Predictability of the stratospheric polar vortex in the ECMWF S2S reforecasts

2021 ◽  
Author(s):  
Rachel Wai-Ying Wu ◽  
Daniela I.V. Domeisen
Keyword(s):  
Lead Time ◽  
Polar Vortex ◽  
Onset Date ◽  
Strong Wind ◽  
Sudden Stratospheric Warming ◽  
Stratospheric Polar Vortex ◽  
Zonal Winds ◽  
Surface Weather ◽  
Acceleration And Deceleration ◽  
Strong Vortex

<p>Extreme stratospheric events, e.g strong vortex events and sudden stratospheric warming (SSW) events, are often the main focus of stratospheric predictability studies. Other than strong vortex and SSW events, strong vortex acceleration and deceleration events are related but less studied events. A better understanding of the mechanisms of acceleration and deceleration events would also contribute to the understanding of SSWs and strong vortex events in the stratosphere. As SSWs tend to be less predictable than strong vortex events, it is hypothesized that the predictability of acceleration and deceleration events might differ as they are related to opposite mechanisms. We identify wind acceleration and deceleration events using the daily mean of the zonal mean zonal winds at 60°N and 10 hPa from the ERA-interim reanalysis for the winters of 1998/99-2018/19. Acceleration and deceleration events are defined as a wind change over a 10-day window above the 60th percentile of the magnitude of all identified events. To evaluate the predictability of the events, the ECMWF S2S hindcasts are verified against ERA-interim data. As expected, the predictability of the events increases with decreasing lead time (as the model initialisation date approaches the event onset date). We also find that all 4 types of events, namely acceleration, deceleration, strong vortex and SSW events, show the same predictability behavior, that is, that the predictability of an event is independent of its nature but dependent only on its magnitude. We discuss the difficulties of the model in predicting events associated with strong wind changes by investigating the heat flux-wind relationship in the model. A better understanding of the predictability and dynamical variability in the stratospheric polar vortex by the model could provide a better understanding of the mechanisms of stratospheric events, thus potentially also improving surface weather predictability.</p>

Sensitivity of the Tropospheric Circulation to Changes in the Strength of the Stratospheric Polar Vortex

2006 ◽  
Vol 134 (8) ◽  
pp. 2191-2207 ◽  
Author(s):  
Thomas Jung ◽  
Jan Barkmeijer
Keyword(s):  
North Atlantic ◽  
Polar Vortex ◽  
Stratospheric Polar Vortex ◽  
The North ◽  
European Area ◽  
Extended Range ◽  
Tropospheric Circulation ◽  
The North Atlantic Oscillation ◽  
Ecmwf Model ◽  
Strong Vortex

Abstract The sensitivity of the wintertime tropospheric circulation to changes in the strength of the Northern Hemisphere stratospheric polar vortex is studied using one of the latest versions of the ECMWF model. Three sets of experiments were carried out: one control integration and two integrations in which the strength of the stratospheric polar vortex has been gradually reduced and increased, respectively, during the course of the integration. The strength of the polar vortex is changed by applying a forcing to the model tendencies in the stratosphere only. The forcing has been obtained using the adjoint technique. It is shown that, in the ECMWF model, changes in the strength of the polar vortex in the middle and lower stratosphere have a significant and slightly delayed (on the order of days) impact on the tropospheric circulation. The tropospheric response shows some resemblance to the North Atlantic Oscillation (NAO), though the centers of action are slightly shifted toward the east compared to those of the NAO. Furthermore, a separate comparison of the response to a weak and strong vortex forcing suggests that to first order the tropospheric response is linear within a range of realistic stratospheric perturbations. From the results presented, it is argued that extended-range forecasts in the European area particularly benefit from the stratosphere–troposphere link.

Tracing North Atlantic Oscillation Forecast Errors to Stratospheric Origins, with a new analysis of the 2021 winter

2021 ◽  
Author(s):  
Erik W. Kolstad ◽  
C. Ole Wulff ◽  
Daniela Domeisen ◽  
Tim Woollings
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Weather Forecasting ◽  
North Atlantic Ocean ◽  
Polar Vortex ◽  
Forecast Errors ◽  
Stratospheric Warming ◽  
Sudden Stratospheric Warming ◽  
Stratospheric Polar Vortex ◽  
Extended Range

