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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年08月 [ 査読有り ]

  掲載種別: 研究論文（学術雑誌）

  　概要を見る

  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年03月 [ 査読有り ]

  掲載種別: 研究論文（学術雑誌）

  　概要を見る

  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年11月 [ 査読有り ]

  掲載種別: 研究論文（学術雑誌）

  　概要を見る

  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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• Millennial-Scale Climate Oscillations Triggered by Deglacial Meltwater Discharge in Last Glacial Maximum Simulations

  Yvan M. Romé, Ruza F. Ivanovic, Lauren J. Gregoire, Sam Sherriff-Tadano, Paul J. Valdes

  Paleoceanography and Paleoclimatology   37 ( 10 )   2022年10月 [ 査読有り ]

  掲載種別: 研究論文（学術雑誌）

  　概要を見る

  Our limited understanding of millennial-scale variability in the context of the last glacial period can be explained by the lack of a reliable modeling framework to study abrupt climate changes under realistic glacial backgrounds. In this article, we describe a new set of long-run Last Glacial Maximum experiments where such climate shifts were triggered by different snapshots of ice-sheet meltwater derived from the early stages of the last deglaciation. Depending on the location and the magnitude of the forcing, we observe three distinct dynamical regimes and highlight a subtle window of opportunity where the climate can sustain oscillations between cold and warm modes. We identify the Eurasian Arctic and Nordic Seas regions as being most sensitive to meltwater discharge in the context of switching to a cold mode, compared to freshwater fluxes from the Laurentide ice sheets. These cold climates follow a consistent pattern in temperature, sea ice, and convection, and are largely independent from freshwater release as a result of effective AMOC collapse. Warm modes, on the other hand, show more complexity in their response to the regional pattern of the meltwater input, and within them, we observe significant differences linked to the reorganization of deep water formation sites and the subpolar gyre. Broadly, the main characteristics of the oscillations, obtained under full-glacial conditions with ice-sheet reconstruction derived meltwater patterns, share similar characteristics with δ18O records of the last glacial period, although our experiment design prevents detailed conclusions from being drawn on whether these represent actual Dansgaard-Oeschger events.

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• Effect of Climatic Precession on Dansgaard-Oeschger-Like Oscillations

  Yuta Kuniyoshi, Ayako Abe-Ouchi, Sam Sherriff-Tadano, Wing Le Chan, Fuyuki Saito

  Geophysical Research Letters   49 ( 6 )   2022年03月 [ 査読有り ]

  掲載種別: 研究論文（学術雑誌）

  　概要を見る

  Using the climate model MIROC4m, we simulate self-sustained oscillations of millennial-scale periodicity in the climate and Atlantic meridional overturning circulation under glacial conditions. We show two cases of extreme climatic precession and examine the mechanism of these oscillations. When the climatic precession corresponds to strong (weak) boreal seasonality, the period of the oscillation is about 1,500 (3,000) years. During the stadial, hot (cool) summer conditions in the Northern Hemisphere contribute to thin (thick) sea ice, which covers the deep convection sites, triggering early (late) abrupt climate change. During the interstadial, as sea ice is thin (thick), cold deep-water forms and cools the subsurface quickly (slowly), which influences the stratification of the North Atlantic Ocean. We show that the oscillations are explained by the internal feedbacks of the atmosphere-sea ice-ocean system, especially subsurface ocean temperature change and salt advection feedback with a positive feedback between the subpolar gyre and deep convection.

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• 数千年周期の気候変動を駆動する南大洋海氷プロセスの定量的理解

  若手研究

  課題番号: 20K14552

  研究期間: 2020年04月  -  2023年03月 

  代表者: シェリフ多田野 サム 

  直接経費: 3,300,000（円）  間接経費: 4,290,000（円）  金額合計: 990,000（円）

• 数千年周期の気候変動を駆動する南大洋海氷プロセスの定量的理解

  若手研究

  課題番号: 20K14552

  研究期間: 2020年04月  -  2023年03月 

  代表者: シェリフ多田野 サム 

  直接経費: 3,300,000（円）  間接経費: 4,290,000（円）  金額合計: 990,000（円）