Differing responses of the QBO to SO 2 injections in two global models

. Artiﬁcial injections of sulfur dioxide ( SO 2 ) into the stratosphere show in several model studies an impact on stratospheric dynamics. The quasi-biennial oscillation (QBO) has been shown to slow down or even vanish, under higher SO 2 injections in the equatorial region. But the impact is only qualitatively, but not quantitatively consistent across the different studies using different numerical models. The aim of this study is to understand the reasons behind the differences in the QBO response to SO 2 injections between two general circulation models, the Whole Atmosphere Community Climate Model 5 (WACCM-110L) and MAECHAM5-HAM. We show that the response of the QBO to injections with the same SO 2 injection rate is very different in the two models, but similar when a similar stratospheric heating rate is induced by SO 2 injections of different amounts. The reason for the different response of the QBO corresponding to the same injection rate is very different vertical advection in the two models, even in the control simulation. The stronger vertical advection in WACCM results in a higher aerosol burden and stronger heating of the aerosols, and, consequently in a vanishing QBO at lower injection rate than 10 in simulations with MAECHAM5-HAM. Acknowledgements. We thank Andrea Segschneider for their helpful comments. This work is a contribution to the German DFG-funded Priority Program ’Climate Engineering: Risks, Challenges, Opportunities?’ (SPP 1689). UN is supported by the SPP 1689 within the project CEIBRAL and CELARIT and DFG Research Unit VollImpact FOR2820 sub project TI344/2-1 UN got support from SPP 1689 and NCAR for an scientiﬁc exchange in Boulder in 2016, where we discussed and stared the model comparison. The simulations were performed on the 5 computer of the Deutsches Klima Rechenzentrum (DKRZ). This work was supported by the National Center for Atmospheric Research, which is a major facility sponsored by Foundation Cooperative Agreement No. 1852977. WACCM is a component of the Community Earth System Model (CESM), which is supported by NSF and the Ofﬁce of Science of the U.S. of Energy. Computing resources were provided by NCAR’s Climate Simulation Laboratory, sponsored by NSF and other agencies. This research was enabled by the computational and storage resources of 10 NCAR’s Computational and Information Systems Laboratory (CISL). All simulations were carried out on the Yellowstone high-performance computing platform (CISL 2012).

Here, we use the specified chemistry version of WACCM, which uses a monthly varying present-day climatology to prescribe ozone, oxidants, and background stratospheric aerosols. Tropospheric aerosols are prognostically derived using the modal aerosol model (MAM3) (Liu et al., 2012). Direct effects and indirect effects of radiative effects of aerosols are included.
Additionally, geoengineering sulfur injections into the stratosphere are performed similarly to ECHAM. As described in Mills et al. (2016), the lack of interactive stratospheric chemistry, prevents OH values from depleting while reacting with the injected 5 sulfur. This leads to a slightly faster formation of sulfate closer to the injection location, and with that a different lofting of aerosols in the tropics compared to a full chemistry version, as used in Mills et al. (2017). However, while the aerosol distribution is somewhat different than in the fully interactive chemistry version of WACCM, roughly 10% higher burden maximum in the tropics, the response of sulfur injections on the QBO is the same. The SST is prescribed and set to present day values. 10

