Researcher Profile and Settings

Master

Affiliation (Master)

• Faculty of Science　Earth and Planetary Sciences　Earth and Planetary Dynamics

Affiliation (Master)

• Faculty of Science　Earth and Planetary Sciences　Earth and Planetary Dynamics

researchmap

Profile and Settings

Degree

• PhD (Science)（Hokkaido University）

Profile and Settings

• Name (Japanese)

  Sasaki

• Name (Kana)

  Yoshi N

• Name

  201101011669344129

Alternate Names

https://geodynamics.sci.hokudai.ac.jp/poc/sasakiyo/
https://geodynamics.sci.hokudai.ac.jp/poc/sasakiyo/index_en.html

Achievement

Research Interests

• Baiu front   Kuroshio Extension   Kuroshio   North Pacific   Sea Level Rise   Air-Sea Interaction   Physical Oceanography   

Research Areas

• Natural sciences / Atmospheric and hydrospheric science

Research Experience

• 2018/02 - Today Hokkaido University Faculty of Science Associate Professor
• 2013/04 - 2018/01 Hokkaido University Faculty of Science Lecturer
• 2011/04 - 2013/03 Hokkaido University
• 2010/12 - 2011/03 Hokkaido University
• 2008/04 - 2010/11 University of Hawaii

Awards

• 2017 Advances in Atmospheric Sciences Editor's Award
   

  受賞者: 佐々木 克徳
• 2015 日本海洋学会 日本海洋学会岡田賞
   

  受賞者: 佐々木 克徳
• 2012 PICES POC Committee Best Oral Presentation Awards
   

  受賞者: 佐々木 克徳
• 2008 Ocean Sciences Meeting Outstanding Student Poster Awards
   

  受賞者: 佐々木 克徳

Published Papers

• Impact of the Kuroshio large meander on local atmospheric circulation and precipitation in winter

  Yoshi N. Sasaki, Ryunosuke Ito

  Progress in Earth and Planetary Science 2024/03/25 [Refereed]
  
• Two Pathways of Subsurface Spiciness Anomalies in the Subtropical South Pacific

  Yoshi N. Sasaki, Yuma Iwai

  Frontiers in Climate 4 2624-9553 2022/06/02 [Refereed][Not invited]
   

  Subduction and migration of density-compensated (warm/salty or cool/fresh) temperature and salinity water-mass anomalies on isopycnals, referred to as spiciness anomalies, are examined in the subtropical gyre of the South Pacific using an observational dataset. The present results demonstrate that the spiciness anomalies are found to follow two pathways from the subtropical region to the tropical area on the 25–25.5σ_θ isopycnals. The water masses of one pathway subduct south of 20°S and mainly flow westward via the mean geostrophic current to the western boundary region. The water masses of this pathway correspond to the salinity maximum on these isopycnals as well as the bottom of the South Pacific Tropical Water. Positive temperature and salinity trends were prominent along this pathway during the study period. In the other pathway, the water masses subduct north of 20°S and go directly to the tropics through the interior region. Decadal variability of the spiciness anomalies is prominent along this pathway. In both pathways, sea surface salinity variability likely plays an important role in generating the spiciness anomalies on the isopycnals. A passive tracer experiment revealed that the advection by the South Equatorial Countercurrent (SECC) divides these two pathways. Hence, SECC plays a key role in determining whether a spiciness anomaly propagates through the interior region or the western boundary region.
• Decadal vision in oceanography 2021: Mid-latitude ocean

  Fuminori Hashihama, Shinya Kouketsu, Yoshiko Kondo, Yoshi N. Sasaki, Shusaku Sugimoto, Kazutaka Takahashi, Takeyoshi Nagai, Jun Nishioka, Hakase Hayashida, and Junya Hirai

  Oceanography in Japan 30 (5) 127 - 154 0916-8362 2021/11/15
• 海洋学の10年展望2021：中緯度

  橋濱史典, 纐纈慎也, 近藤能子, 佐々木克徳, 杉本周作, 高橋一生, 長井健容, 西岡純, 林田博士, 平井淳也

  海の研究 30 (5) 131 - 158 2021/11 [Refereed]
  
• Rapid Warming of Sea Surface Temperature along the Kuroshio and the China Coast in the East China Sea during the Twentieth Century

  Yoshi N. Sasaki, Chisato Umeda

  JOURNAL OF CLIMATE 34 (12) 4803 - 4815 0894-8755 2021/06 [Refereed][Not invited]
   

  It has been reported that the sea surface temperature (SST) trend of the East China Sea during the twentieth century was a couple of times larger than the global mean SST trend. However, the detailed spatial structure of the SST trend in the East China Sea and its mechanism have not been understood. The present study examines the SST trend in the East China Sea from 1901 to 2010 using observational data and a Regional Ocean Modeling System (ROMS) with an eddy-resolving horizontal resolution. A comparison among two observational datasets and the model output reveals that enhanced SST warming occurred along the Kuroshio and along the coast of China over the continental shelf. In both regions, the SST trends were the largest in winter. The heat budget analysis using the model output indicates that the upper-layer temperature rises in both regions were induced by the trend of ocean advection, which was balanced in relation to the increase of surface net heat release. In addition, the rapid SST warming along the Kuroshio was induced by the acceleration of the Kuroshio. Sensitivity experiments revealed that this acceleration was likely caused by the negative wind stress curl anomalies over the North Pacific. In contrast, the enhanced SST warming along the China coast resulted from the ocean circulation change over the continental shelf by local atmospheric forcing.
• Increasing trend in Japan Sea Throughflow transport

  Shinichiro Kida, Katsumi Takayama, Yoshi N. Sasaki, Hiromi Matsuura, Naoki Hirose

  JOURNAL OF OCEANOGRAPHY 77 (1) 145 - 153 0916-8370 2021/02 [Refereed]
   

