Impacts of high-latitude volcanic eruptions on ENSO
and AMOC
Francesco S. R. Pausata
a,1
, Leon Chafik
a,b,c
, Rodrigo Caballero
a
, and David S. Battisti
d,e
a
Department of Meteorology, Stockholm University and Bolin Centre for Climate Research, 10691 Stockholm, Sweden;
b
National Oceanic and Atmospheric
Administration/National Environmental Satellite, Data, and Information Service Center for Satellite Application and Research, College Park, MD 20740;
c
Cooperative Institute for Climate and Satellites, University of Maryland, College Park, MD 20740;
d
Department of Atmospheric Sciences, University of
Washington, Seattle, WA 98195; and
e
Uni Research, 5008 Bergen, Norway
Edited by Benjamin D. Santer, Lawrence Livermore National Laboratory, Livermore, CA, and approved September 16, 2015 (received for review May 11, 2015)
Large volcanic eruptions can have major impacts on global climate,
affecting both atmospheric and ocean circulation through changes
in atmospheric chemical composition and optical properties. The
residence time of volcanic aerosol from strong eruptions is roughly
2
–3 y. Attention has consequently focused on their short-term
impacts, whereas the long-term, ocean-mediated response has
not been well studied. Most studies have focused on tropical erup-
tions; high-latitude eruptions have drawn less attention because
their impacts are thought to be merely hemispheric rather than
global. No study to date has investigated the long-term effects of
high-latitude eruptions. Here, we use a climate model to show that
large summer high-latitude eruptions in the Northern Hemisphere
cause strong hemispheric cooling, which could induce an El Niño-
like anomaly, in the equatorial Pacific during the first 8
–9 mo after
the start of the eruption. The hemispherically asymmetric cooling
shifts the Intertropical Convergence Zone southward, triggering a
weakening of the trade winds over the western and central equa-
torial Pacific that favors the development of an El Niño-like anom-
aly. In the model used here, the specified high-latitude eruption
also leads to a strengthening of the Atlantic Meridional Overturn-
ing Circulation (AMOC) in the first 25 y after the eruption, followed
by a weakening lasting at least 35 y. The long-lived changes in the
AMOC strength also alter the variability of the El Niño
–Southern
Oscillation (ENSO).
high-latitude volcanic eruptions
|
AMOC
–ENSO interaction
|
volcanism
P
roxy data (1, 2) suggest that the strong reduction of surface
insolation over the tropics associated with tropical volcanic
eruptions may increase the likelihood of the El Niño
–Southern
Oscillation (ENSO) and a consequent reduction of the zonal sea
surface temperature (SST) gradient along the equatorial Pacific.
Modeling studies do not yield consistent results and show both
an El Niño-like (3
–5) or La Niña-like (6, 7) anomalies following
a tropical eruption. Recent studies have also suggested that
volcanic eruptions can have a large imprint on ocean circulation,
affecting the strength of the Atlantic Meridional Overturning
Circulation (AMOC) (8
–12) on 5- to 20-y timescales and in-
ducing ocean heat content (OHC) anomalies (13, 14) that may
persist for decades. However, this slow recovery has been
questioned and may be an artifact of experimental design (15).
Furthermore, all previous work on the climate impact of volcanic
eruptions has focused on tropical volcanoes; no studies have
addressed the potential effects of high-latitude eruptions on
ENSO. Here, we use a coupled atmospheric
–ocean–aerosol model
[Norwegian Earth System Model: NorESM1-M (16, 17)] to identify
the mechanisms by which high-latitude volcanic eruptions can
impact ENSO behavior in both the short term (up to 2
–3 y) and
long term (approximately half-century), the latter being medi-
ated by volcano-induced changes in ocean circulation.
