescribed sea surface temperatures (SSTs). Climate models often simulate dynamical warming of the Arctic stratosphere as a response to global warming in association with a strengthening of the deep branch of the Brewer–Dobson circulation; however, until now, no satisfactory mechanism for such a response has been suggested. This study focuses on December–February (DJF) because this is the period when the troposphere and stratosphere are strongly coupled. When forced by increased SSTs, all the models analyzed here simulate Arctic stratosphere dynamical warming, mostly due to increased upward propagation of quasi-stationary wavenumber 1, as diagnosed by the meridional eddy heat flux. Further, it is shown that the stratospheric warming and increased wave flux to the stratosphere are related to the strengthening of the zonal winds in subtropics and midlatitudes near the tropopause. Evidence presented in this paper corroborate climate model simulations of future stratospheric changes and suggest a dynamical warming of the Arctic polar vortex as the most likely response to global warming.

Record Arctic ozone loss in spring 2011 occurred in concert with record positive values of the tropospheric Northern Annular Mode (NAM) index raising the question about the role of stratospheric driver on this tropospheric climate event. A set of 50 years long simulations by atmospheric general circulation model European Centre/Hamburg version 5 (ECHAM5) is carried out and the responses of the model to observed anomalies in stratospheric ozone (O3) and sea surface temperatures (SST) separately and also the response to combined SST and O3 forcing (ALL) are analyzed. In all three experiments the response is characterized by a strengthening of stratospheric polar vortex in March–April. In the ALL experiment, this strengthening is followed by a significant, long-lasting shift of the tropospheric circulation toward a positive NAM phase and increased probability of occurrence of extremely positive NAM events. The combined effect of the O3 and SST forcings on the stratospheric circulation differs from the sum of the individual O3 and SST responses, most likely due to nonlinear effects, leading to a colder stratosphere in February–March. In the troposphere, the sum of the individual responses is comparable in magnitude to the ALL response, but the individual responses are delayed with respect to that in ALL. In summary, these results suggest that both ozone-induced stratospheric cooling and tropospheric forcing associated with the SST anomalies contributed to the record tropospheric climate anomalies observed in spring 2011.

The leading mode of southern hemisphere (SH) climatic variability, the southern annular mode (SAM), has recently seen a shift towards its positive phase due to stratospheric ozone depletion and increasing greenhouse gas (GHG) concentrations. Here we examine how sensitive the SAM (defined as the leading empirical orthogonal function of SH sea level pressure anomalies) is to future GHG concentrations. We determine its likely evolution for three intergovernmental panel on climate change (IPCC) special report on emission scenarios (SRES) for austral summer and winter, using a multi-model ensemble of IPCC fourth assessment report models which resolve stratospheric ozone recovery. During the period of summer ozone recovery (2000–2050), the SAM index exhibits weakly negative, statistically insignificant trends due to stratospheric ozone recovery which offsets the positive forcing imposed by increasing GHG concentrations. Thereafter, positive SAM index trends occur with magnitudes that show sensitivity to the SRES scenario utilised, and thus future GHG emissions. Trends are determined to be strongest for SRES A2, followed by A1B and B1, respectively. The winter SAM maintains a similar dependency upon GHG as summer, but over the entire twenty-first century and to a greater extent. We also examine the influence of ozone recovery by comparing results to models that exclude stratospheric ozone recovery. Projections are shown to be statistically different from the aforementioned results, highlighting the importance of ozone recovery in governing SAM-evolution. We therefore demonstrate that the future SAM will depend both upon GHG emissions and stratospheric ozone recovery.

We study the response of the Southern Hemisphere circulation to the 1982 eruption of El Chichon and 1991 eruption of Pinatubo volcanoes in a suite of up-to-date coupled climate models. We find a significant response in austral spring and autumn in the years following the eruptions, which consists of a stronger stratospheric polar vortex and lowered sea-level pressure over the Antarctic, both consistent with the positive phase of the Southern Annular Mode. The seasonality of the response may be explained in terms of zonal flow-planetary wave interactions. This dynamical response is inconsistent with the observational reanalyses in the polar stratosphere in spring, but not in the troposphere where the internal variability is large compared to the magnitude of the response.

