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Vol.:(0123456789)
1 3
Clim Dyn (2017) 49:3473–3491
DOI 10.1007/s00382-017-3524-1
Intra-seasonal variability ofextreme boreal stratospheric polar
vortex events andtheir precursors
AdelaidaDíaz-Durán1 · EncarnaSerrano1· BlancaAyarzagüena2· MartaAbalos3·
Alvarode laCámara3
Received: 12 May 2016 / Accepted: 3 January 2017 / Published online: 20 January 2017
© Springer-Verlag Berlin Heidelberg 2017
study suggests that dynamical features preceding extreme
polar vortex events in mid-winter should not be generalized
to other winter sub-periods.
Keywords Intra-seasonal variability· Wave activity·
Stratospheric polar vortex extremes· Tropospheric
precursors· Tropospheric forcing· Stratospheric dynamics
1 Introduction
The boreal stratospheric polar vortex is perturbed by a
variety of forcings, such as the Quasi-Biennial Oscillation
(QBO) (Holton and Tan 1980), El Niño-Southern Oscilla-
tion (ENSO) (Manzini etal. 2006; Taguchi and Hartmann
2006; Butler and Polvani 2011), tropospheric blockings
(Martius et al. 2009; Woollings et al. 2010; Barriopedro
and Calvo 2014; Ayarzagüena et al. 2015) or the Arctic
sea ice content (García-Serrano et al. 2015; Kolstad etal.
2015). All these studies are based on observations apart
from Taguchi and Hartmann (2006) and Ayarzagüena etal.
(2015), which are modelling works and Manzini et al.
(2006), Woollings et al. (2010) and Kolstad et al. (2015)
that combine both. The previously cited forcings modu-
late the upward wave activity entering the stratosphere that
leads to stratospheric polar vortex anomalies (Palmer 1981;
Li et al. 2007; Solomon 2014). In particular, an anoma-
lously weak polar vortex (WPV, hereafter) is preceded by
a strong upward wave activity from the troposphere caus-
ing warming over the stratospheric polar region and weak-
ening of the polar cyclonic circulation (Limpasuvan etal.
2004; Polvani and Waugh 2004). The opposite is true for
strong polar vortex events (SPV hereafter, Christiansen
2001). The most dramatic weak vortex regime in the North-
ern Hemisphere (NH) winter is called major stratospheric
Abstract The dynamical variability of the boreal strato-
spheric polar vortex has been usually analysed consider-
ing the extended winter as a whole or only focusing on
December, January and February. Yet recent studies have
found intra-seasonal differences in the boreal stratospheric
dynamics. In this study, the intra-seasonal variability of
anomalous wave activity preceding polar vortex extremes
in the Northern Hemisphere is examined using ERA-
Interim reanalysis data. Weak (WPV) and strong (SPV)
polar vortex events are grouped into early, mid- or late
winter sub-periods depending on the onset date. Overall,
the strongest (weakest) wave-activity anomalies preced-
ing polar vortex extremes are found in mid- (early) winter.
Most of WPV (SPV) events in early winter occur under
the influence of east (west) phase of the Quasi-Biennial
Oscillation (QBO) and an enhancement (inhibition) of
wavenumber-1 wave activity (WN1). Mid- and late win-
ter WPV events are preceded by a strong vortex and an
enhancement of WN1 and WN2, but the spatial structure
of the anomalous wave activity and the phase of the QBO
are different. Prior to mid-winter WPVs the enhancement
of WN2 is related to the predominance of La Niña and
linked to blockings over Siberia. Mid-winter SPV events
show a negative phase of the Pacific-North America pat-
tern that inhibits WN1 injected into the stratosphere. This
* Adelaida Díaz-Durán
adelaidadiazduran@ucm.es
1 Departamento de Geofísica y Meteorología, Facultad de CC.
Físicas, Universidad Complutense de Madrid, Madrid, Spain
2 College ofEngineering, Mathematics andPhysical Sciences,
University ofExeter, Exeter, UK
3 National Center forAtmospheric Research, Boulder, CO,
USA
3474 A.Díaz-Durán et al.
1 3
warming (MSW), during which polar temperature increases
dramatically in a few days, the equator-pole temperature
gradient reverses and the zonal-mean flow becomes east-
erly (Andrews etal. 1987).
The importance of the occurrence of these stratospheric
extreme events for the tropospheric climate variability has
been long documented (Baldwin and Dunkerton 1999,
2001), and several studies have shown an improvement of
seasonal forecasts based on stratospheric information using
different models or statistical methods (Christiansen 2005;
Scaife etal. 2014).
Typically, changes in the boreal polar vortex and its
dynamics have been analysed by considering the whole
extended winter (from November to March) or only focus-
ing on the most dynamically active months (December,
January and February) as representative of the winter sea-
son (Kodera etal. 2003). Recent studies have consistently
shown intra-seasonal differences in the dynamics of the
extended boreal winter. Using a chemistry-climate model
(CCM), Ayarzagüena et al. (2013) identified a different
polar stratospheric response to future changes in early win-
ter and mid-to-late winter under projected climate change
scenarios. This work showed that, even though there are
no statistically significant future changes in the mean fre-
quency of MSWs, there is a shift in the occurrence of this
phenomenon with more events registered in mid- and late
winter in a future climate. Solomon (2014) classified the
episodes of enhanced wave activity (WAEs) into four cat-
egories in a decreasing sequence according to its magni-
tude (major, minor, final and other WAEs) and identified
their most common timing. Major WAEs are more likely
to occur during mid-winter, minor WAEs are recorded
throughout the extended winter, final WAEs occur most
likely in late winter, and the other WAEs predominate in
early winter. Nevertheless, Solomon (2014) did not explore
the reasons for the variability in wave activity among win-
ter sub-periods and the potential precursors in each type
of episodes. The goal of the present paper is to analyse the
intra-seasonal variability of stratospheric anomalous cir-
culation in the NH, and to gain insight into the dynamical
mechanisms driving the extreme polar vortex events in dif-
ferent periods throughout the extended NH winter.
