ALBERT

Description: aerosol microphysics are used to assess the influence of season of eruption on the aerosol evolution and radiative impacts at the Earth's surface (Toohey et al., 2011). This analysis is presented for eruptions with SO2 injection magnitudes of 17 and 700 Tg, the former consistent with estimates of the 1991 Mt. Pinatubo eruption, the later with estimates of the Los Chocoyos eruption of 84 ka BP from modern-day Guatemala. For each eruption magnitude, simulations are performed with eruptions at the location of the Los Chocoyos eruption site (15° N, 91° W) at four equally spaced times of year. Sensitivity to eruption season of aerosol optical depth (AOD), clear-sky and all-sky shortwave (SW) radiative flux is quantified based on the difference between the maximum and minimum response from the four eruption seasons. Eruption season has a significant influence on AOD and clear-sky SW radiative flux anomalies for both eruption magnitudes, reaching maximum values of ~75 %. All-sky SW anomalies are found to be sensitive to season of eruption for the Los Chocoyos eruption magnitude, but insensitive to season of eruption for the Pinatubo-magnitude eruption experiment. Our estimates of sensitivity to eruption season are larger than previously reported estimates: implications regarding volcanic AOD timeseries reconstructions and their use in climate models are discussed.

Description: Extremely large volcanic eruptions have been linked to global climate change, biotic turnover, and, for the Younger Toba Tuff (YTT) eruption 74,000 years ago, near-extinction of modern humans. One of the largest uncertainties of the climate effects involves evolution and growth of aerosol particles. A huge atmospheric concentration of sulfate causes higher collision rates, larger particle sizes, and rapid fall out, which in turn greatly affects radiative feedbacks. We address this key process by incorporating the effects of aerosol microphysical processes into an Earth System Model. The temperature response is shorter (9–10 years) and three times weaker (−3.5 K at maximum globally) than estimated before, although cooling could still have reached −12 K in some midlatitude continental regions after one year. The smaller response, plus its geographic patchiness, suggests that most biota may have escaped threshold extinction pressures from the eruption.

Description: It has been noted that while major volcanic eruptions in the tropics lead to a global distribution of stratospheric aerosol, the distribution is not necessarily symmetric between the Northern and Southern Hemispheres. Furthermore, ice-core records of volcanic events from the Greenland and Antarctic ice sheets are often in poor agreement, with the best known example being that of the Toba eruption of ~74 ka B.P., which has a strong signal in the Greenland ice cores, but no identified signal in Antarctic ice core records. Using simulations with the MAECHAM5-HAM general circulation model including detailed aerosol microphysics, we examine how the hemispheric asymmetry of volcanic aerosol loading, and of the deposition of sulfate to the surface, depends on the season and magnitude of eruption. A number of paleo-eruptions in the Central American Volcanic Arc (CAVA) are simulated, with different SO2 emission strengths (ranging from 17 to 700 Mt SO2) estimated from field measurements. We show that heating of volcanic aerosols leads to atmospheric circulation changes, affecting the global aerosol transport. Our results indicate that for extremely large volcanic eruptions, such anomalous atmospheric circulation patterns can lead to very large asymmetries in stratospheric aerosol loading and sulfate deposition to the polar ice sheets. This work could be useful in better interpreting volcanic signals in paleo-ice core data and improving the accuracy of estimated aerosol optical depth data sets used in the model simulation of past climate.

Description: One of the most important natural causes of climate change are large volcanic eruptions as they have a significant impact on Earth's global climate system, especially on the tropospheric and stratospheric circulation. The direct injection of gases, aerosols and volcanic ash into the stratosphere has a strong and long lasting radiative influence, which leads to a global reduction in surface temperatures, and a pronounced warming of the stratosphere, for several years or even decades. To evaluate the climate response and feedback to an extremely large volcanic eruption we use the complex MPI-Earth System Model (ESM) by forcing it with a simulated Aerosol Optical Depth (AOD) distribution resulting from a stratospheric injection of 700 Mt SO2, corresponding to the Los Chocoyos eruption (VEI〉7) in Guatemala (84 ka BP). To take into account the unknown season of the eruption, we perform experiments for January and July eruptions, including five ensemble simulations for each. We consider global atmospheric effects as well as changes in the ocean circulation, sea ice and the carbon cycle, which are generated by complex relationships between the radiative forcing and the earth climate system on different time scales. In this study we show that the global surface temperature and especially the tropical precipitation drop rapidly before they recover within 8 years. The ocean response indicates a reduction in ocean heat content, a strengthening of the Meridional Overturning Circulation (MOC) and a sensitivity of the sea ice content on longer time scales. Finally modifications in the CO2 storage of the earth climate system comprising the ocean, land and atmospheric CO2 concentrations are investigated

Description: Simulations of tropical volcanic eruptions using a general circulation model with coupled aerosol microphysics are used to assess the influence of season of eruption on the aerosol evolution and radiative impacts at the Earth's surface. This analysis is presented for eruptions with SO2 injection magnitudes of 17 and 700 Tg, the former consistent with estimates of the 1991 Mt. Pinatubo eruption, the later a near-"super eruption". For each eruption magnitude, simulations are performed with eruptions at 15° N, at four equally spaced times of year. Sensitivity to eruption season of aerosol optical depth (AOD), clear-sky and all-sky shortwave (SW) radiative flux is quantified by first integrating each field for four years after the eruption, then calculating for each cumulative field the absolute or percent difference between the maximum and minimum response from the four eruption seasons. Eruption season has a significant influence on AOD and clear-sky SW radiative flux anomalies for both eruption magnitudes. The sensitivity to eruption season for both fields is generally weak in the tropics, but increases in the mid- and high latitudes, reaching maximum values of ~75 %. Global mean AOD and clear-sky SW anomalies show sensitivity to eruption season on the order of 15–20 %, which results from differences in aerosol effective radius for the different eruption seasons. Smallest aerosol size and largest cumulative impact result from a January eruption for Pinatubo-magnitude eruption, and from a July eruption for the near-super eruption. In contrast to AOD and clear-sky SW anomalies, all-sky SW anomalies are found to be insensitive to season of eruption for the Pinatubo-magnitude eruption experiment, due to the reflection of solar radiation by clouds in the mid- to high latitudes. However, differences in all-sky SW anomalies between eruptions in different seasons are significant for the larger eruption magnitude, and the ~15 % sensitivity to eruption season of the global mean all-sky SW anomalies is comparable to the sensitivity of global mean AOD and clear-sky SW anomalies. Our estimates of sensitivity to eruption season are larger than previously reported estimates: implications regarding volcanic AOD timeseries reconstructions and their use in climate models are discussed.

Description: 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.