An idealized model for the coupled large-scale dynamics of the midlatitude atmosphere and the wind-driven ocean circulation is formulated. In this formulation, the upper ocean and the zonally averaged troposphere are coupled through wind stress and heat-fluxes at the air-sea interface. The model equations are based on global conservation of heat and momentum. Simplified parameterizations are used to relate the atmospheric eddy heat and momentum fluxes to the zonally averaged oceanic and atmospheric variables. In the limit in which the time scale of air-sea heat exchange is much shorter than that of the ocean advection process, it is possible to reduce the evolution of the oceanic temperature to a one-dimensional equation for the zonally averaged component. In this approximation, the role of the meridional heat transport by the gyres is to redistribute heat down the mean oceanic temperature gradient.
The model is used to illustrate, in a very simple fashion and within the frame of large-scale dynamics, the ways in which heat and momentum are transported and exchanged in the ocean-atmosphere system and their role in the generation of low-frequency variability. In particular, a mechanism for coupled modes at decadal time scales is proposed. The base for this mechanism is the change of sign of the atmosphere-ocean feedback produced by the delayed response of the oceanic circulation to changes in the wind-stress, which leads to sustained oscillations. The period of the oscillations is about twice the transit time of long Rossby wave across the ocean basin, and thus is of the order of decades. These oscillations are large-scale in space, extending over most of the midlatitude ocean basin, and small in amplitude, with a root meansquare variance of about 10%. They are characterized by oceanic temperature and velocity anomalies that are antisymmetric with respect to the gyre boundary and about 90 degrees out of phase. Also, the oceanic and atmospheric components of the northward heat transport, as hypothesized by Bjerknes fluctuate out of phase.
An extension of this formulation to the case of two oceanic basins conected through the atmosphere is currently being analyzed in the search for teleconection mechanisms.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics