Home

SearchCollections

Journals

About

Contact us

My IOPscience

Separating the stratospheric and tropospheric pathways of El Niño–Southern Oscillation
teleconnections

This content has been downloaded from IOPscience. Please scroll down to see the full text.
2014 Environ. Res. Lett. 9 024014(http://iopscience.iop.org/1748-9326/9/2/024014)
View the table of contents for this issue, or go to the journal homepage for more

Download details:
IP Address: 128.117.20.47
This content was downloaded on 27/02/2014 at 18:33

Please note that terms and conditions apply.

Environmental Research Letters
Environ. Res. Lett. 9 (2014) 024014 (9pp)

doi:10.1088/1748-9326/9/2/024014

Separating the stratospheric andtropospheric pathways of El Niño–Southern
Oscillation teleconnections
Amy H Butler1,2 , Lorenzo M Polvani3,4,5 and Clara Deser6
1

University of Colorado/Cooperative Institute for Research in Environmental Sciences, Boulder, CO, USA
National Oceanic and Atmospheric Administration/Earth System Research Laboratory/Chemical Sciences
Division, Boulder, CO, USA
3
Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA
4
Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA
5
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA
6
National Center for Atmospheric Research, Boulder, CO, USA
2E-mail: amy.butler@noaa.gov
Received 5 December 2013, revised 27 January 2014
Accepted for publication 29 January 2014
Published 26 February 2014

Abstract

The El Niño–Southern Oscillation (ENSO) is a major driver of Northern Hemisphere wintertime
variability and, generally, the key ingredient used in seasonal forecasts of wintertime surface
climate. Modeling studies have recently suggested that ENSO teleconnections might involve botha tropospheric pathway and a stratospheric one. Here, using reanalysis data, we carefully
distinguish between the two. We first note that the temperature and circulation anomalies
associated with the tropospheric pathway are nearly equal and opposite during the warm (ElNiño) and cold (La Niña) phases of ENSO, whereas those associated with the stratosphericpathway are of the same sign, irrespective of the ENSO phase. We then exploit this fact to isolate
the two pathways. Our decomposition reveals that ENSOs climate impacts over North America
are largely associated with the tropospheric pathway, whereas ENSOs climate impacts over the
North Atlantic and Eurasia are greatly affected by the stratospheric pathway. The stratosphericpathway, which we here define on the basis of the occurrence of one or more sudden stratosphericwarmings in a given winter, and whose signature projects very strongly on the North Atlantic
Oscillation, is found to be present 60% of the time during ENSO winters (of either phase): it
therefore likely plays an important role in improving seasonal forecasts, notably over the North
Atlantic and the Eurasian continent.
Keywords: El Niño-Southern Oscillation, stratospheric variability, sudden stratosphericwarmings, teleconnections, Eurasian climate
S Online supplementary data available from stacks.iop.org/ERL/9/024014/mmedia

1. Introduction

precipitation anomalies in the Northern Hemisphere in wintertime: as a consequence, ENSO is a fundamental component of
most seasonal climate forecasts in the extratropics. Horel andWallace (1981) showed that ENSO teleconnections resemble
tropospheric Rossby wave trains forced in the tropical Pacific
Ocean by slowly evolving anomalous sea surface temperatures(SSTs) via changes in precipitation. Many ENSO induced
climate anomalies are, to first order, linear in character,

It is well established that El Niño–Southern Oscillation(ENSO) teleconnections cause important temperature andContent from this work may be used under the terms of
the Creative Commons Attribution 3.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and thetitle of the work, journal citation and DOI.
1748-9326/14/024014+09$33.00

