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Atmos. Chem. Phys., 19, 10379–10390, 2019
https://doi.org/10.5194/acp-19-10379-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
Revisiting the Agung 1963 volcanic forcing –
impact of one or two eruptions
Ulrike Niemeier1, Claudia Timmreck1, and Kirstin Krüger2
1The Atmosphere in the Earth System, Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany
2Meteorology and Oceanography Section, Department of Geosciences, University of Oslo, Oslo, Norway
Correspondence: Ulrike Niemeier (ulrike.niemeier@mpimet.mpg.de)
Received: 30 April 2019 – Discussion started: 9 May 2019
Revised: 17 July 2019 – Accepted: 18 July 2019 – Published: 15 August 2019
Abstract. In 1963 a series of eruptions of Mt. Agung, In-
donesia, resulted in the third largest eruption of the 20th cen-
tury and claimed about 1900 lives. Two eruptions of this se-
ries injected SO2into the stratosphere, which can create a
long-lasting stratospheric sulfate layer. The estimated mass
flux of the first eruption was about twice as large as the mass
flux of the second eruption. We followed the estimated emis-
sion profiles and assumed for the first eruption on 17 March
an injection rate of 4.7 Tg SO2and 2.3 Tg SO2for the second
eruption on 16 May. The injected sulfur forms a sulfate layer
in the stratosphere. The evolution of sulfur is nonlinear and
depends on the injection rate and aerosol background condi-
tions. We performed ensembles of two model experiments,
one with a single eruption and a second one with two erup-
tions. The two smaller eruptions result in a lower sulfur bur-
den, smaller aerosol particles, and 0.1 to 0.3 Wm−2(10 %–
20 %) lower radiative forcing in monthly mean global aver-
age compared to the individual eruption experiment. The dif-
ferences are the consequence of slightly stronger meridional
transport due to different seasons of the eruptions, lower in-
jection height of the second eruption, and the resulting dif-
ferent aerosol evolution.
Overall, the evolution of the volcanic clouds is different in
case of two eruptions than with a single eruption only. The
differences between the two experiments are significant. We
conclude that there is no justification to use one eruption only
and both climatic eruptions should be taken into account in
future emission datasets.
1 Introduction
In September 2017 Mt. Agung, a volcano on Bali, Indonesia
(8.342◦S, 115.58◦E), became restless. Earthquakes, steam,
ash clouds, and lahars resulted in the evacuation of nearly
150 000 people from the volcano’s environment within a ra-
dius of 9–12 km in November 2017 (Gertisser et al., 2018).
The eruption resulted in an ash cloud reaching up to an alti-
tude of about 9.3 km (Marchese et al., 2018) and about 10 DU
SO2above Bali (Hansen, 2017), which was not large and
high enough to result in a climatic impact. The last climatic
eruption of Mt. Agung dates back more than 50 years. From
February 1963 to January 1964 a series of eruptions from
Mt. Agung are documented (Fontijn et al., 2015). The initial
unrest resulted in the third largest eruption of the 20th cen-
tury global volcano record and claimed about 1900 lives. Re-
vising the literature, it is obvious that not just one of the erup-
tions was strong enough to inject SO2into the stratosphere,
a requirement of creating a long-lasting stratospheric sulfate
layer, but also a second one (Self and Rampino, 2012). The
mass flux of the second eruption on 16 March was about half
the size of the mass flux of the first eruption on 17 March.
Self and Rampino (2012) estimate the volumetric eruption
rate for 17 March to be ∼1.8×104m3s−1over a ∼3.5 h du-
ration. The volumetric eruption rate of the 16 May event was
∼0.9×104m3s−1over a ∼4 h duration. The resulting sul-
fate layer caused a climatic impact by scattering and absorb-
ing solar and terrestrial radiation leading to a temperature
decrease of about 0.4 K in the tropical troposphere (Hansen
et al., 1978).
The sulfate load of the Mt. Agung eruptions was estimated
to 7–7.5 Tg SO2from observations of aerosol optical depth
(AOD) a couple of months after the eruption (Self and King,
Published by Copernicus Publications on behalf of the European Geosciences Union.
10380 U. Niemeier et al.: Revisiting the Agung 1963 volcanic forcing
1996). It was impossible to distinguish between single erup-
tions with this method. This could be the reason that up to
now, recent volcanic forcing datasets assume one large erup-
tion phase of 7 Tg SO2for Mt. Agung, the one in March
1963, but neglect the second one. Within this paper we ex-
amine whether or not it is important to consider both erup-
tion phases individually when simulating sulfate evolution
and transport, as well as the impact on radiative forcing of
the Mt. Agung eruption.
