Decadal variability and predictability in themidlatitude ocean { atmosphere

The coupled ocean{atmosphere interaction and predictability associated with the tropical El Ni~ no phenomenon has motivated researchers to seek analogous phenomena in the midlatitudes as well. Are there midlatitude coupled ocean{atmosphere modes? Is there signiicant predictability in the midlatitudes? The authors address these questions in the broader context of trying to understand the mechanisms behind midlatitude variability, using an idealized model of the ocean{atmosphere system. The atmosphere is represented using a global two-level eddy-resolving primitive equation model with simpliied physical parameterizations. The ocean is represented using a state-of-the-art ocean general circulation model, but conngured in a simple Atlantic-like sector geometry. In addition to a coupled integration using this model, uncoupled integrations of the component oceanic and atmospheric models are also carried out to elucidate the mechanisms behind midlatitude variability. The sea surface temperature in the coupled equilibrium state exhibits two dominant modes of variability: (i) a passive oceanic red noise response to stochastic atmospheric forcing, and (ii) an active oceanic mode of variability that is partially excited by atmospheric forcing, and is associated with a periodicity of 16{20 years. True coupled ocean{ atmosphere modes do not appear to play any quantitatively signiicant role in the midlatitudes, due to the fundamentally diierent nature of atmospheric dynamics in the midlatitudes as compared to the tropics. However, coupling to the atmosphere does play an important role in determining the spatial and temporal characteristics of the oceanic variability. A statistical assessment suggests that midlatitude atmospheric predictability is modest compared to the predictability associated with tropical phenomena such as El Ni~ no. This predictability arises from the atmospheric response to oceanic modes of variability, rather than from coupled modes. There is signiicant oceanic predictability on interannual time scales, but not on decadal time scales.