K. Sasaoka

Japan Agency for Marine-Earth Science and Technology

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Kazuhiko Matsumoto

Japan Agency for Marine-Earth Science and Technology

Masahide Wakita

Japan Agency for Marine-Earth Science and Technology

Makio C. Honda

Japan Agency for Marine-Earth Science and Technology

Tetsuichi Fujiki

Japan Agency for Marine-Earth Science and Technology

Toshiro Saino

Japan Agency for Marine-Earth Science and Technology

Akihiko Murata

Japan Agency for Marine-Earth Science and Technology

Hajime Kawakami

Japan Agency for Marine-Earth Science and Technology

S. Watanabe

Nisshin Seifun Group (Japan)

Minoru Kitamura

Japan Agency for Marine-Earth Science and Technology

Katsunori Kimoto

Japan Agency for Marine-Earth Science and Technology

Cooperative Institutions

Japan Agency for Marine-Earth Science and Technology

Hokkaido University

Nagoya University

Tohoku University

The University of Tokyo

Japan Fisheries Research and Education Agency

Japan Science and Technology Agency

University of Tsukuba

Institute for Global Environmental Strategies

Tokyo University of Marine Science and Technology

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Marine ecosystem simulation in the eastern tropical Pacific with a global eddy resolving coupled physical‐biological model

Geophysical Research Letters (2007)

Yoshikazu Sasai Hideharu Sasaki K. Sasaoka Akio Ishida Yasuhiro Yamanaka

We have simulated the seasonal variability of marine biology in the eastern tropical Pacific using a global eddy‐resolving coupled physical‐biological model. Using high‐resolution satellite wind fields, the model reproduces the seasonal variability of surface chlorophyll influenced by the meso‐scale eddies and upwelling associated with the strong offshore wind jets. In winter, upwelling generated by the wind jets in the Gulfs of Tehuantepec, Papagayo, and Panama brings up cold and nitrate‐rich waters from subsurface layer, where the tropical spring bloom occurs and is transported offshore. In summer, the intertropical convergence zone moves northward, and these jets weaken. The Costa Rica Dome develops with wind fields west of the Gulf of Papagayo. The dome in the open ocean supports high chlorophyll by the nutrient supply with upwelling. The westward expansion of surface chlorophyll of dome is response to the thermocline variation with the westward propagation of Rossby waves.

Eddy

Intertropical Convergence Zone

Wind Stress

Tropical Atlantic

Sea-surface height

10.1029/2007gl031507

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Citations (14)

Seasonal variability of phytoplankton community structure in the subtropical western North Pacific

American Geophysical Union eBooks (2016)

Tetsuichi Fujiki K. Sasaoka Kazuhiko Matsumoto Masahide Wakita Yoshihisa Mino

During 2010–2012 in the northwestern region of the North Pacific subtropical gyre, we examined seasonal variability of the phytoplankton community with respect to structure and photo-physiological status using the chemotaxonomy program CHEMTAX and fast-repetition-rate fluorometry. Total chlorophyll a standing stock in the upper 150 m (ʃTChl a) varied from 25.5 to 89.0 mg m−2 during periods of relatively deep vertical mixing (40–207 m), but was fairly constant (18.6–27.2 mg m−2) during stratification periods. Prymnesiophytes, chlorophytes, chrysophytes, prasinophytes, and/or diatoms comprised major portions of the ʃTChl a during mixing periods. Diatoms became the most abundant group (29–43 %) in February 2011, when large phytoplankton blooms were observed, whereas Prochlorococcus was the dominant component (15–46 %) during stratification periods. During mixing periods, nitrate and nitrite (NO3 + NO2) concentrations occasionally exceeded 0.5 μmol kg−1 in surface waters, and F v/F m (photochemical efficiency of photosystem II) ranged from approximately 0.40–0.50 within the euphotic zone. During stratification periods, however, NO3 + NO2 concentrations were very low (<0.1 μmol kg−1) from the surface to 50–95 m, and F v/F m was in the range of 0.25–0.40 in the upper mixed layer. The seasonal variability of phytoplankton community structure in this region was related to the nutrient supply from deep waters. Additionally, we compared our data to published values at the time-series station ALOHA, located in the eastern region of the NPSG. At Sta. ALOHA in 2011, ʃTChl a was almost constant (19.5–26.4 mg m−2) throughout the year and was consistently dominated by Prochlorococcus (61–81 %). These results suggest that the phytoplankton community in the northwestern region of the NPSG is seasonally variable and composed of diverse groups compared with the eastern region of the NPSG.

