Mitsuhiro Toratani

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.

Correction of atmospheric effect on ADEOS/OCTS ocean color data: Algorithm description and evaluation of its performance

This paper first describes the atmospheric correction algorithm for OCTS visible band data used at NASDA/EOC. Sharing a basic structure with Gordon and Wang’s Sea WiFS algorithm, it uses 10 candidate aerosol models including the “Asian dust model” introduced in consideration of the unique feature of aerosols over the east Asian waters. Based on the observations at 670 and 865 nm bands, the algorithm selects a pair of aerosol models that account best for the observed spectral reflectances, and synthesizes the aerosol reflectance used for the atmospheric correction. Two different schemes for determining the value of the parameter for the aerosol model selection are presented and their anticipated estimation error is analyzed in terms of retrieved water reflectance at 443 nm. The results of our numerical simulation show that the standard deviation of the estimation error of the “weighted average” scheme is mostly within the permissible level of ±0.002, reducing the error by 18% on average compared to the “simple average” scheme. The paper further discusses the expected error under the old CZCS-type atmospheric correction, which assumes constant aerosol optical properties throughout the given image. Although our algorithm has a better performance than the CZCS algorithm, further analysis shows that the error induced by the assumption taken in the algorithm that the water-leaving radiance at 670 nm band is negligibly small may be large in high pigment concentration waters, indicating the necessity for future improvements.

Asian dust aerosol: Optical effect on satellite ocean color signal and a scheme of its correction

The paper first exhibits the influence of the Asian dust aerosol (KOSA) on a coastal zone color scanner (CZCS) image which records erroneously low or negative satellite-derived water-leaving radiance especially in a shorter wavelength region. This suggests the presence of spectrally dependent absorption which was disregarded in the past atmospheric correction algorithms. On the basis of the analysis of the scene, a semiempirical optical model of the Asian dust aerosol that relates aerosol single scattering albedo (ωA) to the spectral ratio of aerosol optical thickness between 550 nm and 670 nm is developed. Then, as a modification to a standard CZCS atmospheric correction algorithm (NASA standard algorithm), a scheme which estimates pixel-wise aerosol optical thickness, and in turn ωA, is proposed. The assumption of constant normalized water-leaving radiance at 550 nm is adopted together with a model of aerosol scattering phase function. The scheme is combined to the standard algorithm, performing atmospheric correction just the same as the standard version with a fixed Angstrom coefficient except in the case where the presence of Asian dust aerosol is detected by the lowered satellite-derived Angstrom exponent. Some of the model parameter values are determined so that the scheme does not produce any spatial discontinuity with the standard scheme. The algorithm was tested against the Japanese Asian dust CZCS scene with parameter values of the spectral dependency of ωA, first statistically determined and second optimized for selected pixels. Analysis suggests that the parameter values depend on the assumed Angstrom coefficient for standard algorithm, at the same time defining the spatial extent of the area to apply the Asian dust scheme. The algorithm was also tested for a Saharan dust scene, showing the relevance of the scheme but with different parameter setting. Finally, the algorithm was applied to a data set of 25 CZCS scenes to produce a monthly composite of pigment concentration for April 1981. Through these analyses, the modified algorithm is considered robust in the sense that it operates most compatibly with the standard algorithm yet performs adaptively in response to the magnitude of the dust effect.

Atmospheric correction algorithms for ADEOS/OCTS ocean colar data: Performance comparison based on ship and buoy measurements

Abstract The paper aims at evaluating the performance of the atmospheric correction algorithm for the Ocean Color and Temperature Scanner (OCTS) visible band data used at Earth Observation Center (EOC) of National Space Development Agency of Japan (NASDA). The algorithm uses 10 candidate aerosol models including “Asian dust model” introduced in consideration of the unique feature of aerosols over the east Asian waters. Based on the observations at 670 and 865 nm bands where the reflectance of the water body can be discarded, the algorithm selects a pair of aerosol models that accounts best for the observed spectral reflectances to synthesize the aerosol reflectance in other bands. Three different schemes of the aerosol model selection are presented and their anticipated estimation error in terms of the retrieved water reflectance at 443 nm is analyzed. The results of our numerical simulation show that the standard deviation of the estimation error of the “weighted average with iteration” scheme is mostly within permissible level of ±0.002 in-water reflectance at 443 nm, reducing the error by roughly a factor of 2 compared to the other schemes. The paper also evaluates the performance of the algorithm by comparing the satellite estimates of “water-leaving radiance” nL W and chlorophyll-a concentration with selected buoy- and ship-measured data. In comparison with the old “CZCS-type” atmospheric correction, the OCTS algorithm records factor 2–3 less error in estimating nL W .

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.

