Researcher Database

Researcher Profile and Settings

Master

Affiliation (Master)

  • Institute of Low Temperature Science Water and Material Cycles

Affiliation (Master)

  • Institute of Low Temperature Science Water and Material Cycles

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Profile and Settings

Degree

  • PhD(University of Bremen)

Profile and Settings

  • Profile

    My research interests are Ocean circulation and dynamics in high latitudes and Ice-Ocean Interactions. We welcome motivated students. 

  • Contact Point

    Yoshihiro.Nakayamalowtem.hokudai.ac.jp
  • Name (Japanese)

    Nakayama
  • Name (Kana)

    Yoshihiro
  • Name

    201901007614414758

Achievement

Research Interests

  • Polar Oceanography, Ice shelf-ocean interaction, sea level variation, global warming, climate change   

Research Areas

  • Natural sciences / Atmospheric and hydrospheric science / Polar Oceanography

Research Experience

  • 2018/11 - Today Hokkaido University Institute of Low Temperature Science Assistant Professor
  • 2016/03 - 2018/10 NASA Jet Propulsion Laboratory NASA Postdoctoral Fellow
  • 2015/10 - 2016/02 University of California Irvine Postdoctoral Scholar
  • 2015/07 - 2015/09 NASA Jet Propulsion Laboratory Research Visitor
  • 2014/12 - 2015/02 Alfred Wegener Institute Postdoctoral Scholar

Education

  • 2011/06 - 2015/02  University of Bremen  Alfred Wegener Institute
  • 2009/04 - 2011/03  Hokkaido University  Faculty of Environmental Earth Science
  • 2005/04 - 2009/03  Nagoya University  School of Engineering  Mechanical and Aerospace Engineering

Awards

  • 2023/01 Inoue Research Foundation Inoue Research Award
  • 2020/04 日本海洋学会 岡田賞
     南大洋における海洋と棚氷の相互作用に関する研究
  • 2016/03 NASA NASA Postdoctoral Fellowship

