Decadal variability and predictability in the midlatitude ocean-atmospheresystem

The coupled ocean-atmosphere interaction and predictability associated with the tropical El Nino phenomenon has motivated researchers to seek analogou s phenomena in the midlatitudes as well. Are there midlatitude coupled ocea n-atmosphere modes? Is there significant predictability in the midlatitudes ? The authors address these questions in the broader context of trying to u nderstand 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 simplified physical parameterizations. The ocean is represented using a state-of-the-a rt ocean general circulation model, but configured in a simple Atlantic-lik e sector geometry. In addition to a coupled integration using this model, u ncoupled integrations of the component oceanic and atmospheric models are a lso carried our 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 variabi lity that is partially excited by atmospheric forcing, and is associated wi th a periodicity of 16-20 vr. True coupled ocean-atmosphere modes do not ap pear to play any quantitatively significant rule in the midlatitudes, due t o the fundamentally different nature of atmospheric dynamics in the midlati tudes compared to the Tropics. However, coupling to the atmosphere does pla y an important role in determining the spatial and temporal characteristics of the oceanic variability. A statistical assessment suggests that midlati tude atmospheric predictability is modest compared to the predictability as sociated with tropical phenomena such as El Nino. This predictability arise s from the atmospheric response to oceanic modes of variability). rather th an from coupled modes. There is significant oceanic predictability on inter annual timescales but not on decadal timescales.