  A long-term increasing trend in the transport of the Japan Sea Throughflow is observed from sea-level differences across the Tsushima Strait. Tidal gauge observations show sea level at Hakata, Japan, increasing at a higher rate than that at Busan, Korea. Numerical modeling results suggest that this increasing trend is forced by a northward shift in the Kuroshio axis. As the Kuroshio axis moves northward, sea level along the southern coast of Japan increases. The signal then propagates anticyclonically along the coast as topographic Rossby waves and Kelvin waves, raising sea level and, thus, increasing transport through the Tsushima Strait.
• Forcing Factors Affecting Sea Level Changes at the Coast

  Philip L. Woodworth, Angélique Melet, Marta Marcos, Richard D. Ray, Guy Wöppelmann, Yoshi N. Sasaki, Mauro Cirano, Angela Hibbert, John M. Huthnance, Sebastià Monserrat, Mark A. Merrifield

  Surveys in Geophysics 40 (6) 1351 - 1397 0169-3298 2019/11/08 [Refereed]
  
• Origin of intraseasonal variability in the eastern equatorial Indian Ocean: intrinsic variability and local and remote wind stress forcings

  Chorong Lee, Shoshiro Minobe, Yoshi N. Sasaki

  Journal of Oceanography 75 (2) 119 - 137 0916-8370 2019/04 [Refereed][Not invited]
  
• Interannual to decadal variability of the upper-ocean heat content in the western North Pacific and its relationship to oceanic and atmospheric variability

  Hanna Na, Kwang-Yul Kim, Shoshiro Minobe, Yoshi N. Sasaki

  Journal of Climate 31 (13) 5107 - 5125 0894-8755 2018/07/01 [Refereed][Not invited]
   

  Three-dimensional oceanic thermal structures and variability in the western North Pacific (NP) are examined on the interannual to decadal time scales and their relationship to oceanic and atmospheric variability is discussed by analyzing observation and reanalysis data for 45 years (1964-2008), which is much longer than the satellite-altimetry period. It is shown that the meridional shift of the Kuroshio Extension (KE) and subarctic frontal zone (SAFZ) is associated with the overall cooling/warming over the KE and SAFZ region (KE-SAFZ mode). It appears, however, that changes in KE strength induce different signs of thermal anomalies to the south and north of the KE, not extended to the SAFZ (KE mode), possibly contributing to noncoherent variability between the KE and SAFZ. Thus, the KE and SAFZ are dependent on each other in the context of the KE-SAFZ mode, while the KE is independent of the SAFZ in terms of the KE mode. This intricate relationship is associated with different linkages to atmospheric variability the KE-SAFZ mode exhibits a relatively fast response to the large-scale wind stress curl forcing in the NP, whereas the KE mode is related to a delayed response to the atmospheric forcing via jet-trapped baroclinic Rossby wave propagation. It is suggested that further knowledge of the underlying mechanisms of the two modes would contribute to understanding ocean-atmosphere feedback as well as potential predictability over the western boundary current region in the NP.
• Atmospheric response to interannual variability of sea surface temperature front in the East China Sea in early summer

  Yoshi N. Sasaki, Yuko Yamada

  Climate Dynamics 1 - 14 1432-0894 2017/11/28 [Refereed][Not invited]
   

  The atmospheric response, especially the response of the meiyu-baiu rainband, to interannual variability of the sea surface temperature (SST) front associated with the Kuroshio in the East China Sea in early summer is examined by using reanalysis, satellite, and rain-gauge datasets from 1982 to 2010. It is revealed that the strong (weak) SST front in the East China Sea is accompanied by the heavy (weak) precipitation over the central East China Sea and the southern Japan. Because the strong SST front largely results from the negative SST anomaly over the continental shelf, the local evaporation change in the East China Sea is not balanced by this enhanced precipitation. The moisture for this enhanced precipitation is supplied by interannual variability of horizontal wind convergence over the central East China Sea. In addition to the precipitation change, the strong SST front is also accompanied by the intensification of weather disturbances in the lower troposphere over the East China Sea. This is probably because the negative SST anomaly over the continental shelf enhances the baroclinicity in the lower troposphere. This intensification of the weather disturbances over the East China Sea can explain the enhanced precipitation over the central East China Sea in response to the interannual variability of the SST front. Because the SST anomaly over the continental shelf, which primarily determines the interannual variability of the SST front, persists for a couple of months, these results imply the predictability of the precipitation associated with the meiyu-baiu rainband.
• Sea Level Variability around Japan during the Twentieth Century Simulated by a Regional Ocean Model

  Yoshi N. Sasaki, Ryosuke Washizu, Tamaki Yasuda, Shoshiro Minobe

  JOURNAL OF CLIMATE 30 (14) 5585 - 5595 0894-8755 2017/07 [Refereed][Not invited]
   

  Sea level variability around Japan from 1906 to 2010 is examined using a regional ocean model, along with observational data and the CMIP5 historical simulations. The regional model reproduces observed inter-decadal sea level variability, for example, high sea level around 1950, low sea level in the 1970s, and sea level rise during the most recent three decades, along the Japanese coast. Sensitivity runs reveal that the high sea level around 1950 was induced by the wind stress curl changes over the North Pacific, characterized by a weakening of the Aleutian low. In contrast, the recent sea level rise is primarily caused by heat and freshwater flux forcings. That the wind-induced sea level rise along the Japanese coast around 1950 is as large as the recent sea level rise highlights the importance of natural variability in understanding regional sea level change on interdecadal time scales.
• Dynamical downscaling of future sea level change in the western North Pacific using ROMS

  Zhao-Jun Liu, Shoshiro Minobe, Yoshi N. Sasaki, Mio Terada

  JOURNAL OF OCEANOGRAPHY 72 (6) 905 - 922 0916-8370 2016/12 [Refereed][Not invited]
   

