ALBERT

Description: During two cruises wiht RV Sonne, SO234-2 from 8 to 19 July 2014 (Durban, South Africa to Port Louis, Mauritius) and SO235 from 23 July to 7 August 2014 (Port Louis, Mauritius to Malé, Maldives), within the SPACES (Science Partnerships for the Assessment of Complex Earth System Processes) and OASIS (Organic very short-lived Substances and their air sea exchange from the Indian Ocean to the Stratosphere) research projects, surface water samples were sampled from a continuous running pump in the hydrographic shaft of RV Sonne at a depth of 5 m. Deep water samples were taken from a Niskin-bottle rosette sampler. The samples were then analyzed for halogenated compounds using a purge and trap system onboard, which was attached to a gas chromatograph with an electron capture detector for surface water samples and a GC/MS Agilent 5975 for the deep water samples. An analytical reproducibility of 10% was determined from measuring duplicate water samples, detection limit was 0.2 pmol /L. Calibration was performed with several dilutions of a mixed-compound standard prepared in methanol.

Description: Between 1733 and 1895, a total of 35 additional volcanic eruptions were detected in the new high-resolution measurements (D4i dataset: "Greenland ice-core non-sea-salt sulfur concentrations and calculated volcanic sulfate deposition (1733-1900 CE)" (PANGAEA, doi:10.1594/PANGAEA.960977) of the D4 ice core (McConnel et al., 2007). For the same time period only 25 volcanic eruptions had previously been detected using an ice-core array from Greenland (including NEEM-2011-S1 and NGRIP) and Antarctica, making up the eVolv2k database [Toohey and Sigl, 2017]. 21 volcanic events in D4i are found to match events in the eVolv2k database, 8 tropical events and 13 Northern Hemisphere extratropical (NHET) events. Based on linear fits of eVolv2k volcanic stratospheric sulfur injections (VSSI) to the cumulative D4i sulfate deposition rates, we derive scaling factors to convert D4i volcanic sulfate depositions to VSSI. Fits are of high quality with R2 values of 0.91 and 0.99 for tropical and extratropical events, respectively. Of the remaining events identified in D4i but not included in eVolv2k, we find 11 that are tentatively attributable to VEI=4 events listed in the Volcanoes of the World [Global Volcanism Program, 2013] (GVP) database (e.g, Soufriere St. Vincent, and Awu in 1812; Suwanosejima in 1813; Mayon 1814; Raung 1817; Colima 1818). Although attribution is not completely certain, for these events we assume the attribution is correct and use the historically dated eruption date and location from Volcanoes of the World (Global Volcanism Program, 2013). Eruptions found in D4i which do not have a corresponding event in the GVP database could result from a number of scenarios. To avoid a potential bias by attributing these signals to either tropical latitudes (0°) or to NHET latitudes (i.e. 45°N), we represent the forcing by these unidentified events as the probability-weighted superposition of tropical and extratropical eruptions based on the measured sulfate flux. For each event we calculate the VSSI associated with the sulfate deposition assuming on the one hand the event was tropical, and on the other hand assuming it was extratropical. These VSSI values are then multiplied by the probability that the event was either tropical or extratropical, based on the proportion of NHET and tropical events in the Greenland records used in eVolv2k. Each unidentified sulfate deposition is then represented in the VSSI file as two injections, with the same eruption time taken from the ice ice-core dating, and different VSSI amounts for default tropical and extratropical regions. The resulting list of "additional" eruptions not included in eVolv2k is merged with eVolv2k, and the resulting eruption list named eVolv2k plus D4i used as input to the EVA forcing generator [Toohey et al., 2016] to generate time series of stratospheric aerosol optical depth (SAOD).

Description: Based on a set of continuous sulfate records from a suite of ice cores from Greenland and Antarctica, the HolVol v.1.0 database includes estimates of the magnitudes and approximate source latitudes of major volcanic stratospheric sulfur injection (VSSI) events for the Holocene (from 9500 BCE or 11500 year BP to 1900 CE), constituting an extension of the previous record by 7000 years. The database incorporates new-generation ice-core aerosol records with sub-annual temporal resolution and demonstrated sub-decadal dating accuracy and precision. By tightly aligning and stacking the ice-core records on the WD2014 chronology from Antarctica we resolve long-standing previous inconsistencies in the dating of ancient volcanic eruptions that arise from biased (i.e. dated too old) ice-core chronologies over the Holocene for Greenland. A long-term latitudinally and monthly resolved stratospheric aerosol optical depth (SAOD) time series is reconstructed from the HolVol VSSI estimates, representing the first such reconstruction Holocene-scale reconstruction constrained by Greenland and Antarctica ice cores. These new long-term reconstructions of past VSSI and SAOD variability confirm evidence from regional volcanic eruption chronologies (e.g., from Iceland) in showing that the early Holocene (9500-7000 BCE) experienced a higher number of volcanic eruptions (+16%) and cumulative VSSI (+86%) compared to the past 2,500 years. This increase is coinciding with then rapidly retreating ice sheets during deglaciation, providing context for potential future increases of volcanic activity in regions under projected glacier melting in the 21st century.