The structure of this paper is as follows. Section 2
describes the data and methodology used in this work to
identify extreme polar vortex events and to study the cor-
responding anomalous tropospheric wave-activity propa-
gation. In Sect. 3 we show and discuss intra-seasonal
differences in the wave-activity climatology, as well as dif-
ferences in the anomalous wave activity and anomalous cir-
culation structures prior to WPV and SPV events in three
winter sub-periods (early, mid- and late winter). Finally, in
Sect.4, we include a summary with the main conclusions
derived from our work.
2 Data andmethodology
We use daily-mean data of the three wind components,
temperature and geopotential height from ERA-Interim
reanalysis (Dee et al. 2011), calculated by averaging the
6-hourly data, for the period 1979–2011. The data extends
from 1000 to 1 hPa (37 levels) and the spatial domain is
the Northern Hemisphere (NH) and the subtropical region
of Southern Hemisphere (20ºS–90ºN, 180ºW–180ºE) on a
1.5º × 1.5º grid.
The analysis has been restricted to post-satellite era in
order not to combine post and pre-satellite data that might
lead to misleading results. For instance, Gomez-Escolar
et al. (2012) found differences in the polar stratospheric
temperature climatology in mid-winter between pre-satel-
lite and post-satellite eras, which might be related to satel-
lite data assimilation. Consequently, ERA-Interim has been
used despite covering a shorter period than other currently
reanalysis datasets (e.g.: JRA-55 or NCEP-NCAR).
2.1 Extreme polar vortex events
In this work, the strength of the stratospheric polar vortex is
quantified by using daily values of zonal mean zonal wind
at 60ºN (denoted as
u60
) and 10hPa. The central date (day
0) of a WPV is defined as the first day that
u60
at 10hPa
falls below its daily 15th percentile according to the corre-
sponding daily climatology over 1979–2011. In the case of
SPV events, its central day is identified as the first day that
u60
at 10hPa reaches out its daily 85th percentile (Fig.1a).
Note that percentiles 15th and 85th are a proxy of the mean
value of minus and plus one standard deviation, respec-
tively, in a Gaussian distribution (although we note this
might not be the case for this data). These events should
persist at least 10 days to be considered as extreme events.
In addition, two consecutive extreme events are taken as
independent events if there are at least 10 days between
the disappearance of the former and the onset of the latter.
The 10-day interval corresponds approximately to the win-
tertime radiative relaxation time scale at 10hPa and 60ºN
(Newman and Rosenfield 1997).
We distinguish WPV and SPV events occurred in Octo-
ber-November-December (OND, early winter), in Janu-
ary–February (JF, mid-winter) and in March–April (MA,
late winter). The reason of including October in early
winter, unlike other studies, is because the polar vortex
has already formed according to the geopotential height at
50hPa. In addition, October has the same number of hours
of sunshine as April, which is usually considered part of
the extended winter (Limpasuvan etal. 2004; Kolstad etal.
2015). With these criteria, we have registered 33 WPV (15
in OND, 9 in JF and 9 in MA) and 32 SPV events (17 in
OND, 11 in JF and 4 in MA) (Table1). Note that 8 out
3475Intra-seasonal variability ofextreme boreal stratospheric polar vortex events andtheir…
1 3
of the 9 WPV events which occurred in mid-winter (JF)
satisfy the MSW conditions, i.e., the meridional tempera-
ture gradient at 10 hPa between 60ºN and the North Pole
reverses and
u60
at 10hPa becomes easterly (Quiroz 1979;
Labitzke 1981). The distribution of WPV and SPV events
under the different ENSO and QBO phase is shown in
Fig. 1b, c, respectively. ENSO and QBO information has
been taken from NOAA webpages, namely http://www.
cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/
ensoyears_ERSSTv3b.shtml and http://www.cpc.noaa.gov/
data/indices/, respectively.
2.2 Wave activity propagation
We investigate the anomalous wave activity propagation
and its effect on the mean stratospheric flow in NH winter
the week (average of the 7 days) prior to the occurrence of
SPVs and WPVs in each winter sub-period, by computing
composites of the corresponding anomalies of Eliassen-
Palm flux and its divergence (Eliassen and Palm 1961).
The analysis of the anomalous upward wave propa-
gation is completed by means of the anomalous meridi-
onal eddy heat flux (vʹTʹ)a at 100 hPa (proportional to
the vertical component Fz of E-P flux) the week prior to
the occurrence of SPV and WPV. Hu and Tung (2003)
demonstrated that (vʹTʹ) averaged over the extratrop-
ics at 100hPa gives a good measure of the tropospheric
wave injection into the stratosphere. We calculate the
(vʹTʹ)a at 100hPa averaged over 45ºN–75ºN for all zonal
Fig. 1 a Daily percentiles of
the zonal mean zonal wind at
10hPa and 60ºN (in ms− 1) for
the period 1979–2011 through
the extended winter (Octo-
ber–April). Bottom and top red
lines indicate the 15th and 85th
percentiles, respectively, and
the blue line represents the 50th
percentile. b Distribution of
WPV events in early, mid- and
late winter (OND, JF and MA,
respectively) on basis of ENSO
conditions and direction of
equatorial zonal wind at 50hPa,
i.e., phase of QBO. c As b but
for SPV events
1 Oct 1 Nov 1 Dec 1 Jan 1 Feb 1 Mar 1 Apr
u (m/s)
-20
-10
0
10
20
30
40
50
60
WPV SPV
El Niño
La Niña
Neutral
U50>0
U50<0
OND
4
411
JF
2 6 1
3 6
MA
1 3 5
6 3
OND
JF
MA
12 5
83
3 1
75 5
3 4 4
1 2 1
6
5
(a)
(b)
(c)
Table 1 Central date of WPV and SPV events identified in ERA-
Interim during the winter period 1979–2011
Central date of WPV (33 events) Central date of SPV (32 events)