The radiative forcing of the sulfate aerosols can be simu-
lated by either calculating the evolution and transport of sul-
fur with an aerosol microphysical model or, much simpler,
prescribing the optical parameters of the volcanic aerosols.
The first needs volcanic injection data and information on
emission strength and altitude, and the latter optical prop-
erties like the aerosol optical depth (AOD). Most datasets
base the estimated global coverage of sulfate aerosols after
the Mt. Agung eruption on ground-based measurements and
on ice core measurements, and provide the AOD (e.g., Sato
et al., 1993; Stenchikov et al., 1998; Ammann et al., 2003;
Crowley et al., 2008; Crowley and Unterman, 2013). Satellite
data were not yet available in 1963. Newer datasets not only
rely on measurements, but they also include simulated sul-
fate distributions, e.g., results of an empirical aerosol forcing
generator like Easy Volcanic Aerosol (EVA) (Toohey et al.,
2016) or complex aerosol models, which simulate the evolu-
tion of the aerosol, e.g., Arfeuille et al. (2014) for the SAGE-
4λdataset. On the other hand new volcanic eruption datasets
are released, providing the SO2injection rate for large
climate-relevant volcanic eruptions, e.g. Volcanic Emissions
for Earth System Models (VolcanEESM) (Neely III and
Schmidt, 2016) and the eVolv2k dataset (Toohey and Sigl,
2017), and sulfur injection data. These newer datasets in-
clude one eruption phase only for Mt. Agung, the main erup-
tion in March 1963, and assume the injected amount of SO2
of 7 Tg following the estimates of Self and King (1996).
The evolution of the volcanic aerosols is strongly nonlin-
ear. In particular, the particle size depends on the erupted
mass (Timmreck et al., 2010; Niemeier and Timmreck, 2015)
and sulfur injected into an existing volcanic sulfate layer
evolves differently than sulfur injected into background con-
ditions (Laakso et al., 2016). Additionally, many chemical
processes depend on particle size and sulfate concentrations.
For example stratospheric OH, NOx, and ozone concentra-
tions change under high sulfur load. SPARC (2010) shows
in Figure 8.20 the temperature response of different strato-
spheric chemistry models to volcanic sulfate aerosols. Mod-
els using full aerosol microphysics or prescribing surface
aerosol density tend to overestimate the measured heating in
the stratosphere after the Mt. Agung eruption. This might be
related to the assumption of one eruption phase only, espe-
cially as for the Mt. Pinatubo eruption in 1991 the models
show a slightly better result.
In this study we would like to address the following ques-
tion: is there a significant difference when simulating two
medium eruptions instead of a single large one? We per-
formed two experiments to provide an answer to this ques-
tion. We describe the model and the simulations in more de-
tail in Sect. 2, and show results in Sect. 3 where we describe
the different burden results of the two experiment ensem-
bles (Sect. 3.1) and the cumulative impact of the eruptions
(Sect. 3.2). Finally, we compare our results to measurements
in Sect. 4 before we conclude in Sect. 5.
2 Model and observation data
2.1 Model setup
The model simulations of this study were performed with
the middle atmosphere version of the general circulation
model (GCM) MAECHAM5 (Giorgetta et al., 2006). The
aerosol microphysical model HAM (Stier et al., 2005) is
interactively coupled to the GCM and was extended to a
stratospheric version (Niemeier et al., 2009). MAECHAM5–
HAM, ECHAM–HAM later in the text, was applied with the
spectral truncation at wave number 42 (T42), a horizontal
grid size of about 2.8◦, and 90 vertical layers (L90) up to
0.01 hPa. The model is not coupled to an ocean model and
shows pure volcanic forcing response only. The sea surface
temperatures (SSTs) are set to monthly mean climatologi-
cal values based on the Atmospheric Model Intercompari-
son Project (AMIP) SST observational dataset (Hurrell et al.,
2008). Thus, the SST does not reflect the historical date but
rather represents a climatological mean.
HAM calculates the evolution of sulfate from the in-
jected SO2to sulfate aerosol, including nucleation, accumu-
lation, condensation, and coagulation, as well as transport
and sink processes like sedimentation and deposition (Stier
et al., 2005). A simple stratospheric sulfur chemistry is ap-
plied above the tropopause (Timmreck, 2001; Hommel et al.,
2011) and the sulfate is radiatively active. The model setup
is described in more detail in Niemeier et al. (2009) and
Niemeier and Schmidt (2017).
