Nonlinear Midlatitude Ocean Adjustment

Abstract

Ocean adjustment on annual to interdecadal scales to variable forcing is considered for a more nonlinear general circulation than has previously been studied. The nature of the response is a strong function of forcing frequency and importantly involves the inertial recirculations rather than linear baroclinic waves. The spatial expression of this variability is concentrated near the separation latitudes of the Gulf Stream extension, a model region corresponding to an area in the real ocean of well-known strong ocean?atmosphere buoyancy exchange. ?Turn-on? cases, periodically forced cases, and stochastically forced cases are considered. The first set of experiments clarifies the adjustment timescales and dynamics of a nonlinear circulation. The second set examines modifications to that adjustment rendered by time-dependent forcing. The last set is perhaps the most realistic in terms of the atmospheric forcing of the ocean, because wind spectra are not strongly peaked beyond a few weeks. Multidecadal forcing is argued both to excite a novel, rapid mode of adjustment and to resonate with a considerably slower, nonlinear mode. Stochastic forcing seems clearly to excite the fast mode and to contribute to the slower mode, although the latter also derives considerable variance from intrinsic sources. These conclusions are based on a suite of distinct spatial and temporal characteristics of the dominant ocean variability patterns under various forcing scenarios and comment on the ocean dynamics likely to be important to decadal timescale midlatitude climate variability.