Currents, Eddies, and a "Fish Story" in the Southwestern Japan/East Sea

Abstract

Abstract : As part of the Japan/East Sea (JES) initiative supported by the U.S. Office of Naval Research, we conducted an observational experiment to understand the physics of the mesoscale circulation in the Ulleung Basin, located in the south-western corner of the JES. The current passing through the Korea Strait divides upon entering the JES, with portions of the current flowing along the Korean and Japanese coasts. The variability of these currents is especially energetic in our study region. Our objectives were to measure the time-varying currents in the upper and deep levels of the JES. We relate the population density of vertically migrating fish or squid to the time-varying locations of fronts. We moored a two-dimensional array of pressure-gauge-equipped inverted echo sounders (PIES) and deep-recording current meters for 24 months starting June 1999. Figure 1 shows the study region, which spanned roughly a 250-km square between the Republic of Korea and Japan. The inverted echo sounder is a bottom-moored instrument that measures the time required for an acoustic pulse to travel from the seafloor to the sea surface and back. Travel-time variations arise primarily from temperature changes in the overlying water column caused by meandering fronts and eddies. The PIES array was used to map the temperature field at a suite of depth levels and the dynamic method was used to calculate profiles of geostrophic current. Bottom pressure and deep current meters moored 25 m above the seafloor were used together to map the deep circulation.