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

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.

The processes and labeled peaks and valleys will be discussed, starting with the longest periods.
The geostrophic surface current can be calculated from the slope of sea surface height (SSH) measured by satellite altimeter (Teague et al., 2004) Our two-year mappings of the upper and deep current systems found more complicated variable circulation patterns than the traditional "three-currents paradigm," and the patterns do not repeat seasonally.bottom to avoid losses.Despite these precautions, we found evidence that several of the PIESs were snagged and dragged short distances during the mooring period.We have no direct information on the types of organisms that refl ected our signals.The 10-kHz acoustic pings from our instruments would refl ect from swim bladders or fl oats of fi sh or siphonophores (Batzler, 1975), but they would also refl ect from a high concentration of squid.Mokrin et al. (2002), referencing other sources, note that squid feed on zooplankton or nekton (myctophid fi sh and small juvenile squid) depending on their size; as a result, squid are more abundant in frontal-zone areas where productivity is higher.The refl ective target strength of fi sh or squid of about 20-cm length (Urick, 1967) would require high concentrations (10 -3 m -3 ) such that 10 3 to 10 4 targets would contribute to the echo from a 10-kHz, 6-msec pulse at Approximately a billion squid are caught in the JES each year.Thus, squid is an important fi shery in the JES.

CIRCULATION FEATURES AND PATTERNS IN THE JES
Usually, fi shing occurs at nighttime when bright lights attract the Japanese common squid Todarodes pacifi cus to the fi shing boats.The lights on these vessels are suffi ciently powerful that they can be observed from space by satellites.Cho et al. (1999) superimposed the lights from these images onto surface temperature C I h D h ( ) Figure 6.Maps of the 5°C isotherm depth averaged for a fi ve-day period at 30-day intervals throughout the two-year period.Th e percent of early echoes received at each PIES site during each fi ve-day period is indicated by the dots according to the key at the right.Highest percentages occur in the frontal regions between the warmer (yellow, orange, and red shaded areas) and colder waters.

Figure 1 .
Figure 1.Pressure-gauge-equipped inverted echo sounders (PIES, triangles) and recording current meters (circles) were deployed between June 1999 and June 2001 to measure the upper and deep circulation in the Ulleung Basin, southwestern region of the Japan/East Sea (JES) between Korea and Japan.Th e square indicates one (site N2) of several Acoustic Doppler Current Profi ler moorings in the Korea/Tsushima Strait occupied between May 1999 and June 2000.TOPEX/Poseidon (T/P) ground tracks are shown by the dotted blue lines.Two large blue circles labeled N and S indicate locations where spectra were computed for geostrophic currents estimated separately by the T/P and mapped PIES data.Data from labeled sites are discussed in the text.Bathymetry is color shaded and contoured at 500-m intervals.

Figure 2 .
Figure2.Th e amount of kinetic energy associated with processes at diff erent frequencies is illustrated by their spectra calculated from measurements by a variety of instrumentation.A peak in the spectrum indicates more energy at its corresponding frequency.For example, the peak labeled S arises from semidiurnal tidal currents.Th e spectra are shown for the NE/SW component of fl ow, which is perpendicular to the satellite altimeter ground track shown in Figure1.Geostrophic surface currents were derived from satellite altimetric measurements as well as from the mapped PIES data at the same two locations (marked by the blue circles labeled N, S in Figure1).Th eir corresponding spectra are shown by the purple and blues lines, respectively.Th e solid and dashed purple lines are spectral estimates generated from the same data sets, but averaged using diff erent segment lengths.To resolve the longest periods (dashed), fewer segments are averaged, resulting in broader confi dence intervals.Measured currents at variable depths are shown for the remaining sites.Error bars show the 95 percent confi dence limits.Th e key in the upper frame applies to all frames and the instrument locations are labeled in Figure 1.(Top) Auto-spectral density.(Bottom) Variance-preserving spectra.Inset is an enlargement of the higher-frequency signals.
Perhaps it is not surprising that, based now on suffi cient observations (discussed byMitchell et al., 2005a), the currents and circulation in the southwestern JES differ from the former paradigm in which three currents branch out from the Korea Strait in seasonally repeating paths.Nevertheless, we reconfi rmed several persistent and/or recurrent features of the upper circulation.In fact, all the features summarized in Figures3 and 4, except the Dok Cold Eddy, had been identifi ed previously, and their recurrence and persistence had apparently contributed to the erroneous concept that the circulation varied in a regular seasonal progression of patterns(Mitchell et al., 2005b).The upper circulation and thermocline depth were mapped at daily intervals by the PIES array(Mitchell et al., 2005a).The measurements by the bottom pressure sensors and deep current meters were used in combination to map the deep absolute currents(Teague et al, 2005).Because of the intense bottom fi shing and crabbing in the Ulleung Basin at depths as great as 2000 m, the instruments were moored close to the

Figure 3 (Figure 4 .
Figure 3 (upper panels) shows the annual averages of the circulation in the Ulleung Basin, mapping the surface current vectors and the thermocline depth (5°C isotherm depth) for two years.The

Figure 3
Figure3also emphasizes that the upper circulation changes from one year to another(Park and Watts, 2006).From June 1999 to June 2000 (Year 1) the EKWC on average protruded north of 38.25°N.The circulation around the UWE was strong, and the DCE had only weak cyclonic mean circulation.Our observations exhibited only the outer portion of the OB fl owing along the southeastern shelf break and onto the Oki Spur.From June 2000 to June 2001 (Year 2), the annual average showed a weaker EKWC that turned eastward offshore to feed the SPF, which meandered during this year along approximately 37.5°N.Mitchell et al. (2005a) concluded that the EKWC was absent from 11 June to 5 November 2000, and confi rmed this absence of the EKWC against bimonthly hydrographic surveys by the National Fisheries Research and Development Institute of Korea on six transects that extended from the coast across the shelf and into the Ulleung Basin.This second year exhibited in the mean a much contracted form of the UWE and a steep cold trough of the SPF dipped far southward into the Ulleung Basin near 131.7°E.A strong OB fl owed along the southeastern shelf break and turned northward between 132°E and 132.5°E following the Oki Spur.Interestingly, the disappearance and reestablishment 1000-2000 m.According to the offi cial web site of the Ministry of Maritime Affairs and Fisheries of Korea, the amount of squid brought to the harbors located on the east coast of Korea during 1999-2001 accounted for over 50 percent of the tonnage of the total seafood catch.

Jong-Hwan Yoon is
Professor, Research Institute for Applied Mechanics, Kyushu University, Japan.Moon-Sik Suk is Principal Research Scientist, Korea Ocean Research and Development Institute, Ansan, Republic of Korea.Kyung-Il Chang is Associate Professor, School of Earth and Environmental Sciences, Seoul National University, Seoul, Republic of Korea.
Figure 5. (Top) Th e blue dots show the acoustic echo returns measured at four PIES sites during June 1999 to June 2001.Th e desired signals are the echoes refl ected off the sea surface.However, many echoes were refl ected as deep as 300 m below the surface; in general, the maximum depth coincided roughly with the depth of the 5°C isotherm depth (orange line).Th e red bar in P34 indicates the time period, which is shown in the bottom panel.(Bottom) Acoustic echoes oscillate between the sea surface and approximately 250 m during nighttime and daylight, respectively, indicating that the echoes refl ected off vertically migrating biota, such as fi sh or squid.Shaded bars, encompassing the hours 1900-0400 local time, indicate nighttime in the Japan/East Sea.