Korea Strait Bottom Cold Water Research Articles

Long-Term Changes in Fish Assemblage Structure in the Korea Strait from 1986 to 2010 in Relation with Climate Change

Climate change is expected to impact on marine-capture fisheries around the world. The Korea Strait showes the highest fisheries production among the adjacent seas of Korea, but studies on changes in the fish community related with climate change are scarce. We evaluated the spatio-temporal changes in fish assemblages, and their relationships with oceanic conditions in the Korea Strait (KS, 126°–129°5′E, 33°5′–35°N) to provide a scientific basis for developing fisheries policies to adapt to climate change and global warming. We used the inflow indices of the Korea Strait Bottom Cold Water and the Tsushima Warm Current, water temperature and salinity, and commercial fisheries catch data from 1986 to 2010. To detect temporal shifts in the oceanic conditions, we applied a sequential t test of regime shift. To evaluate spatio-temporal changes of the fish assemblage structure by year, month, and fishing block, we graphically summarized the species compositions of the commercial fisheries catch data by correspondence analysis (CA). We conducted cross-correlation and canonical CA to evaluate the relationship between the oceanic conditions and fish assemblages. Sardine and filefish dominated in spring and were mainly distributed far away from the coast. Anchovy were mostly caught in summer near the southeast coast of Korea, and their annual catch steadily increased. Squid and hairtail dominated in autumn–winter, and were mainly distributed from the northern part off Jeju Island to the southeast water of Korea. Chub mackerel were relatively constant in annual and seasonal catch. CA detected a shift in the fish assemblage between 1990 and 1991, which we speculated was triggered by the 1988–1989 regime shift in the North Pacific. Surface temperatures at 0–20 m depths, especially with a time lag of 1 year, were significantly correlated with the 1990–1991 shift in fish assemblage structure. We concluded that (1) the KS is an intermediate area between the waters of Ieodo and the East Sea with respect to the timing of shift in fish assemblage structure, (2) the regional shifts were characterized by the replacement of dominant fish species from sardine and filefish to chub mackerel and squid. We hope that further multidisciplinary studies between regional oceanographers and fisheries scientists will contribute to the development of fisheries policies through better understanding of the interactions between oceanographic processes and fishes at the regional scale in adaptation to climate change.

Role of cold water and beta-effect in the formation of the East Korean Warm Current in the East/Japan Sea: a numerical experiment

The contributions of bottom cold water and planetary β-effect to the formation of the East Korean Warm Current (EKWC), the western boundary current in the East/Japan Sea (EJS), were evaluated using an idealized three-dimensional numerical model. The model results suggest that the bottom cold water and, to a lesser extent, the planetary β-effect both contribute to the formation of the EKWC. The cold water functions as the bottom of the upper layer, to control the EKWC via conservation of potential vorticity. It is known that cold waters, such as the North Korean Cold Water and Korea Strait Bottom Cold Water often observed during summer along the southwestern coast of the EJS, originate from the winter convection in the northern area. Observational studies consistently show that the EKWC strengthens in summer when the cold water extends further south along the western boundary.

Internal Tide-Induced Enhancement of Cold Water Intrusion on the Continental Shelf of the Korea Strait

ABSTRACT Park, J.-H.; Kim, Y.S.; Lee, H.J.; Lee, H.-Y.; Ha, H.K.; Park, Y.-G.; Jeon, C., and Hirose, N., 2016. Internal tide-induced enhancement of cold water intrusion on the continental shelf of the Korea Strait. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 542–546. Coconut Creek (Florida), ISSN 0749-0208. The Korea Strait bottom cold water (KSBCW), characterized by its temperature < 10°C, is believed to originate from the deep basin of the southwestern East/Japan Sea with a maximum transport in August/September. Although previous studies have suggested possible causes of the enhanced KSBCW intrusion in summer, its mechanism is not yet clear. Here we investigate if tides can affect the strength of KSBCW intrusion using numerical simulations. We utilize a high-resolution regional ocean model with 1/108° grids, covering the Korea Strait and the...

