Evolution of the Thermohaline Stratification in the Northwestern Sea of Japan: Mesoscale Variability and Intra-annual Fluctuations

This paper presents a statistical analysis of time series obtained from the Aqualog profiler moored at the upper continental slope off the southeast Russian coast in the northwestern Sea of Japan from mid-April through mid–October 2015. The vertical conductivity, temperature and depth (CTD) profiles with a high resolution (6 hours in time and 1 m in depth) were measured between 64 and 300 m, and the buoyancy frequency was estimated from these data. Buoyancy frequency and temperature data were decomposed to empirical orthogonal functions (EOFs) in the depth-time domain. Fluctuations in the vertical stratification revealed by the buoyancy frequency were assessed and linked to the passage of dynamic structures near the mooring. In the anticyclonic structures, the stratification weakened above 120 m due to isopycnal bending down and strengthened below 165 m. Stratification changes occurred on an 8–13-day timescale due to the passage of mesoscale eddies near the mooring. Energetic fluctuations were also found over longer 20–30- and 80–130-day time scales. These processes were accompanied by temperature anomalies associated with horizontal advection above 150 m and with vertical pycnocline fluctuations in the deeper layer. Temperature anomalies moving in the vertical direction with an average speed of 1.7 × 10–2 cm/s (14.7 m/day) on timescales from 4 to 13 days were detected at the border between these layers which exhibited different kinds of stratification changes.