Major Stratospheric Warming Research Articles

Enhanced Polar Vortex Predictability Following Sudden Stratospheric Warming Events

AbstractSudden stratospheric warming (SSW) events can form a window of forecast opportunity for polar vortex predictions on subseasonal‐to‐seasonal time scales. Analyzing numerical ensemble simulations, we quantify the associated enhanced predictability due to reduced upward planetary wave fluxes during the mostly radiatively driven recovery phase following SSWs. Ensembles that predict an SSW show reduced ensemble spread in terms of polar vortex strength for several weeks to follow, as well as a corresponding reduction in forecast errors. This increased predictability is particularly pronounced for strong SSWs and even occurs if not all ensemble members predict a major SSW. Furthermore, we found a direct impact of the occurrence of SSWs on the date of the final warming (FW): the decrease in upward wave fluxes delays the FW significantly. The reduced spread after SSWs and the delay in FW date have potentially further implications for (subseasonal) predictions of the tropospheric and mesospheric circulations.

Direct Observational Evidence of Altered Mesosphere Lower Thermosphere Mean Circulation From a Major Sudden Stratospheric Warming

AbstractSudden stratospheric warmings (SSWs) are large‐scale phenomena characterized by dramatic dynamic disruptions in the stratospheric winter polar regions. Previous studies, especially those employing whole atmosphere models, indicate that SSWs have strong impacts on the circulation of the mesosphere lower thermosphere (MLT) and drive a reversal in the mean meridional circulation (MMC) near 90–125 km altitude. However, the robustness of these effects and the roles of SSW‐induced changes in global‐scale wave activity to drive the reversal have been difficult to observe simultaneously. This work employs horizontal lower thermospheric (∼93–106 km altitude) winds near 10°S‐40°N latitude from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer (ICON) to present observational evidence of a prominent MLT MMC reversal associated with the January 2021 major SSW event and to demonstrate connections to semidiurnal tidal activity and possible associations with a ∼3‐day ultra‐fast Kevin wave.

Influence of Major Stratospheric Sudden Warming on the Unprecedented Cold Wave in East Asia in January 2021

An unprecedented cold wave intruded into East Asia in early January 2021 and led to record-breaking or historical extreme low temperatures over vast regions. This study shows that a major stratospheric sudden warming (SSW) event at the beginning of January 2021 exerted an important influence on this cold wave. The major SSW event occurred on 2 January 2021 and subsequently led to the displacement of the stratospheric polar vortex to the East Asian side. Moreover, the SSW event induced the stratospheric warming signal to propagate downward to the mid-to-lower troposphere, which not only enhanced the blocking in the Urals–Siberia region and the negative phase of the Arctic Oscillation, but also shifted the tropospheric polar vortex off the pole. The displaced tropospheric polar vortex, Ural blocking, and another downstream blocking ridge over western North America formed a distinct inverted omega-shaped circulation pattern (IOCP) in the East Asia–North Pacific sector. This IOCP was the most direct and impactful atmospheric pattern causing the cold wave in East Asia. The IOCP triggered a meridional cell with an upward branch in East Asia and a downward branch in Siberia. The meridional cell intensified the Siberian high and low-level northerly winds, which also favored the invasion of the cold wave into East Asia. Hence, the SSW event and tropospheric circulations such as the IOCP, negative phase of Arctic Oscillation, Ural blocking, enhanced Siberian high, and eastward propagation of Rossby wave eventually induced the outbreak of an unprecedented cold wave in East Asia in early January 2021.

Inter‐Hemispheric Coupling During Sudden Stratospheric Warming Events With Elevated Stratopause

