A Mechanistic Model of Mid-latitude Decadal Climate Variability

Abstract
A simple model of coupled decadal ocean--atmosphere modes in middle latitudes is developed. Previous studies have treated atmospheric intrinsic variability as a linear stochastic process modified by a heuristic deterministic coupling. In contrast, the present paper takes an alternative view, based on observational, as well as process modeling results, and represents this time dependence in terms of irregular transitions between two anomalously persistent, high-latitude and low-latitude jet-stream states; these transitions are governed by an equation analogous to one describing the trajectory of a particle in a double-well potential subject to stochastic forcing. Ocean adjustment to atmospheric jet shifts involves persistent transient circulation anomalies maintained by the action of baroclinic eddies; this process is parameterized in the model as a delayed oceanic response. The associated sea-surface temperature anomalies provide heat fluxes that affect atmospheric statistics by modifying the shape of the potential. If the latter coupling is strong enough, the model's spectrum exhibits a peak at a frequency related to the ocean eddy-driven adjustment time scale. A nearly analytical counterpart of the coupled model is used to study the sensitivity of this behavior to key model parameters.

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