The L90 version of MAECHAM5–HAM interactively
generates a quasi-biennial oscillation (QBO) (Giorgetta
et al., 2006). However, we decided to nudge the QBO in the
tropical stratosphere to the observed monthly mean winds at
the Equator (updated Naujokat, 1986), as described in Gior-
getta and Bengtsson (1999). This allows us to inject the vol-
canic sulfur into the observed QBO phase and still include
the better resolved transport processes of the L90 version,
e.g., a less permeable subtropical transport barrier (Niemeier
and Schmidt, 2017). Nudging the QBO prescribes the feed-
backs of the sulfate aerosol heating in the stratosphere on the
QBO winds as observed. However, the QBO winds are pre-
scribed on a monthly basis. This may suppress very short-
term changes in the transport due to dynamical changes
caused by aerosol heating at the equatorial stratosphere.
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U. Niemeier et al.: Revisiting the Agung 1963 volcanic forcing 10381
2.2 Model simulations
We performed experiments of two scenarios for the 1963
eruption of Mt. Agung. We assumed for the first experiment
one eruption phase at 17 March (AGUNG1) with an injection
of 7 Tg SO2over 3 h. For the second scenario two eruptions
were simulated with a ratio of the injection rate of 2 :1. This
reflects the ratio of the volumetric eruption rate and the mass
flux of 4 and 2 kg s−1given in Table 3 in Self and Rampino
(2012). The altitudes of the eruptions were taken as the aver-
age of the range of estimated altitudes in Self and Rampino
(2012). This resulted for the second experiment in the fol-
lowing assumption of two eruption phases (AGUNG2): the
first on 17 March, over 3 h with an injection rate of 4.7 Tg
SO2at an altitude of 50 hPa, and a second on 16 May, over
4 h with an injection rate of 2.3 Tg SO2at a slightly lower
altitude of 70 hPa. The ECHAM5–HAM input data for the
eruptions are summarized in Table 1.
We performed a set of six ensemble members for each
eruption case. See Supplement for further details. All six
simulations were used to calculate an ensemble mean. Ad-
ditionally, we performed a single simulation where an erup-
tion altitude of 50 hPa was assumed for both eruptions
(AGUNG2-50hPa).
2.3 Observations
We compare our simulation results to observations and vol-
canic datasets provided for the Climate Model Intercompari-
son Project (CMIP). The AOD prescribed for the years 1963
to 1964 in the CMIP5 simulations of the Max Planck In-
stitute for Meteorology is based on Sato et al. (1993) and
Stenchikov et al. (1998). The data rely mainly on astronom-
ical observations summarized by Dyer and Hicks (1968), as
no satellite data are available for the period. The AOD data
for CMIP6 were taken from the SAGE-3λdatabase (Luo,
2016; Revell et al., 2017). They combine ice core data (Gao
et al., 2008) and AOD data (Stothers, 2001) with aerosol mi-
crophysical model simulations and include only one eruption
of Mt. Agung in their preparatory model simulations (Ar-
feuille et al., 2014).
Stothers (2001) assembled a revised chronology of ob-
served AOD after the Mt. Agung eruption. The data con-
tain mainly measurements of atmospheric attenuation of
starlight and direct sunlight. Stothers (2001) provides values
of monthly mean AOD data as an average over different mea-
surement sites, between 20 and 40◦N and 20 and 40◦S and
results of monthly mean data of single measurement sites.
Stothers (2001) excluded data of tropical measurement sites
because they were not reliable.
Radiosonde temperature measurements provide informa-
tion on the heating of the stratospheric aerosol layer after
the eruption. This heating causes changes in stratospheric dy-
namics (Aquila et al., 2014; Toohey et al., 2013) and is, there-
fore, an important value which should be taken into account
correctly. Free and Lanzante (2009) provide vertical tem-
perature anomalies after volcanic eruptions from radiosonde
data (RATPAC). The carefully examined dataset contains
data of 85 radiosonde stations, 32 of them in the tropics (Free
et al., 2005). The QBO and El Niño–Southern Oscillation
(ENSO) signals in the temperature were removed. To calcu-
late the temperature anomalies, the average of the 2 years
before the eruption was subtracted from the average over the
2 years after the eruption. The corresponding model data of
AGUNG1 and AGUNG2 were calculated by averaging over
the 2 years after the first eruption and subtracting an ensem-
ble mean of control simulations with nudged QBO data, but
no volcanic eruption, for the years 1964 to 1965. This pro-
vides the anomaly and removes the QBO signal at the same
time.
3 Results
Figure 1 shows the monthly and zonally averaged sulfate bur-
den of the ensemble mean. Mt. Agung is located at 8◦in
the Southern Hemisphere (SH) tropics. Thus, the main trans-
port direction of the aerosols is southward and, hence, bur-
den values in the Northern Hemisphere (NH) remain small.
The ensemble mean shows for both experiments, AGUNG1
and AGUNG2, two areas with high burden: a maximum in
the southern tropics in the months 1 to 3 after the eruption,
and about four months later in the SH midlatitudes and high
latitudes with a secondary maximum between 30 and 60◦S.
