Interannual variation of the Polar Front in the East/Japan Sea: Observed data and numerical simulations

The western end of the North Pacific Polar Front extends into the East/Japan Sea and its path affects water mass formation, eddy generation, ring formation, and the distributions of physical and chemical properties in the marginal sea. Interannual variability of the Polar Front (PF) within the East/Japan Sea is examined from a merged TOPEX/POSEIDON and ERS-1/2 altimeter dataset and upper water temperature data from 1993 to 2001. Effects of wind stress and warm water volume changes on meridional migration of the front are studied with numerical simulations.; Nonseasonal variability of sea surface height (SSH) is higher in the southern warm water region (5--10 cm) than in the northern cold water region (4 cm). Empirical Orthogonal Function analysis of the sea level shows that the first mode is related to intraseasonal oscillations over the entire East/Japan Sea and the second mode to interannual path variation and meandering of the Tsushima Current (TC). The path change of the TC is responsible for the high variability in the southern region, and is also related to the meridional migration of the PF.; A reduced gravity model is developed based on the spectral finite volume (SFV) method to represent the front in the proper manner. The model uses a two-level grid: a globally unstructured element grid and locally structured grid consisting of N x N quadrilateral cells within each element. The SFV method has a convergence order of N + 1 for smooth problems. Incorporation of the flux corrected transport (FCT) algorithm ensures stability of the computations in regions of vanishing layer thickness.; The PF migrates more widely (37°N--41°N) in the western part than in the eastern part (39°N--40°N). The interannual migration induces large variabilities of upper water temperature and SSH in the western region. Responsible physical mechanisms are studied using the reduced gravity model. Warm water volume change in the warm water region uniformly pushes the front in the meridional direction across its mean position. Interannual variation of wind stress causes wide migration of the front in the western part. Interannual variation in the meridional Ekman transport may be responsible for the wide migration in the western part.