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Mitsudera H.,Institute of Low Temperature Science | Nakamura T.,Institute of Low Temperature Science | Sasajima Y.,Atmosphere and Ocean Research Institute | Hasumi H.,Atmosphere and Ocean Research Institute | Wakatsuchi M.,Institute of Low Temperature Science
Journal of Geophysical Research C: Oceans | Year: 2015

Dense Shelf Water (DSW) formation in the northwestern continental shelf of the Sea of Okhotsk is the beginning of the lower limb of the overturning circulation that ventilates the intermediate layer of the North Pacific Ocean. The upper limb consisting of surface currents in the Okhotsk Sea and the subarctic gyre has not been clarified. Using a high-resolution North Pacific Ocean model with a curvilinear grid as fine as 3 km × 3 km in the Sea of Okhotsk, we succeeded in representing the three-dimensional structure of the overturning circulation including the narrow boundary currents and flows through straits that constitute the upper limb, as well as the lower limb consisting of DSW formation and ventilation. In particular, pathways and time scales from the Bering Sea to the intermediate layer via the ventilation in the Sea of Okhotsk were examined in detail using tracer experiments. Further, we found that the overturning circulation that connects the surface and intermediate layer is sensitive to wind stress. In the case of strong winds, the coastal current under polynyas where DSW forms is intensified, and consequently diapycnal transport from the surface layer to the intermediate layer increases. Strong winds also induce a positive sea surface salinity anomaly in the subarctic region, causing a significant decrease in the density stratification and increase in the DSW salinity (i.e., density). These processes act together to produce intense overturning circulation and deep ventilation, which may subduct even to the bottom of the Sea of Okhotsk if the wind is strong. © 2015. American Geophysical Union. All Rights Reserved.


Thomas A.L.,University of Oxford | Fujita K.,University of Ryukyus | Iryu Y.,Tohoku University | Iryu Y.,Nagoya University | And 29 more authors.
Marine Geology | Year: 2012

We present uranium-thorium chronology for a 102m core through a Pleistocene reef at Tahiti (French Polynesia) sampled during IODP Expedition 310 "Tahiti Sea Level". We employ total and partial dissolution procedures on the older coral samples to investigate the diagenetic overprint of the uranium-thorium system. Although alteration of the U-Th system cannot be robustly corrected, diagenetic trends in the U-Th data, combined with sea level and subsidence constraints for the growth of the corals enables the age of critical samples to be constrained to marine isotope stage 9. We use the ages of the corals, together with δ 18O based sea-level histories, to provide maximum constraints on possible paleo water-depths. These depth constraints are then compared to independent depth estimates based on algal and foraminiferal assemblages, microbioerosion patterns, and sedimentary facies, confirming the accuracy of these paleo water-depth estimates. We also use the fact that corals could not have grown above sea level to place a maximum constraint on the subsidence rate of Tahiti to be 0.39mka -1, with the most likely rate being close to the existing minimum estimate of 0.25mka -1. © 2011 Elsevier B.V.


News Article | November 29, 2016
Site: www.eurekalert.org

Japanese researchers have revealed a relationship between helium levels in groundwater and the amount of stress exerted on inner rock layers of the earth, found at locations near the epicenter of the 2016 Kumamoto earthquake. Scientists hope the finding will lead to the development of a monitoring system that catches stress changes that could foreshadow a big earthquake. Several studies, including some on the massive earthquake in Kobe, Japan, in 1995, have indicated that changes to the chemical makeup of groundwater may occur prior to earthquakes. However, researchers still needed to accumulate evidence to link the occurrence of earthquakes to such chemical changes before establishing a strong correlation between the two. A team of researchers at the University of Tokyo and their collaborators found that when stress exerted on the earth's crust was high, the levels of a helium isotope, helium-4, released in the groundwater was also high at sites near the epicenter of the 2016 Kumamoto earthquake, a magnitude 7.3 quake in southwestern Japan, which caused 50 fatalities and serious damage. The team used a submersible pump in deep wells to obtain groundwater samples at depths of 280 to 1,300 meters from seven locations in the fault zones surrounding the epicenter 11 days after the earthquake in April 2016. They compared the changes of helium-4 levels from chemical analyses of these samples with those from identical analyses performed in 2010. "After careful analysis and calculations, we concluded that the levels of helium-4 had increased in samples that were collected near the epicenter due to the gas released by the rock fractures," says lead author Yuji Sano, a professor at the University of Tokyo's Atmosphere Ocean Research Institute. Furthermore, scientists estimated the amount of helium released by the rocks through rock fracture experiments in the laboratory using rock samples that were collected from around the earthquake region. They also calculated the amount of strain exerted at the sites for groundwater sample collection using satellite data. Combined, the researchers found a positive correlation between helium amounts in groundwater and the stress exertion, in which helium content was higher in areas near the epicenter, while concentrations fell further away from the most intense seismic activity. "More studies should be conducted to verify our correlation in other earthquake areas," says Sano. "It is important to make on-site observations in studying earthquakes and other natural phenomena, as this approach provided us with invaluable insight in investigating the Kumamoto earthquake," he adds. Journal article: Yuji Sano, Naoto Takahata, Takanori Kagoshima, Tomo Shibata, Tetsuji Onoue & Dapeng Zhao, "Groundwater helium anomaly reflects strain change during the 2016 Kumamoto earthquake in Southwest Japan", Scientific Reports URL: https:/ DOI: 10.1038/srep37939 Links: Atmosphere and Ocean Research Institute, The University of Tokyo Research contact: Professor Yuji Sano Atmosphere and Ocean Research Institute, The University of Tokyo 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan Tel: +81-4-7136-6100 Fax: +81-4-7136-6067 Email: ysano@aori.u-tokyo.ac.jp Press officer contact: Yoko Ogawa Press Office, Atmosphere and Ocean Research Institute, The University of Tokyo Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8564, Japan Tel: +81-4-7136-6430 Email:kouhou@aori.u-tokyo.ac.jp Funding: This study was partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program. About the University of Tokyo: The University of Tokyo is Japan's leading university and one of the world's top research universities. The vast research output of some 6,000 researchers is published in the world's top journals across the arts and sciences. Our vibrant student body of around 15,000 undergraduate and 15,000 graduate students includes over 2,000 international students. Find out more at http://www. or follow us on Twitter at @UTokyo_News_en.


