ALBERT

Description: The current generation of Earth system models that participate in the Coupled Model Intercomparison Project phase 5 (CMIP5) does not, on average, produce a strengthened Northern Hemisphere (NH) polar vortex after large tropical volcanic eruptions as suggested by observational records. Here we investigate the impact of volcanic eruptions on the NH winter stratosphere with an ensemble of 20 model simulations of the Max Planck Institute Earth system model. We compare the dynamical impact in simulations of the very large 1815 Tambora eruption with the averaged dynamical response to the two largest eruptions of the CMIP5 historical simulations (the 1883 Krakatau and the 1991 Pinatubo eruptions). We find that for both the Tambora and the averaged Krakatau-Pinatubo eruptions the radiative perturbation only weakly affects the polar vortex directly. The position of the maximum temperature anomaly gradient is located at approximately 30°N, where we obtain significant westerly zonal wind anomalies between 10hPa and 30hPa. Under the very strong forcing of the Tambora eruption, the NH polar vortex is significantly strengthened because the subtropical westerly wind anomalies are sufficiently strong to robustly alter the propagation of planetary waves. The average response to the eruptions of Krakatau and Pinatubo reveals a slight strengthening of the polar vortex, but individual ensemble members differ substantially, indicating that internal variability plays a dominant role. For the Tambora eruption the ensemble variability of the zonal mean temperature and zonal wind anomalies during midwinter and late winter is significantly reduced compared to the volcanically unperturbed period.

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: Interest in stratospheric aerosol and its role in climate have increased over the last decade due to the observed increase in stratospheric aerosol since 2000 and the potential for changes in the sulfur cycle induced by climate change. This review provides an overview about the advances in stratospheric aerosol research since the last comprehensive assessment of stratospheric aerosol was published in 2006. A crucial development since 2006 is the substantial improvement in the agreement between in situ and space-based inferences of stratospheric aerosol properties during volcanically quiescent periods. Furthermore, new measurement systems and techniques, both in situ and space based, have been developed for measuring physical aerosol properties with greater accuracy and for characterizing aerosol composition. However, these changes induce challenges to constructing a long-term stratospheric aerosol climatology. Currently, changes in stratospheric aerosol levels less than 20% cannot be confidently quantified. The volcanic signals tend to mask any nonvolcanically driven change, making them difficult to understand. While the role of carbonyl sulfide as a substantial and relatively constant source of stratospheric sulfur has been confirmed by new observations and model simulations, large uncertainties remain with respect to the contribution from anthropogenic sulfur dioxide emissions. New evidence has been provided that stratospheric aerosol can also contain small amounts of nonsulfate matter such as black carbon and organics. Chemistry-climate models have substantially increased in quantity and sophistication. In many models the implementation of stratospheric aerosol processes is coupled to radiation and/or stratospheric chemistry modules to account for relevant feedback processes

Description: Within the SPARC Data Initiative, the first comprehensive assessment of the quality of 13 water vapor products from 11 limb-viewing satellite instruments (LIMS, SAGE II, UARS-MLS, HALOE, POAM III, SMR, SAGE III, MIPAS, SCIAMACHY, ACE-FTS, and Aura-MLS) obtained within the time period 1978-2010 has been performed. Each instrument's water vapor profile measurements were compiled into monthly zonal mean time series on a common latitude-pressure grid. These time series serve as basis for the ‘climatological’ validation approach used within the project. The evaluations include comparisons of monthly or annual zonal mean cross-sections and seasonal cycles in the tropical and extra-tropical upper troposphere and lower stratosphere averaged over one or more years, comparisons of inter-annual variability, and a study of the time evolution of physical features in water vapor such as the tropical tape recorder and polar vortex dehydration. Our knowledge of the atmospheric mean state in water vapor is best in the lower and middle stratosphere of the tropics and mid-latitudes, with a relative uncertainty of ±2-6% (as quantified by the standard deviation of the instruments’ multi-annual means). The uncertainty increases towards the polar regions (±10-15%), the mesosphere (±15%), and the upper troposphere/lower stratosphere below 100 hPa (±30-50%), where sampling issues add uncertainty due to large gradients and high natural variability in water vapor. The minimum found in multi-annual (1998-2008) mean water vapor in the tropical lower stratosphere is 3.5 ppmv (±14%), with slightly larger uncertainties for monthly mean values. The frequently used HALOE water vapor dataset shows consistently lower values than most other datasets throughout the atmosphere, with increasing deviations from the multi-instrument mean below 100 hPa in both the tropics and extra-tropics. The knowledge gained from these comparisons and regarding the quality of the individual datasets in different regions of the atmosphere will help to improve model-measurement comparisons (e.g. for diagnostics such as the tropical tape recorder or seasonal cycles), data merging activities, and studies of climate variability.

