Analytical prototypes for ocean-atmosphere interaction at midlatitudes, Part I: Coupled feedbacks as a sea surface temperatur dependent stochastic process.

J. David Neelin and Wenjie Weng
J Climate, 12, 697-721, 1999.

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© Copyright 1999 by the American Meteorological Society.

Abstract. Effects of ocean-atmosphere feedback processes and large-scale atmospheric stochastic forcing on the interdecadal climate variability in the North Atlantic and North Pacific oceans are examined in a simple midlatitude ocean-atmosphere model. In the ocean, we consider a linearized perturbation system with quasi-geostrophic shallow-water ocean dynamics and a sea surface temperature (SST) equation for a surface mixed-layer. The atmosphere is represented as stochastic wind stress and heat flux forcing. This includes a multiplicative noise component that depends on SST, as well as an additive component that is independent of SST. The model results in both oceans indicate that large-scale additive atmospheric forcing alone (the uncoupled case) can give coherent spatial patterns in the ocean, and sometimes even a weak power spectral peak at interdecadal periods. Coupling due to the influence of SST on the probability density function of the atmospheric noise can produce a more distinct power-spectral peak relative to the uncoupled ocean. Moreover, the time and spatial scales of the interdecadal mode are insensitive to the standard deviation of multiplicative noise. Thus a deterministic feedback limit can be used to simplify the coupled model for further investigation of the physical mechanisms of the interdecadal mode.

In both uncoupled and coupled cases, the period of the interdecadal oscillation is determined by the zonal length scale of atmospheric wind stress feedback and oceanic Rossby wave dynamics. The atmospheric spatial pattern sets the length scale of large-scale wave motion in the ocean. This wave propagates to the west due to oceanic Rossby wave dynamics and is dissipated at the western boundary. However, in the coupled case, the SST anomalies generated by geostrophic current can feed back to the atmosphere, which, in turn, brings some information back to the east and re-excites oceanic waves there. Although the magnitude of the feedback of SST on the atmosphere is much smaller than atmospheric internal variability, its effects are significant.

Citation. Weng, W., and J. D. Neelin, 1999: Analytical prototypes for ocean- atmosphere ineraction at midlatitudes. Part I: Coupled feedbacks as multiplicative noise stochastic forcing. J Climate, 1999, 12, 697-721.

Acknowledgments. This work was supported by Natinal Science Foundation Grant ATM-9521389. NCAR Scientific Computing Division provided the computer resources. This work was motivated by discussions with Mojib Latif during a sabbatical (JDN) at MPIM in 1993. We wish to thank Drs. Michael Ghil, James C. McWilliams, Cecil Penland, and Andrew W. Robertson for their suggestions and comments.

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