23 Nov 1979
28 Feb 1980
01 Dec 1981
16 Apr 1982
24 Feb 1984
27 Dec 1984
24 Mar 1985
20 Mar 1986
20 Jan 1987
19 Nov 1987
20 Feb 1989
22 Mar 1992
02 Apr 1994
11 Apr 1996
16 Nov 1996
18 Dec 1997
11 Dec 1998
25 Feb 1999
11 Nov 2000
06 Feb 2001
21 Dec 2001
17 Apr 2003
26 Dec 2003
25 Oct 2004
11 Mar 2005
21 Oct 2005
07 Jan 2006
17 Oct 2006
10 Oct 2007
23 Jan 2009
06 Nov 2009
26 Jan 2010
05 Apr 2011
16 Jan 1980
28 Nov 1980
17 Apr 1981
18 Nov 1982
03 Jan 1983
27 Oct 1983
21 Jan 1984
09 Dec 1985
19 Oct 1986
24 Jan 1988
07 Oct 1988
19 Dec 1988
22 Dec 1989
26 Oct 1990
17 Dec 1991
23 Jan 1993
14 Feb 1994
29 Nov 1994
01 Mar 1995
05 Feb 1996
27 Jan 1997
22 Oct 1997
06 Jan 2000
03 Apr 2001
26 Oct 2001
07 Nov 2002
12 Jan 2005
10 Dec 2004
23 Dec 2007
30 Dec 2008
27 Mar 2009
08 Feb 2011
3476 A.Díaz-Durán et al.
1 3
wavenumbers and the first two zonal wavenumbers
(k = 1, 2) using Fast Fourier Transform filters.
Since we search for the main regions responsible for
the change in the tropospheric wave injection into the
stratosphere, we also plot not spatially averaged (vʹTʹ),
which corresponds to a rough approximation of the ver-
tical component of the 3D Plumb Flux (Plumb 1985). As
we are interested in dynamical effects of persistent trop-
ospheric circulation structures, daily meridional wind
and temperature values have been smoothed out with
5-day running mean in order to identify quasi-stationary
waves (Nishii etal. 2009).
The anomalous eddy heat flux has been decomposed
into two different components (Nishii etal. 2009; Smith
and Kushner 2012):
The subscripts a and c indicate anomalies and clima-
tological values, respectively. The first right-hand term
of Eq. 1 corresponds to the nonlinear contribution of
anomalous waves, and the sum of the second and the
third term indicates the modulation of climatological
waves by wave anomalies.
2.3 Statistical significance ofresults
The statistical significance of the composites is assessed
with a Monte Carlo-like test of 5000 samples. Because
the composites for each winter sub-period are computed
by considering the central date of the extreme events and
their respective previous 7 days, each sample is defined
as k (number of events) blocks of 8 consecutive days.
Central dates of the k events in each sample are selected
randomly choosing days and years within each winter
sub-period with the condition that the blocks are sepa-
rated at least 10 days from each other (consistently with
our definition of observed strong and weak events).
As an example, to establish the statistical significance
of the composite for WPVs in OND (k = 15 events), we
randomly select 15 non-overlapping blocks from this
winter sub-period, that is, among 92 days (from October
1st to 31st December) of 32 years (from 1979 to 2011).
The average of the data of these random fifteen 8-days
blocks provides one value of the probability distribu-
tion function (PDF), and this procedure is repeated 5000
times to construct the full PDF used to establish the sta-
tistical significance of the “observed” composite. A two-
tailed test is applied with 95% confidence interval based
on the obtained PDF.
(1)
(
v
�
T
�
)
a
=(v
a
�
T
a
�
)
a
+(v
c
�
T
a
�
)+(v
a
�
T
c
�)
3 Results anddiscussion
3.1 Climatological wave activity
We first examine the intra-seasonal variability in the wave
activity by analysing the differences between the three
winter sub-periods and the extended winter climatological
wave activity. Figure 2 displays the cross sections of the
climatological E-P flux, its divergence and the zonal wind
in a (φ, log (p)) plane for the extended winter, early winter,
mid-winter and late winter (i.e. OND, JF and MA).
The climatological E-P flux in the three winter sub-
periods is overall similar to the extended winter, but there
are relevant differences among them. The wave activity
in the subpolar mid-stratosphere is more intense in mid-
winter (JF), and the convergence is stronger in the strato-
sphere (Fig.2c). Consequently, the E-P flux climatology in
the extended winter season (Fig.2a) is mainly dominated
by the mid-winter months (Fig.2c). In early (Fig.2b) and
late winter (Fig.2d), the region of maximum convergence
is restricted to the subpolar mid-stratosphere (~10 hPa,
50ºN–60ºN).
The intraseasonal variability in the climatology of the
meridional eddy heat flux (vʹTʹ) at 100 hPa is shown in
Fig.3. The largest climatological (vʹTʹ) at 100hPa values
for the extended winter in the lower stratosphere are found
between 50ºN and 70ºN (Fig. 3a), in agreement with the
cross-section results of vertical component of the E-P flux
shown in Fig.2a. There are two regions with large positive
meridional eddy heat fluxes, the Eastern Siberia and the
Central Siberia, and only one with large negative fluxes in
Northwest Canada. Some differences appear among winter
sub-periods. The strongest positive (vʹTʹ) at 100hPa values
located in the easternmost part of Siberia are observed dur-
ing mid-winter (Fig.3c) and the positive centres over the
Scandinavian and Central Siberia present the highest values
in late winter (Fig.3d).
3.2 Anomalous wave activity prior toextreme events
3.2.1 Zonal mean wave activity propagation
We next study the intra-seasonal differences of anomalous
wave activity prior to the identified extreme events among
winter sub-periods. Figure 4 (left column) displays the
latitude-height composites of the anomalous E-P flux and
zonal wind over the week before WPV events. In general for
the three winter sub-periods, the wave activity is stronger
than the climatology, with anomalous upward E-P flux
and enhanced eddy-induced deceleration of the mean flow
in the stratosphere, which highlights the role of planetary
waves in forcing WPV events. This interaction between the
anomalous wave activity and the winter stratospheric mean
3477Intra-seasonal variability ofextreme boreal stratospheric polar vortex events andtheir…
1 3
flow leads to a weakening of the zonal wind in the extrat-
ropical stratosphere before the onset of WPV events. Some
differences among winter sub-periods can be observed,
with the strongest anomalies of E-P flux, E-P flux diver-
gence, and zonal wind for WPVs occurring in mid-winter
(Fig. 4c) in agreement with Solomon (2014). In addition,
only prior to mid-winter WPVs the statistically significant
anomalous wave propagation above 10hPa extends into the
subtropics. In the equatorial stratosphere there is an EQBO-
like structure (anomalous easterly winds in the lower strat-
osphere and westerly winds above them) prior to the occur-
rence of WPV events in early winter (Fig.4a). To a lesser
extent, we can also see an EQBO-like structure for events
in mid-winter, and WQBO-like winds for the WPVs in late
winter. Although the anomalous equatorial zonal winds
are not statistically significant at a 95% confidence level,
these vertical structures are in agreement with the statis-
tics on QBO phase and weak vortex events presented in
(Fig.1b). The predominance of EQBO for WPVs in early
winter (11 out of 15 events) and mid-winter (6 in 9 events)
is in agreement with the Holton and Tan (1980) relation-
ship between the QBO and the stratospheric polar vortex
state. However, the dominance of WQBO in late winter (6
out of 9 events) eludes this explanation, probably because
(a) (b)
(c) (d)