Description: Monthly zonal mean climatologies of atmospheric measurements from satellite instruments can have biases due to the non-uniform sampling of the atmosphere by the instruments. We characterize potential sampling biases in stratospheric trace gas climatologies of the Stratospheric Processes and their Role in Climate (SPARC) Data Initiative using chemical fields from a chemistry climate model simulation and sampling patterns from 16 satellite-borne instruments. The exercise is performed for the long-lived stratospheric trace gases O3 and H2O. Monthly sample biases for O3 exceed 10% for many instruments in the high latitude stratosphere and in the upper troposphere/lower stratosphere, while annual mean sampling biases reach values of up to 20% in the same regions for some instruments. Sampling biases for H2O are generally smaller than for O3, although still notable in the upper troposphere/lower stratosphere and Southern Hemisphere high latitudes. The most important mechanism leading to monthly sampling bias is the non-uniform temporal sampling of many instruments, i.e., the fact that for many instruments, monthly means are produced from measurements which span less than the full month in question. Similarly, annual mean sampling biases are well explained by non-uniformity in the month-to-month sampling by different instruments. Non-uniform sampling in latitude and longitude are shown to also lead to non-negligible sampling biases, which are most relevant for climatologies which are otherwise free of sampling biases due to non-uniform temporal sampling.

Description: Reconstructions of the atmospheric sulfate aerosol burdens resulting from past volcanic eruptions are based on ice core-derived estimates of volcanic sulfate deposition and the assumption that the two quantities are directly proportional. We test this assumption within simulations of tropical volcanic stratospheric sulfur injections with the MAECHAM5-HAM aerosol-climate model. An ensemble of 70 simulations is analyzed, with SO2 injections ranging from 8.5 to 700 Tg, with eruptions in January and July. Modeled sulfate deposition flux to Antarctica shows excellent spatial correlation with ice core-derived estimates for Pinatubo and Tambora, although the comparison suggests the modeled flux to the ice sheets is 4–5 times too large. We find that Greenland and Antarctic deposition efficiencies (the ratio of sulfate flux to each ice sheet to the maximum hemispheric stratospheric sulfate aerosol burden) vary as a function of the magnitude and season of stratospheric sulfur injection. Changes in simulated sulfate deposition for large SO2 injections are connected to increases in aerosol particle size, which impact aerosol sedimentation velocity and radiative properties, the latter leading to strong dynamical changes including strengthening of the winter polar vortices, which inhibits the transport of stratospheric aerosols to high latitudes. The resulting relationship between Antarctic and Greenland volcanic sulfate deposition is nonlinear for very large eruptions, with significantly less sulfate deposition to Antarctica than to Greenland. These model results suggest that variability of deposition efficiency may be an important consideration in the interpretation of ice core sulfate signals for eruptions of Tambora-magnitude and larger.