Published Papers

  • Taewook Park, Yoshihiro Nakayama, SungHyun Nam
    Nature Communications 15 (1) 2024/04/11 
    Abstract The Pine Island and Thwaites Ice Shelves (PIIS/TIS) in the Amundsen Sea are melting rapidly and impacting global sea levels. The thermocline depth (TD) variability, the interface between cold Winter Water and warm modified Circumpolar Deep Water (mCDW), at the PIIS/TIS front strongly correlates with basal melt rates, but the drivers of its interannual variability remain uncertain. Here, using an ocean model, we propose that the strength of the eastern Amundsen Sea on-shelf circulation primarily controls TD variability and consequent PIIS/TIS melt rates. The TD variability occurs because the on-shelf circulation meanders following the submarine glacial trough, creating vertical velocity through bottom Ekman dynamics. We suggest that a strong or weak ocean circulation, possibly linked to remote winds in the Bellingshausen Sea, generates corresponding changes in bottom Ekman convergence, which modulates mCDW upwelling and TD variability. We show that interannual variability of off-shelf zonal winds has a minor effect on ocean heat intrusion into PIIS/TIS cavities, contrary to the widely accepted concept.
  • Yoshihiro Nakayama, Pat Wongpan, Jamin S. Greenbaum, Kaihe Yamazaki, Shigeru Aoki
    Frontiers for Young Minds 12 2024/03/27 
    In East Antarctica, warm ocean water travels toward the Totten Ice Shelf. This water melts and thins the ice shelf, and speeds up the rate at which ice moves into the sea, leading to sea-level rise. Scientists often get on board ships called icebreakers to study the ice and water in these regions. However, sea ice and icebergs are major obstacles to navigation and scientific operations. For example, American, Australian, and Japanese icebreakers tried but could only observe a small area where sea ice was more broken up. So, we used a helicopter to measure the ocean during one of our research expeditions. Helicopters can travel faster than icebreakers. They can fly over sea ice and icebergs, and trained workers can drop sensors into small gaps in the ice. In 6 days, we observed ocean temperatures at 67 sites, covering a large area that could not be studied before. We identified wide pathways of warm water flowing toward the Totten Ice Shelf.
  • Shuntaro Hyogo, Yoshihiro Nakayama, Vigan Mensah
    Journal of Geophysical Research: Oceans 129 (3) 2169-9275 2024/03/21 
    Abstract The ice shelves in the Bellingshausen Sea are melting and thinning rapidly due to modified Circumpolar Deep Water (mCDW) intrusions carrying heat toward ice‐shelf cavities. Observations are, however, sparse in time and space, and extensive model‐data comparisons have never been possible. Here, using a circulation model of the region and ship‐based observations, we show that the simulated water mass distributions in several troughs traversing mCDW inflows are in good agreement with observations, implying that our model has the skills to simulate hydrographic structures as well as on‐shelf ocean circulations. It takes 7.9 and 11.7 months for mCDW to travel to the George VI Ice Shelf cavities through the Belgica and Marguerite troughs, respectively. Ice‐shelf melting is mainly caused by mCDW intrusions along the Belgica and Marguerite troughs, with the heat transport through the former being ∼2.8 times larger than that through the latter. The mCDW intrusions toward the George VI Ice Shelf show little seasonal variability, while those toward the Venable Ice Shelf show seasonal variability, with higher velocities in summer likely caused by coastal trapped waves. We also conduct particle experiments tracking glacial meltwater. After 2 years of model integration, ∼33% of the released particles are located in the Amundsen Sea, supporting a linkage between Bellingshausen Sea ice‐shelf meltwater and Amundsen Sea upper ocean hydrography.
  • Kalyan Shrestha, Georgy E. Manucharyan, Yoshihiro Nakayama
    Geophysical Research Letters 51 (3) 0094-8276 2024/02/04 
    Abstract Melting of ice shelves can energize a wide range of ocean currents, from three‐dimensional turbulence to relatively large‐scale boundary currents. Here, we conduct high‐resolution simulations of the western Amundsen Sea to show that submesoscale eddies are prevalent inside ice shelf cavities. The simulations indicate energetic submesoscale eddies at the top and bottom ocean boundary layers, regions with sharp topographic slopes and strong lateral buoyancy gradients. These eddies play a substantial role in the vertical and lateral (along‐isopycnal) heat advection toward the ice shelf base, enhancing the basal melting in all simulated cavities. In turn, the meltwater provides strong buoyancy gradients that energize the submesoscale variability, forming a positive loop that could affect the overall efficiency of heat exchange between the ocean and the ice shelf cavity. Our study implies that submesoscale‐induced enhancement of basal melting may be a ubiquitous process that needs to be parameterized in coarse‐resolution climate models.
  • Alessandro Silvano, Sarah Purkey, Arnold L. Gordon, Pasquale Castagno, Andrew L. Stewart, Stephen R. Rintoul, Annie Foppert, Kathryn L. Gunn, Laura Herraiz-Borreguero, Shigeru Aoki, Yoshihiro Nakayama, Alberto C. Naveira Garabato, Carl Spingys, Camille Hayatte Akhoudas, Jean-Baptiste Sallée, Casimir de Lavergne, E. Povl Abrahamsen, Andrew J. S. Meijers, Michael P. Meredith, Shenjie Zhou, Takeshi Tamura, Kaihe Yamazaki, Kay I. Ohshima, Pierpaolo Falco, Giorgio Budillon, Tore Hattermann, Markus A. Janout, Pedro Llanillo, Melissa M. Bowen, Elin Darelius, Svein Østerhus, Keith W. Nicholls, Craig Stevens, Denise Fernandez, Laura Cimoli, Stanley S. Jacobs, Adele K. Morrison, Andrew McC. Hogg, F. Alexander Haumann, Ali Mashayek, Zhaomin Wang, Rodrigo Kerr, Guy D. Williams, Won Sang Lee
    Frontiers in Marine Science 10 2023/12/08 
    Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system.
  • M. Poinelli, Y. Nakayama, E. Larour, M. Vizcaino, R. Riva
    Geophysical Research Letters 50 (18) 0094-8276 2023/09/21 
    Abstract Iceberg A‐68 separated from the Larsen C Ice Shelf in July 2017 and the impact of this event on the local ocean circulation has yet to be assessed. Here, we conduct numerical simulations of ocean dynamics near and below the ice shelf pre‐ and post‐calving. Results agree with in situ and remote observations of the area as they indicate that basal melt is primarily controlled by wintertime sea‐ice formation, which in turn produces High Salinity Shelf Water (HSSW). After the calving event, we simulate a 50% increase in HSSW intrusion under the ice shelf, enhancing ocean heat delivery by 30%. This results in doubling of the melt rate under Gipps Ice Rise, suggesting a positive feedback for further retreat that could destabilize the Larsen C Ice Shelf. Assessing the impact of ice‐front retreat on the heat delivery under the ice is crucial to better understand ice‐shelf dynamics in a warming environment.
  • Yoshihiro Nakayama, Pat Wongpan, Jamin S. Greenbaum, Kaihe Yamazaki, Tomohide Noguchi, Daisuke Simizu, Haruhiko Kashiwase, Donald D. Blankenship, Takeshi Tamura, Shigeru Aoki
    Geophysical Research Letters 50 (17) 0094-8276 2023/09/11 
    Abstract The recent discovery of warm ocean water near the Totten Ice Shelf (TIS) has increased attention to the Sabrina Coast in East Antarctica. We report the result of 6‐day helicopter‐based observations conducted during the 61st Japanese Antarctic Research Expedition (JARE61), revealing warm ocean water (0.5–1°C) occupying a large previously unsampled area of the Sabrina Coast (116.5°E−120°E) below 550–600 m. Along the TIS front, we observe modified Circumpolar Deep Water (mCDW) well above freezing (∼−0.7°C), consistent with previous work. We identify glacial meltwater outflow from the TIS cavity west of 116°E. No signs of mCDW intrusions toward the Moscow University Ice Shelf cavity are observed; however, those observations were limited to only two shallow (∼330 m) profiles. We also highlight the advantages of helicopter‐based observations for accessibility, speed, maneuverability, and cost‐efficiency. The combination of ship‐ and helicopter‐based observations using the JARE61 approach will increase the potential of future polar oceanographic observations.
  • Tetsuya P. Tamura, Daiki Nomura, Daisuke Hirano, Takeshi Tamura, Masaaki Kiuchi, Gen Hashida, Ryosuke Makabe, Kazuya Ono, Shuki Ushio, Kaihe Yamazaki, Yoshihiro Nakayama, Keigo D. Takahashi, Hiroko Sasaki, Hiroto Murase, Shigeru Aoki
    Global Biogeochemical Cycles 37 (9) 0886-6236 2023/09/02 
    Abstract To clarify the impacts of basal melting of the Antarctic ice sheet and biological productivity on biogeochemical processes in Antarctic coastal waters, concentrations of dissolved inorganic carbon (DIC), total alkalinity (TA), inorganic nutrients, chlorophyll a, and stable oxygen isotopic ratios (δ18O) were measured from the offshore slope to the ice front of the Totten Ice Shelf (TIS) during the spring/summer of 2018, 2019, and 2020. Modified Circumpolar Deep Water (mCDW) intruded onto the continental shelf off the TIS and flowed along bathymetric troughs into the TIS cavity, where it formed a buoyant mixture with glacial meltwater from the ice shelf base. Physical oceanographic processes mostly determined the distributions of DIC, TA, and nutrient concentrations. However, photosynthesis and dilution by meltwater from sea ice and the ice shelf base decreased DIC, TA, and nutrient concentrations in surface water near the ice front. These causes also reduced the partial pressure of CO2 (pCO2) in surface water by more than 100 μatm with respect to mCDW in austral summer of 2018 and 2020, and the surface water became a strong CO2 sink for the atmosphere. Phytoplankton photosynthesis changed DIC and TA in a molar ratio of 106:16. Thus, pCO2 decreased mostly as a result of photosynthesis while dilution by glacial and sea ice meltwater had a small effect. The nutrient consumption ratio suggested that photosynthesis was stimulated by iron in the water column, supplied to the surface layer via buoyancy‐driven upwelling and basal ice shelf meltwater in addition to sea ice meltwater.
  • Daisuke Hirano, Takeshi Tamura, Kazuya Kusahara, Masakazu Fujii, Kaihe Yamazaki, Yoshihiro Nakayama, Kazuya Ono, Takuya Itaki, Yuichi Aoyama, Daisuke Simizu, Kohei Mizobata, Kay I. Ohshima, Yoshifumi Nogi, Stephen R. Rintoul, Esmee van Wijk, Jamin S. Greenbaum, Donald D. Blankenship, Koji Saito, Shigeru Aoki
    Nature Communications 14 (1) 2023/08/17 
    Abstract The Totten Glacier in East Antarctica, with an ice volume equivalent to >3.5 m of global sea-level rise, is grounded below sea level and, therefore, vulnerable to ocean forcing. Here, we use bathymetric and oceanographic observations from previously unsampled parts of the Totten continental shelf to reveal on-shelf warm water pathways defined by deep topographic features. Access of warm water to the Totten Ice Shelf (TIS) cavity is facilitated by a deep shelf break, a broad and deep depression on the shelf, a cyclonic circulation that carries warm water to the inner shelf, and deep troughs that provide direct access to the TIS cavity. The temperature of the warmest water reaching the TIS cavity varies by ~0.8 °C on an interannual timescale. Numerical simulations constrained by the updated bathymetry demonstrate that the deep troughs play a critical role in regulating ocean heat transport to the TIS cavity and the subsequent basal melt of the ice shelf.
  • H. C. F. C. Hay, I. Fenty, R. T. Pappalardo, Y. Nakayama
    Journal of Geophysical Research: Planets 128 (3) 2169-9097 2023/03/08 
    Abstract Europa's geologically scarred surface shows significant evidence that the ice shell may have rotated nonsynchronously in the past. The long‐term spin state of the ice shell is controlled by the time‐mean torques acting upon it. A torque that has not been previously considered is exerted due to drag from oceanic currents beneath the ice. We estimate this torque for the first time by performing global, nonhydrostatic, three‐dimensional simulations of Europa's ocean, including nonlinear turbulent boundary layer drag at the seafloor and ice‐ocean interface. Our simulations show that ocean dynamics, which manifest in alternating east‐west jets, result in a net torque on the ice shell. The torque can act to either spin up or spin down the ice shell depending on the strength of convection, suggesting that a torque reversal can occur as Europa's interior thermally evolves. Scaling analysis indicates that an average jet speed of at least ∼1 cm s−1 is required for the ice‐ocean torque to be comparable to the tidal torque acting to spin up the ice shell. Our results suggest that ocean currents may contribute to any nonsynchronous rotation of the ice shell. Consequently, Europa's present‐day spin state may hold information about the dynamics of its subsurface ocean.