Previous studies have shown that simulated sea surface temperature (SST) responses to the southern annular mode (SAM) in phase 3 of the Coupled Model Intercomparison Project (CMIP3) climate models compare poorly to the observed response. The reasons behind these model inaccuracies are explored. The ocean mixed layer heat budget is examined in four of the CMIP3 models and by using observations–reanalyses. The SST response to the SAM is predominantly driven by sensible and latent heat flux and Ekman heat transport anomalies. The radiative heat fluxes play a lesser but nonnegligible role. Errors in the simulated SST responses are traced back to deficiencies in the atmospheric response to the SAM. The models exaggerate the surface wind response to the SAM leading to large unrealistic Ekman transport anomalies. During the positive phase of the SAM, this results in excessive simulated cooling in the 40°–65°S latitudes. Problems with the simulated wind stress responses, which relate partly to errors in the simulated winds themselves and partly to the transfer coefficients used in the models, are a key cause of the errors in the SST response. In the central Pacific sector (90°–150°W), errors arise because the simulated SAM is too zonally symmetric. Substantial errors in the net shortwave radiation are also found, resulting from a poor representation of the changes in cloud cover associated with the SAM. The problems in the simulated SST responses shown by this study are comparable to deficiencies previously identified in the CMIP3 multimodel mean. Therefore, it is likely that the deficiencies identified here are common to other climate models.

An improved stratospheric representation has been included in simulations with the Hadley Centre HadGEM1 coupled ocean atmosphere model with natural and anthropogenic forcings for the period 1979-2003. An improved stratospheric ozone dataset is employed that includes natural variations in ozone as well as the usual anthropogenic trends. In addition, in a second set of simulations the quasi biennial oscillation (QBO) of stratospheric equatorial zonal wind is also imposed using a relaxation towards ERA-40 zonal wind values. The resulting impact on tropospheric variability and trends is described. We show that the modelled cooling rate at the tropopause is enhanced by the improved ozone dataset and this improvement is even more marked when the QBO is also included. The same applies to warming trends in the upper tropical troposphere which are slightly reduced. Our stratospheric improvements produce a significant increase of internal variability but no change in the positive trend of annual mean global mean near-surface temperature. Warming rates are increased significantly over a large portion of the Arctic Ocean. The improved stratospheric representation, especially the QBO relaxation, causes a substantial reduction in near-surface temperature and precipitation response to the El Chichón eruption, especially in the tropical region. The winter increase in the phase of the northern annular mode observed in the aftermath of the two major recent volcanic eruptions is partly captured, especially after the El Chichón eruption. The positive trend in the southern annular mode (SAM) is increased and becomes statistically significant which demonstrates that the observed increase in the SAM is largely subject to internal variability in the stratosphere. The possible inclusion in simulations for future assessments of full ozone chemistry and a gravity wave scheme to internally generate a QBO is discussed. © Crown Copyright 2009.

The southern annular mode (SAM) has a well-established impact on climate in the Southern Hemisphere. The strongest response in surface air temperature (SAT) is observed in the Antarctic, but the SAM’s area of influence extends much farther, with statistically significant effects on temperature and precipitation being detected as far north as 20°S. Here the authors quantify the ability of the Coupled Model Intercomparison Project, phase 3 (CMIP3) coupled climate models to simulate the observed SAT, total precipitation, sea surface temperature (SST), and sea ice concentration responses to the SAM. The models are able to simulate the spatial pattern of response in SAT reasonably well; however, all models underestimate the magnitude of the response over Antarctica, both at the surface and in the free troposphere. This underestimation of the temperature response has implications for prediction of the future temperature changes associated with expected changes in the SAM. The models possess reasonable skill in simulating patterns of precipitation and SST response; however, some considerable regional deviations exist. The simulated precipitation and SST responses are less constrained by the observations than the SAT response, particularly in magnitude, as significant discrepancies are detected between the responses in the reference datasets. The largest problems are identified in simulating the sea ice response to the SAM, with some models even simulating a response that is negatively correlated with that observed.

