Nonlinear Mechanisms of Low-Frequency Variability in Unstable Western Boundary Currents Under Variable Atmospheric Forcing

Western boundary currents (WBCs) are known to have intrinsic modes of nonlinear variability, and also to respond to variable wind forcing. Previous process studies have mostly focused on either the nonlinear dynamics of intrinsic variability under steady forcing, or the linear response to variable forcing. Here we combine these approaches, presenting a detailed survey of the response of a nonlinear, intrinsically time-dependent WBC to a variable wind stress curl in a simple barotropic model and also an idealized two-layer model driven by observed South Pacific winds. A rich variety of complex nonlinear behavior is observed (e.g. phase locking, chaos and hysteresis), depending on the forcing amplitude and frequency. Periodic wind forcing can make an intrinsically periodic WBC chaotic or partially phase-locked, introducing frequencies one or two orders of magnitude lower than those present in the forcing or intrinsic to the current. These effects arise even with weak wind variability of under 1%, less than the annual wind stress curl cycle across most of the mid-latitudes. These dynamics suggest that low-frequency WBC variability may not be attributable purely to the WBC or forcing in isolation, but may also arise from a nonlinear WBC response to variable forcing.