  The future regional sea level (RSL) rise in the western North Pacific is investigated by dynamical downscaling with the Regional Ocean Modeling System (ROMS) with an eddy-permitting resolution based on three global climate models-MIROC-ESM, CSIRO-Mk3.6.0, and GFDL-CM3-under the highest greenhouse-gas emission scenario. The historical run is forced by the air-sea fluxes calculated from Coordinated Ocean Reference Experiment version 2 (COREv2) data. Three future runs-ROMS-MIROC, ROMS-CSIRO, and ROMS-GFDL-are forced with an atmospheric field constructed by adding the difference between the climate model parameters for the twenty-first and twentieth century to fields in the historical run. In all downscaling, the RSL rise along the eastern coast of Japan is generally half or less of the RSL rise maxima off the eastern coast. The projected regional (total) sea level rises along the Honshu coast during 2081-2100 relative to 1981-2000 are 19-25 (98-104), 6-15 (71-80), and 8-14 (80-86) cm in ROMS-MIROC, ROMS-CSIRO, and ROMS-GFDL, respectively. The discrepancies of the RSL rise along the Honshu coast between the climate models and downscaling are less than 10 cm. The RSL changes in the Kuroshio Extension (KE) region in all downscaling simulations are related to the changes of KE (northward shift or intensification) with climate change.
• 西岸境界流およびその続流の変動に関する理論的・解析的研究

  佐々木 克徳

  海の研究 25 (1) 1 - 16 2016 [Refereed][Invited]
  
• Oceanic fronts and jets around Japan: a review

  Shinichiro Kida, Humio Mitsudera, Shigeru Aoki, Xinyu Guo, Shin-ichi Ito, Fumiaki Kobashi, Nobumasa Komori, Atsushi Kubokawa, Toru Miyama, Ryosuke Morie, Hisashi Nakamura, Tomohiro Nakamura, Hideyuki Nakano, Hajime Nishigaki, Masami Nonaka, Hideharu Sasaki, Yoshi N. Sasaki, Toshio Suga, Shusaku Sugimoto, Bunmei Taguchi, Koutarou Takaya, Tomoki Tozuka, Hiroyuki Tsujino, Norihisa Usui

  JOURNAL OF OCEANOGRAPHY 71 (5) 469 - 497 0916-8370 2015/10 [Refereed][Not invited]
   

  This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air-sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air-sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air-sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.
• Climatological mean features and interannual to decadal variability of ring formations in the Kuroshio Extension region

  Yoshi N. Sasaki, Shoshiro Minobe

  JOURNAL OF OCEANOGRAPHY 71 (5) 499 - 509 0916-8370 2015/10 [Refereed][Not invited]
   

  This study examines the climatological mean features of oceanic rings shed from the Kuroshio Extension (KE) jet and their interannual to decadal variability using satellite altimeter observations from October 1992 to December 2010. To objectively capture ring shedding from the KE jet, a new method that consists of the detection of the jet length changes and the tracking of a ring is proposed. Spatial distribution of the ring formations in the KE region indicates that cyclonic (cold-core) rings were most frequently formed in the upstream region between 143A degrees and 147A degrees E around the steady meander of the KE jet. In contrast, most of anticyclonic (warm-core) rings were formed in the downstream region west of the Shatsky Rise. These pinched-off rings in both the upstream and downstream regions generally propagated westward, but about two-thirds of the rings were reabsorbed by the jet. Nevertheless, about one-fourth of the meridional eddy heat transport at the latitude of the KE resulted from the rings that are not reabsorbed by the jet. The number of ring formations showed substantial interannual to decadal variability. In the upstream and downstream KE region, decadal and interannual variability was dominant, respectively. These ring formation fluctuations were negatively correlated with the strength of the KE jet. It is also revealed that the ring formation variations play an important role in sea surface temperature changes north of the KE jet.
• Decadal sea-level variability along the coast of Japan in response to ocean circulation changes

  Yoshi N. Sasaki, Shoshiro Minobe, Yuji Miura

  JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 119 (1) 266 - 275 2169-9275 2014/01 [Refereed][Not invited]
   

  Decadal sea-level variability along the coast of Japan and its relation to large-scale ocean circulation changes from 1993 to 2010 was investigated using tide-gauge and satellite-derived sea-level data. A singular value decomposition (SVD) analysis is performed between coastal sea levels of Japan and sea levels in the western North Pacific. The first SVD mode reveals that the northward shifts of the Kuroshio Extension (KE) jet and the Kuroshio southeast of Japan accompany the coastal sea-level rise in the early 2000s and 2010, and their southward shifts accompany the coastal sea-level fall in the late 1990s and the late 2000s. The shifts of the KE jet are induced by westward propagating Rossby wave from the eastern North Pacific, which is concentrated along the KE jet axis as jet-trapped Rossby waves. The resulting sea-level changes along the coast of Japan show a strong spatial contrast. The sea-level fluctuation is quite large along the southeastern coast of Japan that is under the direct influence of the jet-trapped Rossby waves, and also large in the western coast of Japan, probably due to coastal waves that are excited by the incoming Rossby waves, but is small north of the KE jet latitude. Hence, the nature of the wave trapped by the KE jet produces an active zone and a shadow zone of coastal sea-level variability of Japan. Our results indicate that the correct representation of western boundary currents is necessary for reliable prediction of future coastal sea-level changes. Key Points <list list-type="bulleted"> Decadal sea level change of the coast of Japan shows strong spatial contrast The nature of the Kuroshio Extension jet produces the strong spatial contrast Understanding of western boundary currents is necessary for sea level prediction
• Decadal Response of the Kuroshio Extension Jet to Rossby Waves: Observation and Thin-Jet Theory