The maximum burden is slightly above 14 mg m−2in the en-
semble of AGUNG1 and 10 mg m−2, about 30 % lower, in
the ensemble of AGUNG2, reflecting the ratio of the ini-
tial injection. The initially higher injection in the ensem-
ble of AGUNG1 results in a higher burden over almost all
simulated months and regions. The strongest absolute dif-
ference between the two ensembles occurs in the time pe-
riod between both climatic eruptions, when the injected sul-
fur amount in AGUNG2 is still smaller. The relative differ-
ence highlights that more aerosols are transported into the
NH tropics in AGUNG2 in the first months after the second
eruption (months 4 to 6). The burden of AGUNG2 increases
slightly after the second eruption but, overall, the tropical
maximum of the burden is smaller and occurs later than in
AGUNG1. In the SH extratropics the differences between the
two ensembles are below 20 %, 1 to 2 mg m−2. In contrast,
the burden is up to 50 % larger in AGUNG2 in months 6 to
10 in the NH extratropics, poleward of 30◦N, but with small
absolute values. Also towards NH winter the relative differ-
ence between the two simulations is larger in the NH than in
the SH, which indicates differences in the transport regime
and wind systems.
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10382 U. Niemeier et al.: Revisiting the Agung 1963 volcanic forcing
Table 1. Overview of the performed simulations and information to the eruption details, after Self and Rampino (2012).
Simulation Eruption Eruption Eruption Eruption Ensemble OH limitation
name mass altitude duration date members
AGUNG1 7 Tg SO250 hPa 06:00–09:00 UTC 17 Mar 1963 six no
AGUNG2 4.7 Tg SO250 hPa 06:00–09:00 UTC 17 Mar 1963 six no
2.3 Tg SO270 hPa 17:00–21:00 UTC 16 May 1963
AGUNG2-50hPa 4.7 Tg SO250 hPa 06:00–09:00 UTC 17 Mar 1963 one no
2.3 Tg SO250 hPa 17:00–21:00 UTC 16 May 1963
Figure 1. Ensemble mean of sulfate burden of experiments AGUNG1 (a) and AGUNG2 (b). Absolute (c) and relative differences (d) of the
two ensembles. The xaxis gives the months after the first eruption in March 1963. Stippling indicates non-significant differences at the 99%
level, following a Student’s ttest. The gray dots mark the location and size of the volcanic eruptions.
3.1 Transport of aerosols
The ensemble of AGUNG2 results in a lower sulfate burden
than AGUNG1 in most areas and at most times (Fig. 1). An
exception is the sulfate burden in the extratropics in the first
months after the second eruption. This indicates a stronger
meridional transport in AGUNG2. The second eruption oc-
curs 2 months later and thus in a different season with a
different stratospheric transport pattern. Additionally, the in-
jection altitude is lower, 70hPa instead of 50 hPa. Figure 2
shows the monthly mean zonal wind (shaded) and the resid-
ual stream function for April and June 1963, 1 month after
the eruption each, of AGUNG2 (see Fig. S5 for results of
AGUNG1). Nudging of the QBO at the Equator results in
similar zonal mean zonal winds in the tropics between the
two experiments. Differences of the zonal mean zonal wind
in the extratropics, around ±10 %, are not significant and are
mainly caused by a meridional shift of the higher-latitude
wind systems.
Punge et al. (2009) showed that meridional transport in
the tropics and subtropics depends on the QBO phase. Fig-
ure 2 shows that both eruption phases inject into easterly
zonal wind. Thus, the QBO phase should not play an impor-
tant role in transport characteristics of both experiments. Sea-
sonality seems more important. The streamlines show that
the zero line, indicating the tropical pipe, is shifted north-
ward in June, allowing more sulfate to be transported into
the NH. Additionally, the second eruption occurs at a lower
altitude, 70 hPa, where the subtropical transport barrier is
weaker than at 50 hPa. This allows more meridional transport
than at 50 hPa: the streamline at 70 hPa in June is stronger
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U. Niemeier et al.: Revisiting the Agung 1963 volcanic forcing 10383
Figure 2. Monthly mean zonal wind (ms−1) of AGUNG2 for April (a) and June (b) 1963 in shading. Contour lines show the stream function
(kg s−1). Positive (solid) streamlines describe clockwise circulation, negative (dashed) ones counterclockwise circulation. The gray dots
mark the injection location of the two volcanic eruptions.
than at 50 hPa in April, 100 kgs−1and around 50 kg s−1, re-
spectively.