Fig. 2 Climatology of E-P flux and its divergence in a extended win-
ter (ONDJFMA), b early winter (OND), c mid-winter (JF) and d late
winter (MA) (1979–2011). E-P flux is represented by vectors with a
scale of 5 × 108 m3 s−2. Shading illustrates the climatological diver-
gence of E-P flux in units of ms− 1day− 1 and contours represent cli-
matological zonal mean zonal wind in ms− 1
3478 A.Díaz-Durán et al.
1 3
the transition from WQBO to EQBO occurs primarily in
spring/summer season (April–August) (Dunkerton 1990).
In this regard, Gray (2003) and Gray etal. (2004) showed
in several model experiments that in early winter the polar
stratosphere might be more sensitive to the QBO phase. In
the later stages of winter, the flow becomes more nonlinear
and the influence of subtropical and equatorial upper strato-
sphere might be more important (Gray etal. 2004). More
recently, White et al. (2016) studied the seasonal evolu-
tion of the Holton-Tan effect during the extended Northern
Hemisphere winter and their results agree well with those
of the present study. They showed that under EQBO in
early winter there is a stronger meridional circulation in the
lower stratosphere and a wave convergence at high latitudes
in the middle stratosphere that leads to a weak polar vortex.
In mid-February the polar vortex starts to recover from the
previous weakening and becomes anomalously strong for
that time of the year (White etal. 2016).
The wave activity in the week prior to the SPV onset
(Fig.4, right column) is weaker than the climatology, with
anomalous downward E-P flux and positive anomalous
divergence (i.e. reduced convergence) at mid- and high
latitudes in the three winter sub-periods. As observed for
the WPVs, the strongest anomalies of wave activity prior
to the SPV events are found in mid-winter (Fig.4d) but the
strongest positive anomalies of zonal wind are observed for
late winter SPVs in the extratropical stratosphere (Fig.4f).
However, this last result must be taken with caution due to
the low number of SPV events (only 4) in late winter. Con-
versely to WPVs, Fig.4b shows the presence of a WQBO
structure linked to SPVs in early winter; Fig. 1c already
showed that WQBO is present during SPV events in early
winter (12 out of 17 cases). Although a WQBO structure
is not very evident in mid-winter (Fig.4d) and late winter
(Fig.4f), 8 in 11 cases in mid-winter and 3 out of 4 cases
in late winter occur during WQBO phase. We recognize
Fig. 3 Climatology of the
meridional eddy heat flux at
100hPa (ms− 1 K) in a extended
winter (ONDJFMA), b early
winter (OND), c mid-winter
(JF) and d late winter (MA)
(1979–2011)
(a) (b)
(c) (d)
3479Intra-seasonal variability ofextreme boreal stratospheric polar vortex events andtheir…
1 3
the limitations of trying to draw conclusions on the poten-
tial relation between QBO and extreme vortex events with
a small number of cases, but we do report some tendency
in the reanalysis data for a particular QBO phase during
extreme events identified in early and mid-winter.
3.2.2 Vertical wave activity propagation
In order to identify the geographical regions where the
anomalous wave activity originates during the week prior
to the onset of stratospheric polar vortex extremes, com-
posites of the anomalous meridional eddy heat flux, (vʹTʹ)a,
at 100 hPa are represented in Fig. 5 (WPVs) and Fig. 6
(SPVs). Overall, before the onset of WPV events (Fig.5,
left column) the climatological behaviour of the wave
activity appears reinforced (compare to Fig.3b–d), but with
different magnitude on the main regions of wave propa-
gation depending on the winter sub-periods. The small-
est anomalies are observed prior to early winter WPVs
(Fig. 5a), although statistically significant changes are
identified over Central Siberia, Bering Sea and Greenland/
Northeast Canada regions. In mid-winter, the wave activity
over Central Siberia is enhanced before the onset of WPVs
(Fig.5d). Additionally, there is a strong injection of wave
activity into the stratosphere over the Bering Strait which
in mid-winter extends further towards the east over Canada
with respect to the climatology (Fig.3c). The climatologi-
cal negative centre over Northern Canada is intensified
(Fig.5g) prior to late winter WPV events. Similarly, the cli-
matological positive (vʹTʹ) at 100hPa (Fig.3d) over Bering
Strait and Scandinavian regions significantly increases.
The vʹTʹ at 100hPa pattern in Fig.5a is similar to that
found by Garcia-Serrano et al. (2015) in a study of the
influence of Arctic sea ice interannual variability in autumn
on the winter (DJF) Euro-Atlantic sea level pressure. In
their study, Garcia-Serrano etal. showed that the heat flux
pattern appeared as a dynamical response to Arctic sea ice
forcing, and invoked a stratospheric pathway linking sea
ice forcing in November and sea level pressure anomalies
in DJF. We have plotted a composite map of Arctic sea ice
concentration anomalies for our OND weak vortex events,
and even though there is a decrease of the Artic sea ice in
agreement with García-Serrano etal., the statistical signifi-
cance is very low (not shown). Therefore, our results are
inconclusive about the role of Arctic sea ice forcing the
stratospheric variability in early winter.
The central and right-hand columns of Fig.5 show the
terms of (vʹTʹ)a at 100 hPa associated with the nonlinear
contribution of anomalous Rossby waves and the modula-
tion of climatological waves by wave anomalies, respec-
tively. Overall, the main contribution to the strengthening
of the wave activity prior to the WPVs comes from the
interaction between climatological and anomalous wave
for the three winter sub-periods (Fig. 5, right column).