  Yoshi N. Sasaki, Shoshiro Minobe, Niklas Schneider

  JOURNAL OF PHYSICAL OCEANOGRAPHY 43 (2) 442 - 456 0022-3670 2013/02 [Refereed][Not invited]
   

  This study examines interannual to decadal variability of the Kuroshio Extension (KE) jet using satellite altimeter observations from 1993 to 2010. The leading empirical orthogonal function (EOF) mode of sea level variability in the KE region represents the meridional shift of the KE jet, followed by its strength changes with a few month lag. This shift of the KE jet lags atmospheric fluctuations over the eastern North Pacific by about three years. Broad sea level anomalies (SLAs) emerge in the eastern North Pacific 3-4 years before the upstream KB jet shift, and propagate westward along the KE jet axis. In the course of the propagation, the meridional scale of the SLAs gradually narrows, and their amplitude increases. This westward propagation of SLAs with a speed of about 5 cm s(-1) is attributed to the westward propagation of the meridional shift of the jet, consistent with the thin-jet theory, whose importance has been suggested by previous numerical studies. In addition, the westward-propagating signals tend to conserve their quasigeostrophic potential vorticity anomaly, which may explain the characteristic changes of SLAs during the propagation. After the westward-propagating signals of positive (negative) SLAs reach at the east coast of Japan, the upstream KB jet strengthens (weakens) associated with the strength changes of the northern and southern recirculation gyres. Interestingly, this strength change of the KE jet propagates eastward with a speed of about 6 cm s(-1), suggesting an importance of advection by the current.
• Influence of the Kuroshio in the East China Sea on the Early Summer (Baiu) Rain

  Yoshi N. Sasaki, S. Minobe, T. Asai, M. Inatsu

  JOURNAL OF CLIMATE 25 (19) 6627 - 6645 0894-8755 2012/10 [Refereed][Not invited]
   

  Influence of the Kuroshio in the East China Sea on the baiu rainband is examined using satellite observations, a reanalysis dataset, and a regional atmospheric model from 2003 to 2008. Satellite observations and reanalysis data reveal that precipitation over the Kuroshio is the highest in early summer (June), when the baiu rainband covers the East China Sea. The high rainfall is collocated with the warm sea surface temperature (SST) tongue of the Kuroshio. This locally enhanced precipitation is embedded in the large-scale baiu rainband, so that the amplitude of precipitation over the Kuroshio is twice as large as that in its surrounding area. The Kuroshio is also accompanied by high surface wind speed, energetic evaporation, and wind convergence. This wind convergence likely results from the SST influence on atmospheric pressure through not only temperature changes, but also humidity changes. Furthermore, the Kuroshio anchors the ascent motion and large diabatic heating with a peak in the midtroposphere, suggesting that the influence of the Kuroshio extends to the upper troposphere. It is also found that the East China Sea in June is the region of the strongest deep atmospheric response to western boundary currents along with the Gulf Stream region in summer. The observational results are well reproduced by the regional atmospheric model. The model indicates that when the SST tongue of the Kuroshio is smoothed, the enhanced precipitation, the energetic evaporation, and the wind convergence over the Kuroshio disappear, although the large-scale structure of the baiu rainband is not essentially changed.
• Decadal Shifts of the Kuroshio Extension Jet: Application of Thin-Jet Theory

  Yoshi N. Sasaki, Niklas Schneider

  JOURNAL OF PHYSICAL OCEANOGRAPHY 41 (5) 979 - 993 0022-3670 2011/05 [Refereed][Not invited]
   

  Meridional shifts of the Kuroshio Extension (KE) jet on decadal time scales are examined using a 1960 2004 hindcast simulation of an eddy-resolving ocean general circulation model for the Earth Simulator (OFES). The leading mode of the simulated KE represents the meridional shifts of the jet on decadal time scales with the largest southward shift in the early 1980s associated with the climate regime shift in 1976/77, a result confirmed with subsurface temperature observations. The meridional shifts originate east of the date line and propagate westward along the mean jet axis, a trajectory inconsistent with the traditionally used linear lone Rossby waves linearized in Cartesian coordinates, although the phase speed is comparable to that in the traditional framework. The zonal scale of these westward propagation signals is about 4000 km and much lamer than their meridional scale. To understand the mechanism for the westward propagation of the KE jet shifts, the authors consider the limit of a thin jet. This dynamic framework describes the temporal evolution of the location of a sharp potential vorticity front under the assumption that variations along the jet are small compared to variations normal to the jet in natural coordinates and is well suited to the strong jet and potential vorticity gradients of the K E. For scaling appropriate to the decadal adjustments in the KE, the thin-jet model successfully reproduces the westward propagations and decadal shifts of the jet latitude simulated in OFES. These results give a physical basis for the prediction of decadal variability in the K E.
• Interannual to decadal Gulf Stream variability in an eddy-resolving ocean model

  Yoshi N. Sasaki, Niklas Schneider

  OCEAN MODELLING 39 (3-4) 209 - 219 1463-5003 2011 [Refereed][Not invited]
   

  Meridional shifts of the Gulf Stream (GS) jet on interannual to decadal timescales and the corresponding oceanic changes around the GS are investigated using a near global eddy-resolving ocean model hindcast from 1960 to 2003. The simulated variability in the shifts of the GS jet axis shows good agreement with observations, and lags atmospheric fluctuations characterized by the North Atlantic Oscillation by about 2 years. This lagged response of the GS jet to the atmospheric variations is attributed to the westward propagation of the undulation of the jet axis from 45 degrees W to 75 degrees W, which has a wavelength of about 4000 km and a displacement of 0.5 degrees. The propagation direction and phase speed of about 2.8 cm s(-1) are consistent with the thin-jet theory. The shifts of the jet axis in the downstream region are likely induced by wind fluctuations through Ekman convergence over the central North Atlantic. Associated with the northward (southward) shift of the jet axis, sea surface temperature is warming (cooling) around and north of the jet, and the former warming has a deep and meridionally narrow subsurface structure, consistent with the northward shift of the jet. The meridional shifts of the jet accompany coherent meridional shifts of energetic eddy activity regions around the GS. Our numerical results suggest that the GS jet brings the atmospheric signals from the central to the western North Atlantic, and the resultant meridional shift of the jet induces the notable oceanic changes around the GS. (C) 2011 Elsevier Ltd. All rights reserved.
• Anti-correlated variability in subduction rate of the western and eastern North Pacific Oceans identified by an eddy-resolving ocean GCM

  Ju Chen, Tangdong Qu, Yoshi N. Sasaki, Niklas Schneider

  GEOPHYSICAL RESEARCH LETTERS 37 0094-8276 2010/12 [Refereed][Not invited]
   

  Analysis of results from an eddy-resolving ocean general circulation model has revealed the existence of an anti-correlated variability in subduction rate of the western and eastern North Pacific Oceans, consistent with the leading mode of Pacific Decadal Oscillation (PDO). A composite analysis shows that during the positive phase of PDO, the winter mixed layer depth (MLD) generally gets deeper in the west and shallower in the east, being closely related to the variability in surface heat flux stemming from an enhanced Aleutian Low, and the situation during the negative phase of PDO is reversed. Changes in MLD are primarily responsible for the anti-correlated variability in subduction rate, though changes in subtropical gyre circulation also play a role. The result also provides useful evidence for the interior and western boundary pathways, through which variability in eastern subtropical mode water can be conveyed into the equatorial region. Citation: Chen, J., T. Qu, Y. N. Sasaki, and N. Schneider (2010), Anti-correlated variability in subduction rate of the western and eastern North Pacific Oceans identified by an eddy-resolving ocean GCM, Geophys. Res. Lett., 37, L23608, doi:10.1029/2010GL045239.
• Observational evidence for propagation of decadal spiciness anomalies in the North Pacific

  Yoshi N. Sasaki, Niklas Schneider, Nikolai Maximenko, Konstantin Lebedev

  GEOPHYSICAL RESEARCH LETTERS 37 0094-8276 2010/04 [Refereed][Not invited]
   

  The propagation of density-compensated (warm/salty or cool/fresh) spiciness anomalies in the North Pacific thermocline is investigated using Argo profiles for the period 2001-2008. A cool/fresh spiciness anomaly on 25 < sigma(theta) < 25.5 kg m(-3) isopycnals appears in the eastern subtropical North Pacific at 120 degrees W-150 degrees W in 2003-2004 with a salinity anomaly of about -0.15 PSS-78. This spiciness anomaly migrates southwestward, and arrives in the western tropical North Pacific at 145 degrees E-175 degrees W in 2008 with the salinity anomaly decreasing to about -0.043 PSS-78. Two warm/salty anomalies are observed to propagate along the same path from 2003 to 2005, and after 2005. The propagation path and speed of the anomalies are in good agreement with advection by the mean geostrophic current. In the course of propagation, the anomalies are diffused and are subject to high frequency injection of spiciness anomalies, especially in the eastern subtropical North Pacific. Citation: Sasaki, Y. N., N. Schneider, N. Maximenko, and K. Lebedev (2010), Observational evidence for propagation of decadal spiciness anomalies in the North Pacific, Geophys. Res. Lett., 37, L07708, doi:10.1029/2010GL042716.
• Decadal sea level variability in the South Pacific in a global eddy-resolving ocean model hindcast

  Yoshi N. Sasaki, Shoshiro Minobe, Niklas Schneider, Takashi Kagimoto, Masami Nonaka, Hideharu Sasaki

  JOURNAL OF PHYSICAL OCEANOGRAPHY 38 (8) 1731 - 1747 0022-3670 2008/08 [Refereed][Not invited]
   

  Sea level variability and related oceanic changes in the South Pacific from 1970 to 2003 are investigated using a hindcast simulation of an eddy-resolving ocean general circulation model (OGCM) for the Earth Simulator (OFES), along with sea level data from tide gauges since 1970 and a satellite altimeter since 1992. The first empirical orthogonal function mode of sea level anomalies (SLAs) of OFES exhibits broad positive SLAs over the central and western South Pacific. The corresponding principal component indicates roughly stable high, low, and high SLAs, separated by a rapid sea level fall in the late 1970s and sea level rise in the late 1990s, consistent with tide gauge and satellite observations. These decadal changes are accompanied by circulation changes of the subtropical gyre at 1000-m depth, and changes of upper-ocean zonal current and eddy activity around the Tasman Front. In general agreement with previous related studies, it is found that sea level variations in the Tasman Sea can be explained by propagation of long baroclinic Rossby waves forced by wind stress curl anomalies, if the impact of New Zealand is taken into account. The corresponding atmospheric variations are associated with decadal variability of El Nino-Southern Oscillation (ENSO). Thus, decadal sea level variability in the western and central South Pacific in the past three and half decades and decadal ENSO variability are likely to be connected. The sea level rise in the 1990s, which attracted much attention in relation to the global warming, is likely associated with the decadal cooling in the tropical Pacific.
• Autumn atmospheric preconditioning for interannual variability of wintertime sea-ice in the Okhotsk Sea

  Yoshi N. Sasaki, Yurika Katagiri, Shoshiro Minobe, Ignatius G. Rigor

  JOURNAL OF OCEANOGRAPHY 63 (2) 255 - 265 0916-8370 2007/04 [Refereed][Not invited]
   

