Observationally Estimated Transport of the Kuroshio South of Japan
Shiro Imawaki (Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan)
e-mail: imawaki@riam.kyushu-u.ac.jp
Hiroshi Uchida (Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan)
e-mail: uchida@riam.kyushu-u.ac.jp
The ASUKA Group (ASUKA stands for Affiliated Surveys of the Kuroshio off Cape Ashizuri)
ABSTRACT
Recently we carried out observations of the Kuroshio south of Japan, including direct current measurements and repeated hydrographic surveys. Estimated transports of the Kuroshio are proportional to sea surface height differences across the Kuroshio. Having this relationship and using satellite altimetry data, we obtain a five-year record of the Kuroshio transport. The average is estimated to be 42×106m3/sec, after excluding the recirculation transport. The seasonal signal is much smaller than the theoretical one.
INTRODUCTION
Oceans as well as the atmosphere are canying heat from equatorial regions to polar regions, to maintain the moderate climate of the earth (1). For the Atlantic, the meridional overturning plays an important role in the net meridional heat transport. On the other hand, horizontal surface circulation such as the subtropical gyre are believed to be important for the North Pacific (2). The Kuroshio, the western boundary current of the North Pacific subtropical gyre, plays an important role by carrying relatively warm water to the north in order to compensate the relatively cooler water flowing southward in the interior. This subtropical gyre is maintained mostly by the wind stress over the ocean.
The transport of the Kuroshio, however, has not yet been estimated accurately, because longterm current measurements of strong currents can not be easily carried out. Most information about the Kuroshio transport has been provided by geostrophic calculation under the assumption of no-motion at some abyssal depths; the transport thus estimated is a relative transport, which is missing the vertically-constant component, i. e., the barotropic component. A few exceptions are pioneer studies (3-6), in which direct current measurements were carried out to estimate the absolute transport of the Kuroshio. No previous studies, however, provided us with a long time series of the absolute transport on the basis of direct current measurements.
