Satellite and ship observations of Kuroshio warm-core ring 93A off Sanriku, northwestern North Pacific, in spring 1997

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 chlorophylla (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/m³ during April. The spring bloom in the ring occurred early in May and the relatively high maximum (>1.0 mg/m³) continued from early in May to mid-June. In late June, a ship-observed surface Chl-a concentration of less than 0.4 mg/m³ 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 concentration 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.     

Description of the oceanographic condition off Sanriku, northwestern Pacific, and its relation to spring bloom detected by the Ocean Color and Temperature Scanner (OCTS) images

We describe the oceanographic condition as observed by hydrographic data and phytoplankton spring bloom detected by OCTS images off Sanriku, northwestern Pacific, during the spring bloom period in 1997. The relationship between the two is discussed. OCTS images detected the bloom in early April in the coastal area around the Izu ridge north of the Kuroshio and the eastern coastal area of Hokkaido to the Oyashio front. The bloom areas were seen along the offshore Kuroshio Extension from the end of April, in the upstream region of the Oyashio south of the Kurile Islands, except for a part of coastal area from the end of May, and in the Kuroshio warm-core ring 93A (KWCR 93A) from early June. The temperature difference between the surface and subsurface layer is used as a stratification index. This was large in the upstream region of the Oyashio south of the Kurile Islands and KWCR 93A from early June. Previous research has pointed out that the spring bloom usually corresponds to the development of stratification in the water column due to seasonal warming. In addition to that, we suggest that the transportation of water containing a high chlorophylla concentration by advection due to strong currents, like the Kuroshio and the Oyashio, is important for the formation of an area of high chlorophylla concentration. These results indicate that the OCTS images are useful for a knowledge of the distribution and the change of chlorophylla concentration in the northwestern Pacific region.