This term, (vʹcTʹa) + (vʹaTʹc), also plays an important role
over those regions where the wave-activity injection into
the stratosphere is weakened prior to mid- and late winter
WPVs (Fig.5f, i, respectively). Additionally, (vʹaTʹa)a prior
to mid-winter WPVs has a non-negligible contribution to
the total (vʹTʹ)a; it makes the enhanced wave activity over
Bering Strait region extend further eastward (Fig.5e).
The same analysis of the (vʹTʹ)a at 100hPa preceding the
onset of SPVs is shown in Fig.6. As expected, an overall
reduction of the climatological wave activity injection into
the stratosphere over the main regions of wave propaga-
tion is observed for all SPVs occurring in winter (Fig. 6,
left column). However, intra-seasonal differences in these
anomalies are evident, both in magnitude and/or location.
For instance, for mid-winter SPV events, the strongest
reduction of the wave activity is observed over Alaska and
northern Europe (Fig. 6d). Interestingly, in mid-winter the
spatial structure for SPV events does not match exactly the
location of the climatological centres (compare Figs. 6d,
3c), while in late winter the spatial pattern weakens the cli-
matological structure (compare Figs.6g, 3d).
Similarly to the WPV events, the term that contributes
the most to the (vʹTʹ)a at 100 hPa is associated with the
modulation of climatological waves by wave anomalies
preceding SPV events (Fig.6, right column). Again there
are some differences among winter sub-periods; this term
is more intense in mid (Fig.6f) and late winter (Fig.6i).
In order to further explore the intra-seasonal differences
in the anomalous wave activity associated with the occur-
rence of polar vortex extremes, Fig.7 presents the temporal
evolution around the onset of these events of the (vʹTʹ)a at
100 hPa averaged over 45ºN–75ºN considering the sepa-
rate zonal wavenumber components (wavenumber 1 and
2 (WN1 and WN2, respectively)). In the case of WPVs
(Fig. 7, left), WN1 component (red line) contributes the
most to the preceding enhancement of the extratropical-
averaged (vʹTʹ)a at 100hPa for the three winter sub-periods.
However in mid- and late winter, the WN2 component of
(vʹTʹ)a at 100 hPa also increases in the week prior to the
events (Fig. 7c, e, green line). Consistently, the pattern
of (vʹTʹ)a at 100 hPa displayed in Fig. 5d, g is a mix of
WN1 and WN2 wave patterns; and the pattern of (vʹcTʹa)
+ (vʹaTʹc) resembles a WN2 pattern (Fig.5f, i). One of the
possible modulators of WN2 wave activity prior WPVs in
mid-winter might be the phenomenon of La Niña. Indeed,
6 out 9 WPV events during mid-winter were preceded by
La Niña (Fig.1b). This is consistent with the results of Bar-
riopedro and Calvo (2014) and Li and Lau (2013) regarding
the link between ENSO and the zonal wavenumber compo-
nents prior to weak polar vortex events. In particular, while
WPVs that develop during El Niño events tend to be pre-
ceded by WN1 wave activity, WPVs that take place during
3480 A.Díaz-Durán et al.
1 3
(a) (b)
(c) (d)
(e) (f)
3481Intra-seasonal variability ofextreme boreal stratospheric polar vortex events andtheir…
1 3
La Niña events tend to be preceded by enhanced WN2
wave activity.
Concerning the wave forcing of SPVs (Fig.7, right col-
umn), there is a reduction of the (vʹTʹ)a at 100 hPa aver-
aged over 45ºN–75ºN around the central date of the events
mainly due to the WN1 wave component. For both early
(Fig. 7b) and mid-winter (Fig. 7d) events, the negative
anomalies grow over time before the SPV onset, but the
highest anomalies are observed prior to mid-winter events.
Results for late winter SPVs (Fig. 7f) showed that the
anomalous values always remain negative although they are
not statistically significant (only 4 events). This prolonged
reduction of wave activity may be because 2 out of 4 late
winter SPVs were preceded by WPVs in mid-winter, and
the propagation of tropospheric planetary waves is weak-
ened after WPV events during the so-called “late winter
cooling period” (Labitzke 1981).
3.3 Anomalous tropospheric circulation structures
prior topolar vortex extremes
In this section, we search for a possible link between
anomalous middle tropospheric circulation structures and
the anomalous wave activity preceding the extreme events
of the stratospheric polar vortex already shown. Figure8
shows composites of the anomalous geopotential height
at 500hPa (Z500) the week before the onset of WPVs and
SPVs in the different winter sub-periods. Also, the cli-
matological eddy geopotential height at the same level is
included to identify possible constructive or destructive
interferences between climatological and anomalous waves
associated with the tropospheric circulation anomalies.
3.3.1 WPV events
Once again, intra-winter differences are evident prior to
WPVs in Fig.8 (left column). In early winter, the anoma-
lous circulation pattern shows a WN1-like structure with
two main centres of action, positive over Greenland and
Western Siberia and negative over the Eastern Siberia
(Fig.8a). The two anomalous centres are located close to
the main climatological ones, Eurasian ridge and Pacific
trough, intensifying the climatological structures. The
positive values of the interaction term of the eddy heat
flux near these areas in Fig.5c confirm the constructive
interference between the anomalous and climatologi-
cal waves. Additionally, the Eurasian ridge and Pacific
trough form part of the climatological WN1 wave (Gar-
finkel et al. 2010) and consequently, the constructive
interference of the anomalous and climatological waves
likely leads to an enhancement of WN1 wave as already
shown in Fig.7a. The anomalous pattern is similar to that
shown in Fig.9b of García-Serrano etal. (2015) but for
200hPa associated with the loss of sea ice concentration
at the Barents-Kara Seas region (northeast of Scandina-
via) in agreement with the results of the heat flux.
In mid-winter, the anomalous Z500 is mainly char-
acterized by a combination of WN1 and WN2 patterns
(Fig.8c). As in early winter, the two strongest centres of
anomalies are located over Western and Eastern Siberia-
Pacific, but the mid-winter pattern also has two weak
ones over Canada and Greenland (the latter not statisti-
cally significant). The positive anomalies of Z500 over
Canada are merely in quadrature with the climatological
eddies and thus, do not interfere much with them. How-
ever, they might be responsible for the anomalous wave
activity injection into the stratosphere associated with the
anomalous waves only (Fig.5e) as happened during the
MSW of January 2009 (Ayarzagüena et al. 2011). The
rest of the Z500 anomalies interfere constructively with
the climatological WN1 and WN2 waves (Garfinkel etal.