1

c 2014 IOP Publishing Ltd

Printed in the UK

Environ. Res. Lett. 9 (2014) 024014

A H Butler et ali.e. their sign reverses as the tropical Pacific SST anomalieschange from warm (El Niño) to cold (La Niña). In fact, overthe Pacific and North American sectors, ENSO impacts can belargely accounted for with linear wave theory and simple ray
tracing in the troposphere (Hoskins and Karoly 1981, Horeland Wallace 1981, Sardeshmukh and Hoskins 1988). We note,however, that over Europe and Asia the precise mechanism
for ENSO impacts remains less clear (Greatbatch et al 2004,
Toniazzo and Scaife 2006, Brönnimann et al 2007).
Because ENSO is a large perturbation of the climate
system, it is not surprising that the stratosphere too is affected by anomalous tropical Pacific SSTs. The stratospheric
response, however, is richer in character than the tropospheric
response. On the one hand, the mean seasonal stratospheric response to ENSO is approximately linear: it is well documented
that during El Niño winters the Northern Hemisphere polar
vortex is anomalously weak as a consequence of enhanced
planetary wave fluxes into the stratosphere (Van Loon andLabitzke 1987), resulting in a warmer stratosphere (Garfinkel
and Hartmann 2007), and higher ozone concentrations in thepolar regions (Randel et al 2009). Similar but oppositely
signed anomalies are observed during La Niña winters, albeit
with smaller amplitude (Manzini et al 2006, Garfinkel andHartmann 2007, Free and Seidel 2009).
On the other hand, the extreme stratospheric response to
ENSO is highly non-linear. This is clearly seen in the frequencyof occurrence of so-called sudden stratospheric warmings(SSWs). During these events, the polar vortex is completely
disrupted, the winds reverse from westerly to easterly, andthe polar stratosphere warms by tens of degrees within a few
days (Charlton and Polvani 2007). One might naı̈vely expectthat SSWs would be enhanced during El Niño and suppressed
during La Niña: however, as reported in Butler and Polvani(2011), the frequency of occurrence of SSWs is doubled during
both phases of ENSO, compared with ENSO-neutral winters.
This surprising fact is still poorly understood (Garfinkel et al2012a).
The reason SSWs are important for surface conditions
follows from the observation that significant troposphericcirculation anomalies can persist up to two months following
the events (Baldwin and Dunkerton 2001). These anomalies
resemble the negative phase of the North Atlantic Oscillation(NAO), or an equatorward shift of the mid-latitude jet, andare associated with a strong anomalous warming over theArctic and Greenland, as well as strong cold air outbreaks
over northern Eurasia (Perlwitz and Graf 1995, Thompson
et al 2002, Kolstad et al 2009).
Since the majority of SSWs occur during ENSO winters(Butler and Polvani 2011), it is then natural to imagine theexistence of a stratospheric pathway linking tropical Pacific
SST anomalies to surface conditions over the North Atlantic
and Eurasian regions. Unlike the familiar, quasi-linear, tropospheric Pacific–North American (PNA) teleconnection pattern, the stratospheric pathway consists of planetary waves that
propagate primarily upwards, greatly perturbing the stratosphere and occasionally producing extreme events there (notably, SSWs), which subsequently affect surface conditions
below, primarily in the North Atlantic sector and Eurasia. The

Figure 1. Schematic of possible pathways by which ENSO can

influence the northern extratropics.

sketch in figure 1 visually summarizes these two distinct ENSO
pathways into the extratropics.
While the study of tropospheric ENSO teleconnections
has a long history (see, e.g., Trenberth et al 1998), the notionof a possible stratospheric pathway is relatively recent. It washinted at by Brönnimann et al (2004), in a case study of theextremely cold Eurasian winters in 1940–1942, a strong andlong-lasting El Niño with a highly perturbed stratosphere.
Since then, several modeling studies (Toniazzo and Scaife
2006, Bell et al 2009, Cagnazzo and Manzini 2009, Ineson andScaife 2009, Ortiz Beviá et al 2010) have suggested that the
ENSO impacts onto the North Atlantic and Eurasian regionsare, in fact, dominated by the stratospheric pathway.
The aim of our paper is to offer observational evidence
for these modeling results. We accomplish this by examining,
separately, ENSO teleconnections during winters with andwithout SSWs, and showing that the presence of the stratospheric pathway causes a very significant surface responseover the North Atlantic region and Eurasia. Our results suggestthat seasonal forecasts could be substantially improved if
knowledge of stratospheric conditions were taken into account.
2. Methods

To define ENSO events we use the ERSST.V3B ‘Oceanic NiñoIndex’ (ONI) calculated over the Niño-3.4 region (5◦ N–5◦ S,
170–120◦ W) from the National Center for Environmental
Prediction (NCEP) Climate Prediction Center (CPC). We
define El Niño and La Niña winters following the NCEP/CPC
convention: events must surpass the +0.5 ◦ C (for El Niño)and −0.5 ◦ C (for La Niña) threshold for a minimum of fiveconsecutive overlapping seasons (NDJ, DJF, JFM, etc). Our
results are not sensitive to changes in this definition, such as
using the DJF Nino-3.4 index alone, or using higher thresholds.
To define major, mid-winter, sudden stratospheric warmings we follow Charlton and Polvani (2007) and simply detect
reversals of the climatological westerly winds at 10 hPa and60◦ N in the boreal winter months (November to March), using
NCEP-NCAR Reanalysis from 1958 to 2013 (Kalnay et al1996). To avoid double counting, we require that winds return
to westerly for 20 consecutive days between independent
events. Final warmings, in which the zonal winds become
easterly and do not return to westerly for at least 10 consecutive
days before 30 April, are excluded. The so-called ‘central date’of each SSW is defined to be the day when the zonal mean
zonal winds at 10 hPa and 60◦ N first become easterly.
2Environ. Res. Lett. 9 (2014) 024014

A H Butler et al

To describe tropospheric conditions, we use daily surface
temperature and 500 hPa geopotential height from NCEPNCAR reanalysis, again over the period 1958–2013. To
remove long-term trends and the annual cycle from daily data,
we follow the procedure delineated in Gerber et al (2010).
We also use the monthly mean North Atlantic Oscillation(NAO) index from the NCEP/CPC, standardized using theJanuary–March mean and standard deviation.
Statistical significance for composites of monthly anomalies (figures 3 and 4) is calculated using the two-tailed Student
t-test for the difference in means, and anomalies with p < 0.05
are shaded in all composites. For significance testing in composites of stratospheric events using daily data (figure 2), we
sample 60 consecutive days following randomly selected years
and dates within the November–March timeframe; the dates
must be separated by 60 days. We sample the same number ofdates as are observed for SSWs in each phase of ENSO (15 for
El Niño, 13 for La Niña), then average each sample and take thedifference. We then create a distribution of anomalies for eachgrid cell by repeating this process 500 times in a Monte Carlotest. Significant anomalies in the SSW composites are thendefined for those grid cells exceeding two standard deviations(σ ) of the anomaly distribution at that location.

