What causes the spread of model projections of ocean dynamic sea-level change in response to greenhouse gas forcing?
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What causes the spread of model projections of ocean dynamic sea‑level change in response to greenhouse gas forcing? Matthew P. Couldrey1 · Jonathan M. Gregory1,2 · Fabio Boeira Dias3,4,5,6 · Peter Dobrohotoff4,5 · Catia M. Domingues4,6,7 · Oluwayemi Garuba8 · Stephen M. Griffies9,10 · Helmuth Haak11 · Aixue Hu12 · Masayoshi Ishii13 · Johann Jungclaus11 · Armin Köhl14 · Simon J. Marsland4,5,6 · Sayantani Ojha15 · Oleg A. Saenko16 · Abhishek Savita4,5,6 · Andrew Shao16 · Detlef Stammer14 · Tatsuo Suzuki17 · Alexander Todd18 · Laure Zanna18,19 Received: 26 December 2019 / Accepted: 1 October 2020 © The Author(s) 2020
Abstract Sea levels of different atmosphere–ocean general circulation models (AOGCMs) respond to climate change forcing in different ways, representing a crucial uncertainty in climate change research. We isolate the role of the ocean dynamics in setting the spatial pattern of dynamic sea-level (ζ) change by forcing several AOGCMs with prescribed identical heat, momentum (wind) and freshwater flux perturbations. This method produces a ζ projection spread comparable in magnitude to the spread that results from greenhouse gas forcing, indicating that the differences in ocean model formulation are the cause, rather than diversity in surface flux change. The heat flux change drives most of the global pattern of ζ change, while the momentum and water flux changes cause locally confined features. North Atlantic heat uptake causes large temperature and salinity driven density changes, altering local ocean transport and ζ. The spread between AOGCMs here is caused largely by differences in their regional transport adjustment, which redistributes heat that was already in the ocean prior to perturbation. The geographic details of the ζ change in the North Atlantic are diverse across models, but the underlying dynamic change is similar. In contrast, the heat absorbed by the Southern Ocean does not strongly alter the vertically coherent circulation. The Arctic ζ change is dissimilar across models, owing to differences in passive heat uptake and circulation change. Only the Arctic is strongly affected by nonlinear interactions between the three air-sea flux changes, and these are model specific. Keywords Sea-level rise · Ocean heat uptake · Climate change · Climate modeling
* Matthew P. Couldrey [email protected] 1
2
National Centre for Atmospheric Science, University of Reading, Reading, UK
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Princeton University Program in Atmospheric and Oceanic Sciences, Princeton, USA
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Max Planck Institute for Meteorology, Hamburg, Germany
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National Center for Atmospheric Research, Boulder, USA Meteorological Research Institute, Tsukuba, Japan
Met Office Hadley Centre, Exeter, UK
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Institute for Atmospheric and Earth System Research, University of Helsinki, Helsinki, Finland
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Center für Erdsystemforschung und Nachhaltigkeit, Universität Hamburg, Hamburg, Germany
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Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australi
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