Subsurface Counter Current beneath the Tsushima Warm Current Reproduced by a Data Assimilative Model
Abstract
In the Japan Sea, the Tsushima Warm Current (TWC) dominates the surface layer, and the TWC water flows roughly northeastward along the coast of Japan. From early summer to autumn, a seasonal circulation often forms beneath the TWC. This seasonal circulation is referred to as the subsurface counter current (SSCC) because it flows over the Japanese continental slope in the opposite direction to the surface TWC. The SSCC has been investigated in both observational and numerical studies, and some numerical studies have proposed that subsurface clockwise gyres that form off the Japanese coast induce the SSCC. However, the preceding numerical studies have focused on the climatological representation of the SSCC, and so more realistic year-to-year examination of the SSCC has been needed. Hence we analyzed 13-year data-assimilative products of JADE (JApan sea Data assimilation Experiment), an ocean forecast system operated at Japan Sea National Fisheries Research Institute, Fisheries Research Agency. The JADE products from 1999 to 2011 reproduced the climatological view of the SSCC reported by the preceding numerical studies: Subsurface clockwise gyres appeared off the Japanese coast from May to October, inducing the SSCC over the continental slope; Core of the SSCC (or the gyre) was found around 140-200 m depth. The subsurface gyres showed year-to-year variation in its position, magnitude and duration, thus magnitude and duration of the SSCC at a fixed location notably varied year-to-year. The reproduced SSCC showed 10-20 cm/s velocity that well corresponded to the observed values. Formation of the subsurface gyres was characterized by seasonal enhancement of clockwise vortices that were trapped in embayments of the bottom relief. The vortices were surface-intensified and showed vertically-coherent negative vorticity from the surface to about 300 m depth. Because the vortices were embedded into the more surface-intensified TWC, the clockwise gyre flow (or the SSCC) was found only in the subsurface layer. Mechanism under the seasonal enhancement of the clockwise vortices is still unclear, and so we need more detailed investigation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMOS21A1666O
- Keywords:
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- 4227 OCEANOGRAPHY: GENERAL / Diurnal;
- seasonal;
- and annual cycles;
- 4243 OCEANOGRAPHY: GENERAL / Marginal and semi-enclosed seas;
- 4260 OCEANOGRAPHY: GENERAL / Ocean data assimilation and reanalysis;
- 4520 OCEANOGRAPHY: PHYSICAL / Eddies and mesoscale processes