Table 1. List of 35 central dates of SSWs in El Niño winters,
La Niña winters, and ENSO-neutral winters in NCEP-NCAR
reanalysis from 1958 to 2013, updated from Butler and Polvani(2011).

SSWs (El Niño)

SSWs (La Niña)

SSWs (neutral)

30 Jan 1958
30 Nov 1958
08 Dec 1965
24 Feb 1966
27 Nov 1968
13 Mar 1969
02 Jan 1970
02 Feb 1973
23 Jan 198708 Dec 1987
14 Mar 1988
18 Jan 2003
24 Feb 2007
09 Feb 2010
24 Mar 2010

23 Mar 1965
17 Jan 1971
20 Mar 1971
24 Feb 1984
02 Jan 1985
22 Feb 1989
15 Dec 1998
25 Feb 1999
20 Mar 2000
11 Feb 2001
21 Jan 2006
22 Feb 200824 Jan 200916 Jan 1960
22 Feb 1979
29 Feb 1980
04 Dec 1981
02 Jan 2002
07 Jan 200407 Jan 2013

1–60 days after SSWs, it would be naı̈ve to state that thesepatterns are caused by SSWs. The reason is quite simple:
SSWs are much more frequent during El Niño and La Niña
3. Results
winters than during ENSO-neutral winters (Butler and Polvani
To set the stage, we start by recalling the surface impacts that 2011). In fact, of the 35 SSWs used to construct the left column
are observed following SSW events. In the period 1958–2013, of figure 2(a), 15 events occur during El Niño and 13 during
a total of 35 SSWs are found using the methods detailed La Niña winters (see table 1). Therefore, one would have every
above, and their central dates7 are listed in table 1. The reason to suspect that the anomalies in figure 2(a) might be duecomposite surface temperature and 500 hPa geopotential to a sampling of the ENSO signal, and not to SSWs. Obviously,
height anomalies, averaged over the 60 days following the one needs to disentangle SSWs from ENSO, to bring out thecentral date of each SSW, are shown in figure 2(a) (left column, unique impacts of a perturbed stratosphere.
We do this in two ways. First, to eliminate any ENSO
top and bottom panels, respectively).
signal,we simply limit the composite to those SSWs that occur
The geopotential height anomalies following SSWs closely
in
ENSO-neutral
winters. The result is shown in figure 2(b)resemble the negative phase of the NAO (Baldwin and(middlecolumn):the key point is that both temperature andDunkerton 2001), with positive heights over Greenland andgeopotential
heightanomalies look qualitatively similar to thethe Arctic, and negative heights over the North Atlantic andcomposites
of
all
SSWs.
In particular, the significant warm
extending across Europe; we also note significant positive
anomalies
overeastern
Canadaand the cold anomalies overheights over subtropical Asia. Concomitantly, the temperature
Eurasia,
as
well
as
the
high
geopotential
over the polar cap andpatterns following SSWs (figure 2(a), top panel) likewiselowgeopotential
over
the
North
Atlantic,
are clearly present
resemble what is typically seen in a negative NAO phase, witheven
during
ENSO-neutral
winters.
We
concludethat there
cold anomalies over northern Europe, Asia, and the easternexistsasurfacesignalofstratosphericoriginwhichhas
nothing
United States, and warm anomalies over eastern Canada
to
do
with
ENSO.
and parts of Greenland, and also over much of China, theThe skeptical reader will perhaps object that only seven
Middle East, and North Africa.
SSW
events were used to construct figure 2(b) (see the right
It should be clear to the alert reader that although thecolumnof table 1) and therefore may doubt the robustness
patterns in figure 2(a) are constructed by compositing the fields
our conclusion, in spite of the statistically significant response
7 We note two small differences between these dates and those given
shown in that figure. To alleviate such doubts, we offer a
in table 1 of Butler and Polvani (2011). First, calculations for SSW second way of isolating the stratospheric impacts. Noting that
central dates using the current version of NCEP-NCAR reanalysis no
the period 1958–2013 contains a comparable number of SSWs
longer detect a SSW in January 1968, which also appeared in the table
during El Niño and La Niña years (15 and 13, respectively),of Charlton and Polvani (2007); this is likely due to slight differencesin versions of the data. Second, an additional SSW in March 2010 was and recalling the relative linearity in the tropospheric ENSO
missed in Butler and Polvani (2011) due to an error, but technically response, one would expect large cancellations to occur ifmeets the Charlton and Polvani (2007) criteria; we include it here for only those SSWs that occur in El Niño and La Niña years
completeness. The results herein do not depend on whether this event are composited (additively). Remarkably enough, no suchis included or not.
cancellation occurs.
3Environ. Res. Lett. 9 (2014) 024014

