Sensitivity of regional monsoons to idealised equatorial volcanic eruption of different sulfur emission strengths

The impact of volcanic forcing on tropical precipitation is investigated in a new set of sensitivity experiments within the Max Planck Institute Grand Ensemble framework. Five ensembles are created, each containing 100 realizations for an idealized "Pinatubo-like" equatorial volcanic eruption with emissions covering a range of 2.5 - 40 Tg sulfur (S). The ensembles provide an excellent database to disentangle the influence of volcanic forcing on monsoons and tropical hydroclimate over the wide spectrum of the climate's internal variability. Monsoons are generally weaker for two years after volcanic eruptions and their weakening is a function of emissions. However, only a stronger than Pinatubo-like eruption ($\geq$ 10 Tg S) leads to significant and substantial monsoon changes, and some regions (such as North and South Africa, South America and South Asia) are much more sensitive to this kind of forcing than the others. The decreased monsoon precipitation is strongly tied to the weakening of the regional tropical overturning. The reduced atmospheric net energy input and increased gross moist stability at the Hadley circulation updraft due to the equatorial volcanic eruption, require a slowdown of the circulation as a consequence of less moist static energy exported away from the ITC

Southern Annular Mode response to volcanic eruptions: implications for ice core proxies

Large tropical volcanic eruptions have been observed to have a significant influence on the large-scale circulation patterns of the Northern Hemisphere, through mechanisms related to the radiative effects of the sulfate aerosols resulting from the volcanic injection of sulfur dioxide into the stratosphere. While no such volcanically induced anomalies in Southern Hemisphere circulation have yet been observed, we find that in general circulation model simulations, eruptions with sulfur dioxide injections larger than that of the 1991 Mt. Pinatubo eruption do result in significant circulation changes in the SH, specifically an enhanced positive phase of the Southern Annular Mode (SAM). We explore the mechanisms for such a SAM response, as well as the corresponding changes in SH temperature, sea ice and precipitation. We also explore how the anomalously strong zonal winds characteristic of the positive SAM regime affect the rate of sulfate deposition to the Antarctic ice-sheet. We suggest that the use of ice-core sulfate records as a proxy for past volcanic activity may benefit from including knowledge of, or better assumptions regarding the changes in large scale atmospheric circulation after large tropical eruptions

What is the limit of climate engineering by stratospheric injection of SO2?

The injection of sulfur dioxide (SO2) into the stratosphere to form an artificial stratospheric aerosol layer is discussed as an option for solar radiation management. The related reduction of radiative forcing depends upon the injected amount of sulfur dioxide, but aerosol model studies indicate a decrease in forcing efficiency with increasing injection rate. None of these studies, however, consider injection rates greater than 20 Tg(S) yr−1. But this would be necessary to counteract the strong anthropogenic forcing expected if "business as usual" emission conditions continue throughout this century. To understand the effects of the injection of larger amounts of SO2, we have calculated the effects of SO2 injections up to 100 Tg(S) yr−1. We estimate the reliability of our results through consideration of various injection strategies and from comparison with results obtained from other models. Our calculations show that the efficiency of such a geoengineering method, expressed as the ratio between sulfate aerosol forcing and injection rate, decays exponentially. This result implies that the sulfate solar radiation management strategy required to keep temperatures constant at that anticipated for 2020, while maintaining business as usual conditions, would require atmospheric injections of approximately 45 Tg(S) yr−1 (±15 % or 7 Tg(S) yr−1) at a height corresponding to 60 hPa. This emission is equivalent to 5 to 7 times the Mt. Pinatubo eruption each year