  Relations in year-to-year variability between wintertime Sea-Ice Concentrations (SICs) IN in the Okhotsk Sea and atmospheric anomalies consisting of zonal and meridional 1000-hPa wind speeds and 850-hPa air temperatures are studied using a singular value decomposition analysis. It is revealed that the late autumn (October-Novemher) atmospheric conditions strongly influence sea-ice variability from the same season (late autumn) through late winter (February-March), in which sea-ice extent is at its maximum. The autumn atmospheric conditions for the positive sea-ice anomalies exhibit cold air temperature anomalies over the Okhotsk Sea and wind anomalies blowing into the Okhotsk Sea from Siberia. These atmospheric conditions yield anomalous ocean-to-atmosphere heat fluxes and cold sea surface temperature anomalies in the Okhotsk Sea. Hence, these results suggest that the atmospheric conditions affect the sea-ice through heat anomalies stored in sea-ice and oceanic fields. The late autumn atmosphere conditions are related to large 700-hPa geopotential height anomalies over the Bering Sea and northern Eurasia, which are related to a stationary Rossby wave propagation over the North Pacific and that from the North Atlantic to Eurasia, respectively. In addition, the late autumn atmospheric preconditioning also plays an important role in the decreasing trend in the Okhotsk sea-ice extent observed from 1980 to the mid-1990s. Based on the lagged sea-ice response to the late autumn atmosphere, a simple seasonal prediction scheme is proposed for the February-March sea-ice extent using four-month leading atmospheric conditions. This scheme explains 45% of the variance of the Okhotsk sea-ice extent.
• Seasonally dependent interannual variability of sea ice in the Bering Sea and its relation to atmospheric fluctuations

  YN Sasaki, S Minobe

  JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 110 (C5) 2169-9275 2005/05 [Refereed][Not invited]
   

  Interannual variability of sea ice in the Bering Sea and its relationship to atmospheric variability is analyzed using a singular value decomposition (SVD) analysis of sea ice concentrations (SICs) and 1000 hPa wind speeds in winter and spring seasons. The statistically significant first and second SVD modes, explaining 76.3% and 17.6% in winter and 54.6% and 29.6% in spring of the squared covariance between the two fields, are identified for SICs both in the winter and spring seasons with 1 month leading wind speeds. The spatial structures show that the first (second) SVD mode explains the SIC variability in the northeastern (northwestern) Bering Sea, related to the local northwesterly (northerly) wind anomalies for the positive SIC anomalies both in the winter and spring seasons. A comparison of the first SVD modes between the winter and spring seasons suggests that the difference of dominant patterns of wind anomalies results in the difference of SIC anomaly distributions between two seasons. The relationship between sea ice and atmospheric circulation anomalies indicates that one mode of the leading two SVD modes in each season is related to large-scale atmospheric circulation associated with the Aleutian low and the other mode is related to relatively local atmospheric fluctuations related with pressure anomalies over Alaska. Furthermore, a slight difference of 700 hPa geopotential height anomalies results in the substantially different sea ice anomalies. These results suggest that in order to know the interannual sea ice variability in the Bering Sea, a better understanding of the wind anomalies over the Bering Sea are important.

MISC

• 梅雨期の東シナ海黒潮における大気海洋相互作用の船舶観測(その2)

  佐々木克徳, 森信行, 見延庄士郎, 中村啓彦, 仁科文子, 岩崎慎介, 磯辺篤彦  日本海洋学会大会講演要旨集  2016-  2016
• 梅雨期の東シナ海黒潮における大気‐海洋相互作用の船舶観測

  MORI NOBUYUKI, MINOBE SHOSHIRO, NAKAMURA HIROHIKO, SASAKI YOSHINORI, NISHINA AYAKO, IWASAKI SHINSUKE, ISOBE ATSUHIKO  日本海洋学会大会講演要旨集  2014-  27  2014/03/10  [Not refereed][Not invited]
  
• 2012: Ocean Science Meeting 2012報告

  見延庄士郎, 須賀利雄, 佐々木克徳, 安藤健太郎, 渡邉英嗣, 蒲生俊敬, 齊藤宏明, 池田元美, 鈴木亨  JOS News Letter  2-  (1)  3  -6  2012/05  [Not refereed][Not invited]
  
• Decadal variability in the Okhotsk Sea : Sea-ice and upper ocean temperature

  佐々木 克徳, 見延 庄士郎  低温科学  65-  21  -30  2007/03/23  [Not refereed][Not invited]
   

  オホーツク海の海氷と上層水温の十年変動についての時空間構造と、その大気との関係について解析を行った。冬季の海氷の十年変動はオホーツク海北部で大きい振幅を持ち、1980年代の初めから1990年代中頃までは減少傾向、それ以降は海氷増加の傾向を示す。この海氷変動は秋季の大気変動に強く影響されている。一方、オホーツク海南部の夏期(5-10月)の上層水温の十年変動はクリル海盆付近で大きい振幅を持ち、概ね十年毎に温暖のピークを持つ準十年変動を示す。この上層水温の十年変動はその前の冬季の大気変動に強く影響されている。
• P203 1999-2001年の黒潮親潮続流域における異常昇温の構造・メカニズム・インパクト(太平洋・インド洋域における大気海洋結合現象)

  浅川 尚吾, 見延 庄士郎, 佐々木 克徳  大会講演予講集  89-  311  -311  2006

Association Memberships

• The American Geophysical Union   the Meteorological Society of Japan   THE OCEANOGRAPHIC SOCIETY OF JAPAN   

Research Projects

• Ever-increasing atmospheric and marine heat waves and unceasing cold spells

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2024/04 -2029/03 

  Author : 小坂 優, 岡島 悟, 佐々木 克徳, 今田 由紀子, 西井 和晃, 野中 正見, 田口 文明
• 北太平洋西部の海洋循環の変化が沿岸水位上昇に与える影響の解明