We simulate an extreme high-latitude multistage eruption
starting on June 1st. We inject 100 Tg of SO
2
and ash
—as an
analog for the ash injection
—mostly into the upper-troposphere/
lower stratosphere over a 4-mo period. The eruption is composed
of eight injections, each lasting for 4 d and spaced out every 15 d
(
SI Appendix, Table S1
). This experimental design was chosen as
analog for one of the strongest high-latitude eruptions in his-
torical time, the 1783 Laki eruption in Iceland. The simulated
volcanic eruption starts from a specific year selected from a tran-
sient historical simulation (1850
–2005). An ensemble of simulations
(ENS
v
) is generated by slightly perturbing the initial conditions
of the day of the eruption. In the same fashion, we generate an
equivalent no-volcano ensemble (ENS
nv
) where the volcanic aerosol
concentration is set to background conditions (
SI Appendix
). The
climate perturbation induced by the volcanic eruption (
Δ
v
) can
be simply expressed as
Δ
v
= STATE
v
– STATE
nv
, where STATE
nv
is the unperturbed climate state, and STATE
v
is the climate state
induced by the eruption. To examine the short-term impact on
ENSO, we analyze the simulations described by Pausata et al. (18)
in which ENS
nv
and ENS
v
are composed of 20 pairs of simulation,
each pair being integrated for 4 y. Here, we extend 10 of these
pairs of simulations out to 60 y after the eruption to investigate its
long-term impact on the AMOC, OHC, and the spatiotemporal
properties of ENSO.
Short-Term Impacts on ENSO
Our results show that the simulated volcanic eruption generates
an aerosol plume that is strictly confined to the Northern
Hemisphere in the months following the eruption, with no direct
radiative forcing on the tropical zone (
SI Appendix, Fig. S1
).
Despite this latitudinally restricted forcing, anomalous El Niño-
like conditions relative to the no-volcano case appear in the tropical
Pacific, peaking between 4 and 9 mo after the beginning of the
Significance
In the model simulations analyzed here, large high-latitude vol-
canic eruptions have global and long-lasting effects on climate,
altering the spatiotemporal characteristic of the El Niño
–Southern
Oscillation (ENSO) on both short (
<1 y) and long timescales and
affecting the strength of the Atlantic Meridional Overturning
Circulation (AMOC). In the first 8
–9 mo following the start of the
eruption, El Niño-like anomalies develop over the equatorial Pa-
cific. The large high-latitude eruptions also trigger a strengthen-
ing of the AMOC in the first 25 y after the eruption, which is
associated with an increase in ENSO variability. This is then
followed by a weakening of the AMOC lasting another 30
–35 y,
associated with decreased ENSO variability.
Author contributions: F.S.R.P. designed and performed research; F.S.R.P. and L.C. analyzed
the data; L.C., R.C., and D.S.B. contributed to the interpretation of the model results;
F.S.R.P. wrote the paper; and L.C., R.C., and D.S.B. contributed to the writing of the
manuscript.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.
1
To whom correspondence should be addressed. Email: francesco.pausata@misu.su.se.
This article contains supporting information online at
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eruption (Fig. 1
B and C). The El Niño-like anomaly is followed
by cold (La Niña-like) anomalies in the second and third year
(Fig. 1
C and
SI Appendix, Fig. S5
). The El Niño-like anomaly is
caused by the strong cooling of the extratropical Northern
Hemisphere following the eruption (Fig. 1
A). It is well estab-
lished that such interhemispherically asymmetric forcing pushes
the Intertropical Convergence Zone (ITCZ) away from the
hemisphere that is cooled (19, 20). Hence, the simulated high-
latitude eruption causes a southward shift of the ITCZ of
∼5–6°
latitude over the Pacific Ocean, bringing the ITCZ closer to the
equator during the fall and winter following the eruption (Fig.
1
A and
SI Appendix, Figs. S2 and S3
). Because surface easterly
winds are weakest in the proximity of the ITCZ, this equator-
ward shift implies a weakening of the easterly winds along the
equator in the central and eastern equatorial Pacific (i.e., a
westerly anomaly; Fig. 1
B and
SI Appendix, Fig. S4
). This leads
via the Bjerknes feedback (21) to a reduction in the east
–west
temperature contrast across the tropical Pacific, thus favoring an
El Niño-like anomaly. The El Niño-like anomaly is a function of
the Northern Hemisphere cooling, but may be influenced by the
preexisting ENSO state: a stronger El Niño-like response may
develop under La Niña compared with El Niño preexisting
conditions (
SI Appendix, Fig. S5
).