Revisiting the Agung 1963 volcanic forcing: Impact of one or two eruptions

In 1963 a series of eruptions of Mt. Agung, Indonesia, resulted in the third largest eruption of the 20th century and claimed about 1900 lives. Two eruptions of this series injected SO2 into 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 emission profiles and assumed for the first eruption on 17 March an injection rate of 4.7 Tg SO2 and 2.3 Tg SO2 for 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 conditions. We performed ensembles of two model experiments, one with a single eruption and a second one with two eruptions. The two smaller eruptions result in a lower sulfur burden, smaller aerosol particles, and 0.1 to 0.3 Wm−2 (10 %–20 %) lower radiative forcing in monthly mean global average compared to the individual eruption experiment. The differences are the consequence of slightly stronger meridional transport due to different seasons of the eruptions, lower injection height of the second eruption, and the resulting different 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

The initial dispersal and radiative forcing of a Northern Hemisphere mid latitude super volcano: a Yellowstone case study

International audienceThe chemistry climate model MAECHAM4/CHEM with interactive and prognostic volcanic aerosol and ozone, was used to study the initial dispersal and radiative forcing of a possible Yellowstone super eruption. Tropospheric climate anomalies are not analysed since sea surface temperatures are kept fix. Our experiments show that the global dispersal of a Yellowstone super eruption is strongly dependent on the season of the eruption. In Northern Hemisphere summer the volcanic cloud is transported westward and preferentially southward, while in Northern Hemisphere winter the cloud is transported eastward and more northward compared to the summer case. Aerosol induced heating leads to a more global spreading with a pronounced cross equatorial transport. For a summer eruption aerosol is transported much further to the Southern Hemisphere than for a winter eruption. In contrast to Pinatubo case studies, strong cooling tendencies appear with maximum values of ?1.6 K/day three months after the eruption in the upper tropical stratosphere. This strong cooling effect weakens with decreasing aerosol density over time and initially prevents the aerosol laden air from further active rising. All-sky net radiative flux changes of more than 32 W/m2 at the surface are about a factor of 6 larger than for the Pinatubo eruption. Large positive flux anomalies of more than 16 W/m2 are found in the first months in the tropics and sub tropics. These strong forcings call for a fully coupled ocean/atmosphere/chemistry model to study climate sensitivity

The Arctic polar vortex response to volcanic forcing of different strengths

Tropical volcanic eruptions injecting sulfur into the stratosphere are assumed to not only scatter radiation and cool Earth’s surface but also to alter atmospheric circulation, and in particular to strengthen the stratospheric polar vortex in boreal winter. The exact impact is difficult to estimate because of the small number of well observed eruptions and the high internal variability of the vortex. We use large (100 member) ensembles of simulations with an Earth system model for idealized volcanic aerosol distributions resulting from sulfur injections between 2.5 and 20 Tg. We suggest the existence of a threshold somewhere between 2.5 and 5 Tg(S) below which the vortex does not show a detectable response to the injection. This nonlinearity is introduced partly through the infrared aerosol optical density which increases much stronger than linear with increasing particle size occurring for increasing injection amount. Additionally, the dynamical mechanism causing the vortex strengthening seems not to set in for small aerosol loading. Furthermore, we add to the recent discussion concerning a possible downward propagation of the circulation response leading to a winter warming in Northern Eurasia. At latitudes northward of about 50°N our simulations do show such an average warming pattern that is statistically significant for injections of 10 Tg(S) or more

