The Arctic is warming at a rate twice the global average and severe winter weather is reported to be increasing across many heavily populated mid-latitude regions, but there is no agreement on ...whether a physical link exists between the two phenomena. We use observational analysis to show that a lesser-known stratospheric polar vortex (SPV) disruption that involves wave reflection and stretching of the SPV is linked with extreme cold across parts of Asia and North America, including the recent February 2021 Texas cold wave, and has been increasing over the satellite era. We then use numerical modeling experiments forced with trends in autumn snow cover and Arctic sea ice to establish a physical link between Arctic change and SPV stretching and related surface impacts.
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
Using 17 CMIP5 and CMIP6 models with a spontaneously generated quasi-biennial oscillation (QBO)-like phenomenon, this study explores and evaluates three dynamical pathways for impacts of the QBO on ...the troposphere: 1) the Holtan–Tan (HT) effect on the stratospheric polar vortex and the northern annular mode (NAM), 2) the subtropical zonal wind downward arching over the Pacific, and 3) changes in local convection over the Maritime Continent and Indo-Pacific Ocean. More than half of the models can reproduce at least one of the three pathways, but few models can reproduce all of the three routes. First, seven models are able to simulate a weakened polar vortex during easterly QBO (EQBO) winters, in agreement with the HT effect in the reanalysis. However, the weakened polar vortex response during EQBO winters is underestimated or not present at all in other models, and hence the chain for QBO, vortex, and tropospheric NAM/AO is not simulated. For the second pathway associated with the downward arching of the QBO winds, 10 models simulate an inconsistent extratropical easterly anomaly center over 20°–40°N in the Pacific sector during EQBO, and hence the negative relative vorticity anomalies poleward of the easterly center is not present in those models, leading to no consensus on the height response over the North Pacific between those models and the reanalysis. However, the other seven models do capture this effect. The third pathway is only observed in the Indo-Pacific Ocean, where the strong climatological deep convection and the warm pool are situated. Seven models can simulate the convection anomalies associated with the QBO over the Maritime Continent, which is likely caused by the near-tropopause low buoyancy frequency anomalies. No robust relationship between the QBO and El Niño–Southern Oscillation (ENSO) events can be established using the JRA55 reanalysis, and 10 models consistently confirm little modulation of the ocean basinwide Walker circulation and ENSO events by the QBO.
Using state-of-the-art models from the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5/6), future changes of sudden stratospheric warming (SSW) events under a moderate emission scenario ...(RCP45/SSP245) and a strong emissions scenario (RCP85/SSP585) are evaluated with respect to the historical simulations. Changes in four characteristics of SSWs are examined in 54 models: the SSW frequency, the seasonal distribution, stratosphere-troposphere coupling, and the persistency of the distorted or displaced polar vortex. The composite results show that none of these four aspects will change robustly. An insignificant (though positive) change in the SSW frequency from historical simulations to RCP45/SSP245 and then to RCP85/SSP585 is consistently projected by CMIP5 and CMIP6 multimodel ensembles in most wintertime months (December-March). This increase in the SSW frequency is most pronounced in mid- (late-) winter in CMIP6 (CMIP5). No shift in the seasonality of SSWs is simulated especially in the CMIP6 future scenarios. Both the reanalysis and CMIP5/6 historical simulations exhibit strong stratosphere-troposphere coupling during SSWs, and the coupling strength is nearly unchanged in the future scenario simulations. The near surface responds immediately after the onset of SSWs in both historical and future scenarios experiments, denoted by the deep downward propagation of zonal-mean easterly anomalies from the stratosphere to the troposphere. On average, the composite circumpolar easterly winds persist for 8 d in the reanalysis and CMIP5/6 historical experiments, which are projected to remain unchanged in both the moderate and strong emissions scenarios, implying the lifecycle of SSWs will not change.
Abstract
Teleconnection patterns associated with the Madden–Julian oscillation (MJO) and El Niño–Southern Oscillation (ENSO) impact weather and climate phenomena in the Pacific–North American region ...and beyond, and therefore accurately simulating these teleconnections is of importance for seasonal and subseasonal forecasts. Systematic biases in boreal midwinter ENSO and MJO teleconnections are found in eight subseasonal to seasonal (S2S) forecast models over the Pacific–North America region. All models simulate an anomalous 500-hPa geopotential height response that is too weak. This overly weak response is associated with overly weak subtropical upper-level convergence and a too-weak Rossby wave source in most models, and in several models there is also a biased subtropical Pacific jet, which affects the propagation of Rossby waves. In addition to this overly weak response, all models also simulate ENSO teleconnections that reach too far poleward toward Alaska and northeastern Russia. The net effect is that these models likely underestimate the impacts associated with the MJO and ENSO over western North America, and suffer from a reduction in skill from what could be achieved.
Using the historical, moderate emission scenario (RCP45/SSP245), and high emission scenario (RCP85/SSP585) experiments provided by the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5/6), ...future changes of stratospheric final warming (SFW) events are explored in this study. Most CMIP5/6 models project a delay of SFWs in the two future scenarios, compared with historical simulation in both hemispheres (4–8 days shift in the multimodel mean). The projected delay in SFWs suggest a later seasonal transition from the climatological wintertime to summertime circulation in both hemispheres. In the Southern Hemisphere (SH), essentially all of the delay in the SFW occurs in the era with strong ozone depletion (1980–2040), and the SFW date is largely unchanged as ozone recovers through the end of the century. Both CMIP5 and CMIP6 multimodel ensemble means (MMEs) do not project any significant change in reversal of westerlies and the stratosphere-troposphere coupling strength during the Northern Hemisphere (NH) SFW. In contrast, both CMIP5 and CMIP6 MMEs project a significant decrease in the strength of SH SFW, but the lower tropospheric response to the SH SFW changes little during 1980–2040. However, the near-surface response to SH SFWs is projected to be significantly stronger during 2040–2100 than during 1980–2040, as well as in CMIP6 than in CMIP5. Biases in SFW over the historical period are generally larger in the NH than in the SH, and show little improvement from CMIP5 to CMIP6.
