Modelling the atmospheric influence of coral reef-derived dimethyl sulfide
AuthorFiddes, Sonya Louise
AffiliationSchool of Earth Sciences
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2022-08-04.
© 2020 Sonya Louise Fiddes
Dimethyl sulfide (DMS) is a naturally occurring aerosol precursor gas which plays an important role in the global sulfur budget, aerosol formation and climate. While DMS is produced predominantly by phytoplankton, recent observational literature has suggested that corals and their symbionts produce a significant amount of DMS, which is currently unaccounted for in modelling studies. It has further been hypothesised that the coral reef source of DMS may modulate the climate. In this thesis, two atmospheric models coupled to online chemistry and aerosol schemes were used for the first time to explore the influence of coral reef-derived DMS on atmospheric composition and meteorology across temporal and spatial scales. A simple non-varying representation of coral reef-derived DMS was developed and added to a common DMS surface water climatology. By comparing the differences between simulations with and without coral reef-derived DMS, the role of coral reef-derived DMS was quantified. The Australian Community Climate Earth System Simulator coupled to the United Kingdom Chemistry and Aerosol model (ACCESS-UKCA) was used to quantify the influence of coral reefs at the global scale. ACCESS-UKCA was evaluated against satellite observations and other global climate models and the sensitivity of aerosol, clouds and radiation to large scale perturbations of DMS was tested. ACCESS-UKCA was found to have similar biases and DMS sensitivity compared to other models and it was estimated that marine DMS contributes 0.45K cooling to the present climate. The influence of coral reef-derived DMS on global to regional scale climate was then investigated. In the Maritime Continent-Australian region, where the highest density of coral reefs exist, a small decrease in nucleation and Aitken mode aerosol was found when coral reefs were removed from the system. However, these small responses were found to have no robust effect on global or regional climate. The Weather Research Forecast model coupled to the CBMZ-MOSAIC (Carbon Bond Mechanism Z - Model for Simulating Aerosol Interactions and Chemistry) chemistry-aerosol scheme (WRF-Chem) was then used to study the same question at higher spatial and temporal scales. WRF-Chem was run to coincide with an October 2016 field campaign over the Great Barrier Reef, Australia, against which the model was evaluated. After halving the DMS surface water climatology, the model performed well for DMS and sulfur processes, though aerosol number was overestimated. The inclusion of coral reef-derived DMS resulted in no compositional change in sulfate aerosol mass or total aerosol number. No direct or indirect aerosol effects were detected. Throughout this work, the complexities of the aerosol-climate system have been emphasised and the limitations of current modelling capabilities highlighted. In conclusion, while total marine DMS was found to have an important climatic influence, this thesis has found no robust link between coral reef-derived DMS and climate or weather. Thus, these results do not support hypotheses around the ability of coral reefs to modulate global or regional climate.
Keywordsaerosol; dimethyl sulfide; DMS; coral reefs; climate; climate change; Great Barrier Reef; atmospheric composition; modelling
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