Description: A comprehensive quality assessment of the ozone products from 18 limb-viewing satellite instruments is provided by means of a detailed inter-comparison. The ozone climatologies in the form of monthly zonal mean time series covering the upper troposphere to lower mesosphere are obtained from LIMS, SAGE I, SAGE II, UARS-MLS, HALOE, POAM II, POAM III, SMR, OSIRIS, SAGE III, MIPAS, GOMOS, SCIAMACHY, ACE-FTS, ACE-MAESTRO, Aura-MLS, HIRDLS, and SMILES within 1978-2010. The inter-comparisons focus on mean biases based on monthly and annual zonal mean fields, on inter-annual variability and on seasonal cycles. Additionally, the physical consistency of the data sets is tested through diagnostics of the quasi-biennial oscillation and the Antarctic ozone hole. The comprehensive evaluations reveal that the uncertainty in our knowledge of the atmospheric ozone mean state is smallest in the tropical middle stratosphere and in the midlatitude lower/middle stratosphere, where we find a 1σ multi-instrument spread of less than ±5%. While the overall agreement among the climatological data sets is very good for large parts of the stratosphere, individual discrepancies have been identified including unrealistic month-to-month fluctuations, large biases in particular atmospheric regions, or inconsistencies in the seasonal cycle. Notable differences between the data sets exist in the tropical lower stratosphere and at high latitudes, with a multi-instrument spread of ±30% at the tropical tropopause and ±15% at polar latitudes. In particular, large relative differences are identified in the Antarctic polar cap during the time of the ozone hole, with a spread between the monthly zonal mean fields of ±50%. Differences between the climatological data sets are suggested to be partially related to inter-instrumental differences in vertical resolution and geographical sampling. The evaluations as a whole provide guidance on what data sets are the most reliable for applications such as studies of ozone variability, model-measurement comparisons and detection of long-term trends. A detailed comparison versus SAGE II data is presented, which can help identify suitable candidates for long-term data merging studies.

Description: We present the first comprehensive intercomparison of currently available satellite ozone climatologies in the upper troposphere/lower stratosphere (UTLS) (300-70hPa) as part of the Stratosphere-troposphere Processes and their Role in Climate (SPARC) Data Initiative. The Tropospheric Emission Spectrometer (TES) instrument is the only nadir-viewing instrument in this initiative, as well as the only instrument with a focus on tropospheric composition. We apply the TES observational operator to ozone climatologies from the more highly vertically resolved limb-viewing instruments. This minimizes the impact of differences in vertical resolution among the instruments and allows identification of systematic differences in the large-scale structure and variability of UTLS ozone. We find that the climatologies from most of the limb-viewing instruments show positive differences (ranging from 5 to 75%) with respect to TES in the tropical UTLS, and comparison to a zonal mean ozonesonde climatology indicates that these differences likely represent a positive bias for p100hPa. In the extratropics, there is good agreement among the climatologies regarding the timing and magnitude of the ozone seasonal cycle (differences in the peak-to-peak amplitude of 〈15%) when the TES observational operator is applied, as well as very consistent midlatitude interannual variability. The discrepancies in ozone temporal variability are larger in the tropics, with differences between the data sets of up to 55% in the seasonal cycle amplitude. However, the differences among the climatologies are everywhere much smaller than the range produced by current chemistry-climate models, indicating that the multiple-instrument ensemble is useful for quantitatively evaluating these models.

Description: First workshop of the Volcanic Impacts on Climate and Society Working Group; Palisades, New York, 6–8 June 2016. To predict and prepare for future climate change, scientists are striving to understand how global-scale climatic change manifests itself on regional scales and also how societies adapt—or don’t—to sometimes subtle and complex climatic changes. In this regard, the strongest volcanic eruptions of the past are powerful test cases, showcasing how the broad climate system responds to sudden changes in radiative forcing and how societies have responded to the resulting climatic shocks.

Description: Radiative forcing from volcanic aerosol impacts surface temperatures; however, the background climate state also affects the response. A key question thus concerns whether constraining forcing estimates is more important than constraining initial conditions for accurate simulation and attribution of posteruption climate anomalies. Here we test whether different realistic volcanic forcing magnitudes for the 1815 Tambora eruption yield distinguishable ensemble surface temperature responses. We perform a cluster analysis on a superensemble of climate simulations including three 30-member ensembles using the same set of initial conditions but different volcanic forcings based on uncertainty estimates. Results clarify how forcing uncertainties can overwhelm initial-condition spread in boreal summer due to strong direct radiative impact, while the effect of initial conditions predominate in winter, when dynamics contribute to large ensemble spread. In our setup, current uncertainties affecting reconstruction-simulation comparisons prevent conclusions about the magnitude of the Tambora eruption and its relation to the “year without summer.”