2010) and might lead to an enhancement of the WN1 and
WN2 wave activity in agreement with Fig. 7c. Compar-
ing with the results of Barriopedro and Calvo (2014), the
centres of positive anomalies of Z500 in mid-winter can
be related to blockings over Siberia sector influenced by
La Niña events. Blockings over this region enhance WN2
wave activity (Nishii et al. 2011), consistently with the
results in Fig.5f.
In late winter, the anomalous structure also shows a com-
bination of WN1 and WN2-like patterns as in mid-winter.
However, in this case the centre of negative anomalies over
Greenland becomes the strongest one and extends further
eastward. As a result, the anomalous low is located close to
the climatological trough over Northeastern America and
intensifies it. The rest of the positive and negative centres
of anomalies of Z500 have weakened and reduced respect
to mid-winter, in particular over the Eastern Siberia-Pacific
that is now restricted to the Pacific. Nevertheless, they still
coincide with the antinodes of the climatological waves
and their constructive interference is statistically significant
according to Fig.5i.
Fig. 4 Latitude-height composites of the anomalous E-P flux
(arrows), its anomalous divergence (shading) and anomalous zonal
mean zonal wind (contours) the week prior to the WPV (left column)
and SPV events (right column) with statistical significance at a 95%
confidence level (Monte Carlo-like test, 5000 random samples). a–b
Composites for 15 WPV and 17 SPV events in early winter, respec-
tively; c–d composites for 9 WPV and 11 SPV events in mid-winter,
respectively; and e–f composites for 9 WPV and 4 SPV events in late
winter, respectively. Arrows with a scale of 3 × 108 m3 s− 2 are drawn
when the value of the vertical component (Fz) of anomalous E-P flux
is statistically significant. Color shading illustrates the statistically
significant anomalous divergence of E-P flux in units of ms− 1day− 1.
Zonal mean zonal wind anomalies (ms− 1) are shown in black con-
tours and those statistically significant are highlighted in yellow con-
tours
◂
3482 A.Díaz-Durán et al.
1 3
Subsequently, we have verified the effects of the
anomalous circulation on the upward propagating WN1
and WN2 wave activity that we have just inferred in
Fig. 8. Figure 9 shows the cross-sections of anomalous
and climatological eddy geopotential height averaged
over 55ºN–75ºN for WN1 (left column) and WN2 (right
column) for WPVs. The averaging latitude band corre-
sponds to the region where the main centres of anomalies
of Z500 and climatological eddy geopotential height are
located. Results do not differ much when selecting a dif-
ferent band such as 45ºN–75ºN (not shown). In early
winter, as anticipated before, the anomalous WN1 wave
is in phase with the climatological wave and propagates
upward as denoted by the westward tilt with the height
(Fig.9a). Thus, the enhancement of the upward-propagat-
ing WN1 wave is verified. At 500hPa the negative and
(V’T’)a (V’aT’a)a (V’cT’a)+(V’aT’c)
(V’T’)a (V’aT’a)a (V’cT’a)+(V’aT’c)
(V’T’)a (V’aT’a)a (V’cT’a)+(V’aT’c)
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
Fig. 5 Composites of the anomalous meridional eddy heat flux
(ms− 1 K) at 100 hPa (left column) and its contributor terms (mid-
dle and right columns) for the week prior to the occurrence of WPV
events. Rows show the results corresponding to the three winter sub-
periods: 15 events in early winter (upper), 9 events in mid-winter
(middle) and 9 events in late winter (bottom). Dotted regions show
statistically significant values at a 95% confidence level (Monte
Carlo-like test, 5000 random samples)
3483Intra-seasonal variability ofextreme boreal stratospheric polar vortex events andtheir…
1 3
positive antinodes of the wave coincide with the locations
of the anomalous Eastern Siberian low and Greenland
and Western Siberian highs identified in Fig.8a. In con-
trast, the anomalous WN2 wave does not interfere with
the climatological wave (Fig.9b). In mid- (Fig.9c, d) and
late winter (Fig. 9e, f), the anomalous WN1 and WN2
waves are in general in phase with climatological waves
and both show westward tilt with height, confirming the
intensification of the upward-propagating WN1 and WN2
waves prior to the occurrence of WPVs.
3.3.2 SPV events
Contrarily to WPV events, before SPV events (Fig.8, right)
in the three winter sub-periods there is a negative centre of
Z500 anomalies over Western Siberia and a positive cen-
tre over Eastern Siberia-Pacific as also shown by Kolstad
and Charlton-Perez (2011) for SPVs in the extended winter
(DJFM). Both centres are close to the climatological Eura-
sian ridge and Pacific trough and they tend to diminish the
climatological wave pattern. Nevertheless, the centres of
(V’cT’a)+(V’aT’c)
(V’aT’a)a
(V’T’)a
(V’T’)a (V’aT’a)a (V’cT’a)+(V’aT’c)
(V’T’)a (V’aT’a)a (V’cT’a)+(V’aT’c)
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
Fig. 6 As Fig.5 but for the SPV events: 17 events in early winter (upper), 11 events in mid-winter (middle) and 4 events in late winter (bottom)
3484 A.Díaz-Durán et al.
1 3
anomalies are much weaker in early winter than in the rest
of the season. The anomalous high over Eastern Siberia is
the main structure leading to the inhibition of wave activity
by interaction with the climatological wave in early winter.
The result is confirmed by the statistically significant nega-
tive values in the interaction term of the heat flux (Fig.6c).
A pseudo-negative phase of Pacific-North American pat-
tern is shown in Fig.8d in mid-winter, which is known to
weaken the climatological Eastern Siberia-Pacific low and
once again, inhibits WN1 component (in agreement with
Fig. 7 results). Previous studies have already shown the
occurrence of a similar pattern over the Western Pacific
prior to a polar stratospheric cooling (Nishii et al. 2010,
2011; Kolstad and Charlton-Perez 2011). In late winter,
the negative anomalies of Z500 over Central Siberia are
more intense than in the other sub-periods (Fig. 8f) and
the anomalous circulation pattern has a WN1-like shape
in opposite phase with the climatological wave (Garfinkel
et al. 2010). This would also explain the suppression of
WN1 wave activity shown in Fig.7f.