Lindsay D.,Japan Agency for Marine - Earth Science and Technology | Yoshida H.,Japan Agency for Marine - Earth Science and Technology | Ishibashi S.,Japan Agency for Marine - Earth Science and Technology | Umetsu M.,Kitazato U. | And 7 more authors.
2013 IEEE International Underwater Technology Symposium, UT 2013 | Year: 2013

A recently developed untethered but remotely operated survey platform, the PICASSO system, is described. This vehicle was designed specifically for surveys of macro- and megazooplankton and marine particulates (maximum depth 1000 m), to link information on gelatinous zooplankton diversity, behaviour and community structure with their function as packagers and producers of marine snow. In addition, an autonomous Visual Plankton Recorder, which is also deployable on the PICASSO vehicle, has been used to investigate particle profiles and plankton distribution vs. depth. Some results from these two systems from eastern Antarctica, the Coral Sea in Australia, and off Japan are introduced. Other techniques for imaging plankton in three dimensions are also introduced. © 2013 IEEE.


Yokoyama Y.,Atmosphere and Ocean Research Institute | Yokoyama Y.,University of Tokyo | Yokoyama Y.,Japan Agency for Marine - Earth Science and Technology | Yokoyama Y.,Australian Nuclear Science and Technology Organisation | Esat T.M.,Australian National University
Oceanography | Year: 2011

Although climate variations and sea level changes are often discussed interchangeably, climate change need not always result in sea level change. Perturbations in Earth's orbit cause major climate changes, and the resulting variations in the amount and distribution of solar radiation at ground level follow cycles lasting for thousands of years. Research done in the last decade shows that climate can change on centennial or shorter time scales. These more rapid changes appear to be related to modifications in ocean circulation initiated during the last glacial period either by injections of fresh meltwater or huge ice discharges into the North Atlantic. When first detected, these rapid climate changes were characterized as episodes decoupled from any significant change in sea level. New data clearly show a direct connection between climate and sea level, and even more surprising, this link may extend to times of glacial-interglacial transitions and possibly also to interglacials. The full extent of this sea level/climate coupling is unknown and is the subject of current research. © 2011 by The Oceanography Society. All rights reserved.


Miyakawa T.,Atmosphere and Ocean Research Institute | Takayabu Y.N.,Atmosphere and Ocean Research Institute | Nasuno T.,Japan Agency for Marine - Earth Science and Technology | Miura H.,Atmosphere and Ocean Research Institute | And 4 more authors.
Journal of the Atmospheric Sciences | Year: 2012

The convective momentum transport (CMT) properties of 13 215 rainbands within a Madden-Julian oscillation (MJO) event simulated by a global nonhydrostatic model are examined. CMT vectors, which represent horizontal accelerations to the mean winds due to momentum flux convergences of deviation winds, are derived for each rainband. The CMT vectors are composited according to their locations relative to the MJO center. While a similar number of rainbands are detected in the eastern and western halves of the MJO convective envelope, CMT vectors with large zonal components are most plentiful between 08 and 208 to the west of the MJO center. The zonal components of the CMT vectors exhibit a coherent directionality and have a well-organized three-layer structure: positive near the surface, negative in the low to midtroposphere, and positive in the upper troposphere. In the low to midtroposphere, where the longitudinal difference in the mean zonal wind across the MJO is 10 m s -1 on average, the net acceleration due to CMT contributes about 216 m s -1. Possible roles of the CMT are proposed. First, the CMT delays the eastward progress of the low- to midtroposphere westerly wind, hence delaying the eastward migration of the convectively favorable region and reducing the propagation speed of the entire MJO. Second, the CMT tilts the MJO flow structure westward with height. Furthermore, the CMT counteracts the momentum transport due to large-scale flows that result from the tilted structure. © 2012 American Meteorological Society.


