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Advective Ocean–Atmosphere Interaction: An Analytical Stochastic Model with Implications for Decadal Variability

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  • 1 National Center for Atmospheric Research, Boulder, Colorado*
  • | 2 Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California
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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 significant stochastic component because of weather excitation. One may expect these patterns to play an important role in ocean–atmosphere interaction. On interannual and longer timescales, horizontal advection in the ocean can also play an important role in such interaction. The authors 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 Hasselmann. The model equations are simple enough that they can be solved analytically, allowing one to fully explore the parameter space. The authors find that the solutions fall into two regimes: (i) a slowshallow regime where local damping effects dominate and (ii) a fastdeep regime where nonlocal advective effects 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 extratropical 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 differences between local thermodynamic feedbacks associated with changes in the air–sea temperature difference and nonlocal dynamic feedbacks associated with horizontal ocean advection.

Corresponding author address: R. Saravanan, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307.

Email: svn@ncar.ucar.edu

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 significant stochastic component because of weather excitation. One may expect these patterns to play an important role in ocean–atmosphere interaction. On interannual and longer timescales, horizontal advection in the ocean can also play an important role in such interaction. The authors 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 Hasselmann. The model equations are simple enough that they can be solved analytically, allowing one to fully explore the parameter space. The authors find that the solutions fall into two regimes: (i) a slowshallow regime where local damping effects dominate and (ii) a fastdeep regime where nonlocal advective effects 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 extratropical 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 differences between local thermodynamic feedbacks associated with changes in the air–sea temperature difference and nonlocal dynamic feedbacks associated with horizontal ocean advection.

Corresponding author address: R. Saravanan, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307.

Email: svn@ncar.ucar.edu

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