Low-Frequency Variability in the Wind-Driven Circulation

Abstract

Abstract The origins of low-frequency variability in a simple homogenous ocean model, forced by a double gyre Ekman suction, are examined numerically. It is found that irregular, large amplitude vacillations in the structure of the circulation typify the behavior of such a model when it is forced sufficiently strongly. These oscillations are associated with order-one changes in the size and transport of the inertial recirculation gyres that lie near the western boundary. It is suggested that this behavior arises as a result of a subcritical homoclinic bifurcation. The aperiodic solutions do not exhibit a strong tendency to linger near any of the simpler unstable solutions that were found for this system. Instead, the latter solutions appear to constrain the aperiodic solutions, confining them to a limited region of phase space for a range of values of the Munk boundary layer scale δM. The form of the aperiodic solutions suggests that there may be an interesting unstable solution that could not be isolated...