STOCHASTICITY AND SPATIAL RESONANCE IN INTERDECADAL CLIMATE FLUCTUATIONS

Ocean-atmosphere interaction plays a key role in climate fluctuations on interdecadal timescales. In this study, different aspects of this i nteraction are investigated using an idealized ocean-atmosphere model, and a hierarchy of uncoupled and stochastic models derived from it. T he atmospheric component is an eddy-resolving two-level global primiti ve equation model with simplified physical parameterizations. The ocea nic component is a zonally averaged sector model of the thermohaline c irculation. The coupled model exhibits spontaneous oscillations of the thermohaline circulation on interdecadal timescales. The interdecadal oscillation has qualitatively realistic features, such as dipolar sea surface temperature anomalies in the extratropics. Atmospheric forcin g of the ocean plays a dominant role in exciting this oscillation. Alt hough the coupled model is in itself deterministic, it is convenient t o conceptualize the atmospheric forcing arising from weather excitatio n as having stochastic time dependence. Spatial correlations inherent in the atmospheric low-frequency variability play a crucial role in de termining the oceanic interdecadal variability, through a form of spat ial resonance. Local feedback from the ocean affects the amplitude of the interdecadal variability. The spatial patterns of correlations bet ween the atmospheric flow and the oceanic variability fall into two ca tegories: (i) upstream forcing patterns, and (ii) downstream response patterns. Both categories of patterns are expressible as linear combin ations of the dominant modes of variability associated with the uncoup led atmosphere.