A one and half year interactive MA/ECHAM4 simulation of Mount Pinatubo Aerosol

The Mount Pinatubo volcanic eruption in June 1991 had significant impact on stratospheric and tropospheric climate and circulation. Enhanced radiative heating caused by the aerosol absorption of solar and terrestrial radiation changed stratospheric temperature and circulation. Using the stratospheric mesospheric version of the Hamburg climate model MA/ECHAM4, we performed an interactive Pinatubo simulation with prognostic stratospheric aerosol. Interactive and noninteractive model results for the years 1991 and 1992 are compared with satellite data and in situ measurements. The on-line calculated heating rates are in good agreement with radiation transfer models indicating maximum heating rates of about 0.3 K/d in October 1991. The dynamic feedback in the MA/ECHAM4 simulation is similar to observations. The model is able to reproduce the strengthening of the polar vortex in winter 1991/1992 and a minor warming in January. The importance of an interactive treatment of the volcanic cloud for the aerosol transport is evidenced by the analysis of effects such as aerosol lifting and meridional transport. In general, the model results agree well with observations from the northern midlatitudes, especially in the first months after the eruption. The MA/ECHAM4 model is successful in reproducing the formation of two distinct maxima in the optical depth but is unable to simulate the persistence of the tropical aerosol reservoir from the end of 1991. Better agreement may be achieved if the influence of the quasi-biennial oscillation and ozone changes is also taken into account

Simulationen zur Bildung und Entwicklung von stratosphärischem Aerosol unter besonderer Berücksichtigung der Pinatuboepisode

Im Rahmen dieser Arbeit ist ein mikrophysikalisches Modell des stratosphärischen Aerosols für das dreidimensionale Zirkulationsmodell ECHAM entwickelt worden. Eine Grundlage des stratosphärischen mikrophysikalischen Modells ist die weitgehend explizite Beschreibung der wesentlichen Parameter des binären Systems in Abhängigkeit von Temperatur und Partialdrücken. In dem Modell selbst werden homogene Nukleation, Kondensation von HzO und H2SOa, Koagulation und Sedimentation berücksichtigt. Für eine Boxversion des mikrophysikalischen Modells sind Sensitivitätsstudien durch- geführt worden. Sie zeigen eine gute Ûbereinstimmung der berechneten Hintergrund- aerosol-Gleichgewichtsverteilung mit den beobachteten Werten. Das mikrophysikalische Boxmodell ist desweiteren in der Lage, das zeitliche Verhalten des stratosphärischen Aerosols nach einer vulkanischen Störung in guter Näherung zu Beobachtungswerten zu simulieren. Es zeigt sich dabei, daß unabhängig von der St¿irke der Störung der Hintergrundwert nach vier bis fünf Jahren wieder erreicht wird. Sensitivitätsstudien in Abhängigkeit von den Eingabedaten zeigen, daß bei einer Temperatur- und einer HzO-Änderung ein anderes Muster in den simulierten Aerosolgrößenverteilungen her- vorgerufen wird als bei einer Änderung der SOz-Konzentration. Für die globale Modellierung ist das mikrophysikalische Modell um ein einfaches Mo- dul für die stratosphärische Schwefelchemie erweitert worden. Darüber hinaus ist das mikrophysikalische Modell mit einem troposphärischen Schwefelkreislauf gekoppelt wor- den, wodurch für das stratosphärische Aerosol global und jahreszeitlich verschiedene tro- posphärische SOz- und SOI--Quellen berücksichtigt werden können. Erste Ergebnisse der dreidimensionalen Modellierung zeigen, daß das Modell die beobachteten Massenkon- zentrationen und Oberflächenverteilungen in der richtigen Größenordnung reproduzieren kann. Die Bildung neuer Teilchen durch homogene Nukleation wird hauptsächlich von der Temperatur bestimmt, und findet in der unteren tropischen Stratosphäre sowie in den polaren Gebieten im Frühjahr statt. In einem weiteren Schwerpunkt dieser Arbeit sind mit der ECHAM4-Ll9-Version tran- siente Pinatuboexperimente mit prognostischem Sulfataerosol durchgeführt worden. Bei den assimilierten Pinatubosimulationen zeigt sich die generelle Schwierigkeit von Gitter- punkts- und spektralen Modellen, den in der Stratosphäre auf isentropen Flächen statt- findenden großräumigen Transport zu simulieren. Durch die Einführung einer Reduzie- rung des vertikalen Transports auf einer isentropen Fläche von 380 K, die die Grenze zwischen stratosphärischer Ober- und Unterwelt markiert, ist jedoch im Rahmen dieser Arbeit eine wesentliche Verbesserung für den stratosphärischen tacertransport erzielt worden.A microphysical model for stratospheric aerosol has been developed and implemented in the climate model ECHAM4. The fundamental basis of the microphysical model is the explicit description of the essen- tial parameters of the binary HzSOa/HzO system dependent on temperature and partial pressure. The following processes are solved: Homogeneous nucleation, condensation, coagulation, water-vapor growth incl. the Kelvin effect and sedimentation For a box version of the microphysical model sensitivity studies were carried out. For the background aerosol the results of the sensitivity studies are in good agreement with observations. F\rrthermore the microphysical box model is able to simulate the temporal development of stratospheric aerosol after an volcanic eruption in good approximation to observed data. Independently of the strength of the volcanic disturbance, four to five years after the eruption the background level is reached again. Sensitivity studies dependent on the initial parameters also show a different pattern in the simulated aerosol distribution for temperature and water vapor changes, than for changes in the SOz and OH concentration. For the global modeling the microphysical model is extended with a module of stra- tospheric sulfur chemistry. Additionally, the microphysical model is coupled to a tro- pospheric sulfur cycle. Due to this combination global and seasonal different SO2- and SO!--sources for stratospheric aerosol could be taken into account. First results of the 3d-simulation show that the model is able to reproduce the observed aerosol mass mixing ratio and the surface concentration by a factor of two. The for- mation of new particles through homogeneous nucleation is mainly determined by the temperature and takes place in the lower stratosphere and in polar spring. A different emphasis of this work is transient Pinatubo simulations with the ECHAM4 L19 model and with prognostic sulfate aerosol. These assimilated Pinatubo simulation shows the general difference of gridpoint and spectral models to simulate the large scale isentropic transport. Due to the introduction of a reduced advective vertical transport through the 380 K isentropic layer, a substantial improvement in the stratospheric tracer transport has been made

