Theoretical and numerical study on shocks and subtropical front in a wind-driven oceanic gyre

Research Project

Project/Area Number	07640567
Research Category	Grant-in-Aid for Scientific Research (C)
Allocation Type	Single-year Grants
Section	一般
Research Field	Meteorology/Physical oceanography/Hydrology
Research Institution	HOKKAIDO UNIVERSITY
Principal Investigator	KUBOKAWA Atsushi Hokkaido Univ., Grad.Sch.of Env.Earth Sci., Pro., 大学院・地球環境科学研究科, 教授 (00178039)
Co-Investigator(Kenkyū-buntansha)	TAKEUCHI Kensuke Hokkaido Univ., Inst.of Low Temp.Sci., Pro., 低温科学研究所, 教授 (00107450)
Project Period (FY)	1995 – 1996
Project Status	Completed (Fiscal Year 1996)
Budget Amount *help	¥2,200,000 (Direct Cost: ¥2,200,000) Fiscal Year 1996: ¥1,200,000 (Direct Cost: ¥1,200,000) Fiscal Year 1995: ¥1,000,000 (Direct Cost: ¥1,000,000)
Keywords	Ocean circulation / Subtropical countercurrent / Ocean general circulation model / Ventilated thermocline model / Subtropical front / Ventilated thermoclineモデル / 定在ロスビー波 / 衝撃波
Research Abstract	In the present study, the generation mechanism of the subtropical countercurrent and associated front is investigated by using an ocean general circulation model (GCM) with a simple geometry. A theory for the countercurrent is also presented. In the ocean GCM,the deep mixed layr occurs in the northern subtropical gyre and is shallows abruptly in the central subtropical gyre. The mixed-layr front, the narrow transition zone of the mixed-layr depth, slants from western central subtropical gyre to northeastern subtropical gyre, and the low potential vorticity fluid is produced at the intersection between the mixed-layr front and the outcrop line. Since the surface density is almost zonally uniform while the mixed-layr front slants northeastward, the minimum potential vorticity fluid in a dense isopycnal is produced in the northeastern region, while that in lighter density isopycnal in the western region. Subducted and advected southwestward, the low potential vorticity fluid in each isopycnal is vertically piled up and makes a thick low potential vorticity pool in the western central subtropical gyre. It is found that the model subtropical countercurrent occurs along the southwestern edge of this pool. Using a ventilated thermocline model, we investigate the effects of the mixed-layr depth distribution on the gyre structure, and it is demonstrated that the mechanism suggested by the numerical experiment can generate the countercurrent. On the other hand, southeastward slanting mixed-layr front can also generate a countercurrent. In the later case, a similar mechanism discussed by Dewar (1992) works. The characteristic equations suggest that the nonlinear steepening may also be important in this system.