Unusual Behavior of the Kuroshio / Kuroshio Extension form Winter 1996 to Summer 1997 Observed by Using ADEOS-OCTS with Some External Conditions
Y. Toba*1,2, H. Murakami*1, J. Kimura*2,3, M. Kim*2, Y. Yoshikawa*2 and K. Shimada*2
*1 Earth Observation Research Center, National Space Development Agency of Japan
*2 Japan Marine Science and Technology Center
*3 Metocean Co. Ltd.
While the Advanced Earth Observing Satellite (ADEOS) was operating, 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 SST and ocean color obtained by ADEOS-OCTS, reinforced by SST images obtained by NOAA-AVHRR. It includes (i) a long lasting very southerly path of the Kuroshio Extension; (ii) an 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 of the Japan Trench; (iii) cutting off of a cold water mass after the February event; and (iv) the formation of a vortex pair after the April event. A new mechanism is suggested for the formation of the alternating-jet flow pattern : a topographically forced alternate-jet instability (AJI). Some external conditions, which might have caused this unusual behavior, have been investigated by using TOPEX / POSEIDON altimeter and other 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. This might have caused the relative , intensification of the deeper layer current, which in turn raised a stronger bottom effect, resulting in the occurrence of AJI.