  • Y. Lin, Q. Yang, Q. Shi, Y. Nakayama, D. Chen
    Geophysical Research Letters 50 (4) 0094-8276 2023/02/15 
    Abstract Accurate estimation of sea‐ice production (SIP) is crucial to understanding the formation of Antarctic Bottom Water (AABW). Existing SIP estimates depend on the heat‐budget method, using atmospheric reanalysis together with satellite‐derived sea‐ice thickness data, and make an unrealistic assumption of no ocean heat flux occurring below the ice. Here, we propose a new method to estimate SIP based on the sea‐ice volume‐conservation (VC) theory. This new method can consider cases with sea‐ice melting (negative SIP). For a latent‐heat polynya—the Ross Sea Polynya—this method captures the synoptic SIP variations affected by warm‐water intrusions, including small‐scale melting and freezing patterns. Using the new VC approach for a sensible‐heat polynya, the Maud Rise Polynya, the sub‐sea‐ice oceanic heat flux was indirectly estimated, and the sea‐ice melting rate induced by warm water uplifted by Ekman pumping was estimated to be 0.15 and 1.44 cm d−1 for 2016 and 2017, respectively.
  • Tiago S. Dotto, Karen J. Heywood, Rob A. Hall, Ted A. Scambos, Yixi Zheng, Yoshihiro Nakayama, Shuntaro Hyogo, Tasha Snow, Anna K. Wåhlin, Christian Wild, Martin Truffer, Atsuhiro Muto, Karen E. Alley, Lars Boehme, Guilherme A. Bortolotto, Scott W. Tyler, Erin Pettit
    Nature Communications 13 (1) 2022/12/21 
    Abstract West Antarctic ice-shelf thinning is primarily caused by ocean-driven basal melting. Here we assess ocean variability below Thwaites Eastern Ice Shelf (TEIS) and reveal the importance of local ocean circulation and sea-ice. Measurements obtained from two sub-ice-shelf moorings, spanning January 2020 to March 2021, show warming of the ice-shelf cavity and an increase in meltwater fraction of the upper sub-ice layer. Combined with ocean modelling results, our observations suggest that meltwater from Pine Island Ice Shelf feeds into the TEIS cavity, adding to horizontal heat transport there. We propose that a weakening of the Pine Island Bay gyre caused by prolonged sea-ice cover from April 2020 to March 2021 allowed meltwater-enriched waters to enter the TEIS cavity, which increased the temperature of the upper layer. Our study highlights the sensitivity of ocean circulation beneath ice shelves to local atmosphere-sea-ice-ocean forcing in neighbouring open oceans.
  • Yoshihiro Nakayama, Toshiki Hirata, Daniel Goldberg, Chad A. Greene
    Geophysical Research Letters 49 (22) 0094-8276 2022/11/16 
    Abstract Ice shelf shape directly controls ocean heat intrusions, melting near the grounding line, and buttressing. Little is known about what determines ice‐shelf shape because ice‐ocean coupled simulations typically aim at projecting Antarctica's contribution to sea‐level rise and they do not resolve small‐scale ice‐ocean interactive processes. We conduct ice‐ocean coupled simulations for an idealized high‐resolution, Pine‐Island‐like model configuration. We show that ocean melting and ice stretching caused by acceleration thin the ice shelf from the grounding line toward the ice shelf front, consistent with previous studies. In the across‐flow direction, ocean melting and ice advection cancel each other out and flatten the ice shelf. More than one‐third of the ice thinning from grounding line to ice front can be attributed to ocean melting at depths shallower than 500 m. Our results emphasize the importance of interactive processes between the entire ice shelf and the ocean for determining the ice shelf shape.
  • Vår Dundas, Elin Darelius, Kjersti Daae, Nadine Steiger, Yoshihiro Nakayama, Tae-Wan Kim
    Ocean Science 18 (5) 1339 - 1359 2022/09/14 
    Abstract. Ice shelves in the Amundsen Sea are thinning rapidly as ocean currents bring warm water into the cavities beneath the floating ice. Although the reported melt rates for the Getz Ice Shelf are comparatively low for the region, its size makes it one of the largest freshwater sources around Antarctica, with potential consequences for, bottom water formation downstream, for example. Here, we use a 2-year-long novel mooring record (2016–2018) and 16-year-long regional model simulations to describe, for the first time, the hydrography and circulation in the vicinity of the ice front between Siple and Carney Island. We find that, throughout the mooring record, temperatures in the trough remain below 0.15 ∘C, more than 1 ∘C lower than in the neighboring Siple and Dotson Trough, and we observe a mean current (0.03 m s−1) directed toward the ice shelf front. The variability in the heat transport toward the ice shelf appears to be governed by nonlocal ocean surface stress over the Amundsen Sea Polynya region, and northward to the continental shelf break, where strengthened westward ocean surface stress leads to increased southward flow at the mooring site. The model simulations suggest that the heat content in the trough during the observed period was lower than normal, possibly owing to anomalously low summertime sea ice concentration and weak winds.
  • Yoshihiro Nakayama, Cilan Cai, Helene Seroussi
    Geophysical Research Letters 48 (18) 0094-8276 2021/09/20
  • Tyler Pelle, Mathieu Morlighem, Yoshihiro Nakayama, Hélène Seroussi
    Geophysical Research Letters 48 (17) 0094-8276 2021/09/08
  • Yoshihiro Nakayama, Chad A. Greene, Fernando S. Paolo, Vigan Mensah, Hong Zhang, Haruhiko Kashiwase, Daisuke Simizu, Jamin S. Greenbaum, Donald D. Blankenship, Ayako Abe‐Ouchi, Shigeru Aoki
    Geophysical Research Letters 48 (17) 0094-8276 2021/09/08 
    The Totten ice shelf (TIS) in East Antarctica has received increasing attention in recent years due to high basal melt rates, which have been linked to a presence of warm modified Circumpolar Deep Water (mCDW) observed at the ice front. We show that mCDW on-shelf intrusions towards the TIS strengthen when the Antarctic Slope Current (ASC) weakens. This demonstrates that the ASC has a blocking effect and ASC weakening leads to on-shelf intrusions, as proposed by previous observational studies. The interannual variability of the ASC is controlled primarily by atmospheric and oceanic conditions beyond our regional model domain. We further show that heat intrusions onto the continental shelf off the TIS are not influenced by off-shelf warming but are enhanced with coastal freshening, suggesting positive feedback whereby ice melt and freshening upstream could start a chain reaction, leading to increased melt, and further coastal freshening.
  • Vigan Mensah, Yoshihiro Nakayama, Masakazu Fujii, Yoshifumi Nogi, Kay I. Ohshima
    Ocean Modelling 165 101843 - 101843 1463-5003 2021/09 
    The formation of Dense Shelf Water (DSW) and Antarctic Bottom Water (AABW) in the Southern Ocean is an essential part of the thermohaline circulation, and understanding this phenomenon is crucial for studying the global climate. AABW is formed as DSW flows down the continental slope and mixes with the surrounding waters. However, DSW formation and its descent remains a poorly resolved issue in many ocean models. We, therefore, simulated the formation and descent of DSW and investigated the model sensitivities to horizontal and vertical grid spacings. The Massachusetts Institute of Technology general circulation model (MITgcm) was used for the region off Cape Darnley in East Antarctica, one of the main AABW production areas, where historical and mooring data are available for comparison. Simulations with coarse horizontal grid resolutions of order (10 km) yielded high volumes of DSW on the shelf. However, the largest part of this DSW was transformed into intermediate water and advected westward. Horizontal model resolutions equal to or higher than 2 km were required to simulate the descent of DSW and a realistic AABW production. Simulated time series at a mooring located at a depth of 2,600 m showed periodic fluctuations in velocity and temperature of 0.3 m.s(-1) and 0.5 degrees C, respectively, consistent with observations. We also found that high-resolution bathymetry datasets are crucial because the newly formed AABW volume was reduced by 20% when a smoother bathymetry was used on a 2-km resolution grid. Vertical resolution had little influence on model performance because the plume was much thicker (> 170 m) than the grids width. Therefore, reproducing the downslope flow of DSW and AABW formation in the Cape Darnley region can be achieved with a high horizontal resolution (<= 2 km) and a relatively coarse vertical resolution (similar to 100 m on the continental slope).
  • Yoshihiro Nakayama, Dimitris Menemenlis, Ou Wang, Hong Zhang, Ian Fenty, An T. Nguyen
    Geoscientific Model Development 14 (8) 4909 - 4924 2021/08/06 
    Abstract. The Antarctic coastal ocean impacts sea level rise, deep-ocean circulation, marine ecosystems, and the global carbon cycle. To better describe and understand these processes and their variability, it is necessary to combine the sparse available observations with the best-possible numerical descriptions of ocean circulation. In particular, high ice shelf melting rates in the Amundsen Sea have attracted many observational campaigns, and we now have some limited oceanographic data that capture seasonal and interannual variability during the past decade. One method to combine observations with numerical models that can maximize the information extracted from the sparse observations is the adjoint method, a.k.a. 4D-Var (4-dimensional variational assimilation), as developed and implemented for global ocean state estimation by the Estimating the Circulation and Climate of the Ocean (ECCO) project. Here, for the first time, we apply the adjoint-model estimation method to a regional configuration of the Amundsen and Bellingshausen seas, Antarctica, including explicit representation of sub-ice-shelf cavities. We utilize observations available during 2010–2014, including ship-based and seal-tagged CTD measurements, moorings, and satellite sea-ice concentration estimates. After 20 iterations of the adjoint-method minimization algorithm, the cost function, here defined as a sum of the weighted model–data difference, is reduced by 65 % relative to the baseline simulation by adjusting initial conditions, atmospheric forcing, and vertical diffusivity. The sea-ice and ocean components of the cost function are reduced by 59 % and 70 %, respectively. Major improvements include better representations of (1) Winter Water (WW) characteristics and (2) intrusions of modified Circumpolar Deep Water (mCDW) towards the Pine Island Glacier. Sensitivity experiments show that ∼40 % and ∼10 % of improvements in sea ice and ocean state, respectively, can be attributed to the adjustment of air temperature and wind. This study is a preliminary demonstration of adjoint-method optimization with explicit representation of ice shelf cavity circulation. Despite the 65 % cost reduction, substantial model–data discrepancies remain, in particular with annual and interannual variability observed by moorings in front of the Pine Island Ice Shelf. We list a series of possible causes for these residuals, including limitations of the model, the optimization methodology, and observational sampling. In particular, we hypothesize that residuals could be further reduced if the model could more accurately represent sea-ice concentration and coastal polynyas.
  • Kaihe Yamazaki, Shigeru Aoki, Katsuro Katsumata, Daisuke Hirano, Yoshihiro Nakayama
    SCIENCE ADVANCES 7 (24) 2375-2548 2021/06 
    The southern boundary (SB) of the Antarctic Circumpolar Current, the southernmost extent of the upper overturning circulation, regulates the Antarctic thermal conditions. The SB's behavior remains unconstrained because it does not have a clear surface signature. Revisited hydrographic data from off East Antarctica indicate full-depth warming from 1996 to 2019, concurrent with an extensive poleward shift of the SB subsurface isotherms (>50 km), which is most prominent at 120 degrees E off the Sabrina Coast. The SB shift is attributable to enhanced upper overturning circulation and a depth-independent frontal shift, generally accounting for 30 and 70%, respectively. Thirty years of oceanographic data corroborate the overall and localized poleward shifts that are likely controlled by continental slope topography. Numerical experiments successfully reproduce this locality and demonstrate its sensitivity to mesoscale processes and wind forcing. The poleward SB shift under intensified westerlies potentially induces multidecadal warming of Antarctic shelf water.
  • Investigation of ice shelf ocean interaction in the Amundsen Sea using numerical modeling and ocean state estimates
    Yoshihiro Nakayama
    Oceanography in Japan 29 (6) 233 - 244 2020/11 [Refereed]
  • Yoshihiro Nakayama, Ralph Timmermann, Hartmut H. Hellmer
    The Cryosphere 14 (7) 2205 - 2216 2020/07/13 [Refereed]
     
