Sei-Ichi Saitoh

Bio-optical characteristics of the western Arctic Ocean: implications for ocean color algorithms

Global ocean color algorithms designed to estimate chlorophyll a concentration (chla) are not accurate at high latitudes. Although a regional Arctic OC4L algorithm was designed to be applicable to high northern latitudes, its applicability remains uncertain. To examine these issues, we investigated the light absorption coefficients of phytoplankton, non-algal particles (NAP), and colored dissolved organic matter (CDOM) and remote sensing reflectance, Rrs(λ), covering most of the western Arctic Ocean. A higher pigment packaging effect was identified relative to that at lower latitudes. The CDOM absorption dominated and accounted for 76% of the total non-water absorption at 443 nm and did not covary with variations in chla. This absorption is significantly higher than those in other marine environments. We also examined the backscattering coefficient of particles obtained from the inversion of Rrs(λ) and found that it covaried well with variation in NAP absorption. Our evaluation shows that when turbid waters (Rrs(670) > 0.00042 sr–1) are excluded, the performance of the OC4L is good and much better than that of the sea-viewing wide field-of-view sensor (SeaWiFS) OC4V4 and the moderate-resolution imaging spectroradiometer (MODIS) OC3M (root mean square error (RMSE) of 0.13, 0.21, and 0.22, respectively). The reason why the OC4L performs well despite strong CDOM absorption is discussed.

Bering Sea cyclonic and anticyclonic eddies observed during summer 2000 and 2001

Abstract Using satellite altimeter and ship data, Bering Sea cyclonic and anticyclonic eddies were observed in summer 2000 and 2001 to examine their biological, chemical and physical structures. Results from the ship transect revealed the interactions between the physical and biological conditions of Bering Sea eddies. At the center of a cyclonic (anticlockwise) eddy, upwelling was transporting nutrient (NO3+NO2) rich water (>25 μM) to the surface, which resulted in relatively high chlorophyll a concentrations (>1.0 mg m−3) developing under the pycnocline. In contrast, in the center of an anticyclonic (clockwise) eddy there was downwelling. This downwelling of surface warm water was destroying a cold layer (at about 150 m depth) caused by winter convection. However, around the periphery of the anticyclonic eddy the isopycnals were tilted up and nutrient-rich water was being transported along with them up into the euphotic zone, so that high chlorophyll a concentrations were being developed above the pycnocline inside the anticyclonic eddy.

Temporal and Spatial Variability of Phytoplankton Pigment Concentrations in the Japan Sea Derived from CZCS Images

Temporal and spatial variability of phytoplankton pigment concentrations in the Japan Sea are described, using monthly mean composite images of the Coastal Zone Color Scanner (CZCS). In order to describe the seasonal changes of pigment concentration from the results of the empirical orthogonal function (EOF) analysis, we selected four areas in the south Japan Sea. The pigment concentrations in these areas show remarkable seasonal variations. Two annual blooms appear in spring and fall. The spring bloom starts in the Japan Sea in February and March, when critical depth (CRD) becomes equal to mixed layer depth (MLD). The spring bloom in the southern areas (April) occurs one month in advance of that in the northern areas (May). This indicates that the pigment concentrations in the southern areas may increase rapidly in comparison with the northern areas since the water temperature increases faster in spring in the southern than in the northern areas. The fall bloom appears first in the southwest region, then in the southeast and northeast regions, finally appearing in the northwest region. Fall bloom appears in November and December when MLD becomes equal to CRD. The fall bloom can be explained by deepening of MLD in the Japan Sea. The pigment concentrations in winter are higher than those in summer. The low pigment concentrations dominate in summer.

The Northward Intruding Eddy along the East Coast of Korea

The current structures and their seasonal variations in the East Korean Warm Current (EKWC) region, which plays a significant role in the northward transport of warm and saline waters, were described by combining the sea surface temperature (SST) data of consecutive satellite inferred (IR) images and hydrographic data. The SST patterns in winter-spring clearly showed that the small meander of thermal front originating from the Tsushima/Korea Strait formed close to the Korean coast and grew an isolated warm eddy with horizontal dimension of order 100 km. Such warm eddy began to intrude slowly northward from spring to summer. At that time, interactions with neighboring synoptic warm eddy [Ks] around the Ulleung Basin were found to have strongly influence the movement of the intruding eddy and its structural change. In autumn, after the northward movement stopped at the north of eddy [Ks], the relative stable northward current along the Korean coast were formed. The evidence from observational results does not support a persistent branching of the EKWC from the Tsushima/Korea Strait, but a seasonal episodic supply of warm and saline waters due to the northward intruding eddy process described above.

