Phenomenology of the low-frequency variability in a reduced-gravity, quasigeostrophic double-gyre model

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. 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.