3.2 Cumulative impact – a sum over time
The zonally averaged cumulative burden (Fig. 3a), time inte-
grated monthly mean values over 21 months, starting with
the month of the first eruption, is roughly 20 % lower for
AGUNG2 than AGUNG1. The difference between the two
results is larger in the tropics than in the secondary maximum
around 50◦S. This results from the stronger meridional trans-
port towards the SH in AGUNG2. Thus, one eruption with
larger SO2injection results in higher burden than the same
injected amount of SO2, but split into two eruptions and in a
slightly stronger tropical confinement of the aerosols. How-
ever, the shaded areas indicate that the variance within each
ensemble is larger than the differences between the ensemble
mean values.
This result is confirmed in the cumulative AOD (Fig. 3b).
But the AOD of AGUNG1 and AGUNG2 differs less (about
10 %) than the burden (about 20 %) in the tropics and even
less in the SH extratropics (about 6 %). The reason for this
is different particle radii. Scattering of sulfate aerosols de-
creases with increasing particle radius. The maximum ef-
fective radii of the sulfate aerosols reach 0.5 µm at 8◦S for
AGUNG1 and stay below 0.45µm for AGUNG2 (Fig. 4a,
b). Sulfate particles are 0.05 µm smaller in AGUNG2. Thus,
they scatter more intensely and the AOD difference between
the two experiments gets smaller. These smaller radii are the
consequence of the lower injection rate of the first eruption
in AGUNG2. Particle sizes increase with increasing injection
rate (Niemeier and Timmreck, 2015). Laakso et al. (2016)
simulated a volcanic eruption into background sulfate level
and into elevated sulfate level from continuous injections for
climate engineering (CE). They show a shorter lifetime and
larger particles under CE conditions. Thus, following Laakso
et al. (2016), we would expect stronger coagulation after the
second eruption, as newly formed particles coagulate fast
with available larger particles. But the second eruption oc-
curs at lower altitude, where sulfate concentration and parti-
cle radii are smaller. Thus, coagulation is less important. We
performed an additional sensitivity study where we injected
the SO2in both eruptions at 50 hPa to differentiate better
between injection rates and emission height. The sensitivity
simulation AGUNG2-50hPa, with both eruptions at the same
altitude, results in particle radii very similar to AGUNG1 and
about 0.05 µm larger than in AGUNG2. This reflects the re-
sults of Laakso et al. (2016). See Sect. 2 in the Supplement
for more information and figures.
The climatic impact can be derived from the aerosol ra-
diative forcing at the top of the atmosphere (TOA), which
was calculated with a radiation double call (Fig. 5a). The
spread of the single ensemble members is large, but the av-
erage of the AGUNG2 ensemble is just outside of the 2σen-
semble variability of AGUNG1. The global monthly mean
TOA forcing of sulfate is about 0.1 to 0.3Wm−2larger
in AGUNG1. The average difference in the short-term vol-
canic forcing over the first 21 post-eruption months is 3 to
10 times larger than the long-term forcing radiative forcing
of stratospheric ozone (−0.033 Wm−2) in CMIP6 (Checa-
Garcia et al., 2018) and comparable to the long-term ra-
diative forcing of the total ozone column (0.28 Wm−2in
CMIP6). The long-term radiative forcing of anthropogenic
sulfate aerosols is assumed to be −0.4 Wm−2(Stocker et al.,
2013).
We estimate surface temperature changes (1Ts) to give
a rough estimate of the consequences of this forcing differ-
ence. 1Tsrelates to forcing as 1Ts=αF , with Fthe radia-
tive forcing and αthe climate sensitivity (Gregory and Webb,
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10384 U. Niemeier et al.: Revisiting the Agung 1963 volcanic forcing
Figure 3. Cumulative values, integral over 21 monthly mean values, of (a) zonally averaged sulfate burden and (b) AOD at 550nm. The
shadings indicate the maximum and minimum values of the single simulations in the ensemble.
Figure 4. Time series of monthly mean effective radius (µm) of sulfate at the grid point corresponding to 8◦S(a, b) and 50◦S(c, d).
2008; Ramaswamy et al., 2001). αis a constant which differs
for each model. Thus in our case
1Ts(AGUNG1)
1Ts(AGUNG2)
=F (AGUNG1)
F (AGUNG2)
=
−1.35 Wm−2
−1.23 Wm−2=1.1,(1)
with F(AGUNG1) and F(AGUNG2) the averages over the
global radiative forcing of months 3 to 9 after the erup-
tion (Fig. 5a). Thus, we overestimate the surface cooling
in AGUNG1 by a factor of 1.1 or 10 %. Both experiments
show the strongest difference of radiative forcing in the trop-
ics (Fig. 5b) and the strongest surface cooling occurs in the
tropics as well.