    Abstract. Previous studies show accelerations of West Antarctic glaciers, implying that basal melt rates of these glaciers were previously small and increased in the middle of the 20th century. This enhanced melting is a likely source of the observed Ross Sea (RS) freshening, but its long-term impact on the Southern Ocean hydrography has not been well investigated. Here, we conduct coupled sea ice–ice shelf–ocean simulations with different levels of ice shelf melting from West Antarctic glaciers. Freshening of RS shelf and bottom water is simulated with enhanced West Antarctic ice shelf melting, while no significant changes in shelf water properties are simulated when West Antarctic ice shelf melting is small. We further show that the freshening caused by glacial meltwater from ice shelves in the Amundsen and Bellingshausen seas can propagate further downstream along the East Antarctic coast into the Weddell Sea. The freshening signal propagates onto the RS continental shelf within a year of model simulation, while it takes roughly 5–10 and 10–15 years to propagate into the region off Cape Darnley and into the Weddell Sea, respectively. This advection of freshening modulates the shelf water properties and possibly impacts the production of Antarctic Bottom Water if the enhanced melting of West Antarctic ice shelves continues for a longer period.
  • Yoshihiro Nakayama, Georgy Manucharyan, Hong Zhang, Pierre Dutrieux, Hector S. Torres, Patrice Klein, Helene Seroussi, Michael Schodlok, Eric Rignot, Dimitris Menemenlis
    SCIENTIFIC REPORTS 9 2045-2322 2019/11 [Refereed][Not invited]
     