The potential of the Arctic stratosphere to sustain the formation of Polar Stratospheric Clouds (PSCs) is a key factor in determining the amount of ozone destroyed each winter, and is often measured as a ‘PSC volume’. The latter quantity has been shown to closely follow a near-linear compact relationship with winter-averaged column ozone loss, and displays a high variability from monthly to decadal time-scales. We examine the connection between meteorological conditions in the troposphere and the variability of lower polar stratospheric temperatures over the last four decades, and specifically, conditions leading to a high PSC volume. In addition to the well-established connection between the North Atlantic Oscillation (NAO) and the polar vortex, we demonstrate the large influence of precursory disturbances over the North Pacific and the Far East, the region of maximum climatological upward wave activity flux. Namely, very high monthly PSC volume (in the top 12%) predominantly follows the development of positive tropospheric height anomalies over the Far East, which lead to a weakening of the background planetary wave trough, and lessened upward wave activity flux into the stratosphere. Precursory anomalies over the Far East are reminiscent of East Asian monsoon amplification episodes. Copyright © 2009 Royal Meteorological Society

Polar temperatures have been warming significantly over the past few decades. A comparison between observational temperature records and model simulations shows that temperature changes in both the Arctic and Antarctic regions can be attributed to human activity. The polar regions have long been expected to warm strongly as a result of anthropogenic climate change, because of the positive feedbacks associated with melting ice and snow1,2. Several studies have noted a rise in Arctic temperatures over recent decades2,3,4, but have not formally attributed the changes to human influence, owing to sparse observations and large natural variability5,6. Both warming and cooling trends have been observed in Antarctica7, which the Intergovernmental Panel on Climate Change Fourth Assessment Report concludes is the only continent where anthropogenic temperature changes have not been detected so far, possibly as a result of insufficient observational coverage8. Here we use an up-to-date gridded data set of land surface temperatures9,10 and simulations from four coupled climate models to assess the causes of the observed polar temperature changes. We find that the observed changes in Arctic and Antarctic temperatures are not consistent with internal climate variability or natural climate drivers alone, and are directly attributable to human influence. Our results demonstrate that human activities have already caused significant warming in both polar regions, with likely impacts on polar biology, indigenous communities2, ice-sheet mass balance and global sea level11.

Balloon-borne water vapour measurements during January and February 2004, which were obtained as part of the LAUTLOS campaign at Sodankylä, Finland, 67° N, were used to analyse the water vapour distribution in the wintertime Arctic lowermost stratosphere. A 2.5 km thick layer (or 30 K in the potential temperature scale) above the tropopause is characterized by a significant water vapour variability on a synoptic timescale with values between stratospheric and tropospheric, which is in good agreement with previously reported measurements. A cross-correlation analysis of ozone and water vapour confirms that this layer contains a mixture of stratospheric and tropospheric air masses. Some of the flights sampled laminae of enhanced water vapour above the tropopause. Meteorological analyses and backward trajectory calculations show that these features were related to filaments that had developed along the flanks of cut-off anticyclones, which had been active at this time over the Northern Atlantic. The role of the filaments was however not to transport water vapour from the troposphere to the stratosphere but rather to transport it within the stratosphere away from regions where intensive two-way stratosphere-troposphere exchange (STE) was identified. Intensive STE occurred around cut-off anticyclones in regions of strong winds, where calculations suggest the presence of clear-air turbulence (CAT). Evidences that CAT contributes to the troposphere-to-stratosphere transport (TST) are presented. However, statistically, relation between TST and CAT during the studied period is weak.