Unusual Behavior of the Kuroshio / Kuroshio Extension from Winter 1996 to Summer 1997 Observed by Using ADEOS-OCTS with Some External Conditions
Yoshiaki Toda (Earth Observation Research Center National Space Development Agency of Japan, 1-9-9 Roppongi, Minato-Ku, Tokyo 106-0032, Japan; and Japan Marine Science and Technology Center, 2-15 Natsushima, Yokosnka 237-0061, Japan)
e-mail: toba@eorc.nasda.go.jp, tobay@jamstec.go.jp
Hiroshi Murakami, (Earth Observation Research Center, National Space Development Agency of Japan)
Junichi Kimura (Japan Marine Science and Technology Center; and Metocean Co. Ltd.)
Myoungsun Kim (Japan Marine Science and Technology Center)
Yasushi Yoshikawa (Japan Marine Science and Technology Center)
Koji Shimada (Japan Marine Science and Technology Center)
EXTENDED ABSTRACT
While the Advanced Earth Observing Satellite (ADEOS) was operaing, the Kuroshio and the Kuroshio Extension exhibited unusual behavior from the winter of 1996 to the summer of 1997. This behavior has been closely studied using a time series of satellite observation images of sea-surface temperature (SST) and ocean color obtained by ADEOS-OCTS, SST images obtained by NOAA-AVHRR, and some hydographic observation data.
The particular behavior includes: (1) A long lasting very southerly path of the Kuroshio Extension; (2) A Kuroshio path very distant from Japan with the following alternating-jet-like north-south flow pattern of the Kuroshio Extension, which occurred twice, once in February and once in April 1997, as independent events, and which was observed to be affected by the bottom topography of the Izu-Ogasawara Ridge and Trench and the Japan Trench, the latter event being more conspicuous; (3) Cutting off of a cold water mass after the February event; (4) The formation of a vortex pair after the April event. A new mechanism is suggested for the formation of the alternating-jet-like flow pattern: a topographically forced alternating-jet instability (AJI). Detailed description of this part of research is referred to Toba and Murakumi (1998).
A close comparison of the ADEOS-OCTS data for upper ocean chlorophll-α data (phytoplankton concentration) and detailed bottom topography chart indicates that, during the April 1997 event, the path of the Kuroshio Extension core exactly followed the eastern side of the Izu-Ogasawara Trench up to the east of the Boso Peninsula, and then flowed to avoid the Daiichi Kashima Seamount. One small branch of it turned around the Seamount. This strongly indicates that the Kuroshio Extension was greatly affected by the bottom topography all the way through the sea surface.
Some external conditions, which might have caused this unusual behavior, have been investigated by using TOPEX/POSEIDON altimeter data. The altimeter data show that the sea-surface gradient across the Kuroshio/Kuroshio Extension diminished greatly at the east and west sides of the Izu-Ogasawara Ridge, as a very temporal phenomenon just prier to the April event: in early to middle April. This means that the upper ocean flow of the Kuroshio became weak at this period by some reason, something like cancellation of eddy motion by coalescence of counter vorticity eddies. As a matter of fact, there were many eddy structures in this area of the ocean before and after this short period.