Observational constraints on the tropospheric and near-surface winter signature of the Northern Hemisphere stratospheric polar vortex

A composite analysis of Northern Hemisphere’s mid-winter tropospheric anomalies under the conditions of strong and weak stratospheric polar vortex was performed on NCEP/NCAR reanalysis data from 1948 to 2013 considering, as additional grouping criteria, the coincidental states of major seasonally relevant climate phenomena, such as El Niño-Southern Oscillation (ENSO), Quasi Biennial Oscillation and strong volcanic eruptions. The analysis reveals that samples of strong polar vortex nearly exclusively occur during cold ENSO states, while a weak polar vortex is observed for both cold and warm ENSO. The strongest tropospheric and near-surface anomalies are found for warm ENSO and weak polar vortex conditions, suggesting that internal tropospheric circulation anomalies related to warm ENSO constructively superpose on dynamical effects from the stratosphere. Additionally, substantial differences are found between the continental winter warming patterns under strong polar vortex conditions in volcanically-disturbed and volcanically-undisturbed winters. However, the small-size samples obtained from the multi-compositing prevent conclusive statements about typical patterns, dominating effects and mechanisms of stratosphere-troposphere interaction on the seasonal time scale based on observational/reanalysis data alone. Hence, our analysis demonstrates that patterns derived from observational/reanalysis time series need to be taken with caution as they not always provide sufficiently robust constraints to the inferred mechanisms implicated with stratospheric polar vortex variability and its tropospheric and near-surface signature. Notwithstanding this argument, we propose a limited set of mechanisms that together may explain a relevant part of observed climate variability. These may serve to define future numerical model experiments minimizing the sample biases and, thus, improving process understanding.This work was supported by the Federal Ministry for Education and Research in Germany (BMBF) through the research program “MiKlip” (FKZ:01LP1158A(DZ):/01LP1130A(CT,MB)).This is the accepted version of an article originally published in Climate Dynamics. The final publication is available at Springer via http://dx.doi.org/10.1007/s00382-014-2101-0