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Journal of Oceanography Vol. 48, pp. 305 to 327. 1992
Phytoplankton Pigment Distributions in Regional Upwelling around the Izu Peninsula Detected by Coastal Zone
Color Scanner on May 1982
J. ISHIZAKA1, H. FUKUSHIMA2, M. KISHINO3, T. SAINO4 and M. TAKAHASHI5
1National Institute for Resources and Environment, Tsukuba 305, Japan 2High-Technology for Human Welfare, Tokai University, Numazu 410-03, Japan
3The Institute of Physical and Chemical Research, Wako 351-01, Japan 4Ocean Research Institute, University of Tokyo, Nakano-ku, Tokyo 164, Japan
5Department of Botany, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
(Received 20 January 1992; in revised form 17 April 1992; accepted 24 April 1992)
Phytoplankton pigment (chlorophyll a + pheopigments) distributions in a regional upwelling around the Izu Peninsula obtained by the Coastal Zone Color Scanner (CZCS) on May 23, 1982, were compared with ship-observed pigment and satellite sea-surface-temperature distributions. Pigment concentrations detected by the CZCS were positively correlated with the ship-observed pigment concentrations. However, they were about factor of 5 smaller when atmospheric correction parameters known for typical oceanic and land aerosol were used and when the parameters were estimated with the “clear water algorithm”. When the atmospheric correction parameters were adjusted so that a pigment concentration derived by CZCS was equivalent to a concentration obtained by the ship at a coincide location, the pattern and magnitudes of the CZCS-derived pigment distributions showed remarkable agreement with ship-observed pigment distributions. Thus, the nor- mal atmospheric correction algorithm may not be suitable for waters around Japan, and the development of better atmospheric correction methods combined with more verification programs is required. The pigment distributions showed patterns that were similar to those observed in sea-surface-temperature distri- butions. Cold water showed higher pigment concentrations, and warm water showed lower pigment concentrations. The Kuroshio, which can be identified by generally warm, low pigment water, showed a large meander which flowed offshore at Shiono-misaki, looped back onshore from Hachijo Island to Omaezaki and then flowed northeast along the Izu and Boso Peninsulas. Locally upwelled water along the Izu Peninsula was seen clearly in the sea-surface-temperature and CZCS pigment distributions as a region of cold water and high pigment concen- trations. Cold upwelled waters were also found at the eastern side of the Izu Islands, but pigment concentrations in these waters was not always high. This difference in the two upwelling regions may be caused by different physical and biological interactions.
1. Introduction Recent advances in remote sensing technology allow measurement of phytoplankton
pigment (chlorophyll a + pheopigments, hereafter called “pigment”) concentrations in the surface layer of the ocean from space. The Coastal Zone Color Scanner (CZCS) was the first
306 J. Ishizaka et al.
sensor specifically designed for this purpose (Hovis et al., 1980; Gordon et al., 1980). This sensor was on the Nimbus-7 satellite which was launched on November 1978 and was operated until June 1986. About 68,000 scenes from the world ocean, including about 4,000 scenes around Japan, were collected by the CZCS and are stored in an archive at the NASA Goddard Space Flight Center (Feldman et al., 1989; Ishizaka and Harashima, 1991). The CZCS sensor covered a region of 1636 km in each scan. This scan length allowed at least one measurement per day of the global ocean if the sensor power was on continuously. However, the sensor was not always continuously operated. The smallest spatial resolution of the sensor was 825 m.
During the CZCS operation, the data were compared with ship observations and accuracy of the CZCS measurements was determined. Most of the verification studies of CZCS mea- surements were conducted in waters around the U.S. coast. Errors in the measurements were on the order of 30–40% but could be as large as a factor of 2 (Smith and Baker, 1982; Gordon et al., 1982, 1983; Gordon and Morel, 1983). Recent studies have focused on applications of CZCS data for variety of ocean processes. These studies include using CZCS data for the statistical analysis of physical-biological interactions (cf. Abbott and Zion, 1987), estimation of primary production (cf. Platt and Sathyendranath, 1988; Sathyendranath et al., 1991a), comparisons to fisheries distributions, verification and improvement of physical-biological models (Ishizaka, 1990a, b, c), and estimation of processes contributing to phytoplankton concentration changes (McClain et al., 1990a). Recent applications of passive remote sensing in oceanography, including ocean color are reviewed by Abbott and Chelton (1992).
