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Published Papers 【 display / non-display 】

• Large-ensemble simulations of the North American and Greenland ice sheets at the Last Glacial Maximum with a coupled atmospheric general circulation–ice sheet model

  Sam Sherriff-Tadano, Ruza Ivanovic, Lauren Gregoire, Charlotte Lang, Niall Gandy, Jonathan Gregory, Tamsin L. Edwards, Oliver Pollard, Robin S. Smith

  Climate of the Past ( Copernicus GmbH )  20 ( 7 ) 1489 - 1512   2024.07 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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  Abstract. The Last Glacial Maximum (LGM) was characterised by huge ice sheets covering the Northern Hemisphere, especially over North America, and by its cold climate. Previous authors have performed numerical simulations of the LGM to better understand coupled climate–ice sheet systems. However, the results of such simulations are sensitive to many model parameters. Here, we perform a 200-member ensemble of simulations of the North American and Greenland ice sheets and climate of the LGM with a coupled ice sheet–atmosphere–slab ocean model (FAMOUS-BISICLES) to explore sensitivities of the coupled climate–ice system to 16 uncertain parameters. In the ensemble of simulations, the global mean surface temperature is primarily controlled by the combination of parameters in the large-scale condensation scheme and the cumulus convection scheme. In simulations with plausible LGM global mean surface temperatures, we find that the albedo parameters have only a small impact on the Greenland ice volume due to the limited area of surface ablation associated with the cold climate. Instead, the basal sliding law controls the ice volume by affecting ice transport from the interior to the margin. On the other hand, like the Greenland ice sheet in future climate change, the LGM North American ice sheet volume is controlled by parameters in the snow and ice albedo scheme. Few of our simulations produce an extensive North American ice sheet when the global temperature is above 12 °C. Based on constraints on the LGM global mean surface temperature, the ice volume and the southern extent of the North American ice sheet, we select 16 acceptable simulations. These simulations lack the southern extent of ice compared to reconstructions, but they show reasonable performance on the ice sheet configuration and ice streams facing Baffin Bay and the Arctic Ocean. The strong sensitivities of the North American ice sheet to albedo at the LGM may imply a potential constraint on the future Greenland ice sheet by constraining the albedo schemes.

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• A multi-model assessment of the early last deglaciation (PMIP4 LDv1): a meltwater perspective

  Brooke Snoll, Ruza Ivanovic, Lauren Gregoire, Sam Sherriff-Tadano, Laurie Menviel, Takashi Obase, Ayako Abe-Ouchi, Nathaelle Bouttes, Chengfei He, Feng He, Marie Kapsch, Uwe Mikolajewicz, Juan Muglia, Paul Valdes

  Climate of the Past   20 ( 4 ) 789 - 815   2024.04 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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  The last deglaciation (~20-11 ka BP) is a period of a major, long-term climate transition from a glacial to interglacial state that features multiple centennial- to decadal-scale abrupt climate variations whose root cause is still not fully understood. To better understand this time period, the Paleoclimate Modelling Intercomparison Project (PMIP) has provided a framework for an internationally coordinated endeavour in simulating the last deglaciation whilst encompassing a broad range of models. Here, we present a multi-model intercomparison of 17 transient simulations of the early part of the last deglaciation (~20-15 ka BP) from nine different climate models spanning a range of model complexities and uncertain boundary conditions and forcings. The numerous simulations available provide the opportunity to better understand the chain of events and mechanisms of climate changes between 20 and 15 ka BP and our collective ability to simulate them. We conclude that the amount of freshwater forcing and whether it follows the ice sheet reconstruction or induces an inferred Atlantic meridional overturning circulation (AMOC) history, heavily impacts the deglacial climate evolution for each simulation rather than differences in the model physics. The course of the deglaciation is consistent between simulations except when the freshwater forcing is above 0.1 Sv - at least 70 % of the simulations agree that there is warming by 15 ka BP in most places excluding the location of meltwater input. For simulations with freshwater forcings that exceed 0.1 Sv from 18 ka BP, warming is delayed in the North Atlantic and surface air temperature correlations with AMOC strength are much higher. However, we find that the state of the AMOC coming out of the Last Glacial Maximum (LGM) also plays a key role in the AMOC sensitivity to model forcings. In addition, we show that the response of each model to the chosen meltwater scenario depends largely on the sensitivity of the model to the freshwater forcing and other aspects of the experimental design (e.g. CO2 forcing or ice sheet reconstruction). The results provide insight into the ability of our models to simulate the first part of the deglaciation and how choices between uncertain boundary conditions and forcings, with a focus on freshwater fluxes, can impact model outputs. We can use these findings as helpful insight in the design of future simulations of this time period.