In light of our results, we find intriguing that the El Niño event
that peaked in January of 1912, 6 mo before the Katmai eruption
in June of 1912 (the largest high-latitude eruption of the 20th
century), was immediately followed by near-normal conditions in
the tropical Pacific rather than the La Niña conditions that
normally occur after El Niño events. Another El Niño event
occurred a year after the eruption (
SI Appendix, Fig. S6
). Fur-
thermore, tree-ring data (22) suggest that the El Niño conditions
preceding the Laki eruption were further strengthened in the
winter of 1783
–1784 (6–9 mo after the beginning of the erup-
tion), in agreement with our findings. However, further evidence
would be needed to test our model results using observations.
Long-Term Impacts on ENSO and AMOC
The impacts of a multistage high-latitude volcanic eruption may
not be limited to the first few years following the eruption: our
model experiment also shows strong effects on SST variability in
the Nino3.4 region and much of the eastern equatorial Pacific
that persist for nearly a half-century following the eruption (Fig.
2). Impacts on ENSO frequency, on the other hand, are weak
and not statistically discernible (
SI Appendix, Tables S3 and S4
).
ENSO variability increases in the first 25 y following the eruption
(Fig. 2
A), whereas it is reduced in the last 35 y of the simulation
(years 26
–60), particularly between 26 and 45 y after the eruption
(Fig. 2
B and
SI Appendix, Table S2
) after which ENSO variability
reverts to normal (Fig. 2
C).
Along with these changes in ENSO variability, we also find
marked changes in the AMOC (Fig. 3
A). After a brief (<6-mo)
weakening of the AMOC, a progressive AMOC strengthening
takes place and peaks with a maximum anomaly of about 1.5
sverdrup (Sv) (1 Sv
= 10
6
m
3
/s) between 5 and 10 y after the
eruption (Fig. 3
A). Thereafter, the AMOC starts to slow down
and reaches a minimum (
∼1 Sv below that in the unperturbed
ensemble) about 35
–40 y after the eruption. Although a slow
recovery is apparent after this period, the AMOC remains sig-
nificantly weaker than in the no-volcano ensemble from 25 y
after the eruption until the end of the analyzed period. The nega-
tive radiative forcing from the volcanic aerosol results in a surface
cooling that develops during the first 1
–3 y after the eruption
(
SI Appendix, Fig. S8A
) and is gradually transferred into the
deep ocean (Fig. 3
B). The surface cooling also causes reduced
-0.2
0.2
-0.4
-0.6
0.6
0.4
0
-0.8
SST and Wind (SONDJF) anomalies
NINO3.4 Index
2 m/s
180°W
150°W
90°W
120°W
JUN01
JUN03
JUN02
JUN04
JUN05
0°
Temperature Anomaly (°C)
Precipitation Anomaly (mm/day)
-4
-3
-2
-1
0
1
2
3
-2
-1.5
-1
-0.5
0.5
1
1.5
-10
0
4
2
10
20
30
40
50
60
70
ITCZ
Latitude (°)
SST
Anomalies (°C)
(°C)
Time(months)
0
15°S
15°N
ITCZ
NV
V
0.8
A
B
C
Fig. 1.
Temperature, precipitation and wind anomalies following the eruption. (A) Ensemble average change (ENS
v
minus ENS
nv
) in the zonal-mean surface
temperature (blue) and precipitation (green) over the Pacific basin (150°E to 90°W), for the period 4
–9 mo following the start of the eruption (September to
February). Shading shows the approximate 95% confidence intervals (twice the SEM) of the change seen in all 20 pairs of experiments. The bold green dashed
lines show the ensemble-averaged position of the ITCZ in the no-volcano and volcano simulations. (B) Ensemble average changes in near-surface wind
(arrows) and SST (shading) 4
–9 mo following the start of the eruption. The box shows the Nino3.4 area. The contours delineate the areas where the SST
anomalies are significant at the 95% confidence level using a Student t test. (C) Ensemble average changes in Nino3.4 index due to the eruption.