    In the Amundsen Sea, modified Circumpolar Deep Water (mCDW) intrudes into ice shelf cavities, causing high ice shelf melting near the ice sheet grounding lines, accelerating ice flow, and controlling the pace of future Antarctic contributions to global sea level. The pathways of mCDW towards grounding lines are crucial as they directly control the heat reaching the ice. A realistic representation of mCDW circulation, however, remains challenging due to the sparsity of in-situ observations and the difficulty of ocean models to reproduce the available observations. In this study, we use an unprecedentedly high-resolution (200 m horizontal and 10 m vertical grid spacing) ocean model that resolves shelf-sea and sub-ice-shelf environments in qualitative agreement with existing observations during austral summer conditions. We demonstrate that the waters reaching the Pine Island and Thwaites grounding lines follow specific, topographically-constrained routes, all passing through a relatively small area located around 104 degrees W and 74.3 degrees S. The temporal and spatial variabilities of ice shelf melt rates are dominantly controlled by the sub-ice shelf ocean current. Our findings highlight the importance of accurate and high-resolution ocean bathymetry and subglacial topography for determining mCDW pathways and ice shelf melt rates.
  • Yoshihiro Nakayama, Dimitris Menemenlis, Hong Zhang, Michael Schodlok, Eric Rignot
    NATURE COMMUNICATIONS 9 2041-1723 2018/08 [Refereed][Not invited]
     