  日本学術振興会：科学研究費助成事業

  Date (from‐to) : 2022/04 -2026/03 

  Author : 佐々木 克徳
• Uncertainties of climate variability and change and future projection in mid-latitudes

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2019/06 -2024/03 

  Author : 見延 庄士郎, 増永 浩彦, 山本 絢子, 杉本 周作, 佐々木 克徳, 時長 宏樹, 釜江 陽一

   

  EUのHighResMIPプロジェクトであるPRIMAVERAのデータを、同プロジェクトと本新学術領域の連携を活かして解析し、爆弾低気圧の個数が高解像度モデルほど増加することを明らかにした論文を出版した。対流放射平衡大気の数値実験でしばしば見られる対流自己凝集化を、衛星観測により検証した。現実大気でも対流自己凝集化に対応する現象が確認されたが、理想化実験に比べ時間スケールが顕著に短いなど注目すべき違いも認められた。大気再解析データより、北米大陸気候へのベーリング海海氷の影響を明らかにし、CMIP6データから同海氷の将来変化について提示した。2017年夏から発生した黒潮大蛇行期間での黒潮続流・大気大循環場の特徴を大気再解析データの統計解析から記述した。20世紀の東シナ海の温暖化を調べるために領域海洋モデルを用いた解析を行った。その結果、温暖化は黒潮流軸と大陸棚上で大きく、そのメカニズムは海洋循環の変化であることが判明した。CMIP6/HighResMIP 高解像度大気海洋結合モデル出力データを解析し、太平洋十年規模振動に伴う大気海洋相互作用の再現性を検証した。その結果、海面熱フラックス偏差の南北ダイポール構造が黒潮続流前線に沿って形成され、アリューシャン低気圧との双方向相互作用を通して、太平洋十年規模振動のライフサイクルに重要な役割を果たしている可能性を突き止めた。高解像度モデル出力を用いた極端現象とそれをもたらす総観規模擾乱の変動に関する研究に取り組んだ。低気圧追跡手法を用いることで、日本上空を通過する二つ玉低気圧とそれに伴う大気の川の流入による降雨や降雪の傾向を明らかにした。
• 東シナ海の海面水温前線の長期変動とその大気への影響の解明

  日本学術振興会：科学研究費助成事業

  Date (from‐to) : 2019/04 -2023/03 

  Author : 佐々木 克徳

   

  2021年度に引き続き東シナ海の長期の海面水温上昇に対する大気応答、特に降水量の応答についての解析を行った。まず観測データと再解析データを用いて1901年以降の6月平均の日本周辺の降水量の線形トレンドを調べたところ、九州南部から東シナ海北部にかけて、統計的に有意な正のトレンドが見られた。これは1901年以降の長期間に梅雨前線に伴う降水量が増加していることを示している。 次にこの九州南部から東シナ海北部にかけての降水量のトレンドに対する東シナ海の海面水温上昇の寄与を調べるために、領域大気モデルWRFを用いた数値シミュレーションを行った。まず側面境界値を再解析データ、海面水温の境界値を衛星データで与えた1991～2010年の6月の標準実験を行った結果、この実験でのモデルの降水量は衛星観測による降水量の気候値をよく再現していた。この実験をベースとして東シナ海の長期の海面水温上昇の影響、特に黒潮流軸付近と大陸棚上で海面水温上昇が大きいことの影響を調べるために、海面水温の境界値を海面水温上昇の空間パターンを元に変化させた感度実験を複数ケース行った。この結果、現在より黒潮流軸付近と大陸棚上の海面水温を上げた実験において、九州南部の6月の降水量に有意な増加が見られた。同様に海面水温を下げた実験においてもおおむね整合的な計算結果が得られた。これらの結果は、観測データと再解析データから得られた降水量の増加トレンドと整合的である。
• Impacts of Climate Variability and Change on Ocean Oxygen

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2018/04 -2023/03 

  Author : 見延 庄士郎, 重光 雅仁, 纐纈 慎也, 小埜 恒夫, 佐々木 克徳, 安中 さやか, 相田 真希

   

  Argoデータの解析から全球における亜表層水塊の収支を明らかにした。この収支をもとに亜表層栄養塩の季節変化の解析を行い、年間の供給に対し冬季の混合が果たす役割の評価を行った。過去のデータを広く集め統合することで、北太平洋表層栄養塩・全炭酸濃度の季節・経年変動を統一的に明らかにした。北太平洋の等密度面上の溶存酸素減少トレンドについて解析を行い、26.6σθ―27.2σθの密度面で有意な溶存酸素濃度の減少トレンドを見出した。特にオホーツク海南部とアリューシャン列島南岸で減少トレンドが顕著である。溶存有機物（難分解性と易分解性）およびバクテリアを導入した物質循環モデルを構築した。また超長期温暖化実験を行う準備を整えた。COCO-NEMUROによる1958年～2018年の結果から、北太平洋亜寒帯域西部と東部では異なった生物応答が見られた。特に親潮域における動植物プランクトン量の変化について観測値と比較検証したところ、クロロフィルaと動物プランクトンは1990年代まではPDOと相関していたが2000年代以降はPDOと有意な関係が認められなかった。日本海浅海域における貧酸素化進行状況に関する解析結果をとりまとめ、論文として刊行した。日本海浅海域の主要水産魚種であるマダイ、タチウオ、ヒラメ、ムシガレイ、ソウハチ、ケンサキイカ等の貧酸素耐性情報の収集を開始した。また初めて北太平洋の東部と西部の生態系指標の総合解析を過去半世紀について行い、特に底魚の減少が共通して明瞭であり、その一因として海洋貧酸素化の影響を指摘した論文を出版した。本論文は2022年発行のIPCC第六次評価報告書(WG2)で引用され国際的にも高い評価を得た。
• Propagation of temperature/salinity variability in the mid-latitude Pacific