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precipitation (
SI Appendix, Figs. S3 and S8B
) and consequently
reduced river runoff at mid-to-high latitudes of the Northern
Hemisphere. Cooler, more saline surface conditions in the first 2
or 3 y (
SI Appendix, Fig. S9
) increase the density of the surface
water in the higher latitudes of the Northern Hemisphere that
act to destabilize the water column, leading to enhanced oceanic
convection in the North Atlantic (
SI Appendix, Fig. S10
) and a
spin-up of the AMOC. The strengthening of the AMOC as well
as the mechanisms involved are similar to those proposed for
tropical eruptions (9, 10): here, we show that the impact on the
AMOC is not limited to the first 10
–20 y and to tropical
eruptions as shown in previous studies (9
–11, 23), but can also
occur in response to high-latitude eruptions, lasting for 50 y
or more.
The changes in ocean circulation are also accompanied by a
decrease in the OHC. The eruption immediately cools the sur-
face (
SI Appendix, Figs. S2 and S8A
), after which the anoma-
lously cold surface water is transferred into the thermocline layer
and deeper in the ocean. Cold anomalies below
∼300 m are
established by year 15, and they persist throughout the re-
mainder of the simulation. In this regard, the evolution of the
global OHC associated with a high-latitude eruption is similar to
A
C
B
D
Fig. 2.
SST SD changes. Ensemble average change (ENS
v
minus ENS
nv
) in the SD of monthly mean SST (in degrees Celsius) for the period 5
–25 y (A), 26–45 y (B),
and 46
–60 y (C) after the eruption and for the entire time series (D). Changes that are significant at the 95% confidence level using an F test are shaded. The
contour interval is 0.03 °C (dashed, negative anomalies; solid, positive anomalies; the zero line is omitted). We discard the first 4 y to remove the hemispheric-
wide cooling due the eruption. The SD is calculated from the concatenated time series using all 10 members in each ensemble. Hence, 210 y of data are used
to calculate the SD in A: 10 members
× 21 y; 200 in B, 150 in C, and 560 y in D. As is commonly done when examining ENSO, we apply a 5-mo running mean on
the SST anomalies to damp high-frequency ocean variability unrelated to ENSO. The box shows the Nino3.4 area.
Fig. 3.
AMOC index and integrated OHC differences. (A) Long-term changes in the AMOC, as indicated by the change in the maximum of the overturning
stream function (in sverdrups). The shading shows approximate 95% confidence intervals (twice the SEM) of the difference in all pairs of experiments that
comprise the ensembles. The vertical lines bound the periods analyzed in Fig. 2. The mean AMOC strength in the ENS
v
is significantly greater (smaller) than
that in the ENS
nv
in the 5
–25 (26–45) y after the eruption. We apply a 5-y running mean to the AMOC time series as commonly done in the literature.
(B) Ensemble average change in OHC (in joules) averaged from the surface to selected depths for the global ocean. Solid lines denote the ensemble average
change, and shading represents the SD of the ensemble difference.
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that for tropical eruptions such as Tambora, Agung, and Pina-
tubo (9, 10).
Discussion and Conclusions
In summary, our results illuminate the mechanism by which large
summer high-latitude eruptions in the Northern Hemisphere
may trigger an El Niño-like anomaly
—relative to the no-volcano
case
—in the first 4–9 mo after the eruption and affect both
AMOC and ENSO variability for decades thereafter. Such
eruptions generate a hemispheric-scale surface cooling and thus
trigger, via energetic constraints (19, 20), a general southward
shift in the ITCZ that is particularly marked in the Pacific basin.
In turn, the southward-shifted ITCZ in the Pacific generates
anomalous surface westerlies over the equatorial western and
central Pacific and anomalous equatorial northerlies in the
eastern Pacific in the first 4
–9 mo following the eruptions (Fig.
1
B); these anomalies constitute an optimal trigger for an El
Niño-like perturbation (24). The processes leading to El Niño-
like anomalies in response to high-latitude eruptions are thus
very different from those hypothesized to act in response to
tropical eruptions (25, 26) and rely on better-understood
mechanisms (19, 20). Only a few modeling studies (27
–30) have
investigated the climate impacts of high-latitude volcanic erup-
tions, and none has looked at a potential influence on ENSO.