Asynchronous responses of fish assemblages to climate-driven ocean regime shifts between the upper and deep layer in the Ulleung basin of the East Sea from 1986 to 2010

Past studies suggested that a basin-wide regime shift occurred in 1988–1989, impacting marine ecosystem and fish assemblages in the western North Pacific. However, the detailed mechanisms involved in this phenomenon are still yet unclear. In the Ulleung basin of the East Sea, filefish, anchovy and sardine dominated the commercial fish catches in 1986–1992, but thereafter common squid comprised > 60% of the total catch in 1993–2010. To illuminate the mechanisms causing this dramatic shift in dominant fisheries species, I related changes in depth-specific oceanographic conditions from 0 to 500 m to inter-annual changes in the fish assemblage structure from 1986 to 2010. In the upper layer of 50–100 m depths, water temperature suddenly increased in 1987–1989, and consequently warm-water epi-pelagic species (anchovy, chub mackerel, and common squid) became dominant, while sardine, relatively cold-water epi-pelagic species, nearly disappeared. An annual index of the volume transport by the Korea Strait Bottom Cold Water, originating from the deep water of the Ulleung Basin, displayed a sudden intensification in 1992–1993, accompanied by decreased water temperature and increased water density in the deep water and replacement of dominant bentho-pelagic species from filefish, warm-water species, to herring and cod, cold-water species. The results suggest that climate-driven oceanic changes and the subsequent ecological impacts can occur asynchronously, often with time lags of several years, between the upper and the deep layer, and between epi-pelagic and deepwater fish assemblages.

기후변화에 따른 제주도 주변 해역 수산 어종 변화(1981-2010)

We compiled and analyzed long-term time-series data collected in Korea to evaluate changes in oceanographic conditions and marine ecosystems near Jeju Island (<TEX>$33^{\circ}00^{\prime}-34^{\circ}00^{\prime}\;N$</TEX>, <TEX>$125^{\circ}30^{\prime}-127^{\circ}30^{\prime}\;E$</TEX>) from 1981 to 2010. Environmental data included depth-specific time series of temperature and salinity that have been measured bimonthly since 1961 in water columns at 175 fixed stations along 22 oceanographic lines in Korean waters by the National Fisheries Research & Development Institute, and time series of estimated volume transport of the Tsushima Warm Current (TWC) and Korea Strait Bottom Cold Water (KSBCW) for the period from 1961 to 2008. We analyzed the species composition in terms of biomass of fish species caught by Korean fishing vessels in the waters near Jeju Island (1981-2010). Data were summarized and related to environmental changes using canonical correspondence analysis (CCA). The CCA detected major shifts in fish community structure between 1982 and 1983 and between 1990 and 1992; the dominant species were a filefish during 1981-1992 and chub mackerel from 1992 to 2007. CCA suggested that water temperature and salinity in the mixed layer and the volume transport of the TWC and the KSBCW were significantly related to the long-term changes in the fish community in the waters off Jeju Island. Fish community shifts seemed to be related to the well-established 1989 regime shift in the North Pacific. Further studies are required to elucidate the mechanisms driving climate change effects on the thermal windows and habitat ranges of commercial species to develop fisheries management plans based on reliable projections of long-term changes in the oceanographic conditions in waters off Jeju Island.

2006~2010년 기간 동안 대한해협 서수도에서 관측된 해수 물성의 변동

Seasonal to inter-annual variations of water properties in the western channel of the Korea Strait are investigated using quasi-monthly hydrographic observations collected during 2006-2010. Weak vertical temperature and salinity gradients are observed during the winter months and these remain until May. At the upper layer, temperature increases from March and reaches a maximum in August, while salinity decreases during the same period. Near-bottom water shows low temperatures during late winter and fall with a minimum peak in September. Korea Strait Bottom Cold Water produces thick layers (>20 m) in 2006 and 2010, while it is observed very near the bottom with relatively high temperature in 2008 and 2009.

