Phenomenology of the Low-Frequency Variability in a Reduced-Gravity, Quasigeostrophic Double-Gyre Model

Abstract

The low-frequency variability of the oceanic wind-driven circulation is investigated by use of a reduced-gravity, quasigeostrophic model with slight variations on the classic double-gyre wind forcing. Approximately 30 eddy-resolving simulations of 100?1000 years duration are analyzed to determine the types of low-frequency variability and to estimate statistical uncertainties in the results. For parameters close to those leading to a stable antisymmetric solution, the system appears to have several preferred phenomenological regimes, each with distinct total energy levels. These states include a high-energy quasi-stable state; a low-energy, weakly penetrating state; and a state of intermediate energy and modest eddy/ring generation. The low-frequency variability of the model is strongly linked to the irregular transitions between these dynamical regimes. For a central set of reference parameters, the behavior of the system is investigated for each period in which the total energy remains in certain ranges. The structure of the time-averaged streamfunction and eddy energy fields are observed to have remarkable repeatability from event to event for each state. A parameter study documents the ways in which the probability distribution function of the total energy depends on the strength and asymmetry of the wind forcing field. As the parameters shift away from those leading to a steady antisymmetric solution, we find that increasing the asymmetry of the wind field or reducing the viscosity decreases the occurrences of the high-energy, quasi-stable state. The low-energy, weakly penetrating state is more robust and exists whenever there is both instability and a certain minimal asymmetry in the forcing. As the wind asymmetry is increased, the distributions shift smoothly (but rapidly) away from the higher-energy states, until only the low-energy state remains.