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: 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: The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) on board the Canadian SCISAT-1 satellite (launched in August 2003) measures over 30 different atmospheric species, including six nitrogen trace gases that are needed to quantify the stratospheric NOy budget. We combine volume mixing ratio (VMR) profiles for NO, NO2, HNO3, N2O5, ClONO2, and HNO4 to determine a zonally averaged NOy climatology on monthly and 3 month combined means (December–February, March–May, June–August, and September–November) at 5° latitude spacing and on 33 pressure surfaces. Peak NOy VMR concentrations (15–20 ppbv) are situated at about 3 hPa (∼40 km) in the tropics, while they are typically lower at about 10 hPa (∼30 km) in the midlatitudes. Mean NOy VMRs are similar in both the northern and southern polar regions, with the exception of large enhancements periodically observed in the upper stratosphere and lower mesosphere. These are primarily due to enhancements of NO due to energetic particle precipitation and downward transport. Other features in the NOy budget are related to descent in the polar vortex, heterogeneous chemistry, and denitrification processes. Comparison of the ACE-FTS NOy budget is made to both the Odin and ATMOS NOy data sets, showing in both cases a good level of agreement, such that relative differences are typically better than 20%. The NOy climatological products are available through the ACE website and are a supplement to the paper. - A middle-atmosphere NOy climatology has been produced using ACE-FTS measurements; - A robust method for quality controlling the input data has been developed - Good agreement is found between ACE-FTS NOy climatology and other climatologies

Description: The goal of the Chemistry-Climate Model Validation (CCMVal) activity is to improve understanding of chemistry-climate models (CCMs) through process-oriented evaluation and to provide reliable projections of stratospheric ozone and its impact on climate. An appreciation of the details of model formulations is essential for understanding how models respond to the changing external forcings of greenhouse gases and ozone-depleting substances, and hence for understanding the ozone and climate forecasts produced by the models participating in this activity. Here we introduce and review the models used for the second round (CCMVal-2) of this intercomparison, regarding the implementation of chemical, transport, radiative, and dynamical processes in these models. In particular, we review the advantages and problems associated with approaches used to model processes of relevance to stratospheric dynamics and chemistry. Furthermore, we state the definitions of the reference simulations performed, and describe the forcing data used in these simulations. We identify some developments in chemistry-climate modeling that make models more physically based or more comprehensive, including the introduction of an interactive ocean, online photolysis, troposphere-stratosphere chemistry, and non-orographic gravity-wave deposition as linked to tropospheric convection. The relatively new developments indicate that stratospheric CCM modeling is becoming more consistent with our physically based understanding of the atmosphere.

Description: In order to validate the reported precision of space-based atmospheric composition measurements, validation studies often focus on measurements in the tropical stratosphere, where natural variability is weak. The scatter in tropical measurements can then be used as an upper limit on single-profile measurement precision. Here we introduce a method of quantifying the scatter of tropical measurements which aims to minimize the effects of short-term atmospheric variability while maintaining large enough sample sizes that the results can be taken as representative of the full data set. We apply this technique to measurements of O(3), HNO(3), CO, H(2)O, NO, NO(2), N(2)O, CH(4), CCl(2)F(2), and CCl(3)F produced by the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS). Tropical scatter in the ACE-FTS retrievals is found to be consistent with the reported random errors (RREs) for H(2)O and CO at altitudes above 20 km, validating the RREs for these measurements. Tropical scatter in measurements of NO, NO(2), CCl(2)F(2), and CCl(3)F is roughly consistent with the RREs as long as the effect of outliers in the data set is reduced through the use of robust statistics. The scatter in measurements of O(3), HNO(3), CH(4), and N(2)O in the stratosphere, while larger than the RREs, is shown to be consistent with the variability simulated in the Canadian Middle Atmosphere Model. This result implies that, for these species, stratospheric measurement scatter is dominated by natural variability, not random error, which provides added confidence in the scientific value of single-profile measurements.

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.