Kakehi S.,Tohoku National Fisheries Research Institute |
Ito S.-i.,Atmosphere and Ocean Research Institute |
Wagawa T.,Japan National Research Institute of Fisheries And Environment of Inland Sea
Journal of Geophysical Research: Oceans | Year: 2017
We used salinity and potential alkalinity data from hydrographic observations to investigate surface mixing ratios in the Kuroshio-Oyashio mixed water region in the western North Pacific. In addition to mixing between the Kuroshio Extension (KE) and Oyashio, we assessed freshwater input/removal. A mixing scenario with three end-members was assumed in the surface layer ≤100 m. The results indicate that water masses near the sea surface in the Kuroshio-Oyashio mixed water region were mainly the result of mixing between the KE and Oyashio. The freshwater contribution was approximately 2.2% at depth 10 m. The volume of freshwater estimated from this percentage was consistent with surface water budgets estimated from reanalysis precipitation and evaporation data. The estimated mixing ratio of the KE (rk) along the quasi-stationary jet in the western North Pacific, which splits from the KE and flows northeastward toward the subarctic region, decreased downstream from 95% to 27% in only 42 days, suggesting that water properties were changed rapidly by mixing. Correlation between rk around the quasi-stationary jet and nutrients concentration was significantly negative in the layer where photosynthesis was negligible, indicating that the mixing between the KE and Oyashio is an important determinant of the horizontal distribution of nutrients in this area. © 2017. American Geophysical Union.
News Article | May 2, 2017
A research group formed by 64 researchers from the National Institute of Polar Research, the University of Tokyo, and other organizations analyzed atmospheric temperatures and dust for the past 720,000 years using an ice core obtained at Dome Fuji in Antarctica. Results indicate that when intermediate temperatures occurred within a glacial period, the climate was highly unstable and fluctuated. A climate simulation was also performed based on the Coupled Atmosphere-Ocean General Circulation Model, which revealed that the major cause of the observed climate instability was global cooling by a decline in the greenhouse effect. Climate instability severely impacts both the Earth's natural environment and human society. In the continued effort for understanding how global warming could affect climate instability, it is important to identify periods in the past that experienced climate instability. These periods need to be studied and modeled to clarify any potential causes of the observed instability. However, little progress has been made in improving our documenting and understanding of climate instability prior to the last glacial period. The research groups of Dr. Kenji Kawamura and Dr. Hideaki Motoyama (National Institute of Polar Research) analyzed the Second Dome Fuji ice core (Fig. 1, left) that were obtained as part of the Japanese Antarctic Research Expedition (JARE) between 2003 and 2007. Their team reproduced fluctuations in the air temperature and dust (solid particulate matter carried by the atmosphere) in the Antarctic for the past 720,000 years (Fig. 1, right). They combined this with data from the Dome C ice core drilled by a European team to obtain highly robust paleoclimate data. They examined these data, discovering that for the past 720,000 years, the intermediate climate within glacial periods was marked by frequent climate fluctuations (Fig. 2). This raised a question: Why does the most instability occur when there is an intermediate climate during a glacial period, rather than during an interglacial period, such as we are currently experiencing, or during the coldest part of a glacial period? The research group of Dr. Ayako Abe-Ouchi (University of Tokyo) used a climate model (MIROC) to first reproduce three types of background climate conditions--the interglacial period, intermediate climate within a glacial period, and the coldest part of a glacial period. They then performed a simulation that added the same quantity of fresh water to the northern part of the North Atlantic Ocean in each of the three climate conditions. This simulation was performed using the Earth Simulator supercomputer at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The simulation results indicated that the response to freshwater inflow is maximized during the intermediate climate that occurs within glacial periods, causing the climate to become unstable (Fig. 3 A-C). An important factor affecting climate instability is the vulnerability of Atlantic deep water circulation during global cooling resulting from a decrease in the atmospheric carbon dioxide concentration (Fig. 3 D-E). Until now, the primary factor for climatic instability was thought to be the existence and instability of continental ice sheets in the Northern Hemisphere, but this experiment has revealed that carbon dioxide is another important factor, determining not only the average state of the climate, but also the long-term stability of the climate. These results also suggest that future stability in the present interglacial period, which has continued for more than 10,000 years, is not guaranteed. Indeed, if significant melting of the Greenland ice sheet occurs due to anthropogenic warming, it might destabilize the climate. According to Dr. Kawamura, "Due to anthropogenic emissions, the atmospheric greenhouse gas concentrations have reached a level not seen over the past million years. Large climate components, such as ice sheets and the oceans that have vast size and longtime scales for variations, will undoubtedly change. It will become even more important to combine the climate reconstructions and numerical simulations for the periods when the global environment was much different than it is today, to understand the Earth system by verifying its mechanisms." The study results have been published in the on-line journal, Science Advances. Atmosphere and Ocean Research Institute, the University of Tokyo
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.
News Article | November 29, 2016
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: email@example.com 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:firstname.lastname@example.org 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.
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.