A H Butler et al

Figure 2. Composite of (top row) surface temperature (K) and (bottom row) 500 hPa geopotential height (gpm; contour interval 10 gpm)
anomalies for days 1–60 following the onset of (a) the 35 major mid-winter SSWs in the historical record, (b) the 7 SSWs that occur during
ENSO-neutral winters, and (c) the sum of 15 SSWs during El Niño winters and 13 SSWs during La Niña winters. The bold black lines (grayshading) indicate anomalies greater than two sigma relative to distributions generated in Monte Carlo simulations (see the text for
details).

El Niño winter versus 0.72 SSWs per La Niña winter), andfar fewer SSWs are found to occur in ENSO-neutral winters(only 0.37 SSWs per ENSO-neutral winter), in agreement withButler and Polvani (2011). Overall, while SSWs only occur
in slightly more than half of all winters, there are more El
Niño and La Niña winters with SSWs than without, and fewer
ENSO-neutral winters with SSWs than without.
Armed with this decomposition, we first recover thecanonical ‘linear ENSO response’, shown in figure 3(a), by
simply subtracting the 18 La Niña winters from the 19 El Niñowinters in the record (irrespective of the stratospheric state).
Note the expected cold United States and warm Canadiansurface conditions (top), and the familiar PNA wave trains in
the height field (bottom). More interesting yet, note that overEurasia the ENSO response in figure 3(a) bears a substantial
resemblance to the SSW composite in figure 2(a): this suggeststhat the stratospheric pathway is playing a role in the ENSO
response in that region.
To bring out this role we next consider the same
linear ENSO response but only for winters in which one
or more SSWs occur, as shown in figure 3(b). Contrastingwith figure 3(a) (top), one can clearly see how the cold
anomalies over Eurasia are much deeper and cover a wider area
when the stratosphere is perturbed. The warm anomalies overGreenland and eastern Canada are also stronger, while the

As shown in figure 2(c), adding opposite ENSO phases
actually amplifies the anomalies following SSWs. Both surface
temperature and geopotential height anomalies in figure 2(c)look qualitatively similar to those in figure 2(b)—from which
all ENSO influence is absent by construction—but haveapproximately double the amplitude: this indicates that the
impacts of SSWs on the troposphere and surface are very
similar, irrespective of the ENSO phase. Figure 2(c), therefore,
is a clear demonstration of the non-linear aspect of the surface
response to ENSO, over the Atlantic and Eurasia, following
SSWs. In the bottom panels of figure 2, furthermore, note theconspicuous absence of any PNA-like wave train originatingin the tropical pacific and extending to North America: thisis yet another confirmation that what is seen in that figureis the impact of greatly perturbed stratospheric conditions.
Figure 2(c), therefore, offers clear evidence that a stratosphericpathway exists.
Having established the existence of that pathway, we next
document its importance. To do so, we now turn our attention
from SSWs to ENSO teleconnections. As summarized intable 2 we find 19 El Niño winters, 18 La Niña winters, and19 ENSO-neutral winters in the 56 winters of the 1958–2013
record. The non-linear aspect of the stratospheric response to
ENSO is also apparent in table 2: the frequency of SSWs in
El Niño and La Niña winters is nearly equal (0.79 SSWs per
4

Environ. Res. Lett. 9 (2014) 024014

A H Butler et al

Figure 3. Top row: surface temperature (K) and bottom row: 500 hPa geopotential height (gpm; contour interval 10 gpm) anomalies
associated with the composite difference of (a) all El Niño winters and all La Niña winters, (b) El Niño and La Niña winters in which at least
one SSW occurs, and (c) El Niño and La Niña winters during which no SSWs occur. The black line (gray shading) indicates anomalies withp < 0.05 for a two-tailed Student t-test.

the Atlantic extend much further into Europe when only
winters with a stratospheric pathway are included.
The role of the stratospheric pathway can be brought out
even more dramatically by simply omitting its signal. Thisis presented in figure 3(c), which shows the linear ENSO
response but only for winters in which no SSW occurs.
When the stratosphere is unperturbed, the cold temperature
anomalies over Eurasia nearly disappear (only a narrow strip
survives over the northernmost edge of the continent), andthe warm anomalies over Greenland and eastern Canada aremuch weaker. Furthermore, the height anomalies no longer
extend over the North Atlantic (contrast the bottom panels of
figures 3(b) and (c)), in a manner that is highly reminiscent ofwhat is seen in model experiments in which the stratosphereis unperturbed (Ineson and Scaife 2009), or of linear inverse
models with stratospheric feedbacks removed (Newman andSardeshmukh 2008).
The lesson from figure 3 is this: with the stratosphericpathway suppressed, what we see in figure 3(c) is simply thetropospheric component of the linear ENSO response. It shows
the familiar PNA wave train and North American temperature
anomalies, but over the North Atlantic and Eurasia it looks
very different from the full ENSO response (figure 3(a)).
At this point it is natural to ask: why not suppress thetropospheric pathway, so as to clearly isolate the stratospheric

