Spatial and temporal variability of satellite-observed Subpolar Front in the East/Japan Sea

Abstract

Spatial and temporal variability of the Subpolar Front (SPF) in the East/Japan Sea are examined based on semi-monthly sea-surface-temperature (SST) gradient maps constructed from declouded SST images over the period 1990–1995. Analyses demonstrate that the SPF experiences distinct seasonal fluctuations as well as strong year-to-year variations in terms of frontal strength, position, and zonal range. Elevated variability of the SPF is attributed mainly to mesoscale eddies at the frontal region. The main SPF is not zonal along 40 ∘ N but trends from the southwest to the northeast during winter–spring, and from the northwest to the southeast in summer. During summer, the SPF west of 135 ∘ E shifts significantly to the north, reaching 41 . 5 ∘ N , which is associated with an enhancement of the East Korea Warm Current (EKWC). However, the surface SPF does not necessarily reflect its subsurface structure. Because of strong baroclinicity in summer, the internal SPF surface is inclined to the south with increasing depth. Northward movement of the western SPF is limited to 20 m depth. Topography may give vorticity constraints on the migration of the SPF, but the maximum SPF is not always located around the shallow Yamato Rise (YR) in the central part of the East/Japan Sea. Relatively strong bathymetrically trapped fronts are confined to the northwestern edge of the YR, within a zonal range of 132.5 – 134 . 5 ∘ E , where steep bathymetry stabilizes the fronts. Topographic steering of SST fronts is weak at the southeastern portion of the YR. The SPF in the EKWC region ( < 132 ∘ E ) is not significantly affected by the bathymetry. Spatial variability of atmospheric wind forcing produces a significant meridional shift of the SPF by generating the northwestern branch of the SPF in November. The strongest frontal zone, of magnitude 0.09 ∘ C / km , near 138 ∘ E shows the largest temporal variability in strength, but the smallest temporal variability in terms of meridional location. A temporal trend of the SPF properties is observed, suggesting that its western (eastern) location migrates to the south (north) reaching - ( + ) 0 . 1 ∘ / year with mean strengthening of 6.8 × 10 - 4 ∘ C / km year during the study period.