For each index and level of the atmosphere, N arbitrary winter days are sampled, where N is the number of events that enter the respective composite. The mean is calculated over these N days. The stippled regions indicate the areas where the composite exceeds 2 standard deviations of the distributions of these mean values. During the cold spell one can identify a signal in the geopotential height index and, in accordance, in the zonal wind and the temperature index also in the stratosphere, but this is rather a consequence of the temperature anomaly at lower levels than the cause of the cold spell.
In all indices there are, to some degree, signals in the troposphere before day 0 of the composite, and effects in the stratosphere occur instantaneously or later in time. In the NAM index the signal in the stratosphere shows up prior to the disturbance in the troposphere. It is evident rather instantaneously throughout the stratosphere, but the persistence of the anomalies is larger in the lower stratosphere.
However, in the regional geopotential height index and the regional wind index, anomalies are present in the troposphere before the main development of the events in the stratosphere.
The tropospheric disturbances could have been, at least partially, the cause of increased wave breaking and deceleration of the vortex in the stratosphere [ Cohen et al. As a consequence, the tropopause is continuously lowered fourth row of Figure 7. The change in the tropopause height during weak vortex events is an immediate consequence of the strong warming of the lower stratosphere, and is further amplified by dynamical feedbacks which cause a cooling in the troposphere.
The theory is developed in the context of a quasigeostrophic model from which a potential vorticity equation is derived. Here again, negative potential vorticity anomalies in the stratosphere result in a lowered tropopause, a compressed tropospheric column below, and consequently a reduced relative vorticity over the polar cap associated with a high pressure signal.
Similar ideas were put forward in Hartley et al. Black and McDaniel  point out that the potential vorticity anomalies need to descend to sufficiently low altitudes within the stratosphere in order to have an effect on the troposphere. Figure 8 left contains a mean over days 30 to 15 before the start of the cold spells, Figure 8 middle a mean over day 15 to day 1 before, and Figure 8 right a mean over day 1 to day 15 of the cold spells.
They intensify during the 15 days before the cold spells, and become evident in the lower stratosphere as well.
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Similarly, Williams  describes how increased temperatures in the stratosphere and an associated lower tropopause height shift the tropospheric jet equatorwards in idealized model simulations. Gerber and Vallis  show that in idealized model experiments the interaction between synoptic eddies retard the motion of the jet, slowing its meridional variation and thereby extending the persistence of annular mode anomalies in the troposphere. During day 30 to day 1 before the cold spells Figures 9 , bottom left, and 9, bottom middle , the poleward flank of the tropospheric jet over the North Atlantic gradually weakens, while the equatorward side strengthens.
The negative zonal wind anomalies of the stratosphere extend downward into the troposphere over the region of the occurrence of the cold spells. The strengthening of the zonal wind at the southern side of the area of surface temperature anomalies is consistent with the geopotential height anomaly pattern Figure 5. The lowering of the tropopause caused by the stratospheric warming over the polar cap produces a tropospheric high pressure anomaly over the polar region which leads to the advection of cold air towards Northern Europe.
The resulting cold air outbreak induces negative geopotential height anomalies over the continent which amplifies the negative NAM signature. At the southern flank of the cold spell area the meridional pressure gradient and consequently the zonal flow are enhanced. In accordance with Charlton et al. Quantities are averaged over the latitudes 65N to 75N, the qualitative picture does however not depend on the exact region considered.
Significant anomalies in the strength of the residual circulation occur mainly at the beginning of the stratospheric vortex disturbance.
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This suggests that the residual mean meridional circulation does not directly play a decisive role in the mechanism through which the stratosphere impacts the troposphere. Rather, in accordance with Cai and Ren  , the apparent downward propagation of stratospheric disturbances into the troposphere is a consequence of the dynamic response to heating and cooling anomalies. However, also in these cases preexisting geopotential height anomalies reminiscent of a negative annular mode pattern are present in the troposphere. In the lower stratosphere and near the tropopause, the temperature anomalies persist due to the relatively low efficiency of radiative cooling at these height levels [ Kiehl and Solomon , ].
Anomalies in the residual mean meridional circulation partly contribute to the adiabatic warming in the lower stratosphere, but the anomaly in the residual circulation mass stream function is mainly restricted to the period of the strongest dynamical disturbance in the stratosphere. Zonal wind anomalies are thus transferred from the stratosphere to the troposphere, and the advection of cold air from the northeast towards Northern Europe further reinforces the negative NAM pattern in the troposphere. The dynamical feedback amplifies the effect of the lower stratospheric perturbation in the troposphere.
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Support by James Anstey, who provided the code for the blocking index computation, as well as the Max Planck Society for the Advancement of Science, is gratefully acknowledged. Volume 4 , Issue 4. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account.
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If the address matches an existing account you will receive an email with instructions to retrieve your username. Open access. Journal of Advances in Modeling Earth Systems. Open Access. Corresponding author: L. Edwin P. Search for more papers by this author. Mark P. Tools Request permission Export citation Add to favorites Track citation.
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Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Abstract  Extreme cold spells over Northern Europe during winter are examined in order to address the question to what degree and in which ways stratospheric dynamics may influence the state of the troposphere. Figure 1 Open in figure viewer PowerPoint. Figure 2 Open in figure viewer PowerPoint. Distribution of monthly winter temperature anomalies over the land area of 48 North to 65 North and 0 East to 40 East based on observations for the period to [ Casty et al.
Figure 3 Open in figure viewer PowerPoint. Blocking occurrence is measured in percentage of days with blocking signature. Figure 4 Open in figure viewer PowerPoint. Forgot password? Keep me logged in. New User. Change Password. Old Password.
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If the address matches an existing account you will receive an email with instructions to reset your password Close. Request Username. Forgot your username? Enter your email address below and we will send you your username. Restricted access. References Andrews D. G, Holton J. B Middle atmosphere dynamics. Baldwin M. Brohan P, Kennedy J. In-situ detection of OH in the lower stratosphere with a balloon borne high repetition rate laser system. Wennberg, P. Aircraft-borne, laser-induced fluorescence instrument for the in situ detection of hydroxyl and hydroperoxyl radicals.
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Estimates of atmospheric hydroxyl radical concentrations from the observed decay of many reactive hydrocarbons in well-defined urban plumes. Williams, J. Variability-lifetime relationship for organic trace gases: A novel aid to compound identification and estimation of HO concentrations. HO cycle in and over the southern Indian Ocean derived from CO, radon, and hydrocarbon measurements made at Amsterdam Island. Rigby, M. Role of atmospheric oxidation in recent methane growth.