Temporal Variation of Phytoplankton Community Related to Water Column Structure in the Korea Strait

Photosynthetic pigments, nutrients, and hydrographic variables were examined in order to elucidate the spatio-temporal variation of water column structure and its effect on phytoplankton community structure in the western channel of the Korea Strait in fall 2006 and spring 2007. High phytoplankton biomass in the spring was associated with high salinity, implying that nutrients were not supplied by coastal waters or the Yangtze-River Diluted water (YRDW) with low salinity. Expansion of the Korea Strait Bottom Cold Water (KSBCW) and a cold eddy observed during the spring season might enhance the nutrient supply from the subsurface layer to the euphotic zone. Chemotaxonomic examination showed that diatoms accounted for 60-70% of total biomass, followed by dinoflagellates. Nutrient supply by physical phenomena such as the expansion of the KSBCW and the occurrence of a cold eddy appears to be the controlling factors of phytoplankton community composition in the Korea Strait. Further study is needed to elucidate the mechanisms by which the KSBCW is expanded, and its role in phytoplankton dynamics.

Interannual variability of the Korea Strait Bottom Cold Water and its relationship with the upper water temperatures and atmospheric forcing in the Sea of Japan (East Sea)

A 35 yearlong temperature data set is analyzed to investigate the long‐term temperature variability in the Korea Strait and its relationship with the temperature variability in the upper layer of the Sea of Japan (East Sea). The second cyclostationary empirical orthogonal function mode of the vertical temperature section in the Korea Strait describes the interannual variability of the Korea Strait Bottom Cold Water (KSBCW). According to the corresponding principal component time series, the strength of the KSBCW fluctuates yearly with a major spectral peak around 3 years. Multiple regression analysis shows that the interannual KSBCW variability is closely linked with the temperature variability in the southwestern region of the Sea of Japan (East Sea) at about 50–100 m depth. Along 40°N, the source of the KSBCW is traced at about 50 m, extending eastward from the east coast of Korea to about 135°E. At 37°N, the source is traced at a deeper level (about 50–100 m), confined more toward the east coast of Korea with a hint of double core characteristics. The interannual KSBCW variability is also related to the southward wind stress along the east coast of Korea. It appears that strong cooling/warming of upper water temperature induced by the basin‐scale wind stress results in the interannual KSBCW variability. This connection is verified by showing a reasonable interannual covariability between the KSBCW and the basin‐scale wind stress.

Seasonal variation of the Korea Strait Bottom Cold Water and its relation to the bottom current

Analysis of high resolution hydrographic and current data revealed that the Korea Strait Bottom Cold Water (KSBCW) flowed into the Korea Strait from May to January and contained two temperature minimums, one in August/September and another in December/January. With the main current flows confined within 70 km of the Korean coast, maximum southwestward bottom currents corresponded with the temperature minimums. Time‐series of bottom current and temperature near the Korean coast clearly showed that the bottom temperature decreased as the bottom current strengthened. Seasonal variation of the KSBCW was linked to the combined effects of the barotropic and baroclinic current variations in the Korea Strait. The annual minimum bottom temperature of the KSBCW was associated with the maximum southwestward baroclinic component. The KSBCW had a secondary maximum southwestward bottom current and secondary minimum temperature in December/January when the Tsushima Warm Current (TWC), flowing into the East/Japan Sea (EJS), had the minimum volume transport.

Southwestward intrusion of korea strait bottom cold water observed in 2003 and 2004

Hydrographic surveys were carried out four times in the western channel of the Korea Strait in March and August 2003 and in June and November 2004. The bottom cold water, which was lower than 10°C, appeared in the channel trough except in March 2003. It flowed southwestward along the shelf of Korean coasts in August 2003 and in November 2004. The width and the maximum speed of the intrusion current were about 20 km and approximately 25 cm s-1, respectively, off Ulsan, Korea. The volume transport of the bottom cold water was estimated 0.019 Sv (Sv≡106 m3 s-1) in August 2003 and 0.026 Sv in November 2004.