4 Comparison to observations
The zonally averaged AOD of AGUNG1 and AGUNG2 dif-
fers mainly in the tropics (Fig. 6a, b) and is rather similar
in the SH extratropics. Our results agree quite well with the
CMIP6 AOD (Fig. 6d), which shows, however, slower trans-
port into the extratropics and no secondary maximum at 40 to
50◦S. Less aerosol reaches the SH high latitudes in CMIP6,
but it has a longer lifetime. The CMIP5 volcanic forcing data
used for the MPI-ESM simulations show a very different evo-
lution (Fig. 6c), which might be related to not reliable mea-
surements in the tropics (Stothers, 2001).
Both AGUNG experiments fit in general well to the mea-
surements of Stothers (2001), which are included as circles
in Fig. 6. The simulated NH AOD is slightly larger than the
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U. Niemeier et al.: Revisiting the Agung 1963 volcanic forcing 10385
Figure 5. Top of the atmosphere (TOA) radiative forcing of sulfate aerosols under all-sky conditions. Aerosol forcing was calculated using a
radiation double call. (a) Global mean TOA forcing over time. The shadings indicate the 2σvariability range for both ensembles. (b) Zonally
averaged radiative forcing as an average over time (21 months).
Figure 6. Monthly mean AOD at 550µm over time. (a, b) AGUNG1 and AGUNG2 ensembles. (c) AOD used for CMIP5 simulations
(Stenchikov et al., 1998) and (d) AOD for CMIP6 simulations Luo (2016). Overlaid as colored circles are measurements of monthly mean
AOD averaged over the regions 20 to 40◦N and 20 to 40◦S, given in Table 3 of Stothers (2001), and single values for the south pole (after
Fig. 2, same paper). The gray circles mark the volcanic eruptions, and the size represents the size of the eruption.
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10386 U. Niemeier et al.: Revisiting the Agung 1963 volcanic forcing
Figure 7. Monthly mean SH AOD (550µm) over time. Colored
lines show model results averaged over 25 to 30◦S (blue) and 35
to 40◦S (orange) each for both ensembles. The black line gives
data of Stothers (2001), the average of measurements between 20
and 40◦S. Single markers show single measurement data, estimated
from Fig. 1 in Stothers (2001), with a similar color code to the
model data.
measurements. Thus, it seems that ECHAM–HAM overes-
timates the northward transport, but we have to take into
account that Stothers (2001) noted the measured NH AOD
is barely above noise level. In the SH the modeled AOD is
smaller than the measurements, but larger than the CMIP6
data. In ECHAM–HAM, meridional exchange within the
subtropics results in lower values between 20 and 30◦S and a
maximum at 50◦S where the meridional transport is blocked
by the edge of the polar vortex. This maximum AOD, above
0.2, is more similar to the measured AOD between 20 and
40◦S which could be an indication of too strong meridional
transport in ECHAM–HAM. The AOD measurements of the
months 19 to 21 and the CMIP6 data may indicate a too short
lifetime of the simulated aerosols at the SH high latitudes.
This is most probably related to too intense sedimentation
at high latitudes (Brühl et al., 2018) in the T42 resolution
of ECHAM5. The consequence is too high wet deposition at
high latitudes, a well-known phenomena of ECHAM–HAM.
A more detailed analysis of the SH extratropics is shown
in Fig. 7. We averaged the model data over latitude bands 25
to 30◦S and 35 to 40◦S to compare those to the single mea-
surement data given in Fig. 1 of Stothers (2001). The sim-
ulated AOD is clearly smaller than the point measurements,
but one should take into account that horizontal gradients in
a volcanic cloud can be large. Thus, a point measurement
should give a higher value than an area mean of a model,
which represents an area of several hundred kilometers. Ad-
ditionally, measurements and simulated values depend not
only on sulfate evolution but also on transport. Further possi-
ble reasons for the differences were stated before. In the first
3 months after the eruption, the simulated AOD agrees well
with the measurements. The onset of the meridional transport
is similar, but the simulated volcanic cloud arrives slightly
later in 25 to 30◦S (see also Fig. 6). The agreement between
the model simulations and the individual stations differs with
time. Between 35 and 40◦S, the model agrees better with the
data of Mt. Bingar (yellow cross) than with the Aspendale
data (red triangle) in the first months after the eruption, where
the sulfate values increase 2 months earlier although the sta-
tion is closer to the tropics. This may indicate the dependency
of the point source on the position of the volcanic cloud. Both
the timing of the maximum and the onset of the decline agree
well in measurements and model results.