Prochlorococcus

Mixed layer

Bloom

Stratification (seeds)

Source

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The Formation of Subtropical Phytoplankton Blooms Is Dictated by Water Column Stability During Winter and Spring in the Oligotrophic Northwestern North Pacific

Journal of Geophysical Research Oceans (2021)

Kazuhiko Matsumoto Yoshikazu Sasai K. Sasaoka Eko Siswanto Makio C. Honda

Abstract Subtropical phytoplankton blooms were observed in winter and late spring (rather than in early spring, as is typical) via shipboard observations in an area south of the Kuroshio Extension in the northwestern North Pacific subtropical gyre. Satellite‐based observations revealed that these submesoscale blooms occurred in warmer water masses in winter and in cooler water masses in late spring. The fact that winter blooms occurred in warmer areas suggests that they depend on water column stratification caused by solar heating. In contrast, the fact that the late‐spring blooms occurred in cooler areas suggests a breakdown of stratification due to a recurrence of convective mixing. Mesoscale blooms occurred at intermediate water temperatures in early spring, suggesting a repeating sequence of stratification and mixing during this period. Wintertime deep convective mixing in the northwestern North Pacific subtropical gyre creates Subtropical Mode Water. The behavior of the wintertime mixed‐layer depth, which determines the thickness of Subtropical Mode Water, also dictates the characteristics of subtropical blooms such as their magnitude and the timing of onset and demise. Based on in situ observations and a model analysis, we conclude that deeper winter mixing, which increases nutrient concentrations, will intensify early‐spring blooms and facilitate the formation of episodic blooms in winter and late spring. On the other hand, shallower winter mixing should increase stratification and thus facilitate the formation of smaller blooms, even in winter.

Stratification (seeds)

Mode water

Spring bloom

Bloom

Mixed layer

Convective mixing

10.1029/2020jc016864

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Seasonal and Interannual Variations in Nitrogen Availability and Particle Export in the Northwestern North Pacific Subtropical Gyre

Journal of Geophysical Research Oceans (2020)

Yoshihisa Mino Chiho Sukigara Makio C. Honda Hajime Kawakami Masahide Wakita

Abstract Time series of particulate nitrogen isotope delta (PN‐ δ 15 N) and flux of trapped particles at 200 m were determined in the northwestern North Pacific subtropical gyre between 2010 and 2014 in order to examine temporal variations in mixed layer nitrate availability and downward particle export. Lower PN‐ δ 15 N (<4‰) seen every winter corresponded to convective nitrate supply triggering phytoplankton blooms. Greater particle fluxes were associated with higher CaCO 3 concentrations (mass fraction >50%), implying that new nitrogen supply stimulated predominantly coccolithophore growth and downward transport of coccoliths. Even in the stratified summer of 2011, a δ 15 N drop associated with a spike in particulate flux was found and was attributed to the uplift of nitrate‐rich deep waters due to a mesoscale cyclonic eddy passing the mooring station. Total nitrate input sustains 86–93% of annual new production in this region while nitrogen fixation accounts for the rest. Trapped particles also showed that the winter δ 15 N decrease appeared earlier in the 2011–2012 blooms than those in 2013–2014, coinciding with 8% higher CaCO 3 concentrations and a 40% lower particulate organic carbon to inorganic carbon export ratio, R (POC:PIC). This reflected stronger convective mixing and hence larger nutrient supply in 2011–2012, caused by larger ocean heat loss related to winter monsoon intensity. Such interannual change of winter R (POC:PIC) can affect CO 2 uptake rate in the northwestern North Pacific subtropical gyre where anthropogenic CO 2 accumulates in subtropical mode waters.

New production

10.1029/2019jc015600

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Detection of regional scale sea-to-air oxygen emission related to spring bloom near Japan by using in-situ measurements of the atmospheric oxygen/nitrogen ratio

Atmospheric chemistry and physics (2008)

Hiroshi Yamagishi Yasunori Tohjima Hirofumi Mukai K. Sasaoka

Abstract. We have been carrying out in-situ monitoring of atmospheric O2/N2 ratio at Cape Ochi-ishi (COI; 43°10' N, 145°30' E) in the northern part of Japan since March 2005 by using a modified gas chromatography/thermal conductivity detector (GC/TCD). The standard deviation of the O2/N2 ratio is estimated to be about ±14 per meg (≈3 ppm) with intervals of 10 minutes. Thus, the in-situ measurement system has a 1σ precision of ± 6 per meg (≈1.2 ppm) for one-hour mean O2/N2 ratio. Atmospheric potential oxygen (APO≈O2+1.1 CO2), which is conserved with respect to terrestrial photosynthesis and respiration but reflects changes in air-sea O2 and CO2 fluxes, shows large variabilities from April to early July 2005. Distribution of satellite-derived marine primary production indicates occurrences of strong bloom in the Japan Sea and the latitudinal band between 30° and 40° N in the western North Pacific in April and in the Okhotsk Sea and northeastern region near Hokkaido Island in the North Pacific in June. Back trajectory analysis of air masses indicates that high values of APO, which last for several hours or several days, can be attributed to the oxygen emission associated with the spring bloom of active primary production.