Simulations
The model simulations for this study follow the same protocol. Both models prescribe a repeating annual cycle of SSTs, present day, and chemical precursors, which are necessary for e.g. sulfur oxidation and radiative processes, on a monthly basis. These fields slightly differ between the two models but are not expected to have much influence on the simulation of the QBO. SO 2 was injected continuously over time into a single grid box at the equator at a height of 60 hPa with three different amounts 15 of sulfur: 2, 4, 8 Tg(S)yr −1 . Simulations were carried out with ECHAM and WACCM for at least 20 years. Exact number of simulation years is shown in Table 1, as well as the number of years used to calculate time averages. ECHAM simulations were carried out longer, however no differences have been found between the results averaged over 20 years and the entire simulation length of ECHAM. Figures show either timeseries or zonal averages over time. Anomalies are calculated relative to an average over a control run of 50 years (ECHAM) and 35 years (WACCM).  Figure 1 shows the zonal mean zonal wind at the equator for the control simulation and two different injection rates for WACCM and ECHAM. Both models simulate the QBO well in the control simulation, without artificial injections of SO 2 (top). The QBO has an observed period of 28 month (Naujokat, 1986) on average. The simulated QBO period is about 27 5 months in WACCM and about 32 month in ECHAM. In WACCM the wind velocity is higher, slightly in the westerly phase but stronger in the easterly phase especially at altitudes below 20 hPa, and the QBO propagates further down as in ECHAM.
After the injection of sulfur into the tropical stratosphere the QBO responds quite differently to the same injection rate in the two models. While ECHAM shows a slower but still existing oscillation of the zonal wind for injections of 4 Tg(S)yr −1 , the oscillation of the zonal wind in WACCM completely vanishes, resulting in constant westerlies in the lower stratosphere, 10 and easterlies above ∼10 hPa (Fig 1, middle). Increasing the injection rate to 8 Tg(S)yr −1 increases slightly the velocity of the westerlies and the vertical extension of the westerly jet in WACCM (Fig 1, bottom). In ECHAM the oscillation vanishes at 8 Tg(S)yr −1 as well but wind velocity and vertical extension of the westerly jet are lower. The stronger westerly jets in WACCM shifts the semi-annual oscillation (SAO) above 5 hPa to higher altitudes. For ECHAM the SAO still reaches 5 hPa for 8 Tg(S)yr −1 injections gets shifted to higher altitudes also when the jets gets stronger with increasing injection rates (Niemeier and Schmidt, 2017). Thus, the QBO disappears in both models as a result of SO 2 injections, but at different injection rates.
3.2 Temperature and heating rate changes advection. This heating differs clearly between WACCM and ECHAM as can be seen in the amplitude of temperature anomalies in the stratosphere for both models (Figure 2). For the same sulfur injection rate, WACCM shows temperature anomaly roughly three times stronger than ECHAM for the 4 and 8 Tg(S)yr −1 injections respectively. Therefore, the different response of the QBO winds to the injection between the models is not surprising, as the thermal wind balance is much more strongly 10 impacted in WACCM than in ECHAM.  rates show that WACCM absorbs more than twice as much in the LW than in SW, while absorption is similar in between LW and SW in ECHAM. Compared to ECHAM, the LW absorption of sulfate in WACCM is three times stronger. We can see different absorption rates in LW and SW between the models, however, the stronger LW heating rate in WACCM corresponds to the stronger temperature anomaly in WACCM. Both models use the same radiation scheme, hence the differences can not be explained by the radiation scheme. Therefore, we can assume that these differences in the LW and SW absorption cannot 5 be the reason for the different temperature anomalies.

Sulfate properties
The zonally averaged sulfate burden, the vertically integrated sulfate concentration, shows at all latitudes a higher burden in WACCM than in ECHAM for the injection rate of 4 Tg(S)yr −1 (Figure 4). WACCM shows a distinct peak at the equator while in ECHAM the distribution is much more even with latitude and the secondary maxima in the extra-tropics are only 10 slightly smaller than the tropical maximum. This three to four times larger tropical sulfate burden in WACCM explains the larger temperature anomaly in WACCM, as more sulfate aerosols can absorb more radiation. The vertical cross section of the zonally averaged sulfate concentrations reveals more details of the differences in distributions of sulfate in the two models ( Figure 5). Not only the tropical concentration is higher in WACCM, in addition, the vertical distribution of aerosols is very different between the two models. In ECHAM the sulfate is vertically advected to 25 hPa, 15 while in WACCM sulfate reaches much higher altitudes and meridional transport mainly occurs mainly below 50 hPa. Vertical advection has to be much stronger in WACCM than in ECHAM to cause the differences. This is likely caused by a combination   heating in WACCM is related to the higher sulfate load. The heating is a consequence of the sulfate burden, and not the source of the differences between the two models.
To further understand differences in the aerosol distribution and the resulting heating between WACCM and ECHAM, we examine the effective radii of aerosols in both models. This comparison ( Figure 6) shows in the tropics twice as large radii for WACCM (0.6 µm) than for ECHAM (0.3 µm). The higher sulfur load results in larger particle radii, less scattering and, less sulfur unit. The different radii are therefore not related to the differences in heating rates between the models. From the larger radii in WACCM we may assume a stronger sedimentation in the tropics in WACCM. But the burden is larger in WACCM. If sedimentation is a major difference between the models, the difference in the tropical burden between the two models should be smaller. An additional process, which determines the lifetime of the aerosols in the tropics is the vertical advection, the residual vertical velocity ω * . 5