Using the Model of an Idealized Moist Atmosphere (MiMA) capable of spontaneously generating a quasibiennial oscillation (QBO), the gradual establishment of the extratropical response to the QBO is ...explored. The period and magnitude of the QBO and the magnitude of the polar Holton–Tan (HT) relationship is simulated in a free-running configuration of MiMA, comparable to that in state-of-the-art climate models. To isolate mechanisms whereby the QBO influences variability outside the tropical atmosphere, a series of branch experiments are performed with nudged QBO winds. When easterly QBO winds maximized around 30 hPa are relaxed, an Eliassen–Palm (E-P) flux divergence dipole quickly forms in the extratropical middle stratosphere as a direct response to the tropical meridional circulation, in contrast to the HT mechanism, which is associated with wave propagation near the zero wind line. This meridional circulation response to the relaxed QBO winds develops within the first 10 days in seasonally varying and fixed-seasonal experiments. No detectable changes in upward propagation of waves in the midlatitude lowermost stratosphere are evident for at least 20 days after branching, with the first changes only evident after 20 days in perpetual midwinter and season-varying runs, but after 40 days in perpetual November runs. The polar vortex begins to respond around the 20th day, and subsequently a near-surface response in the Atlantic Ocean sector forms in mid-to-late winter runs. These results suggest that the maximum near-surface response observed in mid-to-late winter is not simply due to a random seasonal synchronization of the QBO phase, but is also due to the long lag of the surface response to a QBO relaxation in early winter and the short lag of the surface response to a QBO relaxation in mid-to-late winter.
European and eastern United States wintertime weather is strongly influenced by large‐scale modes of variability in the Northern Hemisphere such as the Arctic Oscillation (AO) and North Atlantic ...Oscillation (NAO). The negative phase of the NAO has been linked to both the Madden‐Julian Oscillation (MJO) phase with convection in the West Pacific (phases 6 and 7) and to stratospheric sudden warmings (SSW), but the relative role of each phenomenon is not clear, and the two phenomena are themselves linked, as more than half of SSW events were preceded by phases 6 and 7 of the MJO. Here we disentangle the relative roles of MJO phase 6/7 and stratospheric variability for Northern Hemisphere surface weather during boreal winter. We show that stratospheric variability leads to significantly different North Atlantic anomalies if it is preceded by MJO phase 6/7. Furthermore, MJO phase 6/7 leads to a long‐lived negative AO pattern only if it modulates the stratosphere first. Hence, proper attribution of their respective influence on surface weather needs to take into consideration the linkages between these two phenomena. Finally, MJO phase 6/7 events that lead to SSW can be differentiated from those which do not by their characteristics within the tropics: only MJO phase 6/7 events in which enhanced convection propagates into the South China Sea, which rarely occurs in winter, lead to SSWs.
Key Points
More than half of SSWs were preceded by MJO phase 6/7, and hence the relative role of each phenomenon for NH climate needs clarification
MJO phase 6/7 leads to a strong and long‐lived negative AO only if it modulates the stratosphere first
Some of the surface impacts of the SSW are likely due to the lingering aftereffects of MJO phase 6/7 that helped force the SSW
Sudden Stratospheric Warmings Baldwin, Mark P.; Ayarzagüena, Blanca; Birner, Thomas ...
Reviews of geophysics (1985),
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
A minor sudden stratospheric warming (SSW) happened in September 2019 in the Southern Hemisphere (SH) with winds at 10 hPa, 60°S reaching their minimum value on 18 September. Using multiple data sets ...and real‐time predictions from 11 subseasonal to seasonal (S2S) models, the evolution, favorable conditions, and predictability for this SSW event are explored. The September 2019 SSW happened during several favorable conditions, including easterly equatorial quasi‐biennial oscillation (QBO) winds at 10 hPa, solar minimum, positive Indian Ocean Dipole (IOD) sea surface temperatures (SST), warm SST anomalies in the central Pacific, and a blocking high near the Antarctic Peninsula. With these favorable initial and boundary conditions, the predictive limit to this SSW is around 18 days in some S2S models, and more than 50% of the ensemble members forecast the zonal wind deceleration in reforecasts initialized around 29 August. A vortex slowdown is evident in some initializations from around 22 August, but with a forecast‐reanalysis pattern correlation %3C0.5, while initializations later than 29 August capture the wavelike pattern in the troposphere and the subsequent stratospheric evolution. The ensemble spread in the magnitude of the vortex deceleration during the SSW is mainly explained by the ensemble spread in the magnitude of upward propagation of waves in the troposphere and in the stratosphere, with an underestimated tropospheric wave amplitude leading to a too‐small deceleration of the vortex. The September 2019 SH SSW did not show a near‐instantaneous downward impact on the tropospheric southern annular mode (SAM) in late September and early October 2019. The Australian drought and hot weather in September possibly associated with the positive IOD might have been exacerbated by the negative SAM in October and later months due to the weak stratospheric polar vortex. However, models tend to forecast a near‐instantaneous tropospheric response to the SSW.
Key Points
An SH SSW happened in September 2019, with westerly winds at 10 hPa, 50°S reversed on 16 September
This SH SSW appeared during several favorable conditions, including easterly QBO winds, solar minimum, positive IOD, and warm SST anomalies in the central Pacific
The predictive limit to this SSW is ~18 days in some S2S models, but models forecast a faster tropospheric response to the SSW than observations
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