AbstractSudden stratospheric warmings (SSW) are large‐scale disruptions of the wintertime state of the stratosphere that can affect the circulation at synoptic and global scales, including altitudes up to the mesopause in both winter and summer hemispheres. In this study, the response of the summer mesosphere is analyzed during the SSW in the winter stratosphere. In particular, we focus on major SSW events where the climatological stratopause disappears and subsequently reforms at higher altitude, which we refer to as “extreme SSW” in this article. The summer mesosphere response to such extreme SSW events is analyzed in three different phases: (a) stratosphere warming phase, (b) stratopause discontinuity phase, and (c) stratopause reformation phase. Composites of anomalies with respect to climatology derived from the Microwave Limb Sounder and the extended version of the Whole Atmosphere Community Climate Model with specified dynamics are analyzed. The polar summer mesosphere cools during the stratospheric warming phase and warms in subsequent phases. A detailed lag‐correlation analysis shows strong negative correlation of −0.6 to −0.8 between the summer mesosphere and the winter stratosphere during the stratosphere warming phase, and a positive correlation of 0.4–0.6 in the phases thereafter. An attempt is made to explain the apparent drivers and dynamics responsible for these couplings, supported with evidence from observations and model output.

Formation of a Mesospheric Inversion Layer and the Subsequent Elevated Stratopause Associated With the Major Stratospheric Sudden Warming in 2018/19

AbstractSince 2004, following prolonged stratospheric sudden warming (SSW) events, it has been observed that the stratopause disappeared and reformed at a higher altitude, forming an elevated stratopause (ES). The relative roles of atmospheric waves in the mechanism of ES formation are still not fully understood. We performed a hindcast of the 2018/19 SSW event using a gravity‐wave (GW) permitting general circulation model that resolves the mesosphere and lower thermosphere (MLT) and analyzed dynamical phenomena throughout the entire middle atmosphere. An ES formed after the major warming on January 1, 2019. There was a marked temperature maximum in the polar upper mesosphere around December 28, 2018 prior to the disappearance of the descending stratopause associated with the SSW. This temperature structure is referred to as a mesospheric inversion layer (MIL). We show that adiabatic heating from the residual circulation driven by GW forcing (GWF) causes barotropic and/or baroclinic instability before the MIL formation, causing in situ generation of planetary waves (PWs). These PWs propagate into the MLT and exert negative (westward) forcing, which contributes to the MIL formation. Both GWF and PW forcing (PWF) above the recovered eastward jet play crucial roles in ES formation. The altitude of the recovered eastward jet, which regulates GWF and PWF height, is likely affected by the MIL structure. Simple vertical propagation from the lower atmosphere is insufficient to explain the presence of the GWs observed in this event.

Investigation of the relationship between Euro-Atlantic and West Asia blockings and major stratospheric sudden warmings in the period of 1959–2018

The main aim of this paper is to investigate and compare the possible connection between tropospheric blocking events and major stratospheric sudden warmings (SSWs) in the two periods of 1959−1988 and 1989−2018 to present a dynamical insight into the blocking formation and behaviors. After identifying and characterizing two types of SSWs including wavenumber-1 (W1) and wavenumber-2 (W2) in both the periods, the behaviors of blocking events coincided with major SSWs are examined and compared in the two periods. Then, the relationship between blockings and major SSWs is discussed applying the Empirical Orthogonal Function (EOF) method.In general, 18 and 16 major SSW events were identified in the first and second periods, respectively. The investigation of planetary wave activities indicates that the maximum anomalies of geopotential height amplitude and meridional eddy heat flux in the PRE-SSW phase of both types of SSWs, particularly major W2 warmings, in the second period occurred earlier than those in the first period. The peaks of blocking activities in the second period have also been moved eastward compared with the first period in both prior to and after the onset of SSWs. Moreover, the frequency of blocking event during the PRE-SSW phase of major W1 warmings in Euro-Atlantic and a large part of West Asia in the second period is less than that of the first period, while the occurrence of blocking during the PRE-SSW phase of major W2 warmings in Eastern Europe and West Asia has been increased in the second period. In the POST-SSW phase, blocking activity associated with major W2 warmings is enhanced in West Asia during the second period. In addition, the maximum blocking activities preceding major SSWs in the second period was occurred 5-days prior to the first period. The results of cross-correlation coefficients between blockings and SSWs show significant relationship between them with time lag of about 10-days prior to the onset of warmings in both the periods, especially in the recent years.

