Soyo-maru cruise

Cruise report and initial results from the Soyo-maru Terrence Joyce and Frank Bahr, WHOI

Cruise Overview

The Soyo-maru, operated by the NFRI in Yokohama, departed its homeport on 12 May 1998 and returned there on 18 June with an intermediate port stop in Siogama. On the cruise a total of 196 stations were occupied using a Seabird CTD/rosette (from NFRI) equipped with a dissolved oxygen probe, a fluorometer and a 150 kHz RDI Lowered Acoustic Doppler Current Profiler (LADCP) from WHOI. Personnel were mainly from the NFRI Laboratory with participation from scientists from the University of Tokyo and the Woods Hole Oceanographic Institution. A Vessel-Mounted ADCP (VM-ADCP) aboard the ship was also collecting data during the cruise; this unit was a broad-band 75 kHz unit from RDI. Water samples were collected for analysis of salinity, oxygen, chlorophyll and nutrients, with emphasis in silica and oxygen sampling on the deep (to 3000m depth) stations. Most stations were taken to a depth of 1500m during the cruise

This note is a brief description of our cooperative cruise of Soyo-maru. The Soyo-maru cruise was quite successful with most of the objectives being achieved. Bad weather on the second leg limited the meridional extent of one of the sections (section 6, below) and reduced the station work at the first Kuroshio meander crest. The almost complete set of water property information (CTD, DO, Nutrient) and LADCP and VM-ADCP velocity data will give brand-new results of the hydrographic structure in the Kuroshio-Oyahsio confluence zone. Our primary objectives of this cruise were to

make clear the hydrographic structures using combined data of CTD, chemical tracers and velocity field
make transport and flux estimates of the Oyashio and the Kuroshio source waters and their mixture, which is the North Pacific Intermediate Water (NPIW)
define the NPIW formation site(s) and mixing processes
study the dynamical features of the Oyashio intrusion and exchange process across the Kuroshio Extension
examine the velocity and hydrographic structure of the deep western boundary current along the Japan coast.

The overall cruise track and station locations (Fig. 1) are conveniently grouped into 6 'sections' (labelled 1-6 on the figure) and two meander surveys (labelled 'M'). These sections are briefly described as:

Section-1: Kuroshio off Boso Peninsula to 30N along T/P track
Section-2: 30N zonal, 142.7E-158E, Kuroshio recirculation
Section-3: 158E-meridional section from 30 to 39N crossing the Kuroishio Extension
Section-4: 39N zonal section through the Oyashio recirculation
Section-5: Off Hokkaido- Oyashio inflow
Section-6: 147E-meridional section from 32.3 to 39N
Survey M: Kuroshio Extension meander survey

Section 6 was planned to go to 30N to make a western 'box' within the larger box defined by sections 1-4, but could not be completed due to weather. The inflow from the Tsuguru Strait between the islands of Hokkaido and Honshu was initally planned, but it was decided during a pre-cruise meeting that this section would be given a lower priority in case of time limitations.

Scientific personnel involved in the analysis of data include

Yutaka Hiroe	NFRI
Kosei Komatsu	NFRI
Kiyoshi Kawasaki	NFRI
Ichiro Yasuda	U. Tokyo
Terrence Joyce	WHOI
Frank Bahr	WHOI

Kuroshio Meander Survey, M

Previous observational evidence suggests that the cross-frontal exchange in the Kuroshio Extension (KE) is spatially-correlated with the position of the semi-permanent meanders east of Japan. Warm, salty water is transferred upward and northward across the meander crests with colder, fresher water transferred southward across the meander troughs. This cross-frontal motion can arise due to ageostrophic dynamics along the frontal axis. Our surveys were planned to obtain direct observations of the hydrographic properties of the water column (and hence the geostrophic component of flow), and with the addition of the LADCP, measurements of the actual velocity. Our efforts will be directed towards an analysis of the density and tracer (e.g. salinity) advection in the upper 1000m of the water column within the meandering KE. We show the VM-ADCP data from the near surface layer (Fig. 2, survey M) in the meander crest and trough east of the Izu Ridge. The first crest, near 35.5N, 143E has strong eastward flow which turns southward approaching the first trough, which must lie near 32.5N, 146E where the flow turns to the northeast. The CTD data permit us to estimate the depth of various density surfaces and interpolate the flow at depth on these surfaces using the LADCP data. We show (Fig. 3) the flow at the s_q = 26.6 kgm^-3 surface, which is at depths of 200m to the north of the front and deepens to 700m on the southern side of the KE. This surface has lower flow rates than that at 100m (Fig. 2), but one can see that the stations resolve the meandering flow following this potential density surface. Considerable work (once final CTD data are obtained) will be required to examine the ageostrophic dynamics, which give rise to vertical motions of the water particles as they flow along density surfaces through this meandering structure. This will be the initial focus of the WHOI effort in our collaborative project.

We acknowledge the financial support in the US of the Office of Naval Research, which enabled the LADCP data to be collected on the cruise, and to our collaboration with the Fisheries scientists in the Yokohama and Tohoku Laboratories in Japan as we planned for and participated in the cruise. We also thank the U. of Hawaii for the loan of their LADCP, which was used as a backup (but not needed) for the Soyo-maru cruise.