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 Tracing North Atlantic Oscillation Forecast Errors to Stratospheric Origins

2020 ◽  
Vol 33 (21) ◽  
pp. 9145-9157 ◽  
Author(s):  
Erik W. Kolstad ◽  
C. Ole Wulff ◽  
Daniela I. V. Domeisen ◽  
Tim Woollings
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Weather Forecasting ◽  
North Atlantic Ocean ◽  
Polar Vortex ◽  
Forecast Errors ◽  
Stratospheric Polar Vortex ◽  
The North ◽  
Extended Range ◽  
The North Atlantic Oscillation

AbstractThe 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. 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.

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.

scholarly journals The Wave Geometry of Final Stratospheric Warming Events

2021 ◽  
Author(s):  
Amy H. Butler ◽  
Daniela I. V. Domeisen
Keyword(s):  
North Atlantic ◽  
Polar Vortex ◽  
Stratospheric Warming ◽  
Stratospheric Polar Vortex ◽  
Total Column Ozone ◽  
Planetary Scale ◽  
The North ◽  
Downward Influence ◽  
Column Ozone ◽  
Total Column

Abstract. Every spring, the stratospheric polar vortex transitions from its westerly wintertime state to its easterly summertime state due to seasonal changes in incoming solar radiation, an event known as the final stratospheric warming (FSW). While FSWs tend to be less abrupt than reversals of the boreal polar vortex in midwinter, known as sudden stratospheric warming (SSW) events, their timing and characteristics can be significantly modulated by atmospheric planetary-scale waves. Just like SSWs, FSWs have been found to have predictable surface impacts. While SSWs are commonly classified according to their wave geometry, either by how the vortex evolves (whether the vortex displaces off the pole or splits into two vortices) or by the dominant wavenumber of the vortex just prior to the SSW (wave-1 versus wave-2), little is known about the wave geometry of FSW events. We here show that FSW events for both hemispheres in most cases exhibit a clear wave geometry. Most FSWs can be classified into wave-1 or wave-2 events, but wave-3 also plays a significant role in both hemispheres. Additionally, we find that in the Northern Hemisphere, wave-2 events are more likely to occur later in the spring, while in the Southern Hemisphere, wave-1 or wave-2 events show no clear preference in timing. The FSW enhances total column ozone over the pole of both hemispheres during spring, but the spatial distribution of ozone anomalies can be influenced by the wave geometry and the timing of the event. We also describe the stratosphere's downward influence on surface weather following wave-1 and wave-2 FSW events. Significant differences between the tropospheric response to wave-1 and wave-2 FSW events occur over North America and over the Southern Ocean, while no significant differences are found over the North Atlantic region, Europe, and Antarctica.

Daily North-Atlantic Oscillation (NAO) index: Statistics and its stratospheric polar vortex dependence

2005 ◽  
Vol 14 (6) ◽  
pp. 763-769 ◽  
Author(s):  
Simon Blessing ◽  
Klaus Fraedrich ◽  
Martina Junge ◽  
Torben Kunz ◽  
Frank Lunkeit
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Polar Vortex ◽  
Stratospheric Polar Vortex ◽  
Nao Index

scholarly journals Eurasian autumn snow impact on winter North Atlantic Oscillation depends on cryospheric variability

2019 ◽  
Author(s):  
Martin Wegmann ◽  
Marco Rohrer ◽  
María Santolaria-Otín ◽  
Gerrit Lohmann
Keyword(s):  
Sea Ice ◽  
North Atlantic Oscillation ◽  
Snow Cover ◽  
North Atlantic ◽  
Polar Vortex ◽  
Negative Temperature ◽  
Atmospheric Conditions ◽  
Sea Ice Concentration ◽  
Stratospheric Polar Vortex ◽  
Winter Climate

Abstract. In recent years, many components of the connection between Eurasian autumn snow cover and wintertime North Atlantic Oscillation (NAO) were investigated, suggesting that November snow cover distribution has strong prediction power for the upcoming Northern Hemisphere winter climate. However, non-stationarity of this relationship could impact its use for prediction routines. Here we use snow products from long-term reanalyses to investigate interannual and interdecadal links between autumnal snow cover and atmospheric conditions in winter. We find evidence for a negative NAO tendency after November with a strong west-to-east snow cover gradient, which is valid throughout the last 150 years. This correlation is linked with a consistent impact of November snow on a slowed stratospheric polar vortex. Nevertheless, interdecadal variability for this relationship shows episodes of decreased correlation power, which co-occur with episodes of low variability in the November snow index. We find that the same is also true for sea ice as an NAO predictor. The snow dipole itself is associated with reduced Barents-Kara sea ice concentration, increased Ural blocking frequency and negative temperature anomalies in eastern Eurasia. Increased sea ice variability in recent years is linked to increased snow variability, thus increasing its power in predicting the winter NAO.

The wave geometry of final stratospheric warming events

2021 ◽  
Author(s):  
Amy H. Butler ◽  
Daniela I.V. Domeisen
Keyword(s):  
North Atlantic ◽  
Polar Vortex ◽  
Stratospheric Warming ◽  
Stratospheric Polar Vortex ◽  
Total Column Ozone ◽  
Planetary Scale ◽  
The North ◽  
Downward Influence ◽  
Column Ozone ◽  
Total Column

<p>Every spring, the stratospheric polar vortex transitions from its westerly wintertime state to its easterly summertime state due to seasonal changes in incoming solar radiation, an event known as the "final stratospheric warming" (FSW). While FSWs tend to be less abrupt than reversals of the boreal polar vortex in midwinter, known as sudden stratospheric warming (SSW) events, their timing and characteristics can be significantly modulated by atmospheric planetary-scale waves. Just like SSWs, FSWs have been found to have predictable surface impacts. While SSWs are commonly classified according to their wave geometry, either by how the vortex evolves (whether the vortex displaces off the pole or splits into two vortices) or by the dominant wavenumber of the vortex just prior to the SSW (wave-1 versus wave-2), little is known about the wave geometry of FSW events. We here show that FSW events for both hemispheres in most cases exhibit a clear wave geometry. Most FSWs can be classified into wave-1 or wave-2 events, but wave-3 also plays a significant role in both hemispheres. Additionally, we find that in the Northern Hemisphere, wave-2 events are more likely to occur later in the spring, while in the Southern Hemisphere, wave-1 or wave-2 events show no clear preference in timing. The FSW enhances total column ozone over the pole of both hemispheres during spring, but the spatial distribution of ozone anomalies can be influenced by the wave geometry and the timing of the event. We also describe the stratosphere's downward influence on surface weather following wave-1 and wave-2 FSW events. Significant differences between the tropospheric response to wave-1 and wave-2 FSW events occur over North America and over the Southern Ocean, while no significant differences are found over the North Atlantic region, Europe, and Antarctica. </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 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.

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