SPARC Data Initiative: climatology uncertainty assessment

The SPARC Data Initiative aims to produce trace gas climatologies for a number of species from a number of instruments. In order to properly compare these climatologies, and interpret differences between them, it is necessary to know the uncertainty in each calculated climatological mean field. The inhomogeneous and finite temporal-spatial sampling pattern of each instrument can lead to biases and uncertainties in the mean climatologies. Sampling which is unevenly weighted in time and space leads to biases between a data set's climatology and the truth. Furthermore, the systematic sampling patterns of some instruments may mean that uncertainties in mean fields calculated through traditional methods that assume random sampling may be inappropriate. We aim to address these issues through an exercise wherein high resolution chemical fields from a coupled Chemistry Climate Model are sub-sampled based on the sampling pattern of each instrument. Climatologies based on the sub-sampled data can be compared to those calculated with the full data set, in order to assess sampling biases. Furthermore, investigating the ensemble variability of climatologies based on subsampled fields will allow us to assess the proper methodology for estimating the uncertainty in climatological mean fields

Volcanic stratospheric sulphur injections and aerosol optical depth from 500 BCE to 1900 CE

The injection of sulphur into the stratosphere by explosive volcanic eruptions is the cause of significant climate variability. Based on sulphate records from a suite of ice cores from Greenland and Antarctica, the eVolv2k database includes estimates of the magnitudes and approximate source latitudes of major volcanic stratospheric sulphur injection (VSSI) events from 500 BCE to 1900 CE, constituting an update of prior reconstructions and an extension of the record by 1000 years. The VSSI estimates incorporate improvements to the ice core records in terms of synchronization and dating, refinements to the methods used to estimate VSSI from ice core records, and includes first estimates of the random uncertainties in VSSI values. VSSI estimates for many of the largest eruptions, including Samalas (1257), Tambora (1815) and Laki (1783) are within 10% of prior estimates. A number of strong events are included in eVolv2k which are largely underestimated or not included in earlier VSSI reconstructions, including events in 540, 574, 682 and 1108 CE. The long term annual mean VSSI from major volcanic eruptions is estimated to be ∼ 0.5 Tg [S] yr−1, ∼ 50 % greater than a prior reconstruction, due to the identification of more events and an increase in the magnitude of many intermediate events. A long-term, latitudinally and monthly resolved stratospheric aerosol optical depth (SAOD) time series is reconstructed from the eVolv2k VSSI estimates, and the resulting global mean SAOD is found to be similar (within 33%) to a prior reconstruction for most of the largest eruptions. The long-term (500 BCE–900 CE) average global mean SAOD estimated from the eVolv2k VSSI estimates and including a constant "background" injection of stratospheric sulphur is ∼ 0.014, 30 % greater than a prior reconstruction. These new long-term reconstructions of past VSSI and SAOD variability give context to recent volcanic forcing, suggesting that the 20th century was a period of somewhat weaker than average volcanic forcing, with current best estimates of 20th century mean VSSI and SAOD values being 25 and 14 % less, respectively, than the mean of the 500 BCE to 1900 CE period. The reconstructed VSSI and SAOD data are available at https://doi.org/10.1594/WDCC/eVolv2k_v2>

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

Climatologies from satellite measurements: the impact of orbital sampling on the standard error of the mean

Climatologies of atmospheric observations are often produced by binning measurements according to latitude, and calculating zonal means. The uncertainty in these climatological means is characterized by the standard error of the mean (SEM). However, the usual estimator of the SEM, i.e. the sample standard deviation divided by the square root of the sample size, holds only for uncorrelated randomly sampled measurements. Measurements of the atmospheric state along a satellite orbit cannot always be considered as independent because (a) the time-space interval between two nearest observations is often smaller than the typical scale of variations in the atmospheric state, and (b) the regular time-space sampling pattern of a satellite instrument strongly deviates from random sampling. We have developed an experiment where global chemical fields from a chemistry climate model are sampled according to real sampling patterns of satellite-borne instruments. As case studies, sampling patterns of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and Atmospheric Chemistry Experiment Fourier-Transform Spectrometer (ACE-FTS) satellite instruments are used to iteratively subsample the model O3 fields and produce empirical estimates of the standard error of monthly mean zonal mean model O3 in 5° latitude bins. We find that generally the classic SEM estimator is a conservative estimate of the SEM, i.e. the empirical SEM is often less than the classic estimate. Exceptions occur in instances where the zonal sampling distribution shows non-uniformity with a similar zonal structure as variations in the sampled field, leading to maximum sensitivity to arbitrary phase shifts between the sample distribution and sampled field. The occurrence of such instances is thus very sensitive to slight changes in the sampling distribution, and to the variations in the measured field. This study highlights the need for caution in the interpretation of the oft-used classically computed SEM, and outlines a relatively simple methodology that can be used to assess one component of the uncertainty in monthly mean zonal mean climatologies produced from measurements from satellite-borne instrument

The impact of volcanic aerosol on the Northern Hemisphere stratospheric polar vortex: mechanisms and sensitivity to forcing structure