As for WPVs, we have analysed the upward-propagat-
ing anomalous and climatological WN1 and WN2 waves
of Z (Fig. 10) to confirm the conclusions derived from
the analysis of anomalous Z500. In the three sub-periods
the anomalous WN1 (Fig.10 left column) is in opposite
phase with the climatological waves in the troposphere
-14-70 714-14-70
71
4
-14-70 714-14 -7 0714
-14-70 714
-14-70 714
-4
-2
0
2
4
6
8
10
12
14
16 WPV OND
Heat flux
Heat flux k1
Heat flux k2
-8
-6
-4
-2
0
2
4SPV OND
Heat flux
Heat flux k1
Heat flux k2
-4
-2
0
2
4
6
8
10
12
14
16 WPV JF
Heat flux
Heat flux k1
Heat flux k2
-8
-6
-4
-2
0
2
4SPV JF
Heat flux
Heat flux k1
Heat flux k2
-4
-2
0
2
4
6
8
10
12
14
16
WPV MA
Heat flux
Heat flux k1
Heat flux k2
-8
-6
-4
-2
0
2
4
SPV MA
Heat flux
Heat flux k1
Heat flux k2
(a)
(c)
(e) (f)
(d)
(b)
Fig. 7 Composites of the time evolution of anomalous meridional
heat flux vʹTʹ at 100 hPa averaged over 45ºN–75ºN from 15 days
before to 15 days after the occurrence of WPV (left column) and SPV
events (right column). It has been performed for different zonal wave-
numbers: k = 1 (red line), k = 2 (green line) and the sum of all zonal
wavenumbers (blue line). Rows show the results corresponding to the
three winter sub-periods: early winter (upper), mid-winter (middle)
and late winter (bottom). Asterisk denotes statistically significant val-
ues at a 95% confidence level (Monte Carlo-like test, 5000 random
samples)
3485Intra-seasonal variability ofextreme boreal stratospheric polar vortex events andtheir…
1 3
and stratosphere, indicative of the destructive interfer-
ence of both waves. As for WN2 wave, the anomalous
WN2 is in quadrature with the climatological waves in
early and late winter. In mid-winter, the anomalous WN2
wave seems to be in phase with the climatological one.
In fact, this result is not very surprising since part of the
seven days preceding SPVs show an enhancement of
WN2 heat flux (Fig. 7d). Nevertheless, the suppression
of WN1 wave activity is strong enough to counterbalance
the effects of WN2.
Fig. 8 Composites of anoma-
lous geopotential height (gpm)
at 500hPa (shading) for the
week prior to the occurrence
of WPV (left column) and SPV
events (right column). Rows
show the results corresponding
to the three winter sub-periods:
early winter (upper), mid-
winter (middle) and late winter
(bottom). Dotted regions show
statistically significant values of
geopotential anomalies at a 95%
confidence level (Monte Carlo-
like test, 5000 random samples).
Grey (negative) and orange
(positive) contours represent cli-
matological eddy geopotential
height at the same level during
these winter sub-periods. The
contour interval is 30 gpm
(a) (b)
(c) (d)
(e) (f)
3486 A.Díaz-Durán et al.
1 3
3.4 Temporal evolution ofstratospheric circulation
anomalies
Figure11 shows the time-height composites of the stand-
ardized anomalies of zonal mean zonal wind at 60ºN during
90 days before and after the onset of the extreme events.
Figure 11a, b represent the composites for WPV and
SPV events, respectively, during all the extended winter
similarly to Baldwin and Thompson (2009, their Fig.11).
WPVs composites for the extended winter are preceded
Fig. 9 Composites of WN1
(left column) and WN2 (right
column) components of
anomalous geopotential height
averaged over 55ºN–75ºN the
week prior to the occurrence
of WPV (gpm, shading). Rows
show the results corresponding
to the three winter sub-periods:
early winter (upper), mid-
winter (middle) and late winter
(bottom). Dotted regions show
statistically significant values of
eddy geopotential anomalies at
a 95% confidence level (Monte
Carlo-like test, 5000 random
samples). Dashed black (nega-
tive) and solid black (positive)
contours represent climatologi-
cal WN1 and WN2 components
of geopotential height during
these winter sub-periods
(a) (b)
(c) (d)
(e) (f)
3487Intra-seasonal variability ofextreme boreal stratospheric polar vortex events andtheir…
1 3
by positive anomalies of zonal wind and after the event
onset the zonal wind is reduced (Fig. 11a), in agreement
with Baldwin and Thompson (2009). The same pattern
appears in Fig.11b for SPVs, with the sign of the anoma-
lies inverted and weaker negative anomalies preceding the
event. Although Fig.11a, b do not present statistically sig-
nificant anomalies propagating down to the surface after
the events, the anomalies remain in the upper troposphere
being able to affect the baroclinic activity (Baldwin etal.
2003). The temporal evolution of anomalous zonal wind for
Fig. 10 As Fig.9 but for SPV
events
(a) (b)
(c) (d)
(e) (f)
3488 A.Díaz-Durán et al.
1 3
Fig. 11 Height-time compos-
ites of standardized anomalies
of zonal mean zonal wind at
60ºN around the onset of WPV
(left column) and SPV (right
column) events, from 90 days
before to 90 days after, a–b for
all events through the extended
winter, c–d for early winter
events, e–f for mid-winter
events and g–h for late winter
events. Statistically significant
values at a 95% confidence level
are dotted (Monte Carlo-like
test, 5000 random samples)
(e) (f)
(g) (h)
(c) (d)
(a) (b)
3489Intra-seasonal variability ofextreme boreal stratospheric polar vortex events andtheir…
1 3
WPVs also presents an intra-seasonal variability (Fig.11,
left column). WPVs in early winter (Fig.11c) are preceded
by weak and non-statistically significant negative anoma-
lies, while mid-winter and late winter WPVs (Fig.11e, g)
present a larger previous strengthening of the polar vortex.
