The Effect of Explosive Tropical Volcanism on ENSO

Journal of Geophysical Research, 2005

1] The results from 16 coupled atmosphere/ocean general circulation model (AOGCM) simulations are used to reduce internally generated noise and to obtain an improved estimate of the underlying response of 20th century global mean temperature to volcanic forcing. An upwelling diffusion energy balance model (UD EBM) with the same forcing and the same climate sensitivity as the AOGCM is then used to emulate the AOGCM results. The UD EBM and AOGCM results are in very close agreement, justifying the use of the UD EBM to determine the volcanic response for different climate sensitivities. The maximum cooling for any given eruption is shown to depend approximately on the climate sensitivity raised to power 0.37. After the maximum cooling for low-latitude eruptions the temperature relaxes back toward the initial state with an e-folding time of 29-43 months for sensitivities of 1-4°C equilibrium warming for CO 2 doubling. Comparisons of observed and modeled coolings after the eruptions of Agung, El Chichón, and Pinatubo give implied climate sensitivities that are consistent with the Intergovernmental Panel on Climate Change (IPCC) range of 1.5-4.5°C. The cooling associated with Pinatubo appears to require a sensitivity above the IPCC lower bound of 1.5°C, and none of the observed eruption responses rules out a sensitivity above 4.5°C. Citation: Wigley, T. M. L., C. M. Ammann, B. D. Santer, and S. C. B. Raper (2005), Effect of climate sensitivity on the response to volcanic forcing,

Geophysical Research Letters, 2003

Nature Climate Change

Volcanic activity plays a strong role in modulating climate variability 1. Most model projections of the twenty-first century, however, under-sample future volcanic e ects by not representing the range of plausible eruption scenarios 2-4. Here, we explore how sixty possible volcanic futures, consistent with ice-core records 5 , impact climate variability projections of the Norwegian Earth System Model (NorESM) 6 under RCP4.5 (ref. 7). The inclusion of volcanic forcing enhances climate variability on annual-to-decadal timescales. Although decades with negative global temperature trends become ∼50% more commonplace with volcanic activity, these are unlikely to be able to mitigate long-term anthropogenic warming. Volcanic activity also impacts probabilistic projections of global radiation, sea level, ocean circulation, and sea-ice variability, the local-scale e ects of which are detectable when quantifying the time of emergence 8. These results highlight the importance and feasibility of representing volcanic uncertainty in future climate assessments. Volcanism has been a major driver of past climate variability 1 and will continue to affect future climate alongside human influences 9. Explosive volcanic eruptions warm the stratosphere 10 , cool the troposphere 11 , cause changes in the hydrological cycle 12,13 , and trigger modifications of atmospheric circulation that give rise to large regional climate responses 14. The instrumental period covering the past 150 years has been relatively volcanically quiescent, and it is therefore tempting to ascribe potential volcanism a minor role in future climate impact and risk assessments. In a millennial perspective, however, there have been periods with considerably stronger volcanic activity 5 (Supplementary Fig. 1). Clustered occurrence of strong tropical eruptions has contributed to sustained cold periods such as the Little Ice Age 15 , where the longerterm climate impacts are mediated through ocean heat content anomalies 16 and ocean circulation changes 17-19 that also affect global and regional sea level 20 and sea-ice conditions 15,17. Because volcanic eruptions are unpredictable events, they have generally been excluded from twenty-first century climate projection protocols. Most recent projections either specify future volcanic forcing as zero or a constant background value 4 , whereas considerations of more realistic volcanic effects have been limited to idealized eruption scenarios, repeating recent volcanic activity in near-future simulations 21,22. Herein we explore whether a more complete representation of volcanic forcing uncertainty that considers a range of volcanic forcing possibilities will have

Geophysical Research Letters, 1997

We test the hypothesis that E1 Nifio/Southern Oscillation (ENSO) events are caused or enhanced by volcanic aerosol perturbations using two methods: 1) A case-by-case comparison of the 16 strongest E1 Nifio events of the last 150 years with the characteristics of the largest concurrent volcanic events, and 2) Comparison of the timing of strong E1 Nifio events and the independently determined record of stratospheric optical depth (x) perturbations. Many eruptions that occurred near times of strong E1 Nifio years produced small amounts of stratospheric aerosols, in many cases the relative timing of the two events argues against triggering. The correlation of 4 of 11 peaks in global stratospheric optical depth (x > 0.025) within 3 years of strong E1 Nifio events over the last 150 years is what would be expected by chance. Moreover, five strong E1 Nifios occurred between 1915 and 1960, when the stratosphere was largely free of volcanic aerosols. Of the three strongest tropical volcanic aerosol perturbations that coincided with or were followed by strong E1 Nifios in the period studied (Krakatau in 1883, E1 Chich6n in 1982, and Pinatubo in 1991), the two modern ones occurred after the earliest SST warming of the E1 Nifio events. The coincidence of these two phenomena indicates no causative relationship.