<div> <div> <div> <div> <p>The North Atlantic Oscillation (NAO) is the main driver of weather variability in parts of Eurasia, Greenland, North America, and North Africa on a range of time scales. Successful extended-range NAO predictions would equate to improved predictions of precipitation and temperature in these regions. It has become clear that the NAO is influenced by the stratosphere, but because this downward coupling is not fully reproduced by all forecast models the potential for improved NAO forecasts has not been fully realized. Here, an analysis of 21 winters of subseasonal forecast data from the European Centre for Medium-Range Weather Forecasts monthly forecasting system is presented. By dividing the forecasts into clusters according to their errors in North Atlantic Ocean sea level pressure 15-30 days into the forecasts, we identify relationships between these errors and the state of the stratospheric polar vortex when the forecasts were initialized. A key finding is that the model overestimates the persistence of both the negative NAO response following a weak polar vortex and the positive NAO response following a strong polar vortex. A case in point is the sudden stratospheric warming in early 2019, which was followed by five consecutive weeks of an overestimation of the negative NAO regime. A consequence on the ground was temperature predictions for northern Europe that were too cold. In this talk, we include a new analysis of the temperature prediction performance following the January 2021 sudden stratospheric warming. Another important finding is that the model appears to misrepresent the gradual downward impact of stratospheric vortex anomalies. This result suggests that an improved representation and prediction of stratosphere-troposphere coupling in models might yield substantial benefits for extended-range weather forecasting in the Northern Hemisphere midlatitudes.</p> </div> </div> </div> </div>

scholarly journals Evolution of the stratospheric polar vortex edge intensity and duration in the Southern hemisphere over the 1979–2020 period

2021 ◽  
Author(s):  
Audrey Lecouffe ◽  
Sophie Godin-Beekmann ◽  
Andrea Pazmiño ◽  
Alain Hauchecorne
Keyword(s):  
Solar Cycle ◽  
Southern Hemisphere ◽  
Southern Oscillation ◽  
Polar Vortex ◽  
Onset Date ◽  
Late Winter ◽  
Stratospheric Polar Vortex ◽  
The Difference ◽  
Lower Variability

Abstract. The intensity and position of the Southern Hemisphere stratospheric polar vortex edge is evaluated as a function of equivalent latitude over the 1979–2020 period on three isentropic levels (475 K, 550 K and 675 K) from ECMWF ERA-Interim reanalysis. The study also includes an analysis of the onset and breakup dates of the polar vortex, which are determined from wind thresholds (e.g. 15.2 m.s−1, 20 m.s−1and 25 m.s−1) along the vortex edge. The vortex edge is stronger in late winter, over September–October – November with the period of strongest intensity occurring later at the lowermost level. A lower variability of the edge position is observed during the same period. Long-term increase of the vortex edge intensity and break-up date is observed over the 1979–1999 period, linked to the increase of the ozone hole. Long-term decrease of the vortex onset date related to the 25 m.s−1wind threshold is also observed at 475 K during this period. The solar cycle and to a lower extent the quasi-biennal oscillation (QBO) and El Niño Southern Oscillation (ENSO) modulate the inter-annual evolution of the strength of the vortex edge and the vortex breakup dates. Stronger vortex edge and longer vortex duration is observed in solar minimum (minSC) years, with the QBO and ENSO further modulating the solar cycle influence, especially at 475 K and 550 K: during West QBO (wQBO) phases, the difference between vortex edge intensity for minSC and maxSC years is smaller than during East QBO (eQBO) phases. The polar vortex edge is stronger and lasts longer for maxSC/wQBO years than for maxSC/eQBO years. ENSO has a weaker impact but the vortex edge is somewhat stronger during cold ENSO phases for both minSC and maxSC years.

scholarly journals Stratosphere–Troposphere Coupling in the Southern Hemisphere

2005 ◽  
Vol 62 (3) ◽  
pp. 708-715 ◽  
Author(s):  
David W. J. Thompson ◽  
Mark P. Baldwin ◽  
Susan Solomon
Keyword(s):  
Climate Variability ◽  
Southern Hemisphere ◽  
Large Scale ◽  
Temporal Evolution ◽  
Polar Vortex ◽  
Climate Impacts ◽  
Sudden Stratospheric Warming ◽  
Stratospheric Polar Vortex ◽  
Tropospheric Circulation ◽  
Stratospheric Variability

