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Published Papers 【 display / non-display 】

• Competing effects of sea ice change control the pace and amplitude of millennial-scale climate oscillations

  Brooke Snoll, Ruza Ivanovic, Lauren J. Gregoire, Sam Sherriff-Tadano, Yvan Romé

  Critical Insights in Climate Change ( Informa UK Limited )  1 ( 1 )   2025.10 [ Peer Review Accepted ]

  Type of publication: Research paper (scientific journal)

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• Impact of the temperature-cloud phase relationship on the simulated Arctic warming during the last interglacial

  Nozomi Arima, Masakazu Yoshimori, Ayako Abe-Ouchi, Ryouta O'ishi, Wing-Le Chan, Sam Sherriff-Tadano, Tomoo Ogura

  ( Copernicus GmbH )    2025.09

  Type of publication: Research paper (other science council materials etc.)

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  Abstract. The Arctic during the last interglacial period (LIG) was considered warmer than today. While a recent proxy-based study suggests the disappearance of summer sea ice in the Arctic at the LIG, many climate models fail to capture this feature. It is thus essential to investigate sources of uncertainty in numerical models. The current study examines the impact of the temperature-cloud phase relationship. Sensitivity studies are conducted for the first time to explore the potential importance of this relationship in simulating the LIG climate. Two different cloud parameter sets are used for an atmosphere-ocean general circulation model with and without the dynamic vegetation feedback. The model with cloud parametrization permitting liquid water at a lower temperature and a larger fraction of supercooled liquid water at the same temperature simulates a warmer preindustrial (PI) climate, larger annual mean Arctic warming at the LIG, and substantially reduced sea ice cover during summer at the LIG. It is demonstrated that the low-level clouds play a crucial role in controlling the Arctic response via the greenhouse effect. The result indicates the importance of the temperature-cloud phase relationship in simulating the Arctic climate at the LIG. It also highlights the importance of accurately simulating modern sea ice thickness and representing the processes that affect the fraction of supercooled liquid water in clouds.

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• Climate and ocean circulation changes toward a modern snowball Earth

  Takashi Obase, Takanori Kodama, Takao Kawasaki, Sam Sherriff-Tadano, Daisuke Takasuka, Ayako Abe-Ouchi, Masakazu Fujii

  ( Copernicus GmbH )    2025.04

  Type of publication: Research paper (other science council materials etc.)

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  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.

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•  Southern Ocean processes maintain Ice Age millennial-scale climate variability

  Sam Sherriff-Tadano, Ayako Abe-Ouchi, Wing-Le Chan, Takahito Mitsui, Akira Oka, Takashi Obase, Yuta Kuniyoshi, Yvan Romé, Christo Buizert

  ( Copernicus GmbH )    2025.03

  Type of publication: Research paper (other science council materials etc.)

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  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.

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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 parameters

  Violet L. Patterson, Lauren J. Gregoire, Ruza F. Ivanovic, Niall Gandy, Stephen Cornford, Jonathan Owen, Sam Sherriff-Tadano, Robin S. Smith

  ( Copernicus GmbH )    2025.02

  Type of publication: Research paper (other science council materials etc.)

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  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.

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Grant-in-Aid for Scientific Research 【 display / non-display 】

Grant-in-Aid for Scientific Research 【 display / non-display 】

• Challenging research (sprout)

  Project Year: 2023.06  -  2025.03 

  Direct: 5,000,000 (YEN)  Overheads: 6,500,000 (YEN)  Total: 1,500,000 (YEN)