Bloom

10.5194/acp-8-3325-2008

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Citations (15)

Basin‐scale pCO₂ distribution using satellite sea surface temperature, Chl a, and climatological salinity in the North Pacific in spring and summer

Global Biogeochemical Cycles (2006)

V. V. S. S. Sarma Toshiro Saino K. Sasaoka Yukihiro Nojiri Tsuneo Ono

An empirical method is presented for the estimation of basin‐scale distribution of partial pressure of carbon dioxide (pCO 2 ) in the North Pacific using satellite‐derived sea surface temperature (SST), chlorophyll‐ a concentrations (chl a ), and climatological sea surface salinity (SSS). In this approach, multiple regression equations were developed to compute mixed layer dissolved inorganic carbon (DIC) based on SST, SSS and Chl a , whereas mixed layer total alkalinity (TA) was linearly regressed with SSS. The DIC‐SST relation exhibited three different slopes at SST < 20°, 20° < SST < 27.5° and SST>27.5°C. Therefore data have been grouped with reference to SST. Regression equations were developed for two seasons (spring and summer). The regression errors for DIC and TA were 10.5 and 5 μmol kg −1 , respectively. The pCO 2 was computed from the estimated DIC and TA using dissociation constants given by Mehrbach et al. (1973), refit by Dickson and Millero (1987). The derived pCO 2 agreed with the shipboard pCO 2 observations within an error of 17–23 μatm. The sensitivity test on the regression equations for DIC estimation indicated that SSS is the most influencing parameter, followed by SST and Chl a . Using the monthly average SST and Chl a fields derived from the Advanced Very High Resolution Radiometer (AVHRR) and SeaWiFS (Sea‐viewing Wide Field of view Sensor), respectively, and climatological SSS, monthly basin‐scale pCO 2 fields were computed. The statistical model derived pCO 2 results are in agreement with underway pCO 2 in the North Pacific. This study strongly suggests that satellite‐based techniques are promising tools for estimation of pCO 2 fields on a basin scale but the associated error bars are larger than required to study anthropogenic carbon uptake by the oceans. Incorporation of more in situ shipboard data may help in refining the estimating equations and reducing the errors further.

SeaWiFS

Mixed layer

Advanced very-high-resolution radiometer

Temperature salinity diagrams

SSS*

10.1029/2005gb002594

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Citations (70)

Modulation of Surface Seawater CO₂ System at 80°E: Impacts of the Positive IOD in 2019

Journal of Geophysical Research Oceans (2024)

Akihiko Murata Shinya Kouketsu K. Sasaoka K. Arulananthan

Abstract To elucidate impacts of an Indian Ocean Dipole (IOD) event on surface seawater CO 2 dynamics, we analyzed data collected along the World Ocean Circulation Experiment Hydrographic Program I08 N line (latitudes 20°S–6°N at 80°E) in December 2019, when a strong positive IOD (pIOD) event occurred. After removing the effects of anthropogenic CO 2 accumulation, we examined anomalies of the surface seawater CO 2 fugacity (fCO 2 ) from the climatology in relation to other marine properties. At latitudes 11°S–6°S, where horizontal advection of upwelled water off Sumatra was observed, dissolved inorganic carbon and total alkalinity, both normalized to a salinity of 35 (nTCO 2 and nTA) showed positive anomalies of +11.4 and +8.1 μmol kg −1 , respectively. At latitudes 5°S–5°N, where distinct low‐salinity water was observed because of the pIOD, nTCO 2 and nTA showed negative anomalies of −4.0 and ‒0.5 μmol kg −1 , respectively. The combined effects of the nTCO 2 and nTA anomalies on fCO 2 made the observed fCO 2 anomalies small, +3.2 and −6.6 μatm for 6°S–11°S and 5°S–5°N, respectively, because the direction of the Revelle factor for TCO 2 is opposite to that for TA. We estimated that the pIOD modulated the air–sea CO 2 flux by +0.45 and −0.55 mmol m −2 d −1 on average within11°S–6°S and 5°S–5°N, respectively. The impacts of the pIOD on the CO 2 dynamics could be explained by the anomalous salinity conditions associated with upwelled water and the freshwater balance.

