Journal of Oceanography, Vol. 54, pp. 495 to 508. 1998 495 Copyright  The Oceanographic Society of Japan. Keywords: ⋅ OCTS, ⋅ Kuroshio warm- core ring 93A, ⋅ chlorophyll a, ⋅ spring bloom, ⋅ double spiral structure, ⋅ streamer, ⋅ off Sanriku, ⋅ Oyashio. Satellite and Ship Observations of Kuroshio Warm-Core Ring 93A off Sanriku, Northwestern North Pacific, in Spring 1997 SEI-ICHI SAITOH 1 , DENZO INAGAKE 2 *, KOUSEI SASAOKA 1 , JOJI ISHIZAKA 3 **, YUKIHIRO NAKAME 4 and TOSHIRO SAINO 5 1 Faculty of Fisheries, Hokkaido University, 3-1-1, Minato-cho, Hakodate 041-8611, Japan 2 Fisheries Oceanographic Division, Tohoku National Fisheries Research Institute, 3-27-5, Shinhama-cho, Shiogama, Miyagi 985-0001, Japan 3 National Institute of Resources and Environment, 16-3, Onogawa, Tsukuba 305-8569, Japan 4 Hokkaido Kushiro Fisheries Experimental Station, 2-6, Hama-cho, Kushiro 085-0024, Japan 5 Institute for Hydrospheric-Atmospheric Sciences, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan (Received 31 March 1998; in revised form 27 July 1998; accepted 29 July 1998) Synoptic ship and satellite observations were performed of the Kuroshio warm-core ring (KWCR) 93A and its adjacent waters, off Sanriku, northwestern North Pacific, between early April and late June 1997. The temporal and spatial distribution of chlorophyll a (Chl-a) and sea surface temperature in the study area were analyzed using data from ADEOS Ocean Color and Temperature Scanner (OCTS) and NOAA Advanced Very High Resolution Radiometer (AVHRR). The objective of this study was to describe the temporal and spatial variability of the spring bloom and understand its relationship with the changes in the hydrographic structure of these waters in and around KWCR 93A. The maximum value of Chl-a concentration in the ring was less than 1 mg/m 3 during April. The spring bloom in the ring occurred early in May and the relatively high maximum (>1.0 mg/m 3 ) continued from early in May to mid-June. In late June, a ship-observed surface Chl-a concentration of less than 0.4 mg/m 3 suggests that the spring bloom had already declined in and around KWCR 93A. Double spiral structures of warm and cold streamers appeared from late April to mid-May, which may have an influence on the occurrence of the spring bloom in and around the ring. In this episodic event, the warm streamer can maintain the available potential energy of the ring and the strength of upwelling around the ring. The cold streamer provided water with a high Chl-a con- centration to the surface layer of the ring. In order to understand the temporal and spatial variability of Chl-a distribution in the ring, the behavior of the warm and cold streamers needs to be taken into consideration. 1. Introduction By virtue of their unique physical characteristics, Kuroshio warm-core rings (KWCRs) that detach from the Kuroshio Extension play an important role in regulating biological processes in the waters off Sanriku, northwestern North Pacific. Several oceanographic studies undertaken in the past have revealed the unique characteristics of the KWCR’s (e.g. Kawai, 1972; Hata, 1974; Kitano, 1975; Saitoh et al., 1984; Kawai and Saitoh, 1986; Kawamura et al., 1986; Okada and Sugimori, 1986; Tomosada, 1986; Saitoh et al., 1986). The hydrographic characteristics asso- ciated with the KWCR 86B and their seasonal variability were revealed by the studies carried out in 1980s (e.g. Yasuda et al., 1992; Tsuda and Nemoto, 1992; Terazaki, 1992; Sugimoto and Tameishi, 1992; Sugimoto et al., 1992). Recently, Inagake (1997) described the year-to-year vari- ability of the Kuroshio warm-core ring migration using historical oceanographic data sets from 1981 to 1996. There are only a few studies on chlorophyll a (hereafter Chl-a) distribution and primary productivity in the KWCRs (Yokouchi et al., 1993; Shiomoto et al., 1996). Using a three- *Present address: Oceanography and Southern Ocean Re- sources Division, National Research Institute of Far Seas Fisher- ies, 5-7-1, Orido, Shimizu, Shizuoka 424-8633, Japan. **Present address: Faculty of Fisheries, Nagasaki Univer- sity, 1-14, Bunkyo, Nagasaki 852-8521, Japan.