The expansion of wind fields observed at fixed times (four times daily) in complex empirical orthogonal functions is performed for the Japan Sea area (34°–53° N, 127°–143° E). The wind fields are taken from the 1998–2004 NCEP/NCAR Reanalysis data with better spatial resolution (1° × 1°) than the standard product, which are publicly available on the Internet. Major modes of wind variability in the Japan Sea area are identified. The modes determine a general direction of air-mass transport throughout a year, zonal and meridional modulation, and a cyclonic and an anticyclonic eddy component. Objective classification of wind fields with respect to the prevailing flow direction is performed, and wind stress and wind-curl patterns are obtained for major events in the cold and warm periods of the year. The pattern obtained can be used in hydrodynamic numerical models of the general circulation of the Japan Sea.

The variability of the geostrophic eddy kinetic energy (EKE) in the Sea of Japan derived from weekly altimetric sea level anomalies spanning from 1992 through 2009 is studied. Nonorthogonal modes of variability are revealed accounting for more than 60% of the total variance. They capture the seasonal variation of the mesoscale energetics in the entire Sea of Japan with the EKE growing in the warm season up to the maximum in October through November and diminishing in the cold season down to the minimum in March through April. In the northern Sea of Japan (northward of the Subarctic Front), where the mean EKE is several times less than in the southern sea, areas of considerable variability are detected. Quasi-biennial EKE oscillations are revealed but not the trends covering the whole record.

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.

The variability of sea surface temperature (SST) in the Japan Sea is investigated using the complex EOF analysis of daily data produced at Tohoku University, Japan (New Generation SST; 2002–2006)....