Oman et al. (29), using an atmospheric model coupled to a
mixed-layer ocean, found a weakening of the summer monsoon
circulation and precipitation over Africa and Asia in the summer
of the eruption, consistent with our findings. In our model,
Northern Hemispheric cooling also gives rise to a southward shift
in the Pacific ITCZ and subsequently to an El Niño-like anomaly
via a dynamical (Bjerknes) feedback, which is precluded in a
mixed-layer ocean model.
Our results also suggest that a large high-latitude eruption has
global-scale, long-lasting effects owing to changes in the OHC
and the AMOC, which in turn affect ENSO variability. Several
modeling and observation-based studies have found a causal link
between the AMOC strength and tropical Pacific climate (31
–
36) through large-scale atmospheric circulation teleconnections
associated with SST gradient changes in the tropical Atlantic (
SI
Appendix, Fig. S12
). In addition to this atmospheric bridge, a
readjustment of the global ocean circulation by oceanic waves
also transmits thermocline signals from the North Atlantic to the
tropical Pacific (37
–39). However, the timescales associated with
these teleconnections are very different: the atmospheric influence
can be transmitted from the tropical Atlantic into the tropical
Pacific in few weeks, whereas the oceanic teleconnections have a
timescale of a few decades (36
–38).
Studies do not agree on whether ENSO variance is positively
or negatively correlated to the AMOC strength. For example,
Dong and Sutton (31) and Timmerman et al. (36) examined the
response of five climate models to the imposition of freshwater
forcing (
“hosing”) of the North Atlantic Ocean and found a
significant increase in ENSO variability when the AMOC was
substantially weakened. On the other hand, Timmerman et al.
(39) and Atwood (35) show that, in two other models, ENSO
variance decreases in response to a hosing of the North Atlantic.
The positive correlation between AMOC and ENSO variance is
also supported by proxy reconstructions of the impact of the 8.2-ky
BP freshwater discharge into the North Atlantic, which shows
that the variance of ENSO was reduced for several decades after
the freshwater pulse (35). Physically, a stronger AMOC may
cause an increase in ENSO variability by shoaling and flattening
the Pacific thermocline along the equator (
SI Appendix, Fig.
S13
), which enhances the strength of the Bjerknes feedback (
SI
Appendix, Fig. S14
) (40).
The lack of robustness of modeled ENSO responses to
changes in AMOC is likely the result of the inability to correctly
simulate the climatology of the tropical Pacific atmosphere
–ocean
system, compromising the physics and feedbacks governing the
modeled ENSO (41). Analyses of the climate models used in the
past decade or so show that in most of them
—including in all
models (or very similar versions) examined by Timmermann
et al. (36)
—the spatiotemporal structure of ENSO and the key
feedbacks have large biases compared with those observed (42).
In contrast, the ENSO simulated by the climate model used here
compares favorably to observations [figure 13 in the study by
Bellenger et al. (42)] both in terms of mean state (amplitude,
spatial structure, frequency spectrum, and the seasonality) and
the strength of the feedbacks acting throughout a typical ENSO
cycle (the Bjerknes feedback, the heat flux, shortwave and latent
heat feedbacks). Our model
’s more realistic portrayal of key
features of ENSO
—compared with most climate models—may
be related to the fact that the double ITCZ bias over the tropical
Pacific
—ubiquitous in climate models—is less pronounced in
NorESM1-M (16, 17): the simulated ITCZ is more confined to
the Northern Hemisphere in NorESM1-M, as observed, rather
than being symmetric around the equator as in most of the models.
Our results highlight the potential for large high-latitude
eruptions to affect global climate through long-lasting changes in
ocean circulation and heat content beyond the lifetime of the
injected stratospheric aerosols. Our study also provides new in-
sights for a better understanding of volcanic impacts on ENSO
variability, which is of importance also in view of the potential
role played by the tropical Pacific in the global warming hiatus
(43
–47). More generally, our results suggest that multidecadal
changes in the AMOC
—owing to either natural internal vari-
ability or forcing (such as volcanic eruptions)
—may modulate
the statistics of ENSO for several decades into the future. Fur-
ther modeling studies, possibly at a community level (48) such as
those planned in the Volcano Model Intercomparison Project
(49), will be necessary to better assess the robustness and the
mechanisms behind the AMOC
–ENSO relationship, given the
very different AMOC sensitivity to external forcing shown by
climate models (14). The potential impact of AMOC modifica-
tions on tropical Pacific climate introduces additional challenges
in attributing future changes in ENSO variability to changes in
human activity.