Table 2. The number of winters that occur from 1958 to 2013 thatare classified as El Niño, La Niña, and ENSO-neutrala . Second

column: the frequency of SSWs (SSWs/winter). Third and fourthcolumns: the number of winters in each grouping that have at least
one SSW; and the number of winters that have no SSWs.

All years
El Niño
La Niña
Neutral# of winters

Frequency ofSSWs (# perwinter)#ofwinters
with
SSWs

# of
winters
without
SSWs

56
19
18
19

0.63
0.79
0.72
0.37

29
11
11
7

27
8
7
12

a The 19 El Niño winters are 1957/58, 1958/59, 1963/64, 1965/66,

1968/69, 1969/70, 1972/73, 1976/77, 1977/78, 1982/83, 1986/87,
1987/88, 1991/92, 1994/95, 1997/98, 2002/03, 2004/05, 2006/07,
2009/10. The 18 La Niña winters are 1964/65, 1970/71, 1971/72,
1973/74, 1974/75, 1975/76, 1983/84, 1984/85, 1988/89, 1995/96,
1998/99, 1999/00, 2000/01, 2005/06, 2007/08, 2008/09, 2010/11, and2011/12.

warm anomalies over western North America appear weakerand less significant. The 500 hPa height anomalies in figures
3(a) and (b) (bottom) in general look similar, but one key
difference needs to be highlighted: the height anomalies over5

Environ. Res. Lett. 9 (2014) 024014

A H Butler et alpathway? As it happens, this is easily accomplished by taking
the sum of El Niño and La Niña winters, rather than the
difference. The results are shown in figure 4, and it represents
the ‘non-linear ENSO response’, since any linear behavior ishere eliminated by construction. Recall that the stratosphericpathway has the same sign response in El Niño and La Niña(figure 2(c)), whereas the tropospheric pathway has nearly
equal and opposite responses during these phases: taking thesum of opposite ENSO phases, therefore, is a simple way toremove the tropospheric pathway and reveal the stratosphericpathway alone. (Similar results can also be obtained bylooking at ENSO-neutral winters, with and without SSWs; notshown.)The non-linear ENSO response in figure 4(a) appears to
be largely non-existent. There is weak cooling over Eurasia
and weak warming over northern North America, but theseanomalies are not statistically significant; the same goes forthe height anomalies. This result is in contrast to those of someearlier studies which show significant ENSO non-linearities(Hoerling et al 1997, 2001). Probably, the differences arise
because: (a) we use a lower threshold for ENSO events, so thesample size is doubled in our study; (b) whereas the previous
studies consider only December–February, we average overNovember–March, reducing non-linearities due to differencesin the timing of ENSO SST anomalies; and (c) whereas theprevious studies consider La Niña events mostly prior to 1975,
we use ENSO events spread evenly over the observational
record, which are less likely to be influenced by Pacific decadal
variability (DeWeaver and Nigam 2002).
Now the key question becomes, why does figure 4(a) notshow the stratospheric pathway, as one might expect from thelogic above? The answer is simple: while roughly 60% of
El Niño and La Niña winters (table 2) have SSWs, the other
40% do not. Since ENSO winters with no SSWs often havestrong polar vortices, and since the impacts of a strong polar
vortex are essentially equal and opposite (in sign) to theimpacts of a weak polar vortex (Limpasuvan et al 2005), all
we are seeing in figure 4(a) is a large cancellation. To bring out
the stratospheric pathway we need to separate the non-linear
ENSO response into winters with SSWs and winters without
SSWs, as was done in figure 3.
The stratospheric pathway then emerges with great clarity. It is shown in figure 4(b), from the composite of the
non-linear ENSO response during winters with SSWs: notice
how the anomalies are very similar to those in the SSW
composite in figure 2(a), confirming that our method successfully isolates the stratospheric pathway by removing thetropospheric pathway. We also note some differences in theheight anomalies: in the 60 days following SSWs (figure 2(a),bottom) the heights resemble the negative NAO pattern andare centered over the North Atlantic and European regions,whereas in figure 4(b) (bottom) the heights are positive overthe polar cap and negative over the North Atlantic, but there
are also significant height anomalies over the Pacific sector.
These are likely a result from precursor patterns of SSWs,
which generally involve low height anomalies over the North
Pacific and high height anomalies over the central Pacific(Garfinkel et al 2010, Cohen and Jones 2011). We also note

Table 3. Top rows: the NDJFM mean surface temperature anomaly(K) for northern Eurasia (30–120◦ E, 60–75◦ N). Bottom rows: thestandardized JFM mean North Atlantic Oscillation index for all
winters, El Niño winters, La Niña winters, and ENSO-neutral
winters.
Winters with
Winters without
SSWs
All winters SSWs
NDJFM Eurasian surface temperature anomaly (K)All years
El Niño
La Niña
Neutral

∼0
−0.4
+0.3
+0.1

−0.5
−1.1
+0.2
−0.7

+0.6
+0.5
+0.5
+0.6

JFM NAO index
All years
El Niño
La Niña
Neutral

0.00
−0.26
+0.17
+0.10

−0.17
−0.44
+0.12
−0.22

+0.19
−0.01
+0.24
+0.29

that these anomalies, though weaker in magnitude, look very
similar to ENSO non-linearities found in previous studies(Hoerling et al 1997, DeWeaver and Nigam 2002), perhaps
providing an alternate interpretation for the non-linear ENSO
response.
Lastly, in the absence of SSWs, figure 4(c) shows that theanomalies are essentially equal and opposite to those during
winters with SSWs. This reflects the downward influence of astrong polar vortex, which is roughly equal and opposite to thatof a weak vortex (Baldwin and Dunkerton 2001). Thus, in thefull non-linear ENSO composite (figure 4(a)), the resulting
weak anomalies are a consequence of both the removal of
the tropospheric pathway and the major cancellation between
ENSO winters with a stratospheric pathway and those without.
However, since ENSO winters with a stratospheric pathway
occur more frequently than those without, the overall patternof anomalies in figure 4(a), though weak, generally resembles
those associated with SSWs (figure 4(b)).
We conclude by quantifying the importance of the stratospheric pathway in table 3. Focusing first on the left column we see that, irrespective of SSWs, the surface temperature over Eurasia (30–120◦ E, 60–75◦ N) is colder andthe NAO index is below normal during El Niño winters.
Oppositely signed responses are observed during La Niña. Thisis the familiar linear ENSO response, graphically illustrated infigure 3(a).
However, the left column in table 3 does not tell thewhole story. If we consider separately winters with SSWs
(middle column) and without SSWs (right column) we seethat the surface climate response varies dramatically depending on whether the stratospheric pathway is present or not.
Irrespective of the ENSO phase, Eurasian temperatures arecolder and the NAO index is more negative during winters with
SSWs compared to winters without SSWs. And the differences
between winters with and without SSWs are in general larger
than the differences between El Niño and La Niña winters. We
note in passing that the surface climate during La Niña varies
less than in the other ENSO phases: the stratosphere does
6

Environ. Res. Lett. 9 (2014) 024014

A H Butler et al

Figure 4. Top row: surface temperature (K) and bottom row: 500 hPa geopotential height (gpm; contour interval 10 gpm) anomalies
associated with the composite sum of (a) all El Niño and La Niña winters, (b) El Niño and La Niña winters in which at least one SSW
occurs, and (c) El Niño and La Niña winters during which no SSWs occur. The black line (gray shading) indicates anomalies with p < 0.05
for a two-tailed Student t-test.

not appear to impact the surface climate as strongly during
La Niña winters.
The key message here is that while the ENSO phase isa robust climate predictor for much of North America, thestate of the stratosphere is a greater predictor for the rest of
the Northern Hemisphere. In parts of Eurasia (figures 3(b),(c)), this could mean the difference between a wintertime
average of −1.5 ◦ C below normal during a winter with SSWs
and a wintertime average of nearly +2 ◦ C above normal
during a winter without SSWs. And over eastern Canada andGreenland, it could determine the extent and magnitude of iceloss or growth.

we have shown, these anomalies only arise in those wintersin which the stratosphere is severely perturbed; when no
SSW occurs, the temperature and height anomalies over theseregions are quite different.
We acknowledge that the sample size in our study isrelative small (only 56 winters and only 35 SSWs), andthat we have not accounted for other possible factors. For
instance, the Madden–Julian Oscillation has been found tohave impacts on Northern Hemisphere extratropical climate
and the stratosphere itself (Garfinkel et al 2012b, Riddle
et al 2013). The Quasi-biennial Oscillation (Baldwin et al2001), Eurasian snow cover extent (Cohen et al 2007), andvolcanic eruptions (Brönnimann et al 2007) also influence
surface climate during boreal winter and could be affecting
our analysis. Nonetheless, the strong stratospheric signature
documented above is statistically significant in our analysis.
And, more importantly, the importance of the stratosphericpathway has already been suggested by a number of modelingstudies (e.g. Ineson and Scaife 2009): hence our findings are,in some sense, observational confirmation of earlier modelingwork.
One might also wonder whether our strategy of focusing
on SSWs for identifying the stratospheric pathway’s influence
over the North Atlantic and Eurasia is the best way ofproceeding, given that SSWs are extreme events, and thus

4. Summary and discussion

We have demonstrated that the presence—or absence—of thestratospheric pathway during ENSO winters makes a verysignificant difference in the surface climate response overlarge portions of the Arctic, North Atlantic and Eurasia.
Because SSWs occur more frequently during ENSO winters,
the warm anomalies over Greenland and eastern Canada andthe cold anomalies over Eurasia, as well as a negative NAO-likepattern, could erroneously be associated with the troposphericpathways of El Niño (and vice versa for La Niña). But, as
7

Environ. Res. Lett. 9 (2014) 024014

A H Butler et al

statistically more difficult to capture. An alternative way ofproceeding would use, e.g. polar cap geopotential heightanomalies, as a proxy for an anomalously strong or weak polar
vortex, as was done in Thompson et al (2002). The advantageof that method is that more events, albeit of smaller amplitude,
can be included, possibly resulting in better statistics. We
have, in fact, carried out that calculation, and the results
are very close to the ones presented here. For the interested
reader, we include three figures describing those results in the
supplementary material (available at stacks.iop.org/ERL/9/02
4014/mmedia).
Before closing we also wish to clarify that the causality
link between ENSO and the stratospheric pathway is notcompletely understood. The simple fact that SSWs are known
to occur during ENSO-neutral winters is clear evidence that
ENSO is not the only cause of SSWs (their precise cause
remains an open research question). It is thus not inconceivablethat what we have been interpreting as the stratospheric
ENSO pathway may be a totally unrelated mode of variability
which has accidentally overlapped with ENSO in the lastfew decades: this, however, is highly unlikely. From table 1
one can see that the frequency of SSWs nearly doubles
during ENSO years compared to ENSO-neutral years. The
observational record is short, unfortunately, but a doublingin frequency seems to us a large enough anomaly to justify
our interpretation. Nonetheless, to the main conclusion of thisstudy, we add the caveat that the quantitative estimate of thestratospheric ENSO pathway reported above may, in reality,
represent only an upper bound, since some fraction of the
SSWs occurring during ENSO winters may be unrelated to
ENSO itself.
Lastly, we recall that stratospheric anomalies associatedwith SSWs do not always propagate all the way down to thetroposphere, and this adds to the difficulty in incorporatingsuch information into actual forecasts. Nonetheless, sinceseasonal prediction is currently based largely on the state of
ENSO, it is important to emphasize that stratospheric variability can both modulate ENSO impacts during active ENSO
winters, and exert unique influences on surface climate during
ENSO-neutral winters when predictive skill may be lacking.
A recent modeling study by Sigmond et al (2013), in which
dynamical forecast simulations were initialized during SSWs
while sea surface temperatures were relaxed to climatology,
showed surface impacts similar to those shown above as aconsequence of SSWs during ENSO-neutral winters.

Baldwin M P et al 2001 The Quasi-biennial Oscillation Rev.
Geophys. 39 179–229
Bell C J, Gray L J, Charlton-Perez A J, Joshi M M and Scaife A A
2009 Stratospheric communication of El Niño teleconnections toEuropean winter J. Clim. 22 4083–96
Brönnimann S, Luterbacher J, Staehelin J, Svendby T M, Hansen G
and Svenøe T 2004 Extreme climate of the global troposphereand stratosphere in 1940–42 related to El Niño Nature
431 971–4
Brönnimann S, Xoplaki E, Casty C, Pauling A and Luterbacher J
2007 ENSO influence on Europe during the last centuries Clim.
Dyn. 28 181–97
Butler A H and Polvani L M 2011 El Niño, La Niña, andstratospheric sudden warmings: a reevaluation in light of theobservational record Geophys. Res. Lett. 38 L13807
Cagnazzo C and Manzini E 2009 Impact of the stratosphere on thewinter tropospheric teleconnections between ENSO and theNorth Atlantic and European region J. Clim. 22 1223–38
Charlton A J and Polvani L M 2007 A new look at stratosphericsudden warmings. Part I: climatology and modeling benchmarks
J. Clim. 20 449–69
Cohen J, Barlow M, Kushner P J and Saito K 2007 Stratosphere–
troposphere coupling and links with Eurasian land surface
variability J. Clim. 20 5335–43
Cohen J and Jones J 2011 Tropospheric precursors and stratosphericwarmings J. Clim. 24 6562–72
DeWeaver E and Nigam S 2002 Linearity in ENSOs atmospheric
response J. Clim. 15 2446–61
Free M and Seidel D J 2009 Observed El Niño—Southern
Oscillation temperature signal in the stratosphere J. Geophys.
Res. 114 D23108
Garfinkel C I, Butler A H, Waugh D W, Hurwitz M M andPolvani L M 2012a Why might stratospheric sudden warmings
occur with similar frequency in El Niño and La Niña winters?
J. Geophys. Res. 117 D19106
Garfinkel C I, Feldstein S B, Waugh D W, Yoo C and Lee S 2012b
Observed connection between stratospheric sudden warmings andthe Madden–Julian Oscillation Geophys. Res. Lett. 39 L18807
Garfinkel C I and Hartmann D L 2007 Effects of the
El Niño—Southern Oscillation and the Quasi-biennial
Oscillation on polar temperatures in the stratosphere J. Geophys.
Res.—Atmos. 112 D19112
Garfinkel C I, Hartmann D L and Sassi F 2010 Troposphericprecursors of anomalous Northern Hemisphere stratosphericpolar vortices J. Clim. 23 3282–99
Gerber E P et al 2010 Stratosphere–troposphere coupling andannular mode variability in chemistry–climate models
J. Geophys. Res. 115 D00M06
Greatbatch R J, Lu J and Peterson K A 2004 Nonstationary impact
of ENSO on Euro-Atlantic winter climate Geophys. Res. Lett.
31 L02208
Hoerling M P, Kumar A and Xu T 2001 Robustness of the nonlinear
climate response to ENSOs extreme phases J. Clim.
14 1277–93
Hoerling M P, Kumar A and Zhong M 1997 El Niño, La Niña, andthe nonlinearity of their teleconnections J. Clim. 10 1769–86
Horel J D and Wallace J M 1981 Planetary-scale atmosphericphenomena associated with the Southern Oscillation Mon.
Weather Rev. 109 813–29
Hoskins B J and Karoly D J 1981 The steady linear response of aspherical atmosphere to thermal and orographic forcing J. Atmos.
Sci. 38 1179–96
Ineson S and Scaife A A 2009 The role of the stratosphere in theEuropean climate response to El Niño Nature Geosci. 2 32–6

Acknowledgments

LMP wishes to thank Matthew Newman for a most enlightening conversation at Café Sole, which stimulated the present
line of research. LMP was supported, in part, by a grant fromthe US National Science Foundation to Columbia University.
NCAR is sponsored by the National Science Foundation.
References
Baldwin M P and Dunkerton T J 2001 Stratospheric harbingers ofanomalous weather regimes Science 294 581–4
8

Environ. Res. Lett. 9 (2014) 024014

A H Butler et al

Kalnay E et al 1996 The NCEP/NCAR 40-year reanalysis project
Bull. Am. Meteorol. Soc. 77 437–71
Kolstad E W, Breiteig T and Scaife A A 2009 The association
between stratospheric weak polar vortex events and cold air
outbreaks Q. J. R. Meteorol. Soc. 136 886–93
Limpasuvan V, Hartmann D L, Thompson D W J, Jeev K and
Yung Y L 2005 Stratosphere–troposphere evolution during polar
vortex intensification J. Geophys. Res. 110 D24101
Manzini E, Giorgetta M A, Esch M, Kornblueh L and Roeckner E2006 The influence of sea surface temperatures on the northern
winter stratosphere: ensemble simulations with the MAECHAM5
model J. Clim. 19 3863–81
Newman M and Sardeshmukh P D 2008 Tropical and stratosphericinfluences on extratropical short-term climate variability J. Clim.
21 4326–47
Ortiz Beviá M J, Pérez-González I, Alvarez-Garcı́a F J andGershunov A 2010 Nonlinear estimation of El Niño impact on
the North Atlantic winter J. Geophys. Res. 115 D21123
Perlwitz J and Graf H-F 1995 The statistical connection between
tropospheric and stratospheric circulation of the Northern
Hemisphere in winter J. Clim. 8 2281–95
Randel W J, Garcia R R, Calvo N and Marsh D 2009 ENSO
influence on zonal mean temperature and ozone in the tropicallower stratosphere Geophys. Res. Lett. 36 L15822

Riddle E E, Stoner M B, Johnson N C, L’Heureux M L, Collins D C
and Feldstein S B 2013 The impact of the MJO on clusters of
wintertime circulation anomalies over the North American region
Clim. Dyn. 40 1749–66
Sardeshmukh P D and Hoskins B J 1988 The generation of global
rotational flow by steady idealized tropical divergence J. Atmos.
Sci. 45 1228–51
Sigmond M, Scinocca J F, Kharin V V and Shepherd T G 2013
Enhanced seasonal forecast skill following stratospheric sudden
warmings Nature Geosci. 6 98–102
Thompson D W J, Baldwin M P and Wallace J M 2002
Stratospheric connection to Northern Hemisphere wintertime
weather: implications for prediction J. Clim. 15 1421–8
Toniazzo T and Scaife A A 2006 The influence of ENSO on winterNorth Atlantic climate Geophys. Res. Lett. 33 L24704
Trenberth K E, Branstator G W, Karoly D, Kumar A, Lau N-C and
Ropelewski C 1998 Progress during TOGA in understanding andmodeling global teleconnections associated with tropical sea
surface temperatures J. Geophys. Res.: Oceans
103 14291–324
Van Loon H and Labitzke K 1987 The Southern Oscillation. Part V:
the anomalies in the lower stratosphere of the Northern
Hemisphere in winter and a comparison with the Quasi-biennial
Oscillation Mon. Weather Rev. 115 357–69

9