    Melting of West Antarctic ice shelves is enhanced by Circumpolar Deep Water (CDW) intruding onto the Amundsen and Bellingshausen Seas (ABS) continental shelves. Despite existing studies of cross-shelf and on-shelf CDW transports, CDW pathways onto the ABS originating from further offshore have never been investigated. Here, we investigate CDW pathways onto the ABS using a regional ocean model. Simulated CDW tracers from a zonal section across 67 degrees S (SO4P) circulate along the Antarctic Circumpolar Current (ACC) and Ross Gyre (RG) and travel into ABS continental shelf after 3-5 years, but source locations are shifted westward by similar to 900 km along S04P in 2001-2006 compared to 2009-2014. We find that simulated on- and off-shelf CDW is similar to 0.1-0.2 degrees C warmer in the 2009-2014 case than in the 2001-2006 case together with changes in simulated ocean circulation. These differences are primarily caused by lateral, rather than surface, boundary conditions, implying that large-scale atmospheric and ocean circulations are able to control CDW pathways and thus off- and on-shelf CDW properties.
  • Xylar S. Asay-Davis, Nicolas C. Jourdain, Yoshihiro Nakayama
    CURRENT CLIMATE CHANGE REPORTS 3 (4) 316 - 329 2198-6061 2017/12 [Refereed][Not invited]
     
    Recent advances in both ocean modeling and melt parameterization in ice-sheet models point the way toward coupled ice sheet-ocean modeling, which is needed to quantify Antarctic mass loss and the resulting sea-level rise. The latest Antarctic ocean modeling shows that complex interactions between the atmosphere, sea ice, icebergs, bathymetric features, and ocean circulation on many scales determine which water masses reach ice-shelf cavities and how much heat is available to melt ice. Meanwhile, parameterizations of basal melting in standalone ice-sheet models have evolved from simplified, depth-dependent functions to more sophisticated models, accounting for ice-shelf basal topography, and the evolution of the sub-ice-shelf buoyant flow. The focus of recent work has been on better understanding processes or adding new model capabilities, but a broader community effort is needed in validating models against observations and producing melt-rate projections. Given time, community efforts in coupled ice sheet-ocean modeling, already underway, will tackle the considerable challenges involved in building, initializing, constraining, and performing projections with coupled models, leading to reduced uncertainties in Antarctica's contribution to future sea-level rise.
  • Cilan Cai, Eric Rignot, Dimitris Menemenlis, Yoshihiro Nakayama
    GEOPHYSICAL RESEARCH LETTERS 44 (16) 8396 - 8403 0094-8276 2017/08 [Refereed][Not invited]
     
