Advective Ocean--Atmosphere Interaction: An Analytical Stochastic Model With Implications for Decadal Variability

Advective ocean{atmosphere interaction: an analytical stochastic model with implications for decadal variability Abstract Atmospheric variability on timescales of a month or longer is dominated by a small number of large-scale spatial patterns (\teleconnections"), whose time evolution has a signiicant stochastic component because of weather excitation. One may expect these patterns to play an important role in ocean{atmosphere interaction. On in-terannual and longer timescales , horizontal advection in the ocean can also play an important role in such interaction. We develop a simple one-dimensional stochastic model of the interaction between spatially coherent atmospheric forcing patterns and an advective ocean. The model may be considered a generalization of the zero-dimensional stochastic climate model proposed by Hassel-mann. The model equations are simple enough that they can be solved analytically, allowing us to fully explore the parameter space. We nd that the solutions fall into two regimes: (i) a slow/shallow regime where local damping eeects dominate, and (ii) a fast/deep regime where nonlo-cal advective eeects dominate. An interesting feature of the fast/deep regime is that the ocean{atmosphere system shows preferred timescales , although there is no underlying oscillatory mechanism in the uncoupled ocean or in the uncoupled atmosphere. Furthermore, the existence of the preferred timescale in the ocean does not depend upon a strong atmospheric response to SST anomalies. The timescale is determined by the advective velocity scale associated with the upper ocean and the length scale associated with low-frequency atmospheric variability. For the ex-tratropical North Atlantic basin, this timescale would be of the order of a decade, indicating that advective ocean{ atmosphere interaction could play an important role in decadal climate variability. The solutions also highlight the diierences between local thermodynamic feedbacks associated with changes in the air{sea temperature difference and nonlocal dynamic feedbacks associated with horizontal ocean advection.