하계 대한해협 저층냉수의 물리.화학적인 특성 및 기원

Hydrographic survey in the Korea Strait has long history that has begun in August 1917 at the Busan - Tsushima cross section, still continues to date. However, chemical properties of bottom cold water found exclusively in the western channel of the Korea Strait during summer did not receive much scientific attention. The aim of the study is to decipher the enigmatic origin of the Korea Strait Bottom Cold Water (KSBCW) in terms of chemical properties. The physicochemical properties of the KSBCW are extracted from the CREAHS II hydrographic data. OMP method was applied to analyze origin of the KSBCW quantitatively. The KSBCW is well defined by low temperature below . The cold waters exhibited the local presence near the coast at about 120m depth with a thickness of 20m to 30m. The cold water was characterized by relatively cold, saline and higher chemical concentrations than adjacent waters. The KSBCW seems to have different origin kom that of the coastal upwelled waters at the Ulgi-Gampo because it is saline, denser and contains considerably less dissolved oxygen than upwelled waters. The physicochemical properties are reported to have noticeable annual variations which suggest the complex origin of the KSBCW. OMP analysis show that the KSBCW is a mixture of three water types; TMW (24%), ESIW (36%) and ESPW (40%). Relationship between the KSBCW and the east Sea circulation is traced by mapping the water masses that have similar T, S and DO of KSBCW. The result showed that the KSBCW is most possibly an extension of southward flowing coastal intermediate waters. Front these results, we expect that the monitoring KSBCW will provide us valuable information about the East Sea circulation.

Observations of the Korea Strait bottom cold water

In this study, a set of 13 bottom mounted acoustic Doppler current profilers with temperature and pressure sensors were deployed in the Korea Strait for 11 months in 1999–2000. The overall purpose of the project was to understand the linkages between the East Asian marginal seas. Our focus in this paper is on an intrusion of bottom cold water from the East Sea which appears on the western side of the Korea Strait (Korea Strait Bottom Cold Water, KSBCW). In contrast to previous work, we did not find a yearly cycle in its appearance with maximum in summer and minimum in winter. Rather, sustained intrusions occurred in May/June and again in December/January. Bottom currents during the times of its appearance showed only limited advective intrusion with recycling back to the East Sea. Monthly scale bottom temperature records in the intrusion area were negatively correlated with the cross-strait bottom pressure anomaly, a measure of geostrophic transport through the Strait. Our hypothesis is that when geostrophic transport is low the bottom cold water intrudes, and when it is high the cold water is prevented from intruding. Prediction of the bottom cold water appearance, then, would depend on prediction of the geostrophic transport.

Structure of the Korea strait bottom cold water and its seasonal variation in 1991

Precise CTD data taken bimonthly in 1991 show the structure and variation of the Korea Strait Bottom Cold Water (KSBCW) in detail. The KSBCW appears only near the Korean coast in the strait. It is found within about 50 km of the coast, and is about 20–50 m thick with seasonal variations. Its structure along the strait becomes sharply deeper toward the south throughout the year. The gradient along the strait is 0.7×10 -3 in April and about 0.4×10 -3 in the other months. It moves more than 100 km south in summer when the density of the upper layer is 1.022–1.025 g/cm 3 and retreats in winter when it is 1.026 g/cm 3. Except in August, the structure across the strait is characterized by a rise at its western end. In August it takes on an arch-shape which could not be described due to the deficiency of vertical resolution in the historical data. Using Bernoulli’s equation with the observed values for density stratification and velocity, it can be demonstrated that the East Sea Proper Water in the Ulleung Basin could not flow out to the Korea Strait, whereas the salinity minimum layer water could flow there.