The Global Change Observation Mission-Climate (GCOM-C) satellite, launched in December 2017, is equipped with the Second-generation Global Imager (SGLI) sensor, featuring a moderate spatial resolution of 250 m and 19 spectral bands, including the unique 380 nm band. After six years in orbit, a comprehensive evaluation of SGLI products and their temporal consistency is needed. Remote sensing reflectance (Rrs) is the primary product for monitoring water quality, forming the basis for deriving key oceanic constituents such as chlorophyll-a (Chla) and total suspended matter (TSM). The Japan Aerospace Exploration Agency (JAXA) provides Rrs products through two platforms, G-Portal and JASMES, each employing different atmospheric correction methodologies and assumptions. This study aims to evaluate the SGLI full-resolution Rrs products from G-Portal and JASMES at regional scales (Japan and East Asia) and assess G-Portal Rrs products globally between January 2018 and December 2023. The evaluation employs in situ matchups from NASA’s Aerosol Robotic Network-Ocean Color (AERONET-OC) and cruise measurements. We also assess the retrieval accuracy of two water quality indices, Chla and TSM. The AERONET-OC data analysis reveals that JASMES systematically underestimates Rrs values at shorter wavelengths, particularly at 412 nm. While the Rrs accuracy at 412 nm is relatively low, G-Portal’s Rrs products perform better than JASMES at shorter wavelengths, showing lower errors and stronger correlations with AERONET-OC data. Both G-Portal and JASMES show lower agreement with AERONET-OC and cruise datasets at shorter wavelengths but demonstrate improved agreement at longer wavelengths (530 nm, 565 nm, and 670 nm). JASMES generates approximately 12% more matchup data points than G-Portal, likely due to G-Portal’s stricter atmospheric correction thresholds that exclude pixels with high reflectance. In situ measurements indicate that G-Portal provides better overall agreement, particularly at lower Rrs magnitudes and Chla concentrations below 5 mg/m3. This evaluation underscores the complexities and challenges of atmospheric correction, particularly in optically complex coastal waters (Case 2 waters), which may require tailored atmospheric correction methods different from the standard approach. The assessment of temporal consistency and seasonal variations in Rrs data shows that both platforms effectively capture interannual trends and maintain temporal stability, particularly from the 490 nm band onward, underscoring the potential of SGLI data for long-term monitoring of coastal and oceanic environments.

With the upcoming completion of the Moderate Resolution Imaging Spectroradiometer mission, this study applied 250m-spatial-resolution ocean colour products from the Second- Generation Global Imager (SGLI) on the Global Change Observation Mission-Climate (GCOM-C) platform to continue monitoring red tides in the upper Gulf of Thailand. However, inaccuracies of GCOM-C/SGLI data on red tide detection were observed. To enhance its reliability, we first improved the accuracy of GCOM-C/SGLI remote sensing reflectance (R-rs) data by adjusting it to in situ data using wavelength-specific regression functions. This method significantly reduced the mean absolute percentage difference between in situ and GCOM-C/SGLI data by 25%-87% for single SGLI wavelengths and by 46%-83% for R-rs (lambda)/R-rs (565) of the original data. Subsequently, we employed the corrected data of SGLI R-rs (lambda) at 565 nm and the ratios of R-rs (490)/Rrs(565), R-rs (565), R-rs (530)/R-rs (565), and R-rs (673)/R-rs (565) in local empirical algorithms to estimate chlorophyll-a(chl-a) concentration and identify red tide waters. SGLI chl-a a improved substantially in areas affected by red tides, particularly as chla concentrations increased. Additionally, the corrected GCOMC/SGLI accurately identified red tides caused by green Noctiluca, diatoms, and dinoflagellate blooms consistent with local reports, confirming the GCOM-C/SGLI reliability enhancement. This method may serve as an alternative strategy for monitoring the red tides.