Abstract This study examines the temporal evolution of the tropospheric circulation following large-amplitude variations in the strength of the Southern Hemisphere (SH) stratospheric polar vortex in data from 1979 to 2001 and following the SH sudden stratospheric warming of 2002. In both cases, anomalies in the strength of the SH stratospheric polar vortex precede similarly signed anomalies in the tropospheric circulation that persist for more than 2 months. The SH tropospheric circulation anomalies reflect a bias in the polarity of the SH annular mode (SAM), a large-scale pattern of climate variability characterized by fluctuations in the strength of the SH circumpolar flow. Consistent with the climate impacts of the SAM, variations in the stratospheric polar vortex are also followed by coherent changes in surface temperatures throughout much of Antarctica. The results add to a growing body of evidence that suggests that stratospheric variability plays an important role in driving climate variability at Earth’s surface on a range of time scales.

The mutual impact of weather regimes and the stratospheric circulation on European surface weather

2020 ◽  
Author(s):  
Christian M. Grams ◽  
Remo Beerli ◽  
Dominik Büeler ◽  
Daniela I. V. Domeisen ◽  
Lukas Papritz ◽  
...  
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Polar Vortex ◽  
Stratospheric Polar Vortex ◽  
Weather Regimes ◽  
The North ◽  
Weather Events ◽  
Surface Weather ◽  
The North Atlantic ◽  
Large Scale Flow

<p>Extreme states of the winter stratosphere, such as sudden stratospheric warmings (SSWs) or an extremely strong stratospheric polar vortex (SPV), can affect surface weather over the North-Atlantic European region on subseasonal time scales. Here we investigate the occurrence of Atlantic-European weather regimes during different stratospheric conditions in winter and their link to large-scale weather events in European sub-regions. We further elucidate if the large-scale flow regime in the North Atlantic at SSW onset determines the subsequent downward impact.</p><p>Anomalous stratospheric conditions modulate the occurrence of weather regimes which project strongly onto the NAO and the likelihood of their associated weather events. In contrast weather regimes which do not project strongly onto the NAO are not affected by anomalous stratospheric conditions. These regimes provide pathways to unexpected weather events in extreme stratospheric polar vortex states. For example, Greenland blocking (GL) and the Atlantic Trough (AT) regime are the most frequent large-scale flow patterns following SSWs. While in Central Europe GL provides a pathway to cold and calm weather, AT provides a pathway to warm and windy weather. The latter weather conditions are usually not expected after an SSW. Furthermore, we find that a blocking situation over western Europe and the North Sea (European Blocking) at the time of the SSW onset favours the GL response and associated cold conditions over Europe. In contrast, an AT response and mild conditions are more likely if GL occurs already at SSW onset. An assessment of forecast performance in ECMWF extended-range reforecasts suggests that the model tends to forecast too cold conditions following weak SPV states.</p><p>In summary, weather regimes and their response to anomalous SPV states importantly modulate the stratospheric impact on European surface weather. In particular the tropospheric impact of SSW events critically depends on the tropospheric state during the onset of the SSW. We conclude that a correct representation of weather regime life cycles in numerical models could provide crucial guidance for subseasonal prediction.</p><p> </p><p>References:</p><p>Beerli, R., and C. M. Grams, 2019: Stratospheric modulation of the large-scale circulation in the Atlantic–European region and its implications for surface weather events. Q.J.R. Meteorol. Soc., <strong>145</strong>, 3732–3750, doi:10.1002/qj.3653.</p><p>Domeisen, D. I. V., C. M. Grams, and L. Papritz, 2020: The role of North Atlantic-European weather regimes in the surface impact of sudden stratospheric warming events. Weather and Climate Dynamics Discussions, 1–24, doi:https://doi.org/10.5194/wcd-2019-16.</p>

Investigating the connection between tropospheric blocking and sudden stratospheric warming events using GNSS radio occultation observations

2021 ◽  
Author(s):  
Kamilya Yessimbet ◽  
Andrea Steiner
Keyword(s):  
Radio Occultation ◽  
Polar Vortex ◽  
Satellite System ◽  
Wave Activity ◽  
Stratospheric Warming ◽  
Mean Flow ◽  
Sudden Stratospheric Warming ◽  
Stratospheric Polar Vortex ◽  
First Results ◽  
Global Navigation Satellite

