Oceanographic condition in fall of 1999 along the section between the south of Kyusyu and the Yap Islands

Oceanographic observations using the CTD, XBT and ADCP systems were conducted along a section between the south of Kyusyu to the Yap Islands in the Western North Pacific on October 13-18, 1999. The surface-mixed layer higher than 29 degree C with a thickness of 47-84 m was observed from 13 degree N to 25 degree N. The Subtropical Mode Water occupied the subsurface layer between 200 m and 400 m in the region between 21 degree N and 28 degree N. In the region south of 15 degree N, the isotherm depths increased towards the south from 150 m to 400 m, indicating existence of the North Equatorial Current. The current velocity was measured to be ca. 20 cm/s, nearly equal to its annual mean value, indicating that the North Equatorial Current holds its normal state in fall of 1999. The saline water higher than 34.7 psu occupied the subsurface layer between 100 m and 300 m in almost whole sections. Below the subsurface saline water, there was a halocline between 34.4 psu and 34.7 psu in the entire sections. The intermediate water less than 34.2 psu occupied the layer between 600 m and 850 m in the region between 20 degree N and 29 degree N. At some latitudes in the westward zonal current region south of 20 degree N, some eastward return flows were detected, i.e., at 17 degree N, 18.5 degree N, and 19.5 degree N in the sea surface layer, at 18 degree N, 18.5 degree N and 19.5 degree N in the 50 m depth layer, and at 14.5 degree N similar to 15 degree N and 18 degree N in the 100 m depth layer.