Bifurcation structure of two-dimensional viscous zonal flows on a rotating sphere. (5th June 2015)

• Record Type: Journal Article
• Authors: Sasaki, Eiichi; Takehiro, Shin-ichi; Yamada, Michio
• Is Part Of: Journal of fluid mechanics. Volume 774(2015:Jul.)
• Page Start: 224
• Page End: 244
• Publication Date: 2015-06-05
• Subjects: bifurcation, -- chaos, -- rotating flows; Fluid mechanics -- Periodicals; 532.005
• DOI: 10.1017/jfm.2015.262
• Languages: English
• ISSNs: 0022-1120

Abstract

Abstract : We study the bifurcation structure of zonal flows on a rotating sphere. The setting of our problem is similar to the Kolmogorov problem on a flat torus, where the vorticity forcing is given by a single eigenfunction of the Laplacian. First we prove the global stability of two-jet zonal flow for arbitrary Reynolds number and the rotation rate of the sphere. Then we study the bifurcation structure of steady solutions arising from three-jet zonal flow. In the non-rotating case, we find that two steady travelling-wave solutions bifurcate from a three-jet zonal flow via Hopf bifurcation. As the Reynolds number increases, steady-travelling solutions arise via pitchfork bifurcation from the steady-travelling solutions. On the other hand, in the rotating case, we find saddle-node bifurcations and closed-loop branches. We carry out time integration to study the properties of unsteady solutions at high Reynolds numbers. In the non-rotating case, the unsteady solution is chaotic and it wanders around the steady-travelling solutions bifurcating from three-jet zonal flow. We show that a linear combination of the steady and steady-travelling solutions gives a good approximation of the zonal-mean zonal flow of the unsteady solution, suggesting that the chaotic solution at high Reynolds numbers exists mostly within a relatively low-dimensional space spanned by the steady and steady-travelling solutions, which become unstable at low Reynolds numbers.