<p>Both sudden stratospheric warming (SSW) events and tropospheric blocking events can have a significant influence on winter extratropical surface weather. Upward propagating planetary waves from the troposphere can interact with the stratospheric mean flow and disrupt the stratospheric polar vortex, which is associated with an SSW event. Blocking has often been suggested as one of the tropospheric precursors for anomalous upward propagating wave activity flux. It remains an open question to what extent upward wave activity caused by blocking is related to SSW events. In the present study, we examine the evolution of the Eliassen-Palm fluxes during blocking events that precede SSWs. We use Global Navigation Satellite System radio occultation measurements for this analysis to provide accurate and vertically well-resolved information on the wave coupling between these two phenomena in the upper troposphere and stratosphere. First results will be presented and discussed.</p><p>Keywords: sudden stratospheric warming, Eliassen-Palm flux, blocking</p>

scholarly journals Differences in the Sub-seasonal Predictability of Extreme Stratospheric Events

2022 ◽  
Author(s):  
Rachel Wai-Ying Wu ◽  
Zheng Wu ◽  
Daniela I. V. Domeisen
Keyword(s):  
Extreme Events ◽  
Weather Prediction ◽  
Polar Vortex ◽  
Wave Activity ◽  
Prediction System ◽  
Wave Amplification ◽  
Weather Forecasts ◽  
Surface Weather ◽  
Linear Behaviour ◽  
Strong Vortex

Abstract. Extreme stratospheric events such as sudden stratospheric warming and strong vortex events associated with an anomalously weak or strong polar vortex can have downward impacts on surface weather that can last for several weeks to months. Hence, successful predictions of these stratospheric events would be beneficial for extended range weather prediction. However, the predictability limit of extreme stratospheric events is most often limited to around 2 weeks or less. The predictability also strongly differs between events, and between event types. The reasons for the observed differences in the predictability, however, are not resolved. To better understand the predictability differences between events, we expand the definitions of extreme stratospheric events to wind deceleration and acceleration events, and conduct a systematic comparison of predictability between event types in the European Centre for Medium-Range Weather Forecasts (ECMWF) prediction system for the sub-seasonal predictions. We find that wind deceleration and acceleration events follow the same predictability behaviour, that is, events of stronger magnitude are less predictable in a close to linear relationship, to the same extent for both types of events. There are however deviations from this linear behaviour for very extreme events. The difficulties of the prediction system in predicting extremely strong anomalies can be traced to a poor predictability of extreme wave activity pulses in the lower stratosphere, which impacts the prediction of deceleration events, and interestingly, also acceleration events. Improvements in the understanding of the wave amplification that is associated with extremely strong wave activity pulses and accurately representing these processes in the model is expected to enhance the predictability of stratospheric extreme events and, by extension, their impacts on surface weather and climate.

scholarly journals Learning forecasts of rare stratospheric transitions from short simulations

2021 ◽  
Author(s):  
Justin Finkel ◽  
Robert J. Webber ◽  
Edwin P. Gerber ◽  
Dorian S. Abbot ◽  
Jonathan Weare
Keyword(s):  
Lead Time ◽  
Rare Events ◽  
Rare Event ◽  
Polar Vortex ◽  
Return Time ◽  
Model Complexity ◽  
Stratospheric Polar Vortex ◽  
Weather Patterns ◽  
Optimal Measurements ◽  
The Cost

AbstractRare events arising in nonlinear atmospheric dynamics remain hard to predict and attribute. We address the problem of forecasting rare events in a prototypical example, Sudden Stratospheric Warmings (SSWs). Approximately once every other winter, the boreal stratospheric polar vortex rapidly breaks down, shifting midlatitude surface weather patterns for months. We focus on two key quantities of interest: the probability of an SSW occurring, and the expected lead time if it does occur, as functions of initial condition. These optimal forecasts concretely measure the event’s progress. Direct numerical simulation can estimate them in principle, but is prohibitively expensive in practice: each rare event requires a long integration to observe, and the cost of each integration grows with model complexity. We describe an alternative approach using integrations that are short compared to the timescale of the warming event. We compute the probability and lead time efficiently by solving equations involving the transition operator, which encodes all information about the dynamics. We relate these optimal forecasts to a small number of interpretable physical variables, suggesting optimal measurements for forecasting. We illustrate the methodology on a prototype SSW model developed by Holton and Mass (1976) and modified by stochastic forcing. While highly idealized, this model captures the essential nonlinear dynamics of SSWs and exhibits the key forecasting challenge: the dramatic separation in timescales between a single event and the return time between successive events. Our methodology is designed to fully exploit high-dimensional data from models and observations, and has the potential to identify detailed predictors of many complex rare events in meteorology.