Dynamical changes
The patterns of the heating rates, sulfate concentrations and particle radii hint towards a stronger vertical advection in WACCM.
A proxi for this behaviour is the residual vertical velocity, ω * . Richter et al. (2017) have shown that vertical advection plays a major role in dynamical changes in the tropical stratosphere. Visioni et al. (2018) showed a strong relation between the sulfate lifetime and ω * . Therefore, we compare the residual vertical velocity of the control simulations (Figure 7) to get a more general 10 impression of the behaviour of the two models, independently of additional updraft caused by the aerosol heating. In the altitude of the sulfur injection (60 hPa) and above shows WACCM an up to 70% stronger ω * than ECHAM. This stronger ω * results in a stronger vertical transport of the sulfate aerosols, which increases the tropical sulfate burden in WACCM. Additionally, the minimum of the ω * profile is located at lower altitude in WACCM (70 hPa and 50 hPa in ECHAM), resulting in a stronger tropical confinement of the aerosols at this altitude.

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The consequence is twofold: a) a stronger ω * counteracts more the downward propagation of the QBO shear zones and b) lifts the aerosols to higher altitudes, which increases the burden and, thus causes stronger heating. The heating of the aerosols further increases ω * , which shifts the minimum of ω * downward (Figure 8). This can be seen in both models, but stronger in  in the gravity wave parameterization and the relation how strongly resolved Rossby waves or parameterized gravity waves drive the upward mass flux (Cohen et al., 2014;SPARC, 2010). The better horizontal resolution in WACCM may also play a role. We conclude that the stronger ω * in WACCM to be the main reason for the differences between the QBO response in the two models.

Comparison under the same heating conditions 5
Are differences in ω * between the models the main cause of the difference in QBO impact or does the different heating also play an important role? To answer this question we compare different sulfur injection rates in the models that produce a similar heating rate in the sulfate layer. An injection rate of 2 Tg(S)yr −1 in WACCM and 8 Tg(S)yr −1 in ECHAM fulfills this criterium (Figure 9, top). Both experiments result in a temperature anomaly of ∼4 K in the tropical stratosphere. The heated area is slightly wider in ECHAM because the sulfate concentration (Figure 9, bottom) is slightly higher in the tropics and 10 spreads more meridionally around 50 hPa. However, the maximum burden between the two models is rather similar in the tropics ( Figure 10). The tropical maximum of the sulfate burden in ECHAM is only 2 mgm −2 (12%) higher than in WACCM, despite a factor 4 higher injection rate of sulfur. The differences in the burden are larger in the extratropics (∼50%). But the extratropical differences are not the focus of this study as we focus on the tropical stratosphere and the QBO. The continuous westerly and easterly jets cause a different profile of ω * than oscillating zonal winds under QBO conditions (Fig. 8). The clearly different profile of ω * for the low injection cases to the 8 Tg(S)yr −1 in ECHAM and for all three injection cases to the control in WACCM is a consequence of the disappearance of the QBO. We see a strong correlation of the pattern of 5 the residual vertical velocity to the equatorial zonal wind profiles (Figure 1). The characteristic pattern of the vertical profile of ω * in WACCM becomes similar in ECHAM-HAM when the oscillation of the equatorial jets vanishes at 8 Tg(S)yr −1 ( Figure   8). Consequently, the maximum difference of ω * between the models is only 30% when comparing the 2 Tg(S)yr −1 WACCM and the 8 Tg(S)yr −1 ECHAM injection cases ( Figure 11). Differences occur mostly due to a vertical shift in the profiles. The constant easterly and westerly jets cause distinct maxima and minima of ω * below 50 hPa. This compares well to the theory 10 of the meridional and vertical transport processes within the QBO region and the secondary meridional oscillation (Plumb and Bell, 1982), which is caused by equatorward meridional advection in westerly jets and poleward within easterly jets combined with updraft in easterly shear and downdraft in westerly shear. E.g. the position of the maxima around 30 and 20 hPa are the