Improved Circulation in the Northern Hemisphere by Adjusting Gravity Wave Drag Parameterizations in Seasonal Experiments With ICON‐NWP

AbstractThe stratosphere is one of the main potential sources for subseasonal to seasonal predictability in midlatitudes in winter. The ability of an atmospheric model to realistically simulate the stratospheric dynamics is essential in order to move forward in the field of seasonal predictions in midlatitudes. Earlier studies with the ICOsahedral Nonhydrostatic atmospheric model (ICON) point out that stratospheric westerlies in ICON are underestimated. This is the first extensive study on the evaluation of Northern Hemisphere stratospheric winter circulation with ICON in numerical weather prediction (NWP) mode. Seasonal experiments with the default setup are able to reproduce the basic climatology of the stratospheric polar vortex. However, westerlies are too weak and major stratospheric warmings too frequent in ICON. Both a reduction of the nonorographic, and a reduction of the orographic gravity wave and wake drag lead to a strengthening of the stratospheric vortex and a bias reduction, in particular in January. However, the effect of the nonorographic gravity wave drag scheme on the stratosphere is stronger. Stratosphere‐troposphere coupling is intensified and more realistic due to a reduced gravity wave drag. Furthermore, an adjustment of the subgrid‐scale orographic drag parameterization leads to a significant error reduction in the mean sea level pressure. As a result of these findings, we present our current suggested improved setup for seasonal experiments with ICON‐NWP.

Radiative Relaxation Time Scales Quantified from Sudden Stratospheric Warmings

Abstract Sudden stratospheric warmings (SSWs) are impressive events that occur in the winter hemisphere’s polar stratosphere and are capable of producing temperature anomalies upward of +50 K within a matter of days. While much work has been dedicated toward determining how SSWs occur and their ability to interact with the underlying troposphere, one underexplored aspect is the role of radiation, especially during the recovery phase of SSWs. Using a radiative transfer model and a heating rate analysis for distinct layers of the stratosphere averaged over the 60°–90°N polar region, this paper accounts for the radiative contribution to the removal of the anomalous temperatures associated with SSWs. In total 17 events are investigated over the 1979–2016 period. This paper reveals that in the absence of dynamical heating following major SSWs, longwave radiative cooling dominates and often results in a strong negative temperature anomaly. The polar winter stratospheric temperature change driven by the radiative cooling is characterized by an exponential decay of temperature with an increasing e-folding time of 5.7 ± 2.0 to 14.6 ± 4.4 days from the upper to middle stratosphere. The variability of the radiative relaxation rates among the SSWs was determined to be most impacted by the initial temperature of the stratosphere and the combined dynamic and solar heating rates following the onset of the events. We also found that trace-gas anomalies have little impact on the radiative heating rates and the temperature evolution during the SSWs in the mid- to upper stratosphere.

Modulation by ENSO of the Relationship Between Stratospheric Sudden Warming and the Madden‐Julian Oscillation

AbstractThe ENSO modulation on the relationship between the Madden‐Julian Oscillation (MJO) and major stratospheric sudden warming (SSW) in the Northern Hemisphere winter is analyzed. The results indicate that the statistical relationship between the MJO and SSWs is robust during La Niña years but almost nonexistent during El Niño years. During La Niña years, a significant response of stratospheric temperature appears after certain MJO phases with a distribution consistent with MJO periodicity, whereas the response becomes nonsignificant during El Niño years. Further analysis indicates that ENSO influences the MJO‐related geopotential height anomalies in middle to high latitudes of the upper troposphere, which leads to an increase in wavenumber 2 in La Niña winters. And wavenumber 2 plays a key role in generating the strong heat flux anomalies in the stratosphere. The results may help to explain the more frequent occurrence of extreme events in the stratosphere, such as SSW during La Niña winters.

Low latitude dynamical response to vortex split sudden stratospheric warming: An Eliassen Palm Flux perspective

The present work extensively describes the tropical dynamical changes during the Vortex split Major Stratospheric Warming (V-MSW) event occurred in recent decades. For better visualization of high latitude influence of tropical atmosphere, in this study, we have utilized three successive major vortex split warming events occurred in winters 1984/85, 1988/89 and 2008/09. The realistic visualization of planetary wave activity and its tropical response can be achieved by using Eliassen –Palm flux as a diagnostic tool. The investigation reveals that the tropical weather shows significant changes in temperature, convection and wind circulation during the course of vortex split major stratospheric warming.