Chlorophyll - Specific Absorption Coefficients and Pigments of Phytoplankton off Sanriku, Northwestern North Pacific

The variety in shape and magnitude of thein vivo chlorophyll-specific absorption spectra of phytoplankton was investigated in relation to differences in pigment composition off Sanriku, northwestern North Pacific. Site-to-site variations of the absorption coefficients,aph* (λ), and pigment composition were clearly observed. At warm-streamer stations, higher values ofaph* (440) andaph* (650) were found with relatively high concentrations of chlorophyllb (a green algae marker). At stations located in the Oyashio water (cold streamer),aph* (440) values were lower and fucoxanthin (a diatom marker) concentrations were higher, compared to the other stations. The peak in the absorption spectra at the Oyashio stations was shifted toward shorter wavelengths, which was probably due to the presence of phaeopigments. In a Kuroshio warm-core ring, the magnitude ofaph* (440) was in between those at the warm-streamer and Oyashio stations, and the diagnostic pigment was peridinin (a dinoflagellate marker). These findings indicated that major differences in phytoplankton absorption spectra of each water mass were a result of differences in the phytoplankton pigment composition of each water mass, which was probably related to the phytoplankton community.

Japan Tsunami Current Flows Observed by HF Radars on Two Continents

Quantitative real-time observations of a tsunami have been limited to deep-water, pressure-sensor observations of changes in the sea surface elevation and observations of sea level fluctuations at the coast, which are essentially point measurements. Constrained by these data, models have been used for predictions and warning of the arrival of a tsunami, but to date no detailed verification of flow patterns nor area measurements have been possible. Here we present unique HF-radar area observations of the tsunami signal seen in current velocities as the wave train approaches the coast. Networks of coastal HF-radars are now routinely observing surface currents in many countries and we report clear results from five HF radar sites spanning a distance of 8,200 km on two continents following the magnitude 9.0 earthquake off Sendai, Japan, on 11 March 2011. We confirm the tsunami signal with three different methodologies and compare the currents observed with coastal sea level fluctuations at tide gauges. The distance offshore at which the tsunami can be detected, and hence the warning time provided, depends on the bathymetry: the wider the shallow continental shelf, the greater this time. Data from these and other radars around the Pacific rim can be used to further develop radar as an important tool to aid in tsunami observation and warning as well as post-processing comparisons between observation and model predictions.

A description of temporal and spatial variability in the Bering Sea spring phytoplankton blooms (1997-1999) using satelite multi-sensor remote sensing

Abstract The Bering Sea is well known as a highly productive marginal sea. The objectives of this study were to clarify the interannual variability of spring bloom dynamics of the Bering Sea and to describe the spatial variability of this highly productive area using satellite multi-sensor remote sensing. We used multi-sensor remote sensing data sets of ocean color (OCTS and SeaWiFS), sea surface temperature (AVHRR), sea ice (SSM/I) and sea wind (SSM/I) to understand the complexity of the Bering Sea ecosystem. Phytoplankton biomass depends on the timing of sea ice melting and tends to increase when the melting is delayed. Wind stress is one of the important factors controlling the timing of the spring bloom. In 1997 and 1998, the east–west distribution of phytoplankton biomass exhibited a seesaw pattern, either high in west and low in east or low in west and high in east. We hypothesize that this seesaw pattern results from changes in the position and intensity of the Aleutian Low during spring and its relation to the El Nino–La Nina phenomena. During the El Nino period of 1998, the Aleutian Low shifted to the east of its normal position, and weak wind stresses facilitated the development of stratification and enhancement of spring bloom in the west. Conversely, when the Aleutian Low moved over into the western region in spring 1997, the same situation occurred in the east. Thus, the movements of the Aleutian Low promote a west-east seesaw pattern of sea surface wind stress, and consequently a corresponding seesaw pattern in phytoplankton biomass resulting from the subsequent variations in the depth of the mixed layer.

