JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. C4, PAGES 7639-7650, APRIL 15, 1994 Hydrographic section across the Kuroshio near 35øN, 143øE W. J. Teague Naval Research Laboratory, Stennis Space Center, Mississippi A.M. Shiller Center for Marine Science, University of Southern Mississippi, Stennis Space Center Z. R. Hallock Naval Research Laboratory, Stennis Space Center, Mississippi Abstract. A closely spaced conductivity-temperature-depth/hydrographic sectionwas conducted off the east coast of Japan in July 1992. The southeastwardsection crossed the Japan Trench and the Kuroshio in the vicinity of the Kashima 1 seamount.Vertical sectionsof temperature, salinity, density, oxygen, and nutrients are discussed in conjunctionwith the movement and interleaving of water masses.Complicated vertical and horizontal mixings of water massesare inferred from the temperature and salinity relationships.Mixing processes are patchy and not continuous beneath the front. Warm, salty water found beneath the Kuroshio may result from upward mixing of water from intermediate depths. The main axis of the Kuroshio, indicated by the 14øC isotherm at 200 m, is at 35.7øN, 142.6øE, about 20 km from the north wall surface thermal front. Geostrophic speeds exceed 170 cms-• at the surface; volume transport through thesection is 81 x 10 6 m 3 s -1. 1. Introduction The Kuroshio is the western boundary current associated with the subtropical gyre in the North Pacific Ocean. As such, it is responsible for a significantproportion of pole- ward mass, chemical, and heat transportin that ocean basin. As it crosses the Izu-OgasawaraRidge, it separates from the coastal margin and turns eastward as the Kuroshio Exten- sion. Immediately eastward of the separation area, the current flows generally northeastward[Kawai, 1972]but, on occasion,developsa large, anticyclonicmeanderwhich may detach to form a ring near 37øN, 143øE.This region is also one of intense interaction of the Kuroshio with the Oyashio, resultingin complex frontal structuresand mixing of water masses [Kawai, 1972; Kawai and Saitoh, 1986; Kawamura et al., 1986;Nagata et al., 1986; Yasudaet al., 1992]. The Kuroshio Extension Regional Experiment (KERE) [Mitchell, 1990], begun in 1991 by the Naval Research Laboratory is a study of the Kuroshio Extension consisting of TOPEX/Poseidon altimetry, numerical modeling, and a regional, in situ observational program. The main objectives of the latter are to determine the existence and character of the hypothetical Pacific deep western boundary current (DWBC) and to monitor the Kuroshio Extension near the separation point to determine variability levels and relation to the deep flows. The KERE observationprogram involves direct current measurements, an array of inverted echo sounders (thermocline depth measurements), and a detailed hydrographic section, all along a TOPEX/Poseidon ground Copyright 1994 by the American Geophysical Union. Paper number 94JC00055. 0148-0227/94/94JC-00055505.00 track across the Japan Trench and across the Kuroshio Extension near 35øN, 143øE. In this report we give a first accounting of KERE field measurements with emphasis on upper water column prop- erties and dynamics of the Kuroshio Front. Bathymetry of the region and collection of the hydrographic data are describedin section 2. A general descriptionof the temper- ature-salinity (TS) characteristics and water massproperties are given in section 3. Vertical sectionsof potential temper- ature 0, salinity, potential density o- 0, silica, oxygen, phos- phate, and nitrate are described in section 4, and geostrophic velocities are described in section 5. Implications of these measurements on circulation are discussed in section 6. Propertiesand flows in the deep water column associated with our sectionsare described by Shiller et al. [ 1993]. 2. Data Collection A northwest to southeast section of 18 closely spaced conductivity-temperature-depth (CTD)/hydrographic sta- tions (henceforth, referred to as the KERE section) was occupied from July 8-23, 1992, on a line extending from about 37øN, 142øE to 33øN, 144øE (Figure 1). The section begins on the slope just east of Honshu in about 1000 m of water. Since Japan is an island arc, this slope is properly referred to as the forearc slope rather than the continental slope. The section trends southeastward across the Japan Trench near the Kashima 1 seamount. Station spacingtele- scopes from about 15 km at the landward (i.e., NW) end of the section to about 50 km on the seaward (i.e., SE) end. Most of the stations extend to within 100 m of the bottom. The north wall of the Kuroshio is located at about 35.8øN, between stations 9 and 10 (in the vicinity of the seamount). 7639