The first use of the CZCS for studying the waters around Japan was by Sasaki et al. (1983) who obtained pigment concentrations for the Yellow Sea. More recently, Ogishima et al. (1986) and Hiramatsu et al. (1987) have pointed out problems with using the ordinary atmospheric correction methods for processing CZCS images from the waters around Japan. These studies have indicated the usefulness and problems of using satellite ocean color remote sensing around Japan. However, because of the limited availability of coincident ships and CZCS measurements, there has been no attempt to make comparison between satellite and ship-observed pigments (Ishizaka and Harashima, 1991). Also, although the potential usefulness of CZCS data is clear, there have been few attempts to use CZCS data to understand oceanographic processes in waters around Japan. Matsumura and Fukushima (1988) used CZCS pigment data to categorize the relations between satellite-derived pigment and sea-surface-temperature distributions around Japan. Recent works relating to ocean color studies in the waters around Japan are reviewed in Fukushima and Ishizaka (1992).
There is much evidence that shows that regional upwelling occurs around Japan which results in enhanced biological production (Takahashi et al., 1980, 1986; Takahashi and Kishi, 1984; Toda, 1989). However, the upwelling patterns change rapidly, which make it difficult to observe the spatial distributions of the upwelling and associated phytoplankton variations with ship surveys. Furthermore, regional ship surveys are not sufficient to determine relationships between local upwelling events and larger scale oceanographic phenomena, such as Kuroshio meandering. Satellites are the only tool currently available that can instantaneously observe the spatial distribution of regional upwelling as well as the larger scale ocean patterns. Infrared sea- surface-temperature images provide an overall view of upwelling patterns as regions colder than the surrounding area, and ocean color images give pigment distributions which may be related to upwelling regions. Takahashi et al. (1981) used an airborne infrared sensor to map upwelling regions and found a series of cold vortices associated with upwelling at the eastern side of Oshima
Pigment Distributions in Regional Upwelling 307
Island. Aikoh and Takahashi (1982) used satellite infrared images from the AVHRR/NOAA-6 sensor, and found similar vortices around Oshima Island. Atkinson et al. (1987a) also showed the upwelling regions around the Izu Peninsula and Islands in May, 1982, using the infrared images of AVHRR/NOAA-7.
In this study, CZCS-derived pigment distributions were compared with ship-observed pigment patterns from the regional upwelling around the Izu Peninsula, Japan that were measured on May 23, 1982. The pigment distributions were described in terms of the regional upwelling and associated biological production. An infrared sea-surface-temperature (SST) image for this time was also processed and compared with the pigment image.
The next section introduces the observations of the regional upwelling around the Izu Peninsula that were made on May 1982, and Section 3 briefly reviews the CZCS image pro- cessing procedure. Section 4 describes the results of comparisons between CZCS and ship- observed pigments obtained around the Izu Peninsula on May 23, 1982, and Section 5 describes the oceanographic conditions of the area and the comparisons between satellite-derived pigment and SST fields. Section 6 gives concluding remarks.
2. Regional Upwelling around Izu on May 1982 Oceanographic observations were conducted between the Izu Peninsula and Oshima Island
(Fig. 1) from 22 to 26 May 1982 by the R/V Tansei Maru of the Ocean Research Institute of the University of Tokyo. The observations included simultaneous measurement of SST, pigment, and nitrate plus nitrite with a continuous measuring system (Fig. 2). CTD observations and water sampling with a Rosette sampler were also made. Detailed descriptions of the sampling techniques and the observations are found in Atkinson et al. (1987a) and Takahashi et al. (1986).
During the observation period, the Kuroshio meandered offshore at Shiono-misaki, returned onshore from Hachijo Island to Omaezaki, and flowed northeast along the Izu Peninsula (Atkinson et al., 1987a). Upwelling was observed around Tsumeki Point on the Izu Peninsula. It has been suggested that this local upwelling was related to offshore movement of the Kuroshio in this area on 21 and 22 May subsequent onshore movement of the Kuroshio on 23 and 24 May pushed the upwelled water nearer the coast. A second upwelling event occurred off Tsumeki Point on 25 and 26 May.
Takahashi et al. (1986) explained time changes in the surface chlorophyll a and nitrate plus nitrite in the upwelled water with a simple growth model that was based on simulate