El Niño and La Niña events in the tropical Pacific have significant and disrupting impacts on the global atmospheric and oceanic circulation. El Niño Southern Oscillation (ENSO) impacts also extend ...above the troposphere, affecting the strength and variability of the stratospheric polar vortex in the high latitudes of both hemispheres, as well as the composition and circulation of the tropical stratosphere. El Niño events are associated with a warming and weakening of the polar vortex in the polar stratosphere of both hemispheres, while a cooling can be observed in the tropical lower stratosphere. These impacts are linked by a strengthened Brewer‐Dobson circulation. Anomalous upward wave propagation is observed in the extratropics of both hemispheres. For La Niña, these anomalies are often opposite. The stratosphere in turn affects surface weather and climate over large areas of the globe. Since these surface impacts are long‐lived, the changes in the stratosphere can lead to improved surface predictions on time scales of weeks to months. Over the past decade, our understanding of the mechanisms through which ENSO can drive impacts remote from the tropical Pacific has improved. This study reviews the possible mechanisms connecting ENSO to the stratosphere in the tropics and the extratropics of both hemispheres while also considering open questions, including nonlinearities in the teleconnections, the role of ENSO diversity, and the impacts of climate change and variability.
Plain Language Summary
El Niño and La Niña events, the irregular warming and cooling of the tropical Pacific that occurs every couple of years, have disrupting impacts spanning the entire world. These remote impacts, so‐called “teleconnections”, also reach the stratosphere, the layer of the atmosphere starting at around 10 km above the Earth's surface. El Niño leads to a warming of the stratosphere in both hemispheres, while the lower tropical stratosphere cools. These signatures are linked by a strengthened stratospheric circulation from the tropics to the polar regions. El Niño also leads to more frequent breakdowns of the stratospheric polar vortex, a band of strong eastward winds in the polar stratosphere. For La Niña, these effects tend to be opposite, though they are not always robust, suggesting nonlinear or nonstationary effects, long‐term variability, and trends in the teleconnections. The observational data record is not yet long enough to make conclusions with certainty, and models that try to reproduce the teleconnections indicate that teleconnections might be more linear than the limited number of observations indicate. Further research will be needed to separate the El Niño and La Niña teleconnections from other effects and to determine to what extent nonlinearity and nonstationarity are indeed present.
Key Points
ENSO has a detectable impact on the composition and circulation of the stratosphere in the tropics and extratropics
The changes in stratospheric variability due to ENSO have implications for improving surface prediction
Recent advances in modeling have helped to put the response to the small sample of observed ENSO events in context
Sudden Stratospheric Warmings Baldwin, Mark P.; Ayarzagüena, Blanca; Birner, Thomas ...
Reviews of geophysics,
March 2021, Letnik:
59, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (∼10–50 km) are associated with a complete ...reversal of the climatological wintertime westerly winds. SSWs are caused by the breaking of planetary‐scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid descent and warming of air in polar latitudes, mirrored by ascent and cooling above the warming. The rapid warming and descent of the polar air column affect tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode, making cold air outbreaks over North America and Eurasia more likely. SSWs affect the atmosphere above the stratosphere, producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (nonionized) particles and electron densities, and electric fields. These effects span both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal prediction. This work reviews the current knowledge on the most important aspects of SSWs, from the historical background to dynamical processes, modeling, chemistry, and impact on other atmospheric layers.
Plain Language Summary
The stratosphere is the layer of the atmosphere from ∼10 to 50 km, with pressures decreasing to ∼1 hPa (0.1% of surface pressure) at the top. The polar stratosphere during winter is normally very cold, with strong westerly winds. Roughly every 2 years in the Northern Hemisphere, the quiescent vortex suddenly warms over a week or two, and the winds slow dramatically, resulting in easterly winds that are more similar to the summer. These events, known as sudden stratospheric warmings (SSWs), were discovered in the early 1950s, and today, they are observed in detail by satellites. After several decades researching SSWs, considerable progress has been made in dynamical aspects of SSWs, but our understanding of how they affect both surface weather and the upper atmosphere is incomplete. We observe that variability of the stratospheric circulation (SSWs being an extreme event) is associated with shifts in the jet stream and the paths of storms, with associated effects on rainfall and temperatures. The likelihood of cold weather spells and damaging wind storms is also affected. Almost all SSWs have occurred in the Northern Hemisphere, but there was one spectacular major SSW in 2002 in the Southern Hemisphere.