Dynamic Diagnosis of Stratospheric Sudden Warming Event in the Boreal Winter of 2018 and Its Possible Impact on Weather over North America

In the winter of 2018, a major stratospheric sudden warming (SSW) event occurred in the Northern Hemisphere. This study performs a dynamic diagnosis on this 2018 SSW event and analyzes its possible impact on the weather over North America. The result shows that the ridge over Alaska in the mid-troposphere and the trough over the northeastern North America are the prominent tropospheric precursory signals before the occurrence of this SSW event. The signals appear 10 days before the SSW, which greatly enhances the propagation of the planetary wavenumber 2 from the troposphere to the extratropical stratosphere. The collapse process of stratospheric polar vortex indicates that this SSW is a typical vortex splitting event dominated by planetary wavenumber 2. Additionally, after the SSW onset, no reflection of the stratosphere on the tropospheric planetary waves is observed. Thus, this event can also be classified as an absorbing-type SSW event. A noticeable cold wave occurs in the northwestern North America within 10 days after the 2018 SSW. This cold wave is probably associated with the SSW-related west–east dipole, namely a ridge over Alaska and a trough over the northeastern North America in the mid-troposphere that lasted up to 10 days after the onset date. The composite analysis of the other seven SSW events with an emergence of similar mid-tropospheric circulation pattern after SSW onset date yields coincident 2-meter temperature anomalies in the northwestern North America, which confirms the above conclusion to some extent.

Verification of Subseasonal-to-Seasonal Forecasts for Major Stratospheric Sudden Warmings in Northern Winter from 1998/99 to 2012/13

This study reports verification results of hindcast data of four systems in the subseasonal-to-seasonal (S2S) prediction project for major stratospheric sudden warmings (MSSWs) in northern winter from 1998/99 to 2012/13. This report deals with average features across all MSSWs, and possible differences between two MSSW types (vortex displacement and split types). Results for the average features show that stratospheric forecast verifications, when further averaged among the four systems, are judged to be successful for lead times around 10 d or shorter. All systems are skillful for lead times around 5 d, whereas the results vary among the systems for longer lead times. A comparison between the MSSW types overall suggests larger forecast errors or lower skill for MSSWs of the vortex split type, although the differences do not have strong statistical significance for almost all cases. This limitation is likely to at least partly reflect the small sample size of the MSSWs available.

On the representation of major stratospheric warmings in reanalyses

Abstract. Major sudden stratospheric warmings (SSWs) represent one of the most abrupt phenomena of the boreal wintertime stratospheric variability, and constitute the clearest example of coupling between the stratosphere and the troposphere. A good representation of SSWs in climate models is required to reduce their biases and uncertainties in future projections of stratospheric variability. The ability of models to reproduce these phenomena is usually assessed with just one reanalysis. However, the number of reanalyses has increased in the last decade and their own biases may affect the model evaluation. Here we compare the representation of the main aspects of SSWs across reanalyses. The examination of their main characteristics in the pre- and post-satellite periods reveals that reanalyses behave very similarly in both periods. However, discrepancies are larger in the pre-satellite period compared to afterwards, particularly for the NCEP-NCAR reanalysis. All datasets reproduce similarly the specific features of wavenumber-1 and wavenumber-2 SSWs. A good agreement among reanalyses is also found for triggering mechanisms, tropospheric precursors, and surface response. In particular, differences in blocking precursor activity of SSWs across reanalyses are much smaller than between blocking definitions.