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• De‐Tuning Albedo Parameters in a Coupled Climate Ice Sheet Model to Simulate the North American Ice Sheet at the Last Glacial Maximum

  N. Gandy, L. C. Astfalck, L. J. Gregoire, R. F. Ivanovic, V. L. Patterson, S. Sherriff‐Tadano, R. S. Smith, D. Williamson, R. Rigby

  Journal of Geophysical Research: Earth Surface ( American Geophysical Union (AGU) )  128 ( 8 )   2023.08 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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  Abstract The Last Glacial Maximum extent of the North American Ice Sheets is well constrained empirically but has proven to be challenging to simulate with coupled Climate‐Ice Sheet models. Coupled Climate‐Ice Sheet models are often too computationally expensive to sufficiently explore uncertainty in input parameters, and it is unlikely that values calibrated to reproduce modern ice sheets will reproduce the known extent of the ice at the Last Glacial Maximum. To address this, we run an ensemble with a coupled Climate‐Ice Sheet model (FAMOUS‐ice), simulating the final stages of growth of the last North American Ice Sheets' maximum extent. Using this large ensemble approach, we explore the influence of numerous uncertain ice sheet albedo, ice sheet dynamics, atmospheric, and oceanic parameters on the ice sheet extent. We find that ice sheet albedo parameters determine the majority of uncertainty when simulating the Last Glacial Maximum North American Ice Sheets. Importantly, different albedo parameters are needed to produce a good match to the Last Glacial Maximum North American Ice Sheets than have previously been used to model the contemporary Greenland Ice Sheet due to differences in cloud cover over ablation zones. Thus, calibrating coupled climate‐ice sheet models on one ice sheet may produce strong biases when the model is applied to a new domain.

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• Southern Ocean surface temperatures and cloud biases in climate models connected to the representation of glacial deep ocean circulation

  Sam Sherriff-Tadano, Ayako Abe-Ouchi, Masakazu Yoshimori, Rumi Ohgaito, Tristan Vadsaria, Wing-Le Chan, Haruka Hotta, Maki Kikuchi, Takanori Kodama, Akira Oka, Kentaroh Suzuki

  Journal of Climate ( American Meteorological Society )    1 - 38   2023.03 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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  Abstract Simulating and reproducing the past Atlantic Meridional Overturning Circulation (AMOC) with comprehensive climate models are essential to understanding past climate changes as well as to testing the ability of the models in simulating different climates. At the Last Glacial Maximum (LGM), reconstructions show a shoaling of the AMOC compared to modern climate. However, almost all state-of-the-art climate models simulate a deeper LGM AMOC. Here, it is shown that this paleodata-model discrepancy is partly related to the climate model biases in modern sea surface temperatures (SST) over the Southern Ocean (70°S – 45°S). Analysis of model outputs from three phases of the Paleoclimate Model Intercomparison Project shows that models with warm Southern Ocean SST biases tend to simulate a deepening of the LGM AMOC, while the opposite is observed in models with cold SST biases. As a result, a positive correlation of 0.41 is found between SST biases and LGM AMOC depth anomalies. Using sensitivity experiments with a climate model, we show, as an example, that changes in parameters associated with the fraction of cloud thermodynamic phase in a climate model reduce the biases in the warm SST over the modern Southern Ocean. The less biased versions of the model then reproduce a colder Southern Ocean at the LGM, which increases formation of Antarctic Bottom Water and causes shoaling of the LGM AMOC, without affecting the LGM climate in other regions. The results highlight the importance of sea surface conditions and clouds over the Southern Ocean in simulating past and future global climates.

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• Climate of High-obliquity Exoterrestrial Planets with a Three-dimensional Cloud System Resolving Climate Model

  Takanori Kodama, Daisuke Takasuka, Sam Sherriff-Tadano, Takeshi Kuroda, Tomoki Miyakawa, Ayako Abe-Ouchi, Masaki Satoh

  Astrophysical Journal   940 ( 1 )   2022.11 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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  Planetary climates are strongly affected by planetary orbital parameters such as obliquity, eccentricity, and precession. In exoplanetary systems, exoterrestrial planets should have various obliquities. High-obliquity planets would have extreme seasonal cycles due to the seasonal change of the distribution of the insolation. Here, we introduce the Non-hydrostatic ICosahedral Atmospheric Model (NICAM), a global cloud-resolving model, to investigate the climate of high-obliquity planets. This model can explicitly simulate a three-dimensional cloud distribution and vertical transports of water vapor. We simulated exoterrestrial climates with high resolution using the supercomputer FUGAKU. We assumed aqua-planet configurations with 1 bar of air as a background atmosphere, with four different obliquities (0°, 23.5°, 45°, and 60°). We ran two sets of simulations: (1) low resolution (∼220 km mesh as the standard resolution of a general circulation model for exoplanetary science) with parameterization for cloud formation, and (2) high resolution (∼14 km mesh) with an explicit cloud microphysics scheme. Results suggest that high-resolution simulations with an explicit treatment of cloud microphysics reveal warmer climates due to less low cloud fraction and a large amount of water vapor in the atmosphere. It implies that treatments of cloud-related processes lead to a difference between different resolutions in climatic regimes in cases with high obliquities.

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