    We update observationally based estimates of subaqueous melt, Q(m), beneath Petermann Glacier Ice Shelf (PGIS), Greenland, and model its sensitivity to oceanic thermal forcing, TF, and subglacial runoff, Q(sg), using the Massachusetts Institute of Technology general circulation model (MITgcm), in a two-dimensional domain, with 20m vertical and 40m horizontal resolution at the grounding line. We adjust the drag coefficient to match the observationally based Q(m). With the inclusion of Q(sg), the maximum melt rate (Q(m)(max)) is 2 times larger in summer and 1/3 larger annually than in winter. Q(m)(max) increases above linear with TF and below linear with Q(sg). We estimate that Q(m)(max) increased by 24% (+8.1m/yr) beneath PGIS from the 1990s to the 2000s from a 0.21 degrees C warming in ocean temperature and a doubling in Q(sg), hence contributing to its thinning. If the PGIS is removed, we estimate that the modeled melt rate near the grounding line will increase 13-16 times.
  • Continued retreat of Thwaites Glacier, West Antarctica, controlled by bed topography and ocean circulation.
    H. Seroussi Y. Nakayama E. Larour D. Menemenlis M. Morlighem E. Rignot A. Khazendar
    Geophysical Research Letters 2017/04 [Refereed][Not invited]
  • Amundsen and Bellingshausen Seas simulation with optimized ocean, sea ice, and thermodynamic ice shelf model parameters,
    Y. Nakayama D. Menemenlis M. Schodlok E. Rignot
    Journal of Geophysical Research: Ocean 2017/04 [Refereed][Not invited]
  • On the difficulty of modeling Circumpolar Deep Water intrusions onto the Amundsen Sea continental shelf
    Yoshihiro Nakayama,, Ralph Timmermann, Michael Schorder, Hartmut Hellmer
    Ocean Modeling 2014/12 [Refereed][Not invited]
  • Modeling the spreading of glacial melt water from the Amundsen and Bellingshausen Seas.
    Y. Nakayama R. Timmermann C. B. Rodehacke M. Schröder H. H. Hellmer
    Geophysical Research Letters 2014/11 [Refereed][Not invited]
  • Yoshihiro Nakayama, Kay I. Ohshima, Yoshimasa Matsumura, Yasushi Fukamachi, Hiroyasu Hasumi
    JOURNAL OF PHYSICAL OCEANOGRAPHY 44 (11) 2921 - 2937 0022-3670 2014/11 [Refereed][Not invited]
     
    At several locations around Antarctica, dense water is formed as a result of intense sea ice formation. When this dense water becomes sufficiently denser than the surrounding water, it descends the continental slope and forms Antarctic Bottom Water (AABW). This study presents the AABW formation off the coast of Cape Darnley [Cape Darnley Bottom Water (CDBW)] in East Antarctica, using a nonhydrostatic model. The model is forced for 8 months by a temporally uniform surface salt flux (because of sea ice formation) estimated from Advanced Microwave Scanning Radiometer for Earth Observing System (EOS; AMSR-E) data and a heat budget calculation. The authors reproduce AABW formation and associated periodic downslope flows of dense water. Descending pathways of dense water are largely determined by the topography; most dense water flows into depressions on the continental shelf, advects onto the continental slope, and is steered downslope to greater depths by the canyons. Intense sea ice formation is the most important factor in the formation of AABW off Cape Darnley, and the existence of depressions is of only minor importance for the flux of CDBW. The mechanism responsible for the periodic downslope flow of dense water is further analyzed using an idealized model setup. The period of dense water outflow is regulated primarily by the topographic beta effect.
  • Y. Nakayama, M. Schroeder, H. H. Hellmer
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 77 50 - 62 0967-0637 2013/07 [Refereed][Not invited]
     
    The melting of Pine Island Ice Shelf (PIIS) has increased since the 1990 s, which may have a large impact on ice sheet dynamics, sea-level rise, and changes in water mass properties of surrounding oceans. The reason for the PIIS melting is the relatively warm (similar to 1.2 degrees C) Circumpolar Deep Water (CDW) that penetrates into the PIIS cavity through two submarine glacial troughs located on the Amundsen Sea continental shelf. In this study, we mainly analyze the hydrographic data obtained during ANTXXVI/3 in 2010 with the focus on pathways of the intruding CDW, PIIS melt rates, and the fate of glacial meltwater. We analyze the data by dividing CTD profiles into 6 groups according to intruding CDW properties and meltwater content. From this analysis, it is seen that CDW warmer than 1.23 degrees C (colder than 1.23 degrees C) intrudes via the eastern (central) trough. The temperature is controlled by the thickness of the intruding CDW layer. The eastern trough supports a denser CDW layer than the water mass in Pine Island Trough (PIT). The eastern intrusion is modified on the way into PIT through mixing with the lighter and colder CDW from the central trough. Using ocean transport and tracer transport calculations from the ice shelf front CTD section, the estimated melt rate in 2010 is similar to 30 m yr(-1), which is comparable to published values. From spatial distributions of meltwater content, meltwater flows along the bathymetry towards the west. When compared with earlier (2000) observations, a warmer and thicker CDW layer is observed in Pine Island Trough for the period 2007-2010, indicating a recent thickening of the CDW intrusion. (C) 2013 Elsevier Ltd. All rights reserved.
  • Yoshihiro Nakayama, Kay I. Ohshima, Yasushi Fukamachi
    JOURNAL OF PHYSICAL OCEANOGRAPHY 42 (1) 179 - 192 0022-3670 2012/01 [Refereed][Not invited]
     