Journal of Geophysical Research: Atmospheres, 2001

Several previous studies have attempted to remove the effects of explosive volcanic eruptions and E1 Nifio-Southern Oscillation (ENSO) variability from time series of globally averaged surface and tropospheric temperatures. Such work has largely ignored the nonzero correlation between volcanic signals and ENSO. Here we account for this collinearity using an iterative procedure. We remove estimated volcano and ENSO signals from the observed global mean temperature data, and then calculate trends over 1979-1999 in the residuals. Residual trends are sensitive to the choice of index used for removing ENSO effects and to uncertainties in key volcanic parameters. Despite these sensitivities, residual surface and lower tropospheric (2LT) trends are almost always larger than trends in the raw observational data. After removal of volcano and ENSO effects, the differential warming between the surface and lower troposphere is generally reduced. These results suggest that the net effect of volcanoes and ENSO over 1979-1999 was to reduce globally averaged surface and tropospheric temperatures and cool the troposphere by more than the surface. ENSO and incomplete volcanic forcing effects can hamper reliable assessment of the true correspondence between modeled and observed trends. In the second part of our study, we remove these effects from model data and compare simulated and observed residual trends. Residual temperature trends are not significantly different at the surface. In the lower troposphere the statistical significance of trend differences depends on the experiment considered, the choice of ENSO index, and the volcanic signal decay time. The simulated difference between surface and tropospheric warming rates is significantly smaller than observed in 51 out of 54 cases considered. We also examine multiple realizations of model experiments with relatively complete estimates of natural and anthropogenic forcing. ENSO and volcanic effects are not removed from these integrations. As in the case of residual trends, model and observed raw trends are in good agreement at the surface but differ significantly in terms of the trend differential between the surface and lower troposphere. Observed and simulated lower tropospheric trends are not significantly different in 17 out of 24 cases. Our study highlights the large uncertainties inherent in removing volcano and ENSO effects from atmospheric temperature data. It shows that statistical removal of these effects improves the correspondence between modeled and observed temperature trends over the satellite era. Accounting for volcanoes and ENSO cannot fully explain the observed warming of the surface relative to the lower troposphere, or why this differential warming is not reproduced in the model simulations considered here. 1.

Climate responses to volcanic eruptions include changes in the distribution of temperature and precipitation such as those associated with El Niño Southern Oscillation (ENSO). Recent studies suggest an ENSO-positive phase after a volcanic eruption. In the Atlantic Basin, a similar mode of variability is referred as the Atlantic Niño, which is related to precipitation variability in West Africa and South America. Both ENSO and Atlantic Niño are characterized in the tropics by conjoined fluctuations in sea surface temperature (SST), zonal winds, and thermocline depth. Here, we examine possible responses of the Tropical Atlantic to last millennium volcanic forcing via SST, zonal winds, and thermocline changes. We used simulation results from the National Center for Atmospheric Research Community Earth System Model Last Millennium Ensemble single-forcing experiment ranging from 850 to 1850 C.E. Our results show an SST cooling in the Tropical Atlantic during the post-eruption year accompanied by differences in the Atlantic Niño associated feedback. However, we found no significant deviations in zonal winds and thermocline depth related to the volcanic forcing in the first 10 years after the eruption. Changes in South America and Africa monsoon precipitation regimes related to the volcanic forcing were detected, as well as in the Intertropical Convergence Zone position and associated precipitation. These precipitation responses derive primarily from Southern and Tropical volcanic eruptions and occur predominantly during the austral summer and autumn of the post-eruption year.

