ALBERT

Description: We describe the main differences in simulations of stratospheric climate and variability by models within the fifth Coupled Model Intercomparison Project (CMIP5) that have a model top above the stratopause and relatively fine stratospheric vertical resolution (high-top), and those that have a model top below the stratopause (low-top). Although the simulation of mean stratospheric climate by the two model ensembles is similar, the low-top model ensemble has very weak stratospheric variability on daily and interannual time scales. The frequency of major sudden stratospheric warming events is strongly underestimated by the low-top models with less than half the frequency of events observed in the reanalysis data and high-top models. The lack of stratospheric variability in the low-top models affects their stratosphere-troposphere coupling, resulting in short-lived anomalies in the Northern Annular Mode, which do not produce long-lasting tropospheric impacts, as seen in observations. The lack of stratospheric variability, however, does not appear to have any impact on the ability of the low-top models to reproduce past stratospheric temperature trends. We find little improvement in the simulation of decadal variability for the high-top models compared to the low-top, which is likely related to the fact that neither ensemble produces a realistic dynamical response to volcanic eruptions.

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: Using an Earth System Model, we investigate the potential Southern Hemisphere (SH) climate response to an extremely large volcanic eruption. The volcanic radiative forcing is calculated offline with a global aerosol model taken into account the formation and development of the volcanic aerosol size distribution from an initial stratospheric injection of 700 Mt SO2 corresponding to that estimated for the VEI〉7 volcanic eruption of Los Chocoyos in Guatemala 84 ka BP. Due to the extremely large volcanic radiative forcing, the surface cools over almost the entire SH. A significant positive phase of the Southern Annual Mode (SAM), persisting for at least 12 months, characterizes the simulated posteruption SH atmospheric circulation. Significant changes of surface temperature, precipitation and wind fields result from a distinct increase in magnitude and poleward movement in position of the SH westerlies. This is associated with temporary modifications in the upper ocean circulation in the Antarctic Circumpolar Current region. Due to the propagation of the forced anomalies into the deep ocean layers, the anomalous oceanic state persists well beyond the atmospheric response timescale. Significant negative temperature anomalies in the SH ocean propagate down to ~2000 m during the first ~20-50 post-eruption years, and persist for the entire simulated 200 years. A multicentennial anomaly in the SH ocean heat content represents the longest lived volcanically-forced signal detectable in the simulated climate.

Description: We describe the main differences in simulations of stratospheric climate and variability by models within the fifth Coupled Model Intercomparison Project (CMIP5) that have a model top above the stratopause and relatively fine stratospheric vertical resolution (high-top), and those that have a model top below the stratopause (low-top). Although the simulation of mean stratospheric climate by the two model ensembles is similar, the low-top model ensemble has very weak stratospheric variability on daily and interannual time scales. The frequency of major sudden stratospheric warming events is strongly underestimated by the low-top models with less than half the frequency of events observed in the reanalysis data and high-top models. The lack of stratospheric variability in the low-top models affects their stratosphere-troposphere coupling, resulting in short-lived anomalies in the Northern Annular Mode, which do not produce long-lasting tropospheric impacts, as seen in observations. The lack of stratospheric variability, however, does not appear to have any impact on the ability of the low-top models to reproduce past stratospheric temperature trends. We find little improvement in the simulation of decadal variability for the high-top models compared to the low-top, which is likely related to the fact that neither ensemble produces a realistic dynamical response to volcanic eruptions

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.