Measured data of particle radii are not available, but the
following references provide some estimates, which are help-
ful for a rough comparison. Arfeuille et al. (2014) simulated
for the SAGE-4λdataset effective radii of 0.3 to 0.4 µm for
eruptions of the size of the Agung eruption. Stothers (2001)
estimates a radius of 0.35 µm from the measurement data of
the year 1963 at Bloemfontein, South Africa (29◦S). Our
simulated radii at the Equator (above 0.45 µm) and at 50◦S
(above 0.4 µm) are larger than these values for both exper-
iments (Fig. 4), with a better agreement in AGUNG2. A
smaller radius of the aerosols would cause a larger AOD.
The larger radii in our experiments lead us to the question
of the requirement to include an OH-limitation process for
modeling the sulfate evolution, which is a still open research
question. In the case of high SO2concentrations OH might
be limited for further SO2oxidation (Bekki et al., 1996).
The slower formation of sulfuric acid vapor would result
in smaller particles. Bekki et al. (1996) assumed that OH
limitation occurs only in the case of a super-eruption, but
Mills et al. (2017) show OH limitation after the Mt. Pinatubo
eruption as well. On the other hand, LeGrande et al. (2016)
showed that water vapor in the eruption cloud increases the
amount of available OH. This reaction was not included in
the two studies cited above. The results presented here use
a fixed monthly mean OH concentration which is not influ-
enced by the volcanic cloud. Following Mills et al. (2017),
this missing OH limitation leads to faster formation of sulfate
resulting in larger sulfate particles. Therefore, we performed
one simulation with a simple parameterization of OH limita-
tion; see the Supplement for details. This simulation shows
slower sulfate formation, which agrees less with the mea-
surements, and only a slightly higher AOD half a year after
the eruption (Figs. S4 and S5) for the OH-limited case. As
we use a simplistic parameterization these results are very
limited.
Sulfate aerosol absorbs terrestrial radiation and warms the
stratosphere. This temperature signal depends in the model
on the coupling to radiation. We compare the results of the
ensemble mean temperature data to radiosonde data of Free
and Lanzante (2009). Both model and measurement data are
independent of QBO temperature relations (see Sect. 2.3).
The measurements, averaged over stations between 30◦N
and 30◦S, shows a strong maximum at 70 hPa (Fig. 8). The
average over 30◦N to 30◦S of the model results (black lines)
show a smaller anomaly and a stronger vertical extension of
the heated area, more in AGUNG1 than in AGUNG2. Below
Atmos. Chem. Phys., 19, 10379–10390, 2019 www.atmos-chem-phys.net/19/10379/2019/
U. Niemeier et al.: Revisiting the Agung 1963 volcanic forcing 10387
Figure 8. Profiles of temperature anomaly (K) for the model data
and RATPAC radiosonde measurements taken from Free and Lan-
zante (2009), averaged over 2 years after the Agung eruption(s).
Model results are averaged over 30◦N to 30◦S (black) and 15◦N
to 15◦S (gray), whereas radiosonde data are averaged for the trop-
ics only (30◦N to 30◦S; red line). The horizontal lines mark the
95 % confidence interval for the RATPAC radiosonde data.
the maximum at 70 hPa the simulated temperature is lower
than the measurements. Both features indicate too strong ver-
tical lofting in the model. Figures 6 and 7 indicated a too
low AOD in the model results around 30◦S. Therefore, we
added a second temperature profile, averaged over the main
volcanic cloud at 15◦N to 15◦S (gray line), to Fig. 8. Now,
the maximum is represented better. However, more impor-
tant is the better agreement with the temperature decrease
between 50 hPa and 70 hPa, especially for AGUNG2. The
easterly phase of the QBO is related to downward motion
and suppresses the vertical lofting caused by the heating,
but is also related to stronger vertical transport in the sec-
ondary meridional circulation around 30◦north and south.
This upwelling seems to be stronger in the model than in
the measurements, causing the too high vertical extension of
the simulated volcanic cloud. While these results compare
well to the radiosonde data, simulated temperature anoma-
lies are only half of those observed in the SH (not shown).
This may hint towards a too short lifetime of the simulated
sulfate aerosols (see Fig. 6a, b).
5 Summary and conclusions
We compared results of two scenarios for the Mt. Agung
eruption in 1963: the commonly used one-eruption scenario
with a strength of 7 Tg SO2and a scenario with two erup-
tions. From AOD measurements one single number of the
injected sulfur amount was estimated, but observation and
detection of the mass flux show a scenario of two eruptions.
