About the Project
The historical climate record includes periods of strong cooling after major tropical eruptions, a low-latitude reservoir of volcanic aerosol slowly dispersing to mid-latitudes over several years. The forcings after 1991 Pinatubo, 1982 El Chichon and 1963 Agung are known to dominate decadal forcing trends, yet their uncertainty within historical climate integrations is not currently represented.
While increased solar scattering from the volcanic aerosol cloud is the main way major eruptions force climate, the multi-year perturbation to the stratosphere has a range of other important impacts:
• The volcanic aerosol also absorbs terrestrial radiation, offsetting some of the cooling from the additional solar scattering. The balance of these long-wave and short-wave radiative effects is closely associated with how large the aerosol particles in the volcanic cloud grow (i.e. their size distribution).
• Elevated aerosol surface area accelerates heterogeneous chemistry leading to stratospheric ozone loss via changes in NOy partitioning and chlorine activation (e.g. Hofmann and Solomon, 1989; Fahey et al., 1993).
• Major tropical eruptions warm the stratosphere, causing important circulation changes. The heating increases tropical upwelling, increasing stratospheric water vapour but reducing tropical stratosphere ozone, the vertical redistribution also changing export to mid-latitudes.
• An increase in diffuse radiation reaching the surface can occur after an eruption. This effect was observed after Pinatubo (e.g. Blumenthaler and Ambach, 1994), and likely contributed to the observed pause in CO2 growth rate at that time (Gu et al. 2003), opposing the effects from surface cooling.
This project will investigate the impacts of tropical eruptions on stratospheric composition and the magnitude of the associated radiative effects. The research will involve global composition-climate model experiments to quantify the different volcanic aerosol-chemistry and aerosol-radiation interactions described above, and thereby provide an integrated assessment of the effects of volcanic eruptions.
The potential research strands within the PhD studentship are:
1. Volcanic aerosol-chemistry interactions assessing impacts on stratospheric NOy species and subsequent effects on ozone in mid- and high-latitudes.
2. Exploring how the stratospheric warming within major tropical volcanic plumes influences ozone changes via composition-dynamics interactions.
3. Investigating the impacts of major eruptions on the terrestrial carbon cycle through changes in surface diffuse radiation, temperature and precipitation.
4. Predicting the effects from a hypothetical future major eruption in a low chlorine stratosphere, and contrasting with the 1963 Agung eruption
The GLOMAP aerosol scheme (Mann et al., 2010) is a core component of the joint NERC-Met Office Earth System Model UKESM, and the Leeds team have further extended UKESM capability for volcanic impacts, the evolution of dispersion of major eruption clouds now simulated interactively (see Dhomse et al., 2020).
The project will combine UKESM simulations with offline analysis with the SOCRATES radiative transfer and JULES land surface models to decompose the overall forcing into each individual indirect volcanic forcing and its direct forcing.
• Blumenthaler and Ambach (1994) Changes in solar radiation fluxes after the Pinatubo eruption, Tellus, 46B, 76-78.
• Dhomse et al. (2014) Aerosol microphysics simulations of the Mt. Pinatubo eruption with the UM-UKCA composition-climate model, Atmos. Chem. Phys., 14, 11221–11246.
• Dhomse et al. (2020) Evaluating the simulated radiative forcings, aerosol properties and stratospheric warmings from the 1963 Agung, 1982 El Chichón and 1991 Pinatubo volcanic aerosol clouds, Atmos. Chem. Phys., accepted, https://doi.org/10.5194/acp-2020-344.
• Fahey et al. (1993) In-situ measurements constraining the role of sulphate aerosols in mid-latitude ozone depletion, Nature, 363, 509-514.
• Gu et al., (2003) Response of a Deciduous Forest to the Mount Pinatubo Eruption: Enhanced Photosynthesis, Science, 299, 2035-2038.
• Hofmann and Solomon (1989) Ozone destruction through heterogeneous chemistry following the eruption of El Chichon, J. Geophys. Res., vol. 94, no. D4, pp. 5029-5041.
• Mann et al. (2010) Description and evaluation of GLOMAP-mode: a modal global aerosol microphysics model for the UKCA composition-climate model, Geosci. Mod. Dev., 3, 519–551, 2010.
• Myhre et al. (2013) Anthropogenic and natural radiative forcing: in Climate Change 2013: the Physical Science Basis: Contribution of Working Group 1to the 5th assessment report of the Intergovernmental Panel on Climate Change.
• Timmreck (2012) Modeling the climatic effects of large explosive volcanic eruptions, WIREs Clim Change 2012, 3:545–564.
• Vernier et al. (2011) Major influence of tropical volcanic eruptions on the stratospheric aerosol layer during the last decade, Geophys. Res. Lett., 38, L12807.
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