Modulation (music)

10.1029/2024jc021177

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Validation and applications of GLI ocean product

Hiroshi Murakami K. Sasaoka Kohtaro Hosoda Mayumi Yoshida Yasushi Mitomi

After ADEOS-2 launch on 14 December 2002, GLI ocean group has investigated vicarious calibration, algorithm improvement and product validation, and released version-1 products on 24 December 2003. Although we lost ADEOS-2/GLI on 24 October 2003, we continue product evaluation and algorithm revision toward the future product updates. For example, we are investigating absorptive aerosol correction and 250-m ocean color image processing. The absorptive aerosol correction is very important especially in the East Asia where is surrounded by the fast developing countries and influenced frequently by a kind of carbon aerosols. The 250-m products are made using GLI 250-m channels from blue to near-infrared, and enable to detect fine spatial structures in the coastal areas.

Ocean color

10.1109/igarss.2004.1369870

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Ocean acidification from 1997 to 2011 in the subarctic western North Pacific Ocean

Masahide Wakita S. Watanabe Makio C. Honda Akira Nagano Katsunori Kimoto

Abstract. Rising atmospheric CO2 contents have led to greater CO2 uptake by the oceans, lowering both pH due to increasing hydrogen ions and CaCO3 saturation states due to declining carbonate ion (CO32−). Here, we used previously compiled data sets and new data collected in 2010 and 2011 to investigate ocean acidification of the North Pacific western subarctic gyre. In winter, the western subarctic gyre is a source of CO2 to the atmosphere because of convective mixing of deep waters rich in dissolved inorganic carbon (DIC). We calculated pH in winter mixed layer from DIC and total alkalinity (TA), and found that it decreased at the rate of −0.001 ± 0.0004 yr−1 from 1997 to 2011. This decrease rate is slower than that expected under condition of seawater/atmosphere equilibration, and it is also slower than the rate in the subtropical regions (−0.002 yr−1). The slow rate is caused by a reduction of CO2 emission in winter due to an increase in TA. Below the mixed layer, the calcite saturation horizon (~185 m depth) shoaled at the rate of 2.9 ± 0.9 m yr−1 as the result of the declining CO32− concentration (−0.03 ± 0.01 μmol k−1yr−1). Between 200 m and 300 m depth, pH decline during the study period (−0.0051 ± 0.0010 yr−1) was larger than ever reported in the open North Pacific. This enhanced acidification rate below the calcite saturation horizon reflected not only the uptake of anthropogenic CO2 but also the increase in the decomposition of organic matter evaluated from the increase in AOU, which suggests that the dissolution of CaCO3 particles increased.

Subarctic climate

Ocean Acidification

Alkalinity

Saturation (graph theory)

Mixed layer

10.5194/bgd-10-8283-2013

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Controlling factors of latitudinal distribution of dissolved organic matter in the upper layers of the Indian Ocean

Progress in Earth and Planetary Science (2024)

Masahito Shigemitsu K. Sasaoka Masahide Wakita Taichi Yokokawa T. Hashioka

Abstract We studied chromophoric (CDOM) and fluorescent (FDOM) dissolved organic matter (DOM) and dissolved organic carbon in surface waters to determine the factors controlling the geographical distribution of DOM along two meridional transects in the Indian Ocean. For CDOM, we calculated the absorption coefficients, spectral slope, and absorption coefficient ratio from the observed absorption spectra. For FDOM, we calculated the biological (BIX) and humification (HIX) indices from the excitation emission matrices (EEMs); parallel factor analysis of the EEMs identified three fluorescent components, i.e., two humic-like and one protein-like. Using these DOM parameters, a factor analysis extracted fewer latent variables than the observed variables to account for the geographical distributions. We obtained three factors (F1, F2, and F3), which explained ~ 84% of the variance in the observed data. From the factor loadings, F1, F2, and F3 were interpreted as the effects of net primary production-derived DOM and its horizontal transport, photodegradation, and vertical transport by physical processes. We characterized seven marine biogeochemical provinces by factor scores. F1 scores gradually decreased from the northernmost to the Antarctic province, with a small maximum around the subtropical front. F2 scores were highest in the subtropical province and decreased in both the northward and southward directions. F3 scores were high in the Antarctic and northernmost provinces, and lowest in the subtropical province. Only BIX was insufficiently explained by these factors. BIX was highest in the northern part of the subtropical province, where photodegradation of DOM was the most intense. This suggests that the possible interaction between photodegradation, autochthonous production, and reworking by heterotrophic bacteria of DOM occurs in the subtropical province.

Biogeochemical Cycle

Spectral slope

Photodegradation

10.1186/s40645-024-00623-0

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