Low-Frequency Variability in the Wind-Driven Circulation

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. This solution would consist of an oscillation of the dipolar structure formed by the pair of recirculation gyres. The behavior described appears robust to the addition of a degree of asymmetry in the forcing. Similar large amplitude, low frequency fluctuations in the circulation are also found in a baroclinic double gyre model. In the baroclinic model, these fluctuations occur on a decadal timescale and are sufficiently large that they must be considered a potential component mechanism for some of the decadal climate oscillations seen in the extratropics.