Temporal and spatial variability of coccolithophore blooms in the eastern Bering Sea, 1998-2001

Abstract In the late summer of 1997, aquamarine waters, resulting from a massive bloom of coccolithophore algae, covered most of the continental shelf of the eastern Bering Sea. This was the first such event recorded in the area, but since then, coccolithophore blooms have been common. The objectives of this study were first to determine the threshold value of the Sea Wide Field-of-view Sensor (SeaWiFS) coccolithophore algorithm using a comparison of satellite and ship-based optical observations, and second to analyze temporal and spatial variability using SeaWiFS images of the coccolithophore blooms in the Bering Sea from 1998 to 2001. In late July 1998 and 2000, we made bio-optical measurements of the coccolithophore blooms, which were composed of Emiliania huxleyi. In situ measurements of water leaving radiance (nLw) at characteristic wavelengths yielded values that differed from the standard NASA coccolithophore values. We therefore defined new threshold values for the mask and applied it to a time series of SeaWiFS images from the Bering Sea. Generally, the coccolithophore bloom began in February each year, as melting began along the edge of the sea ice, and then expanded northward. The bloom reached its peak in April, and then from May through August the area covered by the bloom dwindled in size. However, its area increased again in September. Throughout the study period, the blooms were located at depths of 20 to 100 m. There was annual and seasonal variability in the area affected by the coccolithophore bloom. There were large blooms in 1998 and 2000, whereas in 1999 and 2001 the area affected was smaller. These differences might be related to sea surface temperature (SST), since the largest blooms occurred in the warmest years.

The Kyucho in Sukumo Bay Induced by Kuroshio Warm Filament Intrusion

Surface temperature data obtained in and out of the bay all year round from March 1990 through February 1991, except from July through October 1990 were analyzed to investigate seasonal variability of theKyucho in Sukumo Bay, southwest of Shikoku, Japan. TheKyucho periodically occurs in the bay during both the warming period of March through June and the cooding period of November through February. The onset period of theKyucho is 8–15 days during the warming period and 4–14 days during the cooling period, giving an average of about 10 and 8 days, respectively. The position of the Kuroshio axis offshore in the south of Cape Ashizuri-misaki is a significant factor with theKyucho in the bay. Thermal infrared images taken by the NOAA-11 in the sea off east of Kyushu were also analyzed during the two observation periods. It is clearly found that a warm filament derived from the Kuroshio (KWF) advects northeast to Cape Ashizurimisaki along the Kuroshio, then encounters the southwest coast of Shikoku, followed by inducing theKyucho in the bay by the warm water intrusion. The alongshelf dimension of the KWFs is approximately 50–100 km, and the cross-shelf distance from the western edge of the KWFs to that of the body of the east Kuroshio is about 30–50 km. The KWF sometimes closely approaches to the east coast of Kyushu. An onshore meander of the Kuroshio front around Cape Toimisaki might grow into a KWF in the sea off east of Kyushu.

Verification plan of ocean color and temperature scanner atmospheric correction and phytoplankton pigment by moored optical buoy system

A moored optical buoy system has been developed by the National Space Development Agency of Japan (NASDA) for verification of ocean-observing remote sensing sensors of the Advanced Earth Observing Satellite (ADEOS). An underwater spectral radiometer was specifically designed for this optical system. The two upward radiance and downward irradiance correctors were connected with fiber optical system and were placed at the depth of 1.5 m and 6.5 m. The collector parts of the radiometer were extended from the center of the buoy system in order to avoid self shading with 3 m and 1.2 m arms at the depth of 1.5 m and 6.5 m, respectively. Spectral incident irradiance was measured at top of the buoy system. Phytoplankton pigment concentrations were monitored by Aquatracka-III fluorometer placed at the bottom of the surface float. Atmospheric temperature, pressure, humidity, wind speed and direction, water temperature at a depth of 1.2 m and 6.5 m, and wave height and direction were also monitored. In this paper, we present results from test deployment during January 16 to March 15, 1995, in the Suruga Bay, off Shizuoka, Japan, and compare the water-leaving radiance obtained by buoy with one observed by ship. The chlorophyll a concentrations calculated from water-leaving radiance were also compared with chlorophyll fluorescence measured by fluorometer. It was proved that the self shading and wind affected the radiance and irradiance in the sea. It becomes clear that the quality check of the buoy data is very important for verification of the satellite data. We present also the verification plan of atmospheric correction and phytoplankton pigment for ADEOS/ocean color and temperature scanner (OCTS) using the sea truth data obtained from this buoy system and ships.