Climate of the Past, 2014

As the understanding and representation of the impacts of volcanic eruptions on climate have improved in the last decades, uncertainties in the stratospheric aerosol forcing from large eruptions are now linked not only to visible optical depth estimates on a global scale but also to details on the size, latitude and altitude distributions of the stratospheric aerosols. Based on our understanding of these uncertainties, we propose a new model-based approach to generating a volcanic forcing for general circulation model (GCM) and chemistry-climate model (CCM) simulations. This new volcanic forcing, covering the 1600-present period, uses an aerosol microphysical model to provide a realistic, physically consistent treatment of the stratospheric sulfate aerosols. Twenty-six eruptions were modeled individually using the latest available ice cores aerosol mass estimates and historical data on the latitude and date of eruptions. The evolution of aerosol spatial and size distribution after the sulfur dioxide discharge are hence characterized for each volcanic eruption. Large variations are seen in hemispheric partitioning and size distributions in relation to location/date of eruptions and injected SO 2 masses. Results for recent eruptions show reasonable agreement with observations. By providing these new estimates of spatial distributions of shortwave and long-wave radiative perturbations, this volcanic forcing may help to better constrain the climate model responses to volcanic eruptions in the 1600-present period. The final data set consists of 3-D values (with constant longitude) of spectrally resolved extinction coefficients, single scattering albedos and asymmetry factors calculated for different wavelength bands upon request. Surface area densities for heterogeneous chemistry are also provided.

Research Square (Research Square), 2021

Although global and Northern Hemisphere (NH) temperature responses to volcanic forcing have been extensively investigated, knowledge of such responses over Southern Hemisphere (SH) continental regions is still limited. Here we use an ensemble of CMIP5 models to explore SH temperature responses to four major volcanic eruptions: Krakatau (1883), Santa Maria (1902), Agung (1963) and (1991). Focus is on near-surface temperature responses over southern continental landmasses including southern South America (SSA), southern Africa (SAF) and Australia and their seasonal differences. Findings indicate that for all continents, temperature responses were strongest and lasted longest following the Krakatau eruption. Responses in Australia had the shortest lag time, strongest maximum seasonal response, as well as the most signi cant monthly anomalies. In contrast, SSA records the longest lag time, weakest maximum seasonal temperature response, and lowest number of monthly negative anomalies following these eruptions. In most cases, the strongest single-season response occurred in austral autumn or winter, and the weakest in summer or spring. We tentatively propose that cooler temperature responses are likely caused, at least in part, by the intensi cation of the westerlies and associated mid-latitude cyclones and anti-cyclones.

Geophysical Research Letters, 2010

Explosive volcanic eruptions cause episodic negative radiative forcing of the climate system. Using coupled atmosphere-ocean general circulation models (AOGCMs) subjected to historical forcing since the late nineteenth century, previous authors have shown that each large volcanic eruption is associated with a sudden drop in ocean heat content and sea-level from which the subsequent recovery is slow. Here we show that this effect may be an artefact of experimental design, caused by the AOGCMs not having been spun up to a steady state with volcanic forcing before the historical integrations begin. Because volcanic forcing has a long-term negative average, a cooling tendency is thus imposed on the ocean in the historical simulation. We recommend that an extra experiment be carried out in parallel to the historical simulation, with constant time-mean historical volcanic forcing, in order to correct for this effect and avoid misinterpretation of ocean heat content changes.

Nature Geoscience, 2014

Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures have shown slower warming since 1998 than previously 1-5 . Possible explanations for the slow-down include internal climate variability 3,4,6,7 , external cooling influences 1,2,4,8-11 and observational errors 12,13 . Several recent modelling studies have examined the contribution of early twenty-first-century volcanic eruptions 1,2,4,8 to the muted surface warming. Here we present a detailed analysis of the impact of recent volcanic forcing on tropospheric temperature, based on observations as well as climate model simulations. We identify statistically significant correlations between observations of stratospheric aerosol optical depth and satellite-based estimates of both tropospheric temperature and short-wave fluxes at the top of the atmosphere. We show that climate model simulations without the e ects of early twenty-first-century volcanic eruptions overestimate the tropospheric warming observed since 1998. In two simulations with more realistic volcanic influences following the 1991 Pinatubo eruption, di erences between simulated and observed tropospheric temperature trends over the period 1998 to 2012 are up to 15% smaller, with large uncertainties in the magnitude of the e ect. To reduce these uncertainties, better observations of eruption-specific properties of volcanic aerosols are needed, as well as improved representation of these eruption-specific properties in climate model simulations.