Therefore, we assumed a scenario with two climatic erup-
tions of 4.7 Tg SO2and 2.3 Tg SO2respectively. We sim-
ulated a lower burden in the tropics, but slightly stronger
meridional flow into the extratropics in the simulation with
two eruptions, AGUNG2, compared to AGUNG1. We relate
the stronger meridional flow to the lower injection altitude
of the second eruption, where the tropical transport barrier
is weaker. Additionally, the position of the tropical pipe is
further northward in May than in March. This allows more
aerosols to be transported into the NH. The smaller injection
rate and the two different injection altitudes cause the parti-
cles to grow less than in AGUNG1. These processes result in
10 % to 20 % lower radiative forcing, or 0.1 to 0.3 Wm−2in
monthly mean global average, and estimated 10 % less sur-
face cooling in AGUNG2. The strongest signal would occur
in the tropics.
When comparing to the few available measurements we
see that the differences to the measurements are larger than
the differences between the two experiments. We seem to
underestimate the observed AOD and simulate larger parti-
cle radii. The timing of the aerosol evolution in the model
seems to be supported by the measurements. Given the low
number of observations at that time, especially in the trop-
ics, it is difficult to validate the two experiments. Overall, the
smaller particle size and slightly better shape of the temper-
ature anomaly in the vertical profile of AGUNG2 are con-
sequences of different transport and microphysical processes
between the two experiments. These are arguments to include
both climatic eruptions in future emission datasets.
One could also argue that the large model spread, as de-
scribed by Marshall et al. (2018) and Zanchettin et al. (2016),
limits the interpretation of our model results. Other models
may obtain quantitatively different results, but most proba-
bly the simulated difference between the two eruption sce-
narios would be qualitatively similar: a lower radiative forc-
ing of the Agung 1963 eruption when including two eruption
phases. Including a more sophisticated atmospheric chem-
istry (including OH chemistry, water vapor, and ozone) may
increase the differences between the scenarios. Lower SO2
injection rates in AGUNG2 would cause less impact on OH
and ozone, and thus on chemical species in the stratosphere.
Taking two eruption phases into account will be important
for processes in the early evolution of sulfate. Ash and ice
are important species in this early phase. Both were not
taken into account in the simulations presented here but are
planned for the future.
Overall, differences of around 10 % in the global annual
averaged radiative forcing between AGUNG1 and AGUNG2
should justify changes in the volcanic emission datasets. The
more recent volcano datasets are rather detailed. Also, fu-
ture studies using high horizontal and vertical resolution and
more sophisticated models will demand detailed input data.
Details of our assumptions on the Agung eruptions might be
www.atmos-chem-phys.net/19/10379/2019/ Atmos. Chem. Phys., 19, 10379–10390, 2019
10388 U. Niemeier et al.: Revisiting the Agung 1963 volcanic forcing
critically reviewed again but, we recommend including both
eruptions of Mt. Agung in upcoming datasets.
Data availability. Primary data and scripts used in the analysis and
Supplement that may be useful in reproducing the author’s work
are archived by the Max Planck Institute for Meteorology and can
be obtained by contacting publications@mpimet.mpg.de. Model
results are available under https://cera-www.dkrz.de/WDCC/ui/
cerasearch/entry?acronym=DKRZ_LTA_550_ds00002 Niemeier et
al., 2019.
Supplement. The supplement related to this article is available on-
line at: https://doi.org/10.5194/acp-19-10379-2019-supplement.
Author contributions. CT had the main idea for the study. UN, CT,
and KK discussed the experiment design. UN performed the exper-
iments. UN prepared the text with contributions of all co-authors.
Competing interests. The authors declare that they have no conflict
of interest.
Special issue statement. This article is part of the special issue
“The Model Intercomparison Project on the climatic response to
Volcanic forcing (VolMIP) (ESD/GMD/ACP/CP inter-journal SI)”.
It is not associated with a conference.
Acknowledgements. We thank Hauke Schmidt for implement-
ing the QBO nudging into ECHAM5–HAM and Elisa Manzini,
Alan Robock, and the anonymous reviewer for valuable comments.
A discussion about the OH limitation in WACCM with Simone
Tilmes helped to modify ECHAM5–HAM. The simulations were
performed on the computer of the Deutsches Klima Rechenzentrum
(DKRZ). Kirstin Krüger would like to thank Hauke Schmidt and
MPI for Meteorology for the Guest Scientist invitation and UiO for
the sabbatical that allowed the scientific exchange.
Financial support. This research has been supported by FP7
Environment STRATOCLIM (grant no. FP7-ENV.2013.6.1-2), the
Deutsche Forschungsgemeinschaft priority program “Climate En-
gineering: Risks, Challenges, Opportunities?” (grant no. SPP1689,
project CELARIT, UN), the German federal Ministry of Education
research program MiKlip (grant no. FKZ:01LP1517(CT)), and the
DFG Research Unit VollImpact (FOR2820, UN CT).
The article processing charges for this open-access
publication were covered by the Max Planck Society.
Review statement. This paper was edited by Slimane Bekki and re-
viewed by Alan Robock and one anonymous referee.
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