Vicarious calibration of ADEOS-2 GLI visible to shortwave infrared bands using global datasets

We have developed a global vicarious calibration scheme for spaceborne ocean-color sensors, simulating top-of-atmosphere radiance globally using a radiative transfer model, SeaWiFS Level 3 eight-day mean products, and an in-water optical model. This is a relative calibration against two channels used to detect aerosol properties; however, it enables us to determine the spatial and temporal characteristics of the vicarious calibration coefficients (Kvc) without in situ observations. We applied this scheme to the NASDA Global Imager (GLI), which operated from January 25, 2003 to October 24, 2003. Kvc exhibited the following properties: (1) channel characteristics of 1.0-1.1 (GLI was lower than the simulation) in channels 1-9 (380-565 nm), nearly 1.0 in channels 10-19 (625-865 nm), and 0.91-0.98 in channels 24-29 (1050-2210 nm); (2) scan-angle dependency and its temporal changes in channels 1-3; and (3) scan-mirror side differences and temporal changes. Applying Kvc to GLI ocean-color processing produced outputs consistent with the ground observation data. This scheme is also useful for generating consistent products from different ocean-color sensors in orbit.

Interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s

Abstract Surface chlorophyll a concentrations were measured at seven stations located at 1° latitude intervals between 52°30′ N and 58°30′ N along longitude 179°30′ W, in late June and early July from 1991 through 1999. Surface chlorophyll a concentrations at the same locations were estimated from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data from mid June through mid July 2000. Shipboard data collected from 1993–95 showed that the surface chlorophyll a concentration was correlated with chlorophyll a standing stock integrated in the upper 100 m of the water column. Based on the 8-day time series of mean chlorophyll a concentrations at stations on 179°30′ W in 1998, 1999, and 2000 from SeaWiFS data, high concentration and great variability were observed in chlorophyll a in June. When examining interannual differences in phytoplankton biomass, it is preferable to use a month when high chlorophyll a concentrations are high, and show great variability than one when its concentrations are low with little variability. Thus a comparison of surface data from shipboard and satellite observations in June, was considered best to represent the interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s. There were no significant differences in chlorophyll a concentrations among years (1991–2000) or among stations, though high chlorophyll a concentrations were observed occasionally around the shelf edge. Chlorophyll a concentrations rarely exceeded 2 μg l−1 in the Bering Sea basin and the values were usually

Primary production enhancement by typhoon Ketsana in 2003 in western North Pacific

The purpose of this works was to investigate temporal and spatial variation of chlorophyll-a concentration and sea surface temperature before and after the typhoon Ketsana passage in subtropical western North Pacific Ocean and to evaluate primary production enhancement by satellite data. Chlorophyll-a concentration kept higher level (>0.1mg/m3) for one month after typhoon Ketsana passage Maximum value of chlorophyll-a concentration was 3.5 mg/m3 that is 70 times more than a normal condition of the area in the seventh day after the passage. Sea surface temperature decreased from 30 to 22 °C. The lowest Sea surface temperature was recorded in Oct. 25 after two days the passage. Time rag was 4 days between Sea surface temperature minimum and chlorophyll-a concentration maximum. Primary production enhancement by the typhoon was estimated 727 Gg Carbon. Carbon fixation by typhoon Ketsana was estimated about 0.11% of western North Pacifics annual new production.

Optical properties of inorganic suspended solids and their influence on ocean colour remote sensing in highly turbid coastal waters

The influence of the optical properties of inorganic suspended solids (ISS) on in-water algorithms was evaluated using an optical model in highly turbid coastal water, whose ISS concentration reached several hundred grams per cubic metre. The measurements were conducted in the upper Gulf of Thailand. The backscattering coefficient of the ISS was calculated using the Lorenz–Mie scattering theory. On the basis of the measurement, the ISS size distribution was parameterized as a function of ISS concentration, and both the spherical and non-spherical particle shape models were evaluated. For ISS concentrations of 10 g m−3, an estimate of the chlorophyll-a (chl-a) concentration within a factor of 2 on a logarithmic scale is possible in a [chl-a] range of 4–30 mg m−3. The differential coefficient of remote sensing reflectance was calculated to evaluate its respective sensitivities for chl-a and ISS concentrations. The use of radiometric data at 670 nm (700–900 nm) is valid for in-water algorithms used to estimate chl-a (ISS) concentration in highly turbid coastal waters.

Atmospheric correction scheme for ocean color remote sensing in consideration to Asian dust aerosol

The transport of Asian dust particles (KOSA) through the atmosphere to the Northwest Pacific is the most intensive one of this kind on the globe and is important in terms of the geochemical ocean cycle. The dust particles are also known to affect atmospheric correction process of satellite ocean color data, resulting in erroneous estimation of phytoplankton pigment concentration which, in turn, affects the proper evaluation of primary production in the area. Considering this problem, the authors propose a modified atmospheric correction scheme for Nimbus-7/CZCS data, based on the standard Gordons (1983) algorithm. To process the imagery the authors assumed that normalized water-leaving radiance at 550 nm was constant and that aerosol single scattering albedo at 550 nm covaries with the spectral dependency of aerosol scatterance. The single scattering albedo and the optical depth of aerosol at 550 nm are determined from the 550 nm and 670 nm channels by pixel-wise iterative procedures. The scheme processes data for other channels by assuming a spectral dependency of aerosol single scattering albedo modeled after obtaining a typical Asian dust image. The method was applied to selected Japanese CZCS scenes, and the derived phytoplankton pigment concentration was compared with Gordons algorithm. For images with well-behaved atmospheric condition, the results in phytoplankton pigment concentration mere overestimated by Gordons method. The scheme is considered useful to correct for the atmospheric effect of absorptive aerosol.<<ETX>>