In mid -May 2020, a coccolithophore bloom of Gephyrocapsa oceanica Kamptner, 1943 was observed in Sagami Bay, Japan, possibly for the first time in 25 years. This species is a common yet elusive taxon in the waters around Japan, but there has only been one report of a bloom observation in Sagami Bay in 1995. The G. oceanica bloom in 2020 was observed by the combined approach of the JAXA ocean color remote sensing satellite "Shikisai" and field surveys. The true -color RGB images from the satellite showed the initial bloom in Tokyo Bay appears to have advected into Sagami Bay on May 5. On May 15, ad hoc field sampling was conducted based on visual confirmation from the shore. Nitrate, nitrite and phosphate concentrations at the sea surface were 3.28 mu M, 0.11 mu M and 0.29 mu M, respectively, and higher than the median of May from 1997 to 2019. Chlorophyll a concentration was 2.23 mu g L-1 at the sea surface, and cell density was 5.3 X 103 cells mL-1. The large bloom appears to have reached the western shorelines of the bay, and dispersed counterclockwise within the bay. The bloom reached an estimated 1119.02 km2 surface expression on May 17, and completely disappeared by May 24. Further, the median value of 19'-hexanoyloxyfucoxanthin (a signature pigment of haptophytes, including coccolithophores) concentration has been increasing from 2017 to 2020, and the coastal area of Sagami Bay may have changed to a region favorable for haptophytes.

A severe red tide event, caused primarily by dinoflagellate Karenia selliformis, occurred during the autumn of 2021 in the waters off southeast Hokkaido and resulted in damage of $70 million to fishery industries. There is a high demand for early warning methods based on ocean colour observations to respond to future occurrences of red tides. We therefore used a quasi-analytical algorithm to compute the total absorption coefficient (a) from the Second Generation Global Imager (SGLI) remote sensing reflectance and assessed its potential to detect blooms of K. selliformis. We discovered that, within the same range of SGLI-retrieved chlorophyll-a concentrations, the a at a wavelength of 530 nm a(530) tended to be lower during K. selliformis blooms than during diatom blooms. The a(530) observed by the SGLI therefore showed promise as an optical property for detecting K. selliformis blooms. We discuss the reasons for selection of a(530) and the limitations of the current study’s bloom detection method.

This study identifies the characteristics of water regions with negative normalized water-leaving radiance (nLw(λ)) values in the satellite observations of the Second-generation Global Imager (SGLI) sensor aboard the Global Change Observation Mission–Climate (GCOM-C) satellite. SGLI Level-2 data, along with atmospheric and in-water optical properties measured by the sun photometers in the AErosol RObotic NETwork-Ocean Color (AERONET-OC) from 26 sites globally, are utilized in this study. The focus is particularly on Tokyo Bay and the Ariake Sea, semi-enclosed water regions in Japan where previous research has pointed out the occurrence of negative nLw(λ) values due to atmospheric correction with SGLI. The study examines the temporal changes in atmospheric and in-water optical properties in these two regions, and identifies the characteristics of regions prone to negative nLw(λ) values due to atmospheric correction by comparing the optical properties of these regions with those of 24 other AERONET-OC sites. The time series results of nLw(λ) and the single-scattering albedo (ω(λ)) obtained by the sun photometers at the two sites in Tokyo Bay and Ariake Sea, along with SGLI nLw(λ), indicate the occurrence of negative values in SGLI nLw(λ) in blue band regions, which are mainly attributed to the inflow of absorptive aerosols. However, these negative values are not entirely explained by ω(λ) at 443 nm alone. Additionally, a comparison of in situ nLw(λ) measurements in Tokyo Bay and the Ariake Sea with nLw(λ) values obtained from 24 other AERONET-OC sites, as well as the inherent optical properties (IOPs) estimated through the Quasi-Analytical Algorithm version 5 (QAA_v5), identified five sites—Gulf of Riga, Long Island Sound, Lake Vanern, the Tokyo Bay, and Ariake Sea—as regions where negative nLw(λ) values are more likely to occur. These regions also tend to have lower nLw(λ) values at shorter wavelengths. Furthermore, relatively high light absorption by phytoplankton and colored dissolved organic matter, plus non-algal particles, was confirmed in these regions. This occurs because atmospheric correction processing excessively subtracts aerosol light scattering due to the influence of aerosol absorption, increasing the probability of the occurrence of negative nLw(λ) values. Based on the analysis of atmospheric and in-water optical measurements derived from AERONET-OC in this study, it was found that negative nLw(λ) values due to atmospheric correction are more likely to occur in water regions characterized by both the presence of absorptive aerosols in the atmosphere and high light absorption by in-water substances.