Observations and simple theoretical arguments suggest that the Northern Hemisphere (NH) stratospheric polar vortex is stronger in winters following major volcanic eruptions. However, recent studies show that climate models forced by prescribed volcanic aerosol fields fail to reproduce this effect. We investigate the impact of volcanic aerosol forcing on stratospheric dynamics, including the strength of the NH polar vortex, in ensemble simulations with the Max Planck Institute Earth System Model. The model is forced by four different prescribed forcing sets representing the radiative properties of stratospheric aerosol following the 1991 eruption of Mt. Pinatubo: two forcing sets are based on observations, and are commonly used in climate model simulations, and two forcing sets are constructed based on coupled aerosol–climate model simulations. For all forcings, we find that simulated temperature and zonal wind anomalies in the NH high latitudes are not directly impacted by anomalous volcanic aerosol heating. Instead, high-latitude effects result from enhancements in stratospheric residual circulation, which in turn result, at least in part, from enhanced stratospheric wave activity. High-latitude effects are therefore much less robust than would be expected if they were the direct result of aerosol heating. Both observation-based forcing sets result in insignificant changes in vortex strength. For the model-based forcing sets, the vortex response is found to be sensitive to the structure of the forcing, with one forcing set leading to significant strengthening of the polar vortex in rough agreement with observation-based expectations. Differences in the dynamical response to the forcing sets imply that reproducing the polar vortex responses to past eruptions, or predicting the response to future eruptions, depends on accurate representation of the space–time structure of the volcanic aerosol forcing

Climatic and societal impacts of a volcanic double event at the dawn of the Middle Ages

Volcanic activity in and around the year 536 CE led to severe cold and famine, and has been speculatively linked to large-scale societal crises around the globe. Using a coupled aerosol-climate model, with eruption parameters constrained by recently re-dated ice core records and historical observations of the aerosol cloud, we reconstruct the radiative forcing resulting from a sequence of two major volcanic eruptions in 536 and 540 CE. We estimate that the decadal-scale Northern Hemisphere (NH) extra-tropical radiative forcing from this volcanic “double event” was larger than that of any period in existing reconstructions of the last 1200 years. Earth system model simulations including the volcanic forcing show peak NH mean temperature anomalies reaching more than −2 °C, and show agreement with the limited number of available maximum latewood density temperature reconstructions. The simulations also produce decadal-scale anomalies of Arctic sea ice. The simulated cooling is interpreted in terms of probable impacts on agricultural production in Europe, and implies a high likelihood of multiple years of significant decreases in crop production across Scandinavia, supporting the theory of a connection between the 536 and 540 eruptions and evidence of societal crisis dated to the mid-6th century

Stratosphere troposphere coupling: the influence of volcanic eruptions

Stratospheric sulfate aerosols produced by major volcanic eruptions modify the radiative and dynamical properties of the troposphere and stratosphere through their reflection of solar radiation and absorption of infrared radiation. At the Earth's surface, the primary consequence of a large eruption is cooling, however, it has long been known that major tropical eruptions tend to be followed by warmer than usual winters over the Northern Hemisphere (NH) continents. This volcanic "winter-warming" effect in the NH is understood to be the result of changes in atmospheric circulation patterns resulting from heating in the stratosphere, and is often described as positive anomalies of the Northern Annular Mode (NAM) that propagate downward from the stratosphere to the troposphere. In the southern hemisphere, climate models tend to also predict a positive Southern Annular Mode (SAM) response to volcanic eruptions, but this is generally inconsistent with post-eruption observations during the 20th century. We review present understanding of the influence of volcanic eruptions on the large scale modes of atmospheric variability in both the Northern and Southern Hemispheres. Using models of varying complexity, including an aerosol-climate model, an Earth system model, and CMIP5 simulations, we assess the ability of climate models to reproduce the observed post-eruption climatic and dynamical anomalies. We will also address the parametrization of volcanic eruptions in simulations of the past climate, and identify possibilities for improvemen

A New Volcanic Stratospheric Sulfate Aerosol Forcing Emulator (EVA_H): Comparison With Interactive Stratospheric Aerosol Models

Idealized models or emulators of volcanic aerosol forcing have been widely used to reconstruct the spatiotemporal evolution of past volcanic forcing. However, existing models, including the most recently developed Easy Volcanic Aerosol (EVA; Toohey et al., doi: 10.5194/gmd‐2016‐83), (i) do not account for the height of injection of volcanic SO urn:x-wiley:jgrd:media:jgrd55987:jgrd55987-math-0001; (ii) prescribe a vertical structure for the forcing; and (iii) are often calibrated against a single eruption. We present a new idealized model, EVA_H, that addresses these limitations. Compared to EVA, EVA_H makes predictions of the global mean stratospheric aerosol optical depth that are (i) similar for the 1979–1998 period characterized by the large and high‐altitude tropical SO urn:x-wiley:jgrd:media:jgrd55987:jgrd55987-math-0002 injections of El Chichón (1982) and Mount Pinatubo (1991); (ii) significantly improved for the 1998–2015 period characterized by smaller eruptions with a large variety of injection latitudes and heights. Compared to EVA, the sensitivity of volcanic forcing to injection latitude and height in EVA_H is much more consistent with results from climate models that include interactive aerosol chemistry and microphysics, even though EVA_H remains less sensitive to eruption latitude than the latter models. We apply EVA_H to investigate potential biases and uncertainties in EVA‐based volcanic forcing data sets from phase 6 of the Coupled Model Intercomparison Project (CMIP6). EVA and EVA_H forcing reconstructions do not significantly differ for tropical high‐altitude volcanic injections. However, for high‐latitude or low‐altitude injections, our reconstructed forcing is significantly lower. This suggests that volcanic forcing in CMIP6 last millenium experiments may be overestimated for such eruptions