Key Points
Sudden stratospheric warmings are dramatic events of the polar stratosphere that affect the atmosphere from the surface to the thermosphere
Our understanding of sudden stratospheric warmings has accelerated recently, particularly the predictability of surface weather effects
More observations, improved climate models, and big data methods will address uncertainties in key aspects of sudden stratospheric warmings
Various criteria exist for determining the occurrence of a major sudden stratospheric warming (SSW), but the most common is based on the reversal of the climatological westerly zonal-mean zonal winds ...at 60° latitude and 10 hPa in the winter stratosphere. This definition was established at a time when observations of the stratosphere were sparse. Given greater access to data in the satellite era, a systematic analysis of the optimal parameters of latitude, altitude, and threshold for the wind reversal is now possible. Here, the frequency of SSWs, the strength of the wave forcing associated with the events, changes in stratospheric temperature and zonal winds, and surface impacts are examined as a function of the stratospheric wind reversal parameters. The results provide a methodical assessment of how to best define a standard metric for major SSWs. While the continuum nature of stratospheric variability makes it difficult to identify a decisively optimal threshold, there is a relatively narrow envelope of thresholds that work well—and the original focus at 60° latitude and 10 hPa lies within this window.
Major, sudden midwinter stratospheric warmings (SSWs) are large and rapid temperature increases in the winter polar stratosphere are associated with a complete reversal of the climatological westerly ...winds (i.e., the polar vortex). These extreme events can have substantial impacts on winter surface climate, including increased frequency of cold air outbreaks over North America and Eurasia and anomalous warming over Greenland and eastern Canada. Here we present a SSW Compendium (SSWC), a new database that documents the evolution of the stratosphere, troposphere, and surface conditions 60 days prior to and after SSWs for the period 1958–2014. The SSWC comprises data from six different reanalysis products: MERRA2 (1980–2014), JRA-55 (1958–2014), ERA-interim (1979–2014), ERA-40 (1958–2002), NOAA20CRv2c (1958–2011), and NCEP-NCAR I (1958–2014). Global gridded daily anomaly fields, full fields, and derived products are provided for each SSW event. The compendium will allow users to examine the structure and evolution of individual SSWs, and the variability among events and among reanalysis products. The SSWC is archived and maintained by NOAA's National Centers for Environmental Information (NCEI, doi:10.7289/V5NS0RWP).
Stratospheric conditions are increasingly being recognized as an important driver of North Atlantic and Eurasian climate variability. Mindful that the observational record is relatively short, and ...that internal climate variability can be large, the authors here analyze a new 10-member ensemble of integrations of a stratosphere-resolving, atmospheric general circulation model, forced with the observed evolution of sea surface temperature (SST) during 1952–2003. Previous studies are confirmed, showing that El Niño conditions enhance the frequency of occurrence of stratospheric sudden warmings (SSWs), whereas La Niña conditions do not appear to affect it. However, large differences are noted among ensemble members, suggesting caution when interpreting the relatively short observational record. More importantly, it is emphasized that the majority of SSWs are not caused by anomalous tropical Pacific SSTs. Comparing composites of winters with and without SSWs in each ENSO phase separately, it is demonstrated that stratospheric variability gives rise to large and statistically significant anomalies in tropospheric circulation and surface conditions over the North Atlantic and Eurasia. This indicates that, for those regions, climate variability of stratospheric origin is comparable in magnitude to variability originating from tropical Pacific SSTs, so that the occurrence of a single SSW in a given winter is able to completely alter seasonal climate predictions based solely on ENSO conditions. These findings, corroborating other recent studies, highlight the importance of accurately forecasting SSWs for improved seasonal prediction of North Atlantic and Eurasian climate.
DEFINING SUDDEN STRATOSPHERIC WARMINGS Butler, Amy H.; Seidel, Dian J.; Hardiman, Steven C. ...
Bulletin of the American Meteorological Society,
11/2015, Letnik:
96, Številka:
11
Journal Article
Recenzirano
Odprti dostop
Sudden stratospheric warmings (SSWs) are large, rapid temperature rises in the winter polar stratosphere, occurring predominantly in the Northern Hemisphere. Major SSWs are also associated with a ...reversal of the climatological westerly zonal-mean zonal winds. Circulation anomalies associated with SSWs can descend into the troposphere with substantial surface weather impacts, such as wintertime extreme cold air outbreaks. After their discovery in 1952, SSWs were classified by the World Meteorological Organization. An examination of literature suggests that a single, original reference for an exact definition of SSWs is elusive, but in many references a definition involves the reversal of the meridional temperature gradient and, for major warmings, the reversal of the zonal circulation poleward of 60° latitude at 10 hPa.