On the value of reanalyses prior to 1979 for dynamical studies of stratosphere–troposphere coupling

Abstract. Studies of stratosphere–troposphere coupling, particularly those seeking to understand the dynamical processes underlying the coupling following extreme events such as major stratospheric warmings, suffer significantly from the relatively small number of such events in the “satellite” era (1979 to present). This limited sampling of a highly variable dynamical system means that composite averages tend to have large uncertainties. Including years during which radiosonde observations of the stratosphere were of sufficiently high quality substantially extends this record, reducing this sampling uncertainty by up to 20 %. Moreover, many open questions in this field involve aspects of tropospheric dynamics likely to be better constrained by “conventional” (i.e. radiosonde and surface-based) observations. Based on an intercomparison of reanalyses, a quantitative case is made that for many purposes the improved sampling obtained by including this period outweighs the reduced precision of the reanalyses in the Northern Hemisphere. Studies of stratosphere–troposphere coupling should therefore consider the use of this period when using reanalysis data. These results also support continued attention on this period from centres producing reanalyses.

On the variability of tides during a major stratospheric sudden warming in September 2002 at Southern hemispheric extra-tropical latitude

Investigation of the middle atmospheric tidal response to the September 2002 sudden stratospheric warming (SSW) is carried out from a Southern hemispheric extra-tropical location Cachoeira Paulista (22.7°S, 45°W) using meteor radar observations and reanalysis database. Significant diurnal and semidiurnal tidal amplitudes are found in the mesosphere and lower thermosphere (MLT) a few days prior to the warming onset consistent with the high latitude observations. In the troposphere and stratosphere (TS) the tidal amplitudes attain higher value during the main phase of the warming which decrease subsequently. The simultaneous enhancement of the tidal amplitudes in the TS and diminution in the MLT may imply strong dissipation of the upward propagating tides during the SSW at present location. An evident signature of the tide-planetary wave coupling due to nonlinear interactions is found. A preferential modulation of the tidal amplitudes by long/short period planetary waves are observed in the MLT/TS. The migrating diurnal tide (DW1) and nonmigrating semidiurnal tide (SW1) at 10 hPa pressure level show considerable variability during the warming episode.

The Stratospheric Sudden Warming Event in February 2018 and its Prediction by a Climate System Model

AbstractA major stratospheric sudden warming (SSW) event was observed in February 2018 after a 4‐year absence since the winter of 2013/2014. Based on the reanalysis data, the polar night jet changed from a very strong state to a moderate state during 12–19 January, and the moderate westerlies directly reversed to easterlies during 5–15 February. The intensified East Asian trough, Alaskan blocking, and East U.S. trough amplified the extratropical climatological wave 2, which propagated upward into the stratosphere, leading to a vortex‐splitting SSW event. Predictions of the February 2018 SSW event are explored in hindcasts initialized 0–4 weeks in advance by the Beijing Climate Center Climate System Model. Less than 20% of the 28 ensemble members predict the reversal of [U]60°N, 10hPa in hindcasts initialized 3 or 4 weeks in advance if a 5‐day error is allowed, while this ratio increases to 43% in hindcasts initialized 1 week in advance. Based on the climatological occurrence of SSW events in the forecast system, the maximum deterministic predictable limit of this event is 1–2 weeks in this forecast system. The eddy heat flux and its domination by wave 2 can only be predicted within the predictable time limit. A comparison between hindcast members initialized 2 weeks in advance suggests that the extratropical troughs and blockings are responsible for the upward propagation of waves from the troposphere to the stratosphere. The predictable limit of the stratospheric circulation pattern for the February 2018 SSW, 1–2 weeks, also generalizes to other vortex split SSW events such as the January 2009 and February 1999 cases.

Comparison of Subseasonal‐to‐Seasonal Model Forecasts for Major Stratospheric Sudden Warmings

AbstractThis study explores forecasts for five distinctive major stratospheric sudden warmings (MSSWs) using subseasonal‐to‐seasonal prediction data with lead times up to about 2 weeks. Results reveal model‐to‐model variability of the forecasts, as some models, such as the European Centre for Medium‐Range Weather Forecasts and National Centers for Environmental Prediction models, forecast the MSSWs better. Results also demonstrate greater difficulty of forecasting the vortex split MSSWs (three of the five) than the vortex displacement MSSWs (the other two). It is also shown that the variable forecasts among different lead times and models for each MSSW are characterized by how well the models reproduce the weakenings and deformations of the stratospheric polar vortex around the MSSWs and the intensifications of tropospheric planetary waves before them. The variability is well captured by planetary wave activity, or poleward heat flux, in the lower stratosphere. This underlines the importance of enhanced planetary waves in the troposphere and lower stratosphere for successful forecasts of the forecasting MSSWs, although the causality between the MSSWs and enhanced planetary waves will deserve further investigation.

