Abstract
A subsurface thermohaline front semi-permanently formed in association with near-bottom cyclonic circulation in the northern East China Sea was newly found from detailed hydrographic data collected during two cruises in February 2017 (winter) and April 2018 (spring) along with supplementary satellite remote sensing and historical hydrographic data. An alternate intruding frontal structure in water properties was observed across the cyclonic circulation in both seasons as formed by two contrasting water masses—low-temperature and low-salinity (i.e., low spiciness) water transported by the East China Sea Current and high-temperature and high-salinity (i.e., high spiciness) water transported by the Tsushima Warm Current. Consistent structures were confirmed from current observations during the two cruises, historical hydrographic observations, and satellite altimetry-derived sea surface height and surface frontal structure, indicative of retroflection of the Cheju Warm Current as deemed by the seasonal development of thermal stratification in spring. Our results reveal significant heat and material exchanges between the open Pacific and the broad shelf, particularly via diapycnal mixing and cross-front transports associated with across-front flow and cyclonic circulation, in the northern East China Sea.
Highlights
Understanding the structure of the thermohaline front and associated circulation in the northern East China Sea is of great importance because the northern East China Sea is located in the middle of heat and material exchange between the open Pacific and the neighbouring marginal seas such as the Yellow Sea, East China Sea, and coastal waters around Korea[1,2,3]
The currents observed at a depth of 40 m during these cruises after extracting tidal currents are broadly consistent with satellite altimetry-derived surface geostrophic currents averaged over the observation periods, yielding known circulation patterns, namely, the Tsushima Warm Current (TWC) and Cheju Warm Current (CWC), except for a significant difference in the western entrance of Cheju Strait and the line C05–C10 (Fig. 1b,c)
Surface geostrophic currents www.nature.com/scientificreports are smeared owing to enhanced thermal stratification in April, the alternate intruding frontal structures produced by the cyclonic circulation are still maintained along the C-line (Figs. 2f, and 3g,h)
Summary
Understanding the structure of the thermohaline front and associated circulation in the northern East China Sea is of great importance because the northern East China Sea is located in the middle of heat and material exchange between the open Pacific and the neighbouring marginal seas such as the Yellow Sea, East China Sea, and coastal waters around Korea[1,2,3]. The ECSC and KCC transporting low-temperature and low-salinity water and the CWC and YSWC transporting high-temperature and high-salinity water converge in the northern East China Sea to form a strong thermohaline front. This thermohaline front is called the Cheju-Yangtze Front (CYF) because it takes the form of a slanted S-shape connecting Cheju Island and the Changjiang river mouth[2,13]. The CYF is strongest in the winter when it consists of a northern warm-tongue in the east where high-temperature and high-salinity water of TWW origin expands northwestward and a southern cold-tongue in the west where low-temperature and low-salinity water is transported southeastward by the ECSC14,15 (Fig. 1a). In this study, we aim to (1) analyse the structures of the southern front of the warm-tongue and (2) identify relevant frontal circulation from the perspective of heat and material exchanges based on in-situ hydrographic observations during two recent cruises, historical (over 50 years) hydrographic observations, and supplementary satellite observations
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