Materials and Methods
Model Description and Experiment Design. We use the coupled atmospheric
–
ocean
–aerosol model NorESM1-M (16, 17) with horizontal resolution 1.9°
(latitude)
× 2.5° (longitude) and 26 vertical levels. The model includes
treatment of the direct effect of aerosols, and the first and second indirect
effects of aerosols on warm clouds. NorESM1 is an Earth System Model that
uses a modified version of CAM4, CAM4
–Oslo, for the atmospheric part of
the model, with an updated module that simulates the life cycle of sea salt,
mineral dust, particulate sulfate, black carbon, and primary and secondary
organics. CAM4
–Oslo is coupled to an updated version of the isopycnal
ocean model MICOM. A more detailed description is provided in
SI Appendix
.
The multistage high-latitude eruption is simulated by injecting, mostly in
the upper-troposphere/lower stratosphere, 100 Tg of SO
2
and dust over a
4-mo period. The eruption is comprised of eight injections, each lasting for
4 d (
SI Appendix, Table S1
). The simulated volcanic eruption starts from a
specific year selected from a transient 156-y historical (1850
–2005) simula-
tion. We generate the individual ensemble members by perturbing the ini-
tial conditions of the specific year in which the eruption is simulated;
perturbations are constructed by replacing the state of the atmosphere on
June 1st with that from days immediately preceding or following the
eruption. Twenty integrations are performed, each 4 y long. Ten of these
integrations are extended to 60 y; together, they constitute the volcanic
ensemble, ENS
v
. An equivalent ensemble is generated from a control run
that has volcanic aerosols set to background conditions (ENS
nv
): historical
aerosol emissions are taken from Intergovernmental Panel on Climate
Change AR5 datasets (50). The eruption year selected is the model year 1934
(eruption year: number 01), which is roughly in middle of the climatology
period, and it presents El Niño conditions as it was before the Laki eruption
(
SI Appendix
). A detailed examination of NorESM performance in interactively
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simulating the Laki eruption and a comparison with other modeling studies
is available in the study by Pausata et al. (18).
We analyze monthly mean model output. We assess the statistical sig-
nificance of differences in mean state and variability (at a stipulated 95%
significance level) using t and F tests.
Analyses.
ENSO. The ENSO index used in this study consists of monthly mean SST
anomalies spatially averaged over the Nino3.4 region (5°N to 5°S and 170°W
to 120°W). A 5-mo running mean is applied to damp uncoupled intra-
seasonal variations in SST. El Niño events are defined as the periods during
which the 5-mo running mean of the SST index anomaly is greater than
+0.4 °C for at least 6 consecutive mo. Changes in the ENSO variability are
measured as changes in the SST SD in the Nino3.4 area. The SD is calculated
from the concatenated time series using all 10 members in each ensemble.
The concatenation does not change the variance in ENS
v
and ENS
nv
, and only
slightly affects the threshold for statistical significance.
AMOC. The AMOC index is the maximum in the zonally averaged overturning
stream function in the North Atlantic between 30°N and 60°N and between
500- and 2,000-m depth. NorESM simulates a vigorous AMOC compared with
other models, being in the upper range of AMOC strengths simulated by
CMIP3 models (17). Measured by the maximum in the overturning stream
function in North Atlantic, the AMOC in the NorESM is about 30 Sv at 26.5°N,
whereas the observed AMOC is about 18
–20 Sv (1).
ACKNOWLEDGMENTS. We thank A. Hannachi, A. Grini, M. Gaetani, and
U. Ninnemann for discussions and suggestions, and J. Carton, A. Robock, and
two anonymous reviewers for insightful comments on the manuscript. The
simulations were performed on resources provided by the Swedish National
Infrastructure for Computing at the National Supercomputer Centre.
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