Kawasaki T.,Japan National Institute of Polar Research | Hasumi H.,Atmosphere and Ocean Research Institute
Journal of Geophysical Research: Oceans | Year: 2016

The heat influx of the Atlantic water and its interannual variability through the Fram Strait toward the Arctic Ocean are examined by using a realistically configured ice-ocean general circulation model. The modeled routes of the Atlantic water and high eddy activity around the Fram Strait are consistent with many observations. Two-thirds of the heat transported by the Atlantic water passing through the Fram Strait (78°N) is lost by the westward transport and the sea surface cooling, and the other one-third is injected to the Arctic Ocean. The contribution of oceanic eddy to the westward heat transport is 5% of that of mean current. The variability of sea level pressure anomaly centered at the Nordic Seas explains the interannual variability of the heat passing through the Fram Strait, transported westward, and cooled at the sea surface in the north of the Fram Strait. The interannual variabilities of these heat fluxes have significant correlations with the NAO. The interannual variability of heat transported by the Atlantic water and entering the Arctic Ocean is caused by the variability of the Siberian high. © 2015. American Geophysical Union. All Rights Reserved.


Hargreaves J.C.,Research Institute for Global Change | Annan J.D.,Research Institute for Global Change | Yoshimori M.,Atmosphere and Ocean Research Institute | Abe-Ouchi A.,Atmosphere and Ocean Research Institute
Geophysical Research Letters | Year: 2012

We investigate the relationship between the Last Glacial Maximum (LGM) and climate sensitivity across the PMIP2 multi-model ensemble of GCMs, and find a correlation between tropical temperature and climate sensitivity which is statistically significant and physically plausible. We use this relationship, together with the LGM temperature reconstruction of Annan and Hargreaves (2012), to generate estimates for the equilibrium climate sensitivity. We estimate the equilibrium climate sensitivity to be about 2.5C with a high probability of being under 4C, though these results are subject to several important caveats. The forthcoming PMIP3/CMIP5 models were not considered in this analysis, as very few LGM simulations are currently available from these models. We propose that these models will provide a useful validation of the correlation presented here. © 2012. American Geophysical Union. All Rights Reserved.


Tanaka Y.,Atmosphere and Ocean Research Institute | Tanaka Y.,University of Tokyo | Yasuda I.,Atmosphere and Ocean Research Institute | Hasumi H.,Atmosphere and Ocean Research Institute | And 3 more authors.
Journal of Climate | Year: 2012

Diapycnal mixing induced by tide-topography interaction, one of the essential factors maintaining the global ocean circulation and hence the global climate, is modulated by the 18.6-yr period oscillation of the lunar orbital inclination, and has therefore been hypothesized to influence bidecadal climate variability. In thisstudy, the spatial distribution of diapycnal diffusivity together with its 18.6-yr oscillation estimated from a global tide model is incorporated into a state-of-the-art numerical coupled climatemodel to investigate its effects on climate variability over the North Pacific and to understand the underlying physical mechanism. It is shown that a significant sea surface temperature (SST) anomaly with a period of 18.6 years appears in the Kuroshio-Oyashio Extension region; a positive (negative)SST anomaly tends to occur during strong (weak) tidal mixing. This is first induced by anomalous horizontal circulation localized around the Kuril Straits, where enhanced modulation of tidal mixing exists, and then amplified through a positive feedback due to midlatitude air-sea interactions. The resulting SST and sea level pressure variability patterns are reminiscent of those associated with one of the most prominent modes of climate variability in the North Pacific known as the Pacific decadal oscillation, suggesting the potential for improving climate predictability by taking into account the 18.6-yr modulation of tidal mixing. © 2012 American Meteorological Society.


Hiraike Y.,Atmosphere and Ocean Research Institute | Tanaka Y.,Japan Agency for Marine - Earth Science and Technology | Hasumi H.,Atmosphere and Ocean Research Institute
Journal of Geophysical Research: Oceans | Year: 2016

The subduction process of Pacific Antarctic Intermediate Water (PAAIW) in the Pacific is investigated using output from an eddy-resolving ocean model. Focus is on contribution of eddies to the subduction process. To separate the subduction rate into contributions by eddies and mean flows, the temporal residual mean (TRM) velocity is used. In the mean subduction rate, lateral induction caused by the strong eastward flow of the Antarctic Circumpolar Current (ACC) is dominant. The largest rate is located in the Drake Passage. The estimated eddy-induced subduction rate is comparable with the mean subduction rate, and it tends to cancel the vertical mean component in many regions. In the west of the Drake Passage, however, the eddy-induced subduction is larger than the vertical mean component, and this eddy-induced subduction was not detected in previous studies using the thickness diffusion parameterization and an eddy-permitting model. Results of idealized sensitivity studies to model resolution suggest that the subduction rate would be larger using a model with higher vertical resolution. © 2015. American Geophysical Union.

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