Low-Frequency Variability in Shallow-Water Models of the Wind-Driven Ocean Circulation. Part II: Time-Dependent Solutions

• Main Authors: Eric Simonnet Universite, Eric Simonnet, Universite De Paris-sud, Michael Ghil, Kayo Ide, Roger Temam, Shouhong Wang, J. Phys Oceanogr
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.23.1149; http://www.atmos.ucla.edu/tcd/PREPRINTS/part2.ps.gz

Time-dependent model behavior is investigated for both a rectangular and a North-- Atlantic-shaped basin. Multiple steady states and their dependence on various parameters and other model properties were studied in Part I for the rectangular basin. As the wind stress is increased on the rectangular basin, each steady-state branch is destabilized by a Hopf bifurcation. The periodic solutions that arise o# the lower branch have a robust subannual periodicity of 4--5 months. For the upper branch, the period varies between sub- and interannual, depending on the Froude number F 2 defined with respect to the lower active layer's thickness H 2 . As F 2 is lowered, the nearly antisymmetric steady state is destabilized baroclinically, before the pitchfork bifurcation examined in detail in Part I occurs. In this case of low F 2 , as the wind stress is increased further, the baroclinic limit cycle undergoes also a symmetry-breaking bifurcation that is the counterpart of the perturbed pitchfork bifurcation in the high- F 2 case. Two types of mechanisms play a role in the transition to aperiodic behavior. First, as the forcing increases, the upper-branch limit cycle loses its stability and the lowerbranch limit cycle attracts all orbits. Next, this limit cycle increases in amplitude until it touches the nearly antisymmetric steady state that is unstable at these forcing values. Subsequently, the solution varies irregularly between flow patterns where the subtropical or the subpolar gyre dominates. The time scale of this variability is interdecadal. The role of global bifurcations that involve homoclinic and heteroclinic orbits in the interdecadal variability is investigated. Numerical simulations were carried out for the North Atlantic with ETOPO-5 geometry and.