Journal of Oceanography, Vol. 55 (No. 3), pp. 449-462, 1999
Shinya Shimokawa1,2 and Tomonori Matsuura2
1Department of Geography, Swiss Federal Institute of Technology (ETH), Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
2Atmospheric and Hydrospheric Science Division, National Research Institute for Earth Science and Disaster Prevention (NIED), 3-1 Tennodai, Tsukuba 305-0006, Japan
(Received 19 June 1998; in revised form 21 January 1999; accepted 3 February 1999)
Abstract: Using an eddy-resolving two layer primitive-equation model forced by symmetric wind stress, we investigate the asymmetry of the recirculation of a double gyre (subpolar gyre and subtropical gyre). In the case where the layer thickness change is large, cyclonic recirculation becomes unstable and splits into meso-scale vortices more easily than anti-cyclonic recirculation in their developing stage. The subpolar gyre is, therefore, filled with more vortices than the subtropical gyre. Moreover, the effect of the upper layer potential vorticity on the lower layer potential vorticity in the subpolar gyre is stronger than that in the subtropical gyre. The characteristic of turbulence in the subpolar gyre is different from that found in the subtropical gyre and, therefore, the asymmetry of the recirculation of the double gyre is maintained by this difference. The asymmetry can not be produced in a quasi-geostrophic model because it ignores the nonlinearity associated with layer thickness change. Moreover, we investigate the effects of layer thickness and lateral viscosity on the asymmetry of the recirculation of the double gyre. In the case of realistic physical parameters, the asymmetry of the recirculation of the double gyre is noticeable from the view of the activities of the eddies. In the case with the shallowest upper layer, the position of separation of the western boundary current moves further southward.