Change Observation Mission -Climate (GCOM-C, Shikisai) satellite was launched by Japan Aerospace eXploration Agency (JAXA) on December 23, 2017, to observe the carbon cycle and radiation budget for climate models predicting global warming.Since January 1, 2018, the SGLI has acquired high-quality ocean color and surface temperature data, which have now accumulated over more than 4 years.SGLI measures solar and terrestrial radiance in 13 visible and near-infrared channels with 2 polarizations, 4 shortwave and 2 thermal infrared channels.Observations are made at 250 m resolution in most channels twice every four days.Because of the high spatial and temporal resolutions, the data are expected to be useful in coastal investigations, although they cover the global ocean including polar seas.The ocean standard products, i.e., chlorophyll-a concentration (Chl-a), total suspended matter, and colored dissolved organic matter (CDOM), as well as photosynthetically active radiation, remote sensing reflectance, and sea surface temperature, are freely available from the JAXA Global Portal System (G-portal, https:// gport al. jaxa.jp) and the JAXA Satellite Monitoring for Environmental Studies (JASMES, https:// www.eorc.jaxa.jp/ JASMES/ index_j.html).We anticipate that more oceanographers will be interested in using SGLI data for their studies in numerical modeling, carbon cycle, biogeochemical cycle, marine ecology, and management of marine resources and coastal environments.

Sediment plumes, released to the Bering Sea from the delta front of the Yukon River, Alaska, are initiated mainly by glacier-melt sediment runoffs in the glacierized regions of the Yukon River drainage basin. The surface sediment plumes are extended around the fan-shaped Yukon River delta, which is followed by the northwestward dispersion. During continuous measurements of the Yukon River discharge and sediment load, behaviors of the sediment plumes were explored by shipboard and coastal observations in the Bering Sea. At the high river sediment load of ca. 2500 kg/s, the plume partially plunged into the sea bottom layer. The plunging probably originated in the nepheloid-layer formation from the flocculation of river-suspended sediment, of which more than 90% wt. is silt and clay (grain size d < 63 μm). In order to numerically obtain the area of the surface sediment plumes, a satellite image analysis was performed by using three near-infrared bands in MODIS/Aqua or MODIS/Terra. The plume area was significantly correlated (R2 = 0.735, p < 0.01) to the sediment load averaged for the two days with time lags of 20 days and 21 days to the date of a certain satellite image. Hence, the dispersion of plume-suspended sediment appears to be controlled by the sediment runoff events in the Yukon River rather than the northward “Alaskan Coastal Water”.

In the Bering Sea around and off the Yukon River delta, surface sediment plumes are markedly formed by glacier-melt and rainfall sediment runoffs of the Yukon River, Alaska, in June– September. The discharge and sediment load time series of the Yukon River were obtained at the lowest gauging station of US Geological Survey in June 2006–September 2010. Meanwhile, by coastal observations on boat, it was found out that the river plume plunges at a boundary between turbid plume water and clean marine water at the Yukon River sediment load of more than ca. 2500 kg/s. Grain size analysis with changing salinity (‰) for the river sediment indicated that the suspended sediment becomes coarse at 2 to 5‰ by flocculation. Hence, the plume’s plunging probably occurred by the flocculation of the Yukon suspended sediment in the brackish zone upstream of the plunging boundary, where the differential settling from the flocculation is considered to have induced the turbid water intrusion into the bottom layer.

沿岸域における海色衛星による水質リモートセンシングでは，水質推定の前処理にあたる大気補正計算の誤差が水質推定誤差の主な原因の1つになっている．そこで本研究では，気候変動観測衛星GCOM-Cの多波長光学放射計SGLIを利用したリモートセンシング反射率（𝑅𝑟𝑠）の推定精度の改善を目的として，東京湾の固有光学特性の実測値に基づき水中モデルを構築し，エアロゾル反射率の推定精度を向上させることで，東京湾に適した大気補正アルゴリズムを作成した．その結果，従来の大気補正アルゴリズムの計算結果と比較して，正規化平均バイアスが380nmで498.3%から68.6%，412nmで447.7%から117.0%，443nmで338.3%から113.9%，490nmで137.6%から32.6%，530nmで64.5%から11.2%，565nmで32.8%から14.2%，673.5nmで50.1%から10.1%に改善した．