scholarly journals Surface impacts of the Quasi Biennial Oscillation

2018 ◽  
Vol 18 (11) ◽  
pp. 8227-8247 ◽  
Author(s):  
Lesley J. Gray ◽  
James A. Anstey ◽  
Yoshio Kawatani ◽  
Hua Lu ◽  
Scott Osprey ◽  
...  
Keyword(s):  
Polar Vortex ◽  
Late Winter ◽  
Quasi Biennial Oscillation ◽  
Stratospheric Polar Vortex ◽  
Zonal Winds ◽  
Orthogonal Function ◽  
The North ◽  
The Pacific ◽  
Biennial Oscillation ◽  
The North Pacific

Abstract. Teleconnections between the Quasi Biennial Oscillation (QBO) and the Northern Hemisphere zonally averaged zonal winds, mean sea level pressure (mslp) and tropical precipitation are explored. The standard approach that defines the QBO using the equatorial zonal winds at a single pressure level is compared with the empirical orthogonal function approach that characterizes the vertical profile of the equatorial winds. Results are interpreted in terms of three potential routes of influence, referred to as the tropical, subtropical and polar routes. A novel technique is introduced to separate responses via the polar route that are associated with the stratospheric polar vortex, from the other two routes. A previously reported mslp response in January, with a pattern that resembles the positive phase of the North Atlantic Oscillation under QBO westerly conditions, is confirmed and found to be primarily associated with a QBO modulation of the stratospheric polar vortex. This mid-winter response is relatively insensitive to the exact height of the maximum QBO westerlies and a maximum positive response occurs with westerlies over a relatively deep range between 10 and 70 hPa. Two additional mslp responses are reported, in early winter (December) and late winter (February/March). In contrast to the January response the early and late winter responses show maximum sensitivity to the QBO winds at ∼ 20 and ∼ 70 hPa respectively, but are relatively insensitive to the QBO winds in between (∼ 50 hPa). The late winter response is centred over the North Pacific and is associated with QBO influence from the lowermost stratosphere at tropical/subtropical latitudes in the Pacific sector. The early winter response consists of anomalies over both the North Pacific and Europe, but the mechanism for this response is unclear. Increased precipitation occurs over the tropical western Pacific under westerly QBO conditions, particularly during boreal summer, with maximum sensitivity to the QBO winds at 70 hPa. The band of precipitation across the Pacific associated with the Inter-tropical Convergence Zone (ITCZ) shifts southward under QBO westerly conditions. The empirical orthogonal function (EOF)-based analysis suggests that this ITCZ precipitation response may be particularly sensitive to the vertical wind shear in the vicinity of 70 hPa and hence the tropical tropopause temperatures.

scholarly journals Diagnostic Comparison of Tropospheric Blocking Events with and without Sudden Stratospheric Warming

2015 ◽  
Vol 72 (6) ◽  
pp. 2227-2240 ◽  
Author(s):  
Stephen J. Colucci ◽  
Michael E. Kelleher
Keyword(s):  
Geopotential Height ◽  
Onset Time ◽  
Polar Vortex ◽  
Outer Edge ◽  
Heat Fluxes ◽  
Necessary Condition ◽  
Stratospheric Warming ◽  
Sudden Stratospheric Warming ◽  
Stratospheric Polar Vortex ◽  
Upper Troposphere

Abstract Tropospheric blocking events over the Northern Hemisphere during 1980–2012 were composited and contrasted according to whether they coincided in time with a sudden stratospheric warming (SSW). Those that coincided with an SSW were associated with significantly greater poleward eddy heat fluxes in the upper troposphere near the block onset time than were those blocking events not coinciding with an SSW. Furthermore, the heat fluxes in the SSW–blocking composites were concentrated inside the stratospheric polar vortex (i.e., within an area enclosed by the outer edge of an objectively defined polar vortex). Thermally forced stratospheric geopotential height rises were also significantly larger near block onset time inside the stratospheric polar vortex in the SSW–blocking composites than in the non-SSW–blocking cases. Although all the SSW events during the investigated period coincided with tropospheric blocking, the reverse was not true since there were many more blocking events than SSWs. Therefore, blocking itself was not a sufficient condition for an SSW. It is conjectured that blocking may not be a necessary condition for an SSW if persistently anomalous tropospheric heat fluxes and thermally forced, stratospheric geopotential height rises, concentrated inside the stratospheric vortex, occur in the absence of blocking.

Sign in / Sign up

Export Citation Format

Share Document