Albers and Birner (2014) showed a similar vortex precon-
ditioning prior to vortex split MSWs, which is in agreement
with the important contribution of WN2 wave activity pre-
ceding WPVs in the present study. In particular, Albers and
Birner (2014) found that split MSWs are preceded by an
anomalously strong vortex without vertical tilt that is wider
in the upper stratosphere and narrower in the lower strato-
sphere. According to their study, the enhancement of grav-
ity and planetary wave drag constrains this former strong
vortex about the pole and weakens it. In early winter and
mid-winter WPVs significant anomalies reach the tropo-
sphere, and this happens earlier for mid-winter anomalies.
In late winter negative anomalies remain in the upper trop-
osphere, probably due to the high variability of the spring-
time troposphere.
Figure 11 (right) shows the evolution of zonal wind
anomalies corresponding to SPV events. In contrast to
WPV events, prior to SPV events there are no clear oppo-
site-sign anomalies in early and mid-winter (Fig.11d, f).
This difference is probably due to the fact that the SPV
events have a longer time-scale as they are driven by radia-
tive relaxation while wave forcing drives WPV events.
However, late winter SPVs are preceded by negative anom-
alies (Fig.11h), which is consistent with the occurrence of
2 out 4 SPVs after WPVs and the associated reduction of
planetary wave propagation (Fig.7f).
4 Summary andconclusions
In this study we have examined the intra-seasonal variabil-
ity of the anomalous wave activity injection into the strat-
osphere preceding polar vortex extremes in the NH using
ERA-Interim reanalysis data (1979–2011). Particularly,
weak (WPV) and strong (SPV) polar vortex events have
been grouped in winter sub-periods depending on the date
of the onset: early winter (OND), mid-winter (JF) or late
winter (MA).
We have identified the patterns prior to the occurrence
of polar vortex extremes in each winter sub-period. In
general, WPV (SPV) events are associated with construc-
tive (destructive) interference of anomalous wave activity
with the climatological patterns in each winter sub-period.
The results show clear intra-seasonal differences in the
anomalous wave activity preceding both types of extreme
events. Overall, mid-winter extreme events are preceded by
the strongest wave activity anomalies, whereas the weak-
est anomalies are observed prior to early winter extreme
events. We summarize here the specific features observed
for each winter sub-period.
In early winter, the upward wave propagation preced-
ing WPVs (SPVs) is under the influence of a WQBO-like
(EQBO-like) structure. The anomalous geopotential pat-
tern at 500 hPa before WPVs (SPVs) represents a WN1-
like structure in phase (out of phase) with the climatologi-
cal wave that increases (inhibits) the upward WN1 wave
propagation. The spatial patterns of the anomalous wave
injection into the stratosphere could be related to the strat-
ospheric pathway of the autumn Arctic sea ice influence
on the winter Euro-Atlantic climate as recently shown by
García-Serrano et al. (2015), although our results are not
conclusive in this respect.
Both mid- and late winter WPV events are preceded by
a strong vortex with a specific geometry that favours pole-
ward planetary wave propagation, as discussed by Albers
and Birner (2014). However, only mid-winter WPVs are
influenced by an EQBO-like structure and it is at a lesser
extent than in early winter. The anomalous pattern of the
vertical wave injection into the stratosphere before WPVs
in mid- and late winter has a contribution of WN1 and
WN2 wave activity. This is due to the constructive inter-
ference between climatological and anomalous waves over
the Eastern Siberia-Pacific region and Western Siberia that
produces an increase of the upward WN1 and WN2 wave
propagation. For mid-winter WPV events the increase in
WN2 wave activity might be related to the predominance
of La Niña over El Niño conditions, linked to enhanced
blockings over Siberia and WN2 wave activity polar strato-
sphere perturbations (Barriopedro and Calvo 2014). There
is a non-linear eddy-eddy interaction that is responsible
for the extension of wave activity over Northern Canada in
mid-winter.
Regarding SPVs, mid- and late winter events are pre-
ceded by a WQBO-like structure, although it is not very
evident in Fig.4d, f. We also observe that late winter SPV
events are preceded by WPV, in agreement with the late
winter cooling proposed by Labitzke (1981). The modula-
tion of climatological patterns by anomalous eddies is the
main responsible for the reduction in wave activity preced-
ing mid- and late winter SPVs. In both sub-periods, the
suppression of wave activity is mainly due to the inhibition
of the WN1 component. For instance, the mid-winter SPV
events show a negative phase of the Pacific-North Ameri-
can pattern that has been traditionally related to the men-
tioned reduction of WN1 wave activity (Nishii etal. 2010,
2011; Kolstad and Charlton-Perez 2011).
It is important to note that the relatively short time
period covered by the data and the subsequent small
number of extreme polar events might be a weakness for
the robustness of our results. In order to overcome this
shortcoming, we have applied a non-parametric statistical
3490 A.Díaz-Durán et al.
1 3
significance test (namely, a Monte Carlo-like test) to pro-
vide confidence to our conclusions. Nevertheless, the
number of SPV events is particularly small in late win-
ter (only four) and thus we have avoided deriving any
important conclusion for this group of events. The use of
a much longer dataset might have reduced this problem,
but we prefer not to mix pre- and post-satellite data (more
details in Sect.2).
Our results support the conclusions of previous studies
concerning the different forcings of extreme polar vortex
regime and their most effective timing. To the best of our
knowledge, the present work constitutes the first analysis of
the intra-seasonal variability of the triggering mechanisms
of these events. An important implication of this study is
that the dynamical behaviour associated with events occur-
ring in mid-winter months cannot be generalized to other
winter sub-periods, in particular regarding precursors and
wave activity preceding extreme polar vortex events. Our
results suggest that considering the intra-seasonal variabil-
ity of polar vortex extremes can help improve the represen-
tation of stratosphere-troposphere interactions in climate
models and seasonal forecast systems.
Acknowledgements The authors are grateful to two anonymous
reviewers, whose comments help improve this manuscript. This work
was supported by the Spanish Ministry of Economy and Competitive-
ness (grant number CGL2012-34997). BA is supported by the Natural
Environment Research Council (grant number NE/M006123/1). MA
acknowledges funding from the NASA ACMAP program. The ERA-
Interim reanalysis data were obtained online at http://data-portal.
ecmwf.int/data/d/interim full moda/. ENSO and QBO indices have
been taken from http://www.cpc.ncep.noaa.gov/products/analysis_
monitoring/ensostuff/ensoyears_ERSSTv3b.shtml and http://www.
cpc.noaa.gov/data/indices/, respectively.
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