Though versions of this definition are still commonly used to detect SSWs, the details of the definition and its implementation remain ambiguous. In addition, other SSW definitions have been used in the last few decades, resulting in inconsistent classification of SSW events. We seek to answer the questions: How has the SSW definition changed, and how sensitive is the detection of SSWs to the definition used? For what kind of analysis is a “standard” definition useful? We argue that a standard SSW definition is necessary for maintaining a consistent and robust metric to assess polar stratospheric wintertime variability in climate models and other statistical applications. To provide a basis for, and to encourage participation in, a communitywide discussion currently underway, we explore what criteria are important for a standard definition and propose possible ways to update the definition.
The stratosphere, the layer of the atmosphere at heights between 10-50 km, is an important source of variability for the weather and climate at the Earth’s surface on timescales of weeks to decades. ...Since the stratospheric circulation evolves more slowly than that of the troposphere below, it can contribute to predictability at the surface. Our synthesis of studies on the coupling between the stratosphere and the troposphere reveals that the stratosphere also contributes substantially to a wide range of climate-related extreme events. These extreme events include cold air outbreaks and extreme heat, air pollution, wildfires, wind extremes, and storm clusters, as well as changes in tropical cyclones and sea ice cover, and they can have devastating consequences for human health, infrastructure, and ecosystems. A better understanding of the vertical coupling in the atmosphere, along with improved representation in numerical models, is therefore expected to help predict extreme events on timescales from weeks to decades in terms of the event type, magnitude, frequency, location, and timing. With a better understanding of stratosphere-troposphere coupling, it may be possible to link more tropospheric extremes to stratospheric forcing, which will be crucial for emergency planning and management.
Coupling between the stratosphere and the troposphere contributes to extreme events at the Earth’s surface, and can help with predictability on timescales from weeks to decades, according to a synthesis of the influence of the stratosphere on surface climate.
The winter of 2019–2020 was dominated by an extremely strong stratospheric polar vortex and positive tropospheric Arctic Oscillation (AO). Here, we analyze forecasts from six different prediction ...systems contributing to the C3S seasonal forecast database. Most performed very strongly, with consistently high skill for January–March 2020 from forecasts launched through October–December 2019. Although the magnitude of the anomalies was underestimated, the performance of most prediction systems was extremely high for a positive AO winter relative to the common hindcast climate. Ensemble members which better predicted the extremely strong stratospheric vortex better predicted the extreme tropospheric state. We find a significant relationship between forecasts of the anomalous midlatitude tropospheric wave pattern in early winter, which destructively interfered with the climatological stationary waves and the strength of the stratospheric vortex later in the winter. Our results demonstrate a strong interdependence between the accuracy of stratospheric vortex and AO forecasts.
Plain Language Summary
Westerly winds during the winter of 2019–2020 were unusually strong and long lasting through a deep layer of the atmosphere. We investigate how well this was predicted months ahead of time. We find that seasonal weather forecast systems predicted the winter pattern very well, especially when compared with previous winters. Forecasts which better predicted the strength of the winds higher in the atmosphere did better overall. We find that there was a link between predictions of the weather patterns lower down in the atmosphere and how they suppressed large‐scale atmospheric waves in the midlatitudes, which likely helped the winds remain stronger higher up.
Key Points
Forecasts from six seasonal prediction systems consistently predicted the large‐scale winter patterns with unusually high accuracy
Ensemble members which better predicted the extreme stratospheric state also better predicted the extreme tropospheric state
Accurate prediction of the midlatitude tropospheric wave pattern was associated with more accurate stratospheric forecasts
The El Niño-Southern Oscillation (ENSO) is a major driver of Northern Hemisphere wintertime variability and, generally, the key ingredient used in seasonal forecasts of wintertime surface climate. ...Modeling studies have recently suggested that ENSO teleconnections might involve both a tropospheric pathway and a stratospheric one. Here, using reanalysis data, we carefully distinguish between the two. We first note that the temperature and circulation anomalies associated with the tropospheric pathway are nearly equal and opposite during the warm (El Niño) and cold (La Niña) phases of ENSO, whereas those associated with the stratospheric pathway are of the same sign, irrespective of the ENSO phase. We then exploit this fact to isolate the two pathways. Our decomposition reveals that ENSOs climate impacts over North America are largely associated with the tropospheric pathway, whereas ENSOs climate impacts over the North Atlantic and Eurasia are greatly affected by the stratospheric pathway. The stratospheric pathway, which we here define on the basis of the occurrence of one or more sudden stratospheric warmings in a given winter, and whose signature projects very strongly on the North Atlantic Oscillation, is found to be present 60% of the time during ENSO winters (of either phase): it therefore likely plays an important role in improving seasonal forecasts, notably over the North Atlantic and the Eurasian continent.
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