Yoshiaki Toba
Chief Scientist (Earth Observation)
Japan Space Exploration Agency

A combination of simultaneous images of ocean color and sea surface temperature (SST) by OCTS can reveal oceanic conditions more clearly than the use of SST images alone. A good example is the finding of unusual paths of the Kuroshio in the spring of 1997.

The Kuroshio, the western boundary current of the North Pacific Ocean, has two major groups of paths: those with large meander and those without large meander. The time periods of these two groups do not switch periodically but with time scales of a few years. When there is no large meander, the Kuroshio flows along Honshu. When there is a large meander, the Kuroshio flows off Honshu usually south of Kii Peninsula to the southeast then turns to the north, usually along the west slope of the Izu-Ogasawara Ridge, to flow over the Ridge around 34N32N to the east. There is a cold water mass off Kii Peninsula or Enshunada, surrounded by the large meander.

Since December 1996, the Kuroshio was observed to take a very southerly path. In particular, the OCTS ocean color and SST images of 26 April revealed that the Kuroshio path reached 31N on the Izu-Ogasawara Ridge. At the same time, the northern cold water mass reached down to 32N to the east of the Izu-Ogasawara Ridge, the Kuroshio flowed south of this cold water, then flowed straight to the north at 142E along the east slope of the Japan Trench, reaching 37N. From that point, the Kuroshio Extension did not flow eastward immediately, but flowed like a pin curl south down to 34N then eastward. Further to the east, at 146E, a cold water mass was seen flowing straight south.

It is very unusual for the Kuroshio to take such a southerly path around the Izu-Ogasawara Ridge, with many meridional flow patterns. Also, it is unusual that the Kuroshio Extension flowed south of 35N for such a long period (since December 1996). The cause of these rather unusual flow patterns is to be investigated oceanographically. However, there might be some possibility that it is related to the recent global warming.

The OCTS data of 25-27 May 1997 revealed that the Kuroshio cut off a conspicuous cold water mass to the south of the Kuroshio Extension to the east of the Izu-Ogasawara Ridge. A similar phenomenon also occurred from February to March.

The SST pattern as observed by satellite is the temperature at the very surface of the oceans of about 20 micro meters thickness. This is affected by the short time-scale variability of the SST due to air-sea boundary processes including solar radiation, evaporation and other factors, so the pattern does not necessarily correspond to the current pattern. The ocean color data, however, represents the average phytoplankton concentration of the upper ocean of the order of a few tens of meters in thickness. Consequently, the combination of the SST and ocean color images is very useful for studying the oceanic conditions, though the phytoplankton concentration is not a conservative quantity since it includes chemical and biological processes of some time scales.

It is also noted in the ocean color image of 25 May 1997 that the cut-off cold water mass to the southeast of the Boso Peninsula corresponds to a very good fishing area of bonitos. OCTS data is thus very useful for fisheries purposes also.

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