Summary and Discussion
We performed here simulations with different injection rates of SO 2 at the equator to compare the impact on the QBO in two different general circulation models (WACCM and ECHAM). The QBO typically consists of alternating easterly and westerly 5 zonal mean zonal winds, however in the presence of sulfur injections, the QBO sometimes vanishes, and turns into persistent westerlies in the lower stratosphere and persistent easterlies in the upper stratosphere. Both models used in the study had similar setup (e.g. prescribed SSTs and present day chemical precursors like OH or ozone) and were coupled to an aerosol microphysical model with three modes in WACCM and four modes in ECHAM. Both models qualitatively simulate an impact on the QBO of sulfur injections similar to what was found in previous studies (Niemeier and Schmidt, 2017;Richter et al., 10 2017), however WACCM shows a disappearance of the QBO at an injection rate of 2 Tg(S)yr −1 whereas ECHAM shows the disappearance of the QBO for an injection rate of 8 Tg(S)yr −1 .
We have shown that this difference results from different tropical vertical advection and different tropical residual vertical velocity, ω * , in the two models. ω * differs not only in the simulations with SO 2 injections, but also in the control simulations without any sulfur injection. ω * is 70% larger than in ECHAM near the altitude of the SO 2 injection. Additionally, the minimum 15 of ω * is located at a lower altitude. At altitudes with a small ω * meridional transport is enhanced, while a strong ω * causes an enhanced tropical confinement of the aerosols. This confinement is stronger in WACCM above 50 hPa. Thus, the stronger ω * results in a stronger vertical lifting, higher sulfate burden and, consequently, stronger heating of the stratosphere caused by aerosol absorption. This heating disturbs the thermal wind balance and causes an additional westerly momentum. Finally, this results in the disappearance of the QBO at lower SO 2 injection rates than in ECHAM. This result partly opposes the 20 assumptions of Kleinschmitt et al. (2017), who assumed the heating as main cause for different vertical advection in two models. It would be interesting to compare our results to the ω * of their control simulation.
The reason for the different ω * in the two models is complex. ω * , or the speed of the upwelling in the Brewer Dobson circulation is driven by a combination of larger scale (Rossby and synoptic-scale waves) and parameterized waves. The propagation of waves and deposition of wave momentum by larger scale waves is impacted by numerous aspects of the model such as horizontal and vertical resolution, diffusion parameterization, physics parameterizations, which all differ between WACCM and ECHAM. Gravity wave parameterization contributions to driving the Brewer-Dobson circulation also vary between models (Butchart et al., 2011). WACCM and ECHAM have very different gravity wave parameterizations. It would be very difficult hence to isolate the reason for the different ω * between WACCM and ECHAM, but simulations with different horizontal resolution shown in Section 3.6 have shown that horizontal resolution difference between WACCM and ECHAM contributed to 10 the differences in ω * . Additionally, sedimentation may differ between the models as might be concluded from the difference in deposition when simulation a Tambora like volcanic eruption (Marshall et al., 2018). Sedimentation is a very important sink process for aerosols, especially at the poles, but three dimensional fields of sedimentation velocities were not available for both models. As we concentrated on the tropical stratosphere only, we leave this topic for further studies.
Finally we conclude that the difference in tropical upwelling even under present climate conditions between two models has 15 a major impact on the projected effects of SO 2 injections on the QBO in WACCM and ECHAM. This is worrisome in terms of level of certainty of effects of SO 2 injections on stratospheric circulation in future climates, especially as the changes in the Brewer-Dobson circulation are uncertain, and in addition changes in gravity waves, which are a big driver of the QBO are even more uncertain in changing climate. Hence, a lot more research is needed before agreement is reached on how SO 2 injections could affect the QBO. The reasons for the differences in this variable are too complex to give a recipe for a better agreement 20 of the results.
Code and data availability. Primary data and scripts used in the analysis and other supplementary information that may be useful in