Submesoscale eddies in Peter the Great Bay of the Japan/East Sea in winter

Abstract

Cold-core (T 27.24 kg/m3 were observed in the near-bottom layer at the shelf break in Peter the Great Bay (PGB) of the Japan/East Sea in March 2010. The anomalously cold water was 0.4 ml/l richer in oxygen than was the surrounding water, and it contained more suspended particles. The observations were carried out by using a moored automatic mobile Aqualog profiler. Profiling occurred as frequent as every hour, allowing us to obtain data with high temporal resolution. The Aqualog profiler delivered vertical profiles of the ocean current velocity, acoustic backscatter at 2 MHz, temperature, and salinity between the depths of 20 and 105 m. Other oceanographic instruments were mounted at fixed depths on the mooring line to measure current velocity, temperature, conductivity, dissolved oxygen, chlorophyll-a fluorescence, and turbidity. Complementary data included ship-borne CTD casts and satellite-borne imagery and scatterometry as well as coastal weather station records. The Regional Ocean Modeling System (ROMS) was employed to study the origin and evolution of the cold water. The model simulated the ocean dynamics at a 600-m horizontal resolution in PGB from 2009 to 2010. The model was forced by the surface momentum, heat and fresh water fluxes of the NCEP-DOE Reanalysis 2. The lateral boundary condition of the model was obtained from nesting into a Japan Coastal Ocean Predictability Experiment model data set. According to the ROMS simulation, the circulation in Ussuri Bay in the northeastern bay of PGB was anticyclonic in February–March 2010. The submesoscale cyclonic vortices generated around the anticyclonic gyre. The submesoscale cyclones tended to move southward out of the bay, and they transported the anomalously cold water towards the outer shelf. As a result, the cold water anomalies were often observed to persist for as long as 2 days near 42.5°N, 132°E. Lagrangian analysis confirmed that this cold water observed by the Aqualog profiler originated in Ussuri Bay. The model simulation showed that the submesoscale cyclonic eddies played a specific role in supplying the densest water from the northern part of PGB to the outer shelf, where the dense water was then entrained by the mesoscale eddies in the Primorye Current zone and could cascade down the continental slope into the deep northern basin of the sea. This transport of the densest water by the submesoscale eddies was estimated to be 5–10·10−3 Sv in February–March 2010.