    Wind factor, the ratio of sea ice drift speed to surface wind speed, is a key factor for the dynamics of sea ice and is generally about 2%. In some coastal oceans, however, the wind factor tends to be larger near the coast. This study proposes the enhancement mechanism of the sea ice drift caused by the dynamical coupling between sea ice and a coastal ocean. In a coastal ocean covered with sea ice, wind-forced sea ice drift excites coastal trapped waves (shelf waves) and generates fluctuating ocean current. This ocean current can enhance sea ice drift when the current direction is the same as that of the wind-driven drift. The authors consider a simplified setting where spatially uniform oscillating wind drifts sea ice parallel to the coast. When a barotropic long shelf wave is assumed for the ocean response, sea ice drifts driven by wind and ocean are obtained analytically. The ratio of ocean-driven to wind-driven sea ice drifts is used for the evaluation of the oceanic contribution to the enhancement of sea ice drift. The enhancement is mostly determined by the characteristics of the shelf waves, and sea ice drift is significantly enhanced close to the coast with lower-frequency wind forcing. Comparison with the observation off the Sakhalin coast shows that the degree of enhancement of sea ice drift and its characteristic such that larger enhancement occurs near the coast are mostly consistent with our theoretical solution, suggesting that this mechanism is present in the real ocean.

Research Projects

  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2021/04 -2024/03 
    Author : 中山 佳洋
     
    本課題は、アムンゼン海を含む西南極モデル、トッテン棚氷を含む東南極モデルの両者について開発を進めている。両地域において、棚氷の融解が南極大陸から海への氷の流出を促進し、海面上昇へ寄与することが危惧されている。海洋モデルを用いて、温かい水塊の棚氷下部への流入経路の特定と、その変動要因の理解が求められている。
    西南極モデルについては、アジョイント法を用いたデータ同化を実施し、ある程度良い観測データとの一致が再現された。この課題については、すでに論文として投稿し、Geoscientific Model Development誌に掲載済みである。同時に、データ同化を実施しても、経年変動の再現性が改善されず、現モデルに含まれていないプロセスが棚氷融解や海洋場の再現性に影響を与えていることが示唆された。そこで、棚氷下部から海へと流出する氷河融解水の影響を見積もった(すでにGeophysical Research Lettersに掲載済み)。現在では、さらに、他のプロセスの寄与を調査するために、感度実験、データ解析等を進めている。
    東南極モデルについては、モデルを開発した(Geophysical Research Lettersに掲載)。さらに、このモデルについて、2019年に新しく取得された海底地形データへのモデル地形の更新を行った。現在は、観測データと数値モデルの綿密な比較を行い、グリーン関数法を用いたデータ同化の準備を行っている。
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2021/04 -2024/03 
    Author : 板木 拓也, 関 宰, 中山 佳洋, 井尻 暁, 菅沼 悠介, 池原 実
     
    近年、南極では外洋の温暖な深層水が棚氷の下面に入り込むことで棚氷の融解/氷床の海への流出の原因となっていることが示されており、巨大氷冠を有する東南極ではトッテン氷河の融解にともなう海水準上昇が懸念されている。しかし、今後起こるとされている融解の加速を現在の海洋観測データだけで検証することは難しく、過去の融解と海洋変動の記録を合わせて評価する必要がある。第61次日本南極地域観測では、2019年11月から2020年3月に砕氷船「しらせ」を用いた採泥調査が実施され、世界に先駆けて東南極トッテン氷河の前縁域から海底コアが採取された。本研究では、海底コアから精密かつ多様な古環境情報を抽出することで、過去の気候変動によって引き起こされた氷河融解の影響を明らかにし、コンピュータによるモデル計算でそのメカニズムに関する理解を得ることを目的とする。調査の空白域である氷河前縁域の理解が進むことによって、南極氷床と海洋がかかわる気候変動・海水準変動の将来予測の精度向上が期待される。本年度は、採取された表層堆積物およびコアの基礎データを取得し、当該海域における堆積物の性状に関する分析を行った。表層堆積物に関しては、1編の論文が受理され、更に1編が執筆中である。今後、これらの情報をコアに応用して、過去の環境変動を検討する。また、モデル計算にかかる境界条件などについて検討を行った。様々な条件での実験と古環境データと比較することで、より確からしいメカニズムを検討する予定である。
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2019/08 -2021/03 
    Author : NAKAYAMA Yoshihiro
     
    East Antarctica's Totten Glacier has recently been melting at an alarming rate. In this study, we develop a regional East Antarctic configuration of the Massachusetts Institute of Technology general circulation model (MITgcm) and simulate ocean heat intrusions towards the Totten ice shelf. We demonstrate that Antarctic Slope Current (ASC) has a blocking effect and ASC weakening leads to on-shelf intrusions, which may control the Totten ice shelf melt rate.


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