No robust evidence of future changes in major stratospheric sudden warmings: a multi-model assessment from CCMI

Major stratospheric sudden warmings (SSWs) are the largest instance of wintertime variability in the Arctic stratosphere. Due to their relevance for the troposphere-stratosphere system, several previous studies have focused on their potential response to anthropogenic forcings. However, a wide range of results have been reported, from a future increase in the frequency of SSWs to a decrease. Several factors might explain these contradictory results, notably the use of different metrics for the identification of SSWs, and the impact of large climatological biases in single-model studies. Here we revisit the question of future SSWs changes, using an identical set of metrics applied consistently across 12 different models participating in the Chemistry Climate Model Initiative. From analyzing future integrations we find no statistically significant change in the frequency of SSWs over the 21st century, irrespective of the metric used for the identification of SSWs. Changes in other SSWs characteristics, such as their duration and the tropospheric forcing, are also assessed: again, we find no evidence of future changes over the 21st century.

Seasonal Winter Forecasts of the Northern Stratosphere and Troposphere: Results from JMA Seasonal Hindcast Experiments

Abstract This study investigates winter forecasts of the northern stratosphere and troposphere using seasonal hindcast experiments of the Japan Meteorological Agency (JMA). A main focus is placed on the seasonal forecasts of the December–February (DJF)-mean northern annular mode (NAM) when the forecasts are initialized in late fall. Results demonstrate that the hindcast data have significant skill for both ensemble-mean and category (probability) forecasts of the NAM but only in the stratosphere. Probability forecasts for DJF major stratospheric sudden warmings (MSSWs) are also suggested to be significant (higher probabilities for actual MSSW years) near the 90% confidence level. The forecast skill of the stratospheric NAM changes with the observed phase of the quasi-biennial oscillation (QBO), although the QBO is not simulated but is only included in initial conditions. The skill is higher for the easterly phase, characterized by hits of negative NAM states (weaker-than-normal polar vortex), whereas it is lower for the westerly phase, reflecting misses of positive NAM states. It is finally shown that a verification score for category forecasts of the stratospheric and tropospheric NAM tends to covary as a whole. The tropospheric forecast skill is significant when the stratosphere has large NAM anomalies in the real world and they are well forecasted. In contrast, the tropospheric forecasts are sometimes poor when the stratospheric forecasts fail to capture observed NAM conditions. It is speculated that stratospheric and tropospheric forecasts could be improved together through the stratosphere–troposphere coupling for such cases, that is, by successfully forecasting anomalous vortex states in the stratosphere.

Nighttime mesospheric ozone enhancements during the 2002 southern hemispheric major stratospheric warming

Sudden Stratospheric Warmings (SSW) affect the chemistry and dynamics of the middle atmosphere. Major warmings occur roughly every second winter in the Northern Hemisphere (NH), but has only been observed once in the Southern Hemisphere (SH), during the Antarctic winter of 2002. Observations by the Global Ozone Monitoring by Occultation of Stars (GOMOS, an instrument on board Envisat) during this rare event, show a 40% increase of ozone in the nighttime secondary ozone layer at subpolar latitudes compared to non-SSW years. This study investigates the cause of the mesospheric nighttime ozone increase, using the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model with specified dynamics (SD-WACCM). The 2002 SH winter was characterized by several reductions of the strength of the polar night jet in the upper stratosphere before the jet reversed completely, marking the onset of the major SSW. At the time of these wind reductions, corresponding episodic increases can be seen in the modelled nighttime secondary ozone layer. This ozone increase is attributed largely to enhanced upwelling and the associated cooling of the altitude region in conjunction with the wind reversal. This is in correspondence to similar studies of SSW induced ozone enhancements in NH. But unlike its NH counterpart, the SH secondary ozone layer appeared to be impacted less by episodic variations in atomic hydrogen. Seasonally decreasing atomic hydrogen plays however a larger role in SH compared to NH.