University of the Ryukyus · Faculty of Science

Research keywordsSouthern Ocean processes・millennial climate variability・snowball Earth・sea ice feedbacks・ice sheet sensitivity・last glacial maximum・last interglacial warming・cloud phase feedbacks

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• Exploring the sensitivity of the Northern Hemisphere ice sheets at the last two glacial maxima to coupled climate-ice sheet model parameters2025 · Abstract. Simulations of past periods are useful for testing the ability of numerical models to simulate ice sheet changes under significantly different climate conditions to present day. This can help improve projections of future sea level rise made by these same models and avoid over-tuning to particular (e.g. modern) stationary climate conditions. The Last Glacial Maximum (LGM; ~21 thousand years ago (ka)) has been extensively used for this purpose since it is relatively well constrained by empirical evidence. However, less is known about the Penultimate Glacial Maximum (PGM; ~140 ka) and why the vast ice sheets covering much of the Northern Hemisphere (NH), differed to the LGM. The answer likely lies, at least in part, in the different orbital configurations between the two periods, and the resulting impact on climate-ice sheet interactions. Here, we perform and compare the first large ensembles of coupled climate-ice sheet (FAMOUS-BISICLES) simulations of the LGM and PGM to better understand how NH ice sheets interact with the climate and quantify how sensitive the simulations are to the choice of uncertain model inputs, including physical parameter values. Specifically, we vary 12 uncertain parameters that control the model representations of ice sheet albedo, ice dynamics and climate. The ensembles are evaluated against palaeo-evidence of global mean temperature, ice volume and extent to calibrate the model and find combinations of parameters that simultaneously yield plausible ice sheets and climates for both periods. The sensitivity of the North American ice sheet and the Eurasian ice sheet during the LGM and PGM, to each of the 12 parameter values, is explored using Gaussian Process emulators to perform a Sobol sensitivity analysis. From the whole ensemble, we find two simulations that meet our evaluation constraints for the LGM ice sheets. The parameter values that influence the albedo of the ice sheet have the largest influence on the resulting ice sheet volumes, but several other parameters display different sensitivity indices depending on the ice sheet (North American versus Eurasian) and time period (PGM versus LGM). This includes parameters that affect the cloud liquid water, lapse rate, basal sliding and downscaling elevation heights.
• Southern Ocean processes maintain Ice Age millennial-scale climate variability2025 · Millennial-scale climate variability during Pleistocene Ice Ages, known as Dansgaard-Oeschger (DO) cycles, are characterised by abrupt transitions between Greenland cold stadials and warm interstadials, which coincide with gradual warming and cooling over Antarctica, respectively, via the bipolar seesaw. DO cycles are associated with reorganisations of the Atlantic Meridional Overturning Circulation (AMOC), but the mechanisms driving them remain unclear. In this study, from nudging experiments based on intrinsic millennial-scale AMOC variability in a complex climate model, we show that gradual changes in sea ice over the Southern Ocean induced by the bipolar seesaw act as a negative feedback to maintain the millennial-scale AMOC variability. Southern Ocean surface cooling during the interstadial phases enhances regional sea ice-related salt and freshwater fluxes, which eventually weakens the AMOC by strengthening the oceanic stratification over the North Atlantic by increasing and decreasing the salinity of Antarctic bottom water and Antarctic intermediate water, respectively. The Southern Ocean feedback becomes particularly important for DO cycles with long periodicities, such as those occurring during Marine Isotope Stages 5, 4, 2 and those appearing after major Heinrich events. Our results suggest that the Southern Ocean feedback helps drive the DO cycles, demonstrating the globally connected nature of these events.
• Climate and ocean circulation changes toward a modern snowball Earth2025 · Abstract. In the past, Earth experienced snowball events, where its surface became completely covered with ice. Previous studies used general circulation models to investigate the onset and climate of such snowball events. Using the MIROC4m coupled atmosphere-ocean climate model, this study examined the changes in the oceanic circulation during the onset of a modern snowball Earth and elucidated their evolution to steady states under the snowball climate. Abruptly changing the solar constant to 94 % of its present-day value caused the modern Earth climate to turn into a snowball state after 1300 years and initiated rapid increase in sea ice thickness. During onset of the snowball event, extensive sea ice formation and melting of sea ice in the mid-latitudes caused substantial freshening of surface waters and salinity stratification. By contrast, such salinity stratification was absent if the duration necessary for snowball onset was short because of stronger solar constant forcing. After snowball onset, the global sea ice cover reduced air–sea fluxes and caused drastic weakening in the deep ocean circulation. However, as the ocean temperature and salinity fields approached near constant states, the meridional overturning circulation resumed in the steady-state snowball climate. Although the evolution of the oceanic circulation would depend on model setting, particularly regarding the treatment of air–sea fluxes and the continental distribution, our results highlight the importance of the oceanic circulation and associated biogeochemical changes in the climate system feedback and sequence of snowball events.
• Competing effects of sea ice change control the pace and amplitude of millennial-scale climate oscillations2025 · Brooke Snoll, Ruza Ivanovic, Lauren J. Gregoire, Sam Sherriff-Tadano, Yvan Romé

• どうすれば気候モデルは最終氷期最盛期の海洋深層循環を再現できるのか？2025 · 学術変革領域研究(A)
• 最先端の階層的気候モデリングに基づく全球凍結イベントの推移条件解明への挑戦2023 · 本研究では、全球が凍結するスノーボールイベントに着目し、全球凍結時の地形境界条件の設定、突入条件、脱出条件、継続期間に関する4つの小課題のもと、最新の階層的気候モデルをスノーボール気候に適用することで、総合的かつ多角的に、極限古環境の解明を目指す。2023年度は、古大陸分布を精査し、モデルに入力する中央海嶺と海溝の意義を考慮した上で、約6億年前のMarinoanスノーボール時の古地形を復元した。また、大気海洋結合モデルを用いて、現在海陸分布のもとで恒星放射フラックスを現在から91%までの複数通りの数千年長期積分を行い、全球凍結突入の条件を求めた。全球凍結突入後1000年以上にわたって気候計算を続けるための海洋モデルの設定を変更し、全球凍結突入後の海洋深層循環の経過を求めることが可能になった。全球雲解像モデルに関しては、初期段階として、 全球が氷に覆われた低二酸化炭素濃度状態を想定し、高解像度気候シミュレーションを実施した。大気海洋結合モデルと全球雲解像モデルにおけるスノーボール実験ができるように、現在モデルをアップデートしている。また、気候モデルとの結合を目標として氷床下での風化モデルを構築している。海洋生態系の変化については、低次栄養段階生態系モデルの開発の途上である。今後、大気海洋結合モデルでの水循環表現の改善、氷床底面融解による風化量の推定、スノーボール状態での雲形成/分布と全球気候への影響を定量評価、全球凍結中および脱出後における大陸風化率とそれに伴う大気中二酸化炭素の減少率の推定、低水温や低光量環境下での海洋生態系モデル実験を実施する。
• 氷床気候結合モデルを用いた氷期における氷床急発達の検証2022 · ■■■

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