本研究の目的は，東京湾において，GCOM-C SGLI（通称「しきさい」）のSST（表面水温）データの精度を検証することと，月平均水温の特徴を把握することである．この目的を実現するために2019年1月から2019年8月まで，東京湾の3地点で午前11時±30分に同期観測された210個のSGLI SSTと実測SSTデータセットが取得された．その結果，同湾におけるSGLI SSTの精度は，品質フラグによる雲除去を行った場合，標準偏差が0.90℃，バイアスが-0.27℃とそれぞれ計算された．またSGLIから導かれたSSTデータは各月9～33%の取得率であった．さらに月平均SST分布の特徴として，特に１～２月の湾口部において，SGLIのデータはそのフロントをよく捉えている．

GCOM-C/SGLI is a multi-wavelength optical radiometer launched on December 23, 2017. The data provision has started from December 20, 2018. In this research, we briefly introduce standard Level 2 products and their validation results based on in situ data. Mean absolute percentage differences are 16.3 - 69.6% for NLW between 380 - 670 nm, 39 and 64.5% for AOT at 670 nm and 865 nm, 27.9% for CHL and 44.2% for aCDOM. Although the number of in situ validation data are still scare for a few ocean color products, the accuracy of GCOM-C/SGLI data will be improved by our future efforts of calibration/validation activities.

The distributions of seawater dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) in and around a phytoplankton bloom off the east coast of northern Japan were investigated using seawater samples collected at ten stations from 14 to 28 April 2007. The seawater samples were collected from the sea surface to a depth of 100 m at each station, and the concentrations of particulate DMSP (DMSPp), DMS, chlorophyll a (Chl a), phytoplankton pigments, nutrients and suspended particulate matter were measured on board the R/V Tansei-maru (KT-07-7). The stations belonged to three water masses: the Tsugaru warm current water system (TW), the Oyashio water system (OW) and the Kuroshio water system (KW). Differences in the seawater parameters were observed among these water masses. At OW stations, an intensive phytoplankton bloom mainly consisted of large diatoms and characterized by high concentrations in Chl a, DMSP and DMS was observed. In addition, differences in DMSPp and DMS concentrations at OW stations could be attributed to differences in bloom stage. At KW stations, although Chl a was low, high DMSPp and DMS concentrations were observed at Stn 11 in this water mass, presumably due to the presence of high DMSP producers such as prymnesiophytes. At TW stations, several low DMSP producers were relatively abundant, resulting in low DMSPp and DMS concentrations. Using these data obtained from ten stations located in these three water masses, the relationship between DMS and Chl a/mixed layer depth (MLD) were tested for six MLD criteria. The best relationship was obtained when the same criteria of MLD as being adopted by Simo and Dachs (2002). Then, we proposed the new Fp ratios representing the fractions of high and low DMSP producers among phytoplankton to predict the DMSPp: Chl a ratio, and our results showed an improvement when comparing with the results using the traditional Fp ratio.

Japan Aerospace Exploration Agency (JAXA) will launch an Earth observing satellite for climate studies named "Global Change Observation Mission – Climate (GCOM-C)" in 2017 which carries a multi-spectral optical sensor named Second-Generation Global Imager (SGLI). The GCOM-C satellite will observe various geophysical variables such as vegetation, land surface temperature, aerosol, clouds, ocean color, sea surface temperature, snow cover extent, snow grain size and so on. The objectives of the SGLI observations are to elucidate the roles of the geophysical variables in the recent changing Earth's climate system and to establish long-term satellite data record of the variables. Three kinds of target accuracies to be achieved for the SGLI observations are defined for evaluating the success of the GCOM-C satellite mission. First one is the minimum thresholds to be achieved for the first SGLI data release at one year after the launch of GCOM-C. Second and third thresholds are the standard and goal accuracies to be achieved at five years after the launch for evaluating the full and extra success of the mission. The target accuracies for individual SGLI products are available at the GCOM-C web site (http://suzaku.eorc.jaxa.jp/GCOM_C/index.html). The quality and accuracies of SGLI products are planned to be evaluated and maintained through validation activities organized by the SGLI validation team consisting of JAXA and SGLI principal investigators (PIs). Uncertainties of the SGLI data products will be characterized through the comparison with in-situ observations, similar products derived from other satellites, climatological data, and/or numerical model simulations. This paper summarizes the overall validation plan for the SGLI geophysical variable products.