Title Variation of oceanographic conditions and sea water exchange associated with northward migration of the temperature front south of Kyushu, Japan
Authers Tsutomu SAITO
Keywords temperature front, Kuroshio frontal eddy, Ohsumi Strait, water exchange, Japanese jack mackerel
Citation Bull. Fish. Res. Agen. No.27, 1-41, 2009
Abstract
As larvae and juveniles of Japanese jack mackerel, whose main spawning ground is the southern East China Sea, are transported to the waters off the southern coast of Japan by currents, they pass the Kuroshio front, south of Kyushu. With interest in the transport of larvae and juveniles, this study focused on the variation of temperature and velocity field south of Kyushu, and sea water exchange as the result.
 The surface temperature front off the southern coast of Kyushu is known to migrate northward repeatedly, having about a 20 day cycle. In connection with the migration of the front, formation and northeastward propagation of a warm tongue-like structure, periodic change in the sea level at Nakanoshima, and formation of the eastward current through the Ohsumi Strait have been reported. Furthermore, it has also been reported that an anticyclonic Kuroshio meander and a cyclonic eddy developed by turns on the western side of the Tokara Strait has similar cycles. Their features, however, are known only fragmentarily. We tried to describe the time evolution of the front in a three-dimensional way. By CTD (Conductivity Temperature Depth recorder)and LADCP(Lowered Acoustic Doppler Current Profiler), we carried out three-dimensional dense observations on temperature and velocity six times in the period 2000-2003. The observations were rearranged to give a sequence of temperature and velocity fields which described the evolution of the front, by using the north-south position of the front as an index. The rearrangement was justified by comparing with successive satellite images of sea surface temperature obtained in a different period.
 The sequence of temperature and velocity fields showed that northward migration of the temperature front was an aspect of the formation, development and dissipation of an anticyclonic Kuroshio frontal eddy on the continental slope south of the Satsuma Peninsula. The following variation of temperature and velocity field then became clear. In the observation area, the anticyclonic meander of the Kuroshio north edge propagated eastward, and was developed as it approached the west coast of Yakushima, then the temperature front migrated northward and reached Yakushima. The further eastward propagation of the meander caused the formation and development of the anticyclonic eddy on the continental slope, south of the Satsuma Peninsula. Since the currents had the structure of reaching a depth of 300 m similar to the Kuroshio north edge, it was appropriate to view the eddy as a Kuroshio frontal eddy. The warm tongue-like structure in the sea surface temperature field was an aspect of the anticyclonic frontal eddy, and the migration of the front on the ferry route from Yakushima to Satamisaki occurred by northeastward propagation of the eddy. The eddy finally dissipated near the Satsuma Peninsula. There, the anticyclonic meander of the Kuroshio north edge propagated to the south of Yakushima. The sea level change at Nakanoshima was accompanied by the eastward propagation of the meander. Moreover, the anticyclonic frontal eddy was accompanied by the cyclonic frontal eddy in the west or southwest. The eastward current had a velocity of more than 100 cm/sec in the Ohsumi Strait.
 The analysis was made on the water types south of Kyushu using CTD/LADCP data and towed ADCP data obtained from the observations when the strong eastward current was formed throughout the Ohsumi Strait. The results showed that the current transported the water which originated in the East China Sea from the southwest of Kyushu to the waters off the southern coast of Japan. The volume transported on the continental shelf around Tanegashima and Yakushima, corresponding to the Ohsumi Strait for the most part, measured by the towed ADCP was comparable with that of the Tsushima Warm Current. If the intensification of the eastward current through the Ohsumi Strait is a part of the velocity variation of the approximate 20 day cycles, it would be interesting that the strength of the variation may affect the quantity of the larva and juvenile of Japanese jack mackerel transported to the waters off the southern coast of Japan from the East China Sea.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull27/saitou.pdf