Title The Oceanic Condition in the Region East of Kyushu I : Intensive Survey in September of 1984

Intensive oceanographic survey was carried out on Sept. 12 and 13 of 1984 in the region east of Kyushu. The CTD observations were made at 14 stations on the two parallel sections ESE of Hyuga and Nobeoka, ca 10 miles apart to each other. In addition, at 29 stations in a small triangular area in this region, the XBT observations were made. The distances of two adjacent CTD stations are mainly ca 5 miles and XBT ca 1 or 2 miles. The preliminary results are as follows. Was found the down intrusion of less saline surface water to a depth of 30 m layer of a point ca 10 miles apart. The isolated saline water was detected on the subsurface layer near the coast. The geostrophic calculation refered to 1000 dbar revealed the existence of coastal counter current with the maximum SSW component of 55 cm/sec. The width of this counter current is more than 30 miles off Nobeoka (nouthern section) and less than 15 miles off Hyuga (southern section). The XBT results show that the widths of small scale cold cores and warm tongues are in the order of two miles. 1 . Introduction It is a well known fact that the Kuroshio flows northeastward in the offshore region east of Kyushu in general, and it changes the direction more or less eastwards in the region south of Cape Ashizurimisaki. In the coastal region east of Kyushu, the oceanographical observations have been performed once a month or two by the Ohita and Miyazaki Prefectural Fisheries Experiment Stations. From these routine observations, some complicate features, i e., the outflow of the Seto Inland Sea Water of less saline and low temperature into the Pacific through the western half of Bungo-suido passage, the coastal current flowing toward SSW, the horizontal intrusion of tonguelike warm water to coastal region, and the upwelling of cold saline water along 200 m isobath, are found to be largely affected in any case by the variation of distance of Kuroshio axis from the coast (these * Laboratory of Physical Oceanography, Faculty of Fisheries, Kagoshima University, 4-50-20, Shimoarata, Kagoshima, 890 Japan. *2 Training Ship Kagoshima-maru, Faculty of Fisheries, Kagoshima University, 4-50-20, Shi moarata, Kagoshima, 890 Japan. 152 Mem. Fac. Fish., Kagoshima Univ. Vol. 34, No.l (1985) results are compiled by Chaen1*). It can be expected that the interleaving thermohaline fine structures may exist in such an offshore region where the mixing or exchange of the Kuroshio water with the coastal water is prominent. However, the routine observations stated above are limited in the coastal region within 30 miles from the coast, besides the observation depth is less than 200 m, and furthermore, are reported the oceanographical data in only standard depths. Therefore, has not been clarified yet the three dimensional distribution of fine structure in the offshore region. The fine structure in this region was investigated by Nagata2) for the first time and Ichikawa et al3) also. Nagata 2) analysed many historical data ofBT (Bathythermograph), showing that the occurrence frequency of shallow temperature inversion in this region is ca 20% and it increases in the Kuroshio region. Unfortunately, the horizontal scale of shallow temperature inversions in this region was not discussed by himself. Ichikawa et 3) al suggested the possibility of intrusion of low salinity Seto Inland Sea Water to the subsurface layer of the Kuroshio region through the density frontal zone at Bungo-suido passage, though their conclusion is rather speculative because it is derived from the CTD (Conductivity, Temperature and Depth Meter) data at only one station. This problem in the region between the eastern coast of Kyushu and the Kuroshio itself deserves intensive study. In order to clarify the three dimensional structure of mixing and exchange processes between the less saline and low temperature coastal water and the saline high temperature Kuroshio water, we carried out the CTD and XBT (Expendable Bathythermograph) observations intensively on Sept. 12 and 13 in 1984. The preliminary results of these observations are presented in .this paper. 2 . Observation Fig. 1 shows the locations of our intensive CTD and XBT observations in the region east of Kyushu carried out on Sept. 12 and 13 in 1984 on board the training ship Kagoshima-maru of Faculty of Fisheries, Kagoshima University. The CTD observations were made sequentially at 14 stations on two parallel sections ESE off Hyuga and off Nobeoka, ca 10 miles apart to each other. These two sections were selected as a northern boundary region where the bottom topography and the configulation of coastal line are quite simple. The distances between two adjacent stations are ca 5 miles with a few exception (10 miles). The CTD casts were made to the maximum depth of 1000 dbar. The water temperature and conductivity for any pressure were obtained by a Neil Brown System Inc., Mark III CTD System and they were recorded directly on a computer compatible magnetic tape at every 250 milli-seconds, which corresponds to the vertical interval of ca 25 cm. The CTD date were checked, using the materials collected by a General Oceanics Inc., Rosset Multi-bottled Water Sampler at several depths, before and after the sequential 14 CTD casts. The interpolated values of water temperature and salinity, which are estimated from the running means of 5 points, are tabulated in Appendix 1 together with the station data.