Publications by authors named "Kay I Ohshima"

5 Publications

  • Page 1 of 1

Age distribution of Antarctic Bottom Water off Cape Darnley, East Antarctica, estimated using chlorofluorocarbon and sulfur hexafluoride.

Sci Rep 2022 05 19;12(1):8462. Epub 2022 May 19.

Institute of Low Temperature Science, Hokkaido University, Sapporo, 060-0819, Japan.

Chlorofluorocarbon (CFC) and sulfur hexafluoride (SF) were used to investigate the timescale of Antarctic Bottom Water (AABW) that spreads off Cape Darnley (CD) in East Antarctica. The age of the AABW was estimated based on the observed SF/CFC-12 ratio while taking into account tracer dilution by Lower Circumpolar Deep Water. Along the western canyons off CD and the ~ 3000 to 3500 m isobaths, the bottom water age was < 5 years, reflecting the spread of newly formed CD Bottom Water. Higher ages of ~ 8 years obtained for areas east of CD and > 20 years in the northwestern offshore region indicate inflows of AABW through the Princess Elizabeth Trough and Weddell Sea Deep Water, respectively. This study determined the age distribution in the region off CD, where three different types of AABW spread.
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http://dx.doi.org/10.1038/s41598-022-12109-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC9120186PMC
May 2022

Weakened overturning and tide control the properties of Oyashio Intermediate Water, a key water mass in the North Pacific.

Sci Rep 2021 07 15;11(1):14526. Epub 2021 Jul 15.

Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan.

The western subarctic Pacific exhibits major biological productivity fed by the Oyashio Current and its two source waters: Western Subarctic Water, which supplies nutrients from the subarctic Pacific, and cold Okhotsk Sea Intermediate Water (OSIW), which supplies iron from the Sea of Okhotsk. We created seasonal climatologies of water properties to understand how the long-term trend (~ 50 years) and 18.6-year tidal cycle affect the Oyashio Intermediate Water (OYW). We found that over the trend, decreased OSIW outflow due to weakening of North Pacific overturning modifies OYW in winter. Meanwhile, OSIW outflow increases (decreases) in strong (weak) tide years. We predict that the opposite effects of the trend and strong tide will lead to stagnation of OYW properties until the mid-2020s, followed by accelerated warming until the mid-2030s (weak tide). A predicted 1 °C increase in OYW temperature and 50% decrease in OSIW content between 1960 and 2040 potentially have significant impact on biological productivity and carbon drawdown in the North Pacific.
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http://dx.doi.org/10.1038/s41598-021-93901-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8282869PMC
July 2021

Strong ice-ocean interaction beneath Shirase Glacier Tongue in East Antarctica.

Nat Commun 2020 08 24;11(1):4221. Epub 2020 Aug 24.

Institute of Low Temperature Science, Hokkaido University, Kita-19 Nishi-8, Kita-ku, Sapporo, 060-0819, Japan.

Mass loss from the Antarctic ice sheet, Earth's largest freshwater reservoir, results directly in global sea-level rise and Southern Ocean freshening. Observational and modeling studies have demonstrated that ice shelf basal melting, resulting from the inflow of warm water onto the Antarctic continental shelf, plays a key role in the ice sheet's mass balance. In recent decades, warm ocean-cryosphere interaction in the Amundsen and Bellingshausen seas has received a great deal of attention. However, except for Totten Ice Shelf, East Antarctic ice shelves typically have cold ice cavities with low basal melt rates. Here we present direct observational evidence of high basal melt rates (7-16 m yr) beneath an East Antarctic ice shelf, Shirase Glacier Tongue, driven by southward-flowing warm water guided by a deep continuous trough extending to the continental slope. The strength of the alongshore wind controls the thickness of the inflowing warm water layer and the rate of basal melting.
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http://dx.doi.org/10.1038/s41467-020-17527-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7445286PMC
August 2020

Estimation of sea-ice thickness and volume in the Sea of Okhotsk based on ICESat data.

Ann Glaciol 2018 Jul 5;59(76 Pt 2):101-111. Epub 2018 Apr 5.

Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo 060-0819, Japan.

Sea-ice thickness in the Sea of Okhotsk is estimated for 2004-2008 from ICESat derived freeboard under the assumption of hydrostatic balance. Total ice thickness including snow depth ( ) averaged over 2004-2008 is 95 cm. The interannual variability of is large; from 77.5 cm (2008) to 110.4 cm (2005). The mode of varies from 50-60 cm (2007 and 2008) to 70-80 cm (2005). Ice thickness derived from ICESat data is validated from a comparison with that observed by Electromagnetic Induction Instrument (EM) aboard the icebreaker near Hokkaido, Japan. Annual maps of reveal that the spatial distribution of is similar every year. Ice volume of 6.3 × 10 m is estimated from the ICESat derived and AMSR-E derived ice concentration. A comparison with ice area demonstrates that the ice volume cannot always be represented by the area solely, despite the fact that the area has been used as a proxy of the volume in the Sea of Okhotsk. The ice volume roughly corresponds to that of annual ice production in the major coastal polynyas estimated based on heat budget calculations. This also supports the validity of the estimation of sea-ice thickness and volume using ICESat data.
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http://dx.doi.org/10.1017/aog.2018.8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7365270PMC
July 2018

Evidence for ice-ocean albedo feedback in the Arctic Ocean shifting to a seasonal ice zone.

Sci Rep 2017 Aug 15;7(1):8170. Epub 2017 Aug 15.

International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, Alaska, 99775-7340, USA.

Ice-albedo feedback due to the albedo contrast between water and ice is a major factor in seasonal sea ice retreat, and has received increasing attention with the Arctic Ocean shifting to a seasonal ice cover. However, quantitative evaluation of such feedbacks is still insufficient. Here we provide quantitative evidence that heat input through the open water fraction is the primary driver of seasonal and interannual variations in Arctic sea ice retreat. Analyses of satellite data (1979-2014) and a simplified ice-upper ocean coupled model reveal that divergent ice motion in the early melt season triggers large-scale feedback which subsequently amplifies summer sea ice anomalies. The magnitude of divergence controlling the feedback has doubled since 2000 due to a more mobile ice cover, which can partly explain the recent drastic ice reduction in the Arctic Ocean.
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http://dx.doi.org/10.1038/s41598-017-08467-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5557900PMC
August 2017
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