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2016/04 -2020/03 

  Author : SASAKI YOSHINORI

   

  Subduction and migration of density-compensated temperature and salinity water-mass anomalies on isopycnals are examined in the subtropical gyre of the South Pacific using an observational dataset. The present results demonstrate that density-compensated anomalies are found to follow two pathways from the subtropical region to the tropical region. It is revealed that the advection by the South Equatorial Countercurrent (SECC) divides these two pathways. Hence, SECC plays a key role in determining whether a density-compensated anomaly propagates through the interior region or the western boundary region.
• Sea level rise due to the global warming for Japan and East Asia: Mechanisms and future projection

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2014/04 -2019/03 

  Author : Minobe Shoshiro, KUROGI Masao, TERADA Mio, LIU Zhao-Jun

   

  Important results are as follows. 1) The world-leading sea level rise predicted east off Japan is mainly caused by the local warming of the ocean carried by the water mass called subtropical mode water. 2) The reason why the sea level at the coast of Japan does not rise as much as sea level east off Japan is due to the shield effect caused by the coastal trapped wave propagating from the subarctic region to the south along the coast. 3) Compared with the dynamical downscale results, the climate model results are generally reliable for the coastal sea-level rise of the four main islands of Japan. 4) Uncertainty upper limits of sea-level rise of the four main islands of Japan and coast of East Asia are larger than those for the global ocean average. 5) The past high sea-level along the Japan around 1950 was caused the change of wind accompanied by natural variation.
• A study on mean features and variability of meandering and ring formations in the Kuroshio Extension and Gulf Stream regions

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2013/04 -2016/03 

  Author : Sasaki Yoshinori

   

  A new method that consists of the detection of the jet path changes and the tracking of a mesoscale eddy is proposed to objectively capture eddy shedding from an oceanic jet based on sea surface height data. This method is applied to the Kuroshio Extension and Gulf Stream jets. Based on these results, it is revealed that the pinched-off mesoscale eddies play an important role in across-jet transports of heat and nutrients. In addition, the fluctuations of the eddy formation are negatively correlated with the strength of the jets, suggesting deceleration of the jets by eddy formation.
• Basin scale atmosphere and ocean interaction

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2010/04 -2016/03 

  Author : Minobe Shoshiro, KOMORI NOBUMASA, INATSU MASARU, YOSHIDA AKIRA, SASAKI YOSHINORI, OFUCHI WATARU, ISOGUCHI OSAMU, KAWAMOTO KAZUAKI, KACHI MISAKO, Schneider Niklas, O'Reilly Christopher H., Small Richard Justin

   

  By using numerical modelling and data analysis, we have discovered the mid-latitude ocean influences on a wide range of atmospheric phenomena, including termination of Baiu rain season over the northwestern North Pacific, the route and strength of tropical cyclones, Europe blocking and its impact on cold spells. We have found that the pressure adjustment mechanism works over major western boundary currents in the mid-latitudes globally. We have developed a new diagnostic method that enable us to determine mechanisms near surface (10 m altitude) winds for time-mean field, and have showed that both the pressure adjustment mechanism and the vertical mixing mechanism working but differently. Also, we have reported for the first time that observed decadal variability of the Kuroshio Extension is due to the jet-trapped Rossby wave, and that the decadal Kuroshio Extension changes impact Japanese coastal sea-level.
• Understanding the mechanism for decadal variability of oceanic jets in the North Pacific and the North Atlantic

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2011 -2012 

  Author : SASAKI Yoshinori

   

  Mechanisms for decadal variability of the Kuroshio Extension and the Gulf Stream jets are examined. It is revealed that the meridional shifts of these jets on decadal timescales are induced by westward propagating signals that are caused by atmospheric fluctuations. These signals do not propagate as a traditional linear long Rossby wave, but propagate as a jet-trapped Rossby wave, which was proposed by our previous study. Our results indicate a predictability of the decadal variability of the Kuroshio Extension and the Gulf Stream jets for several years, and give a physical basis for the prediction.
• 北太平洋の経年〜数十年変動メカニズム:0.1°数値計算結果と観測データー解析

  日本学術振興会：科学研究費助成事業

  Date (from‐to) : 2006 -2007 

  Author : 佐々木 克徳

   

  本年度は地球シミュレーター上で数値積分された海洋大循環モデルOFESの出力結果と,衛星海面高度計データーを用いた海表面高度の海盆スケールの変動の解析と,衛星海面高度計データーを用いた海洋中の中規模渦などの小規模な空間スケールの変動の解析を行った.中規模渦は海洋中の熱輸送,運動量輸送,物質輸送等について重要な役割を担っている.これら中規模渦の挙動について明らかにするために,海表面高度偏差データーから中規模渦の移動ベクトルを計算するアルゴリズムを提案し,この手法を用いて全球における中規模渦の移動ベクトルの格子化データーを作成した.この格子化データーを用いて中規模渦の移動速度の変動について解析を行い,変動の振幅は黒潮続流域等の西岸境界流域付近で大きいことを明らかにした.さらに中規模渦の移動速度の変動と,空間スケールの大きい背景流の変動との関係について解析を行い,両者に強い関係があることを明らかにした.この結果は背景流の変動が中規模渦の移動距離や移動経路に影響を与えることを示唆する.実際に黒潮続流域における中規模渦の挙動の解析から,黒潮続流下流域の流速が早い時期と遅い時期では,中規模渦の移動距離,移動経路,存在期間が大きく異なるということを明らかにした.また,OFESの出力結果と衛星高度計データーを用いた南太平洋の海盆スケールの海表面高度変動については国際学術雑誌であるJournal of Physical Oceanographyへと投稿し受理されている.