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Paola Cessi

Abstract

A model that isolates the interaction between midlatitude ocean gyres and the wind stress due to atmospheric baroclinic eddies is formulated. The ocean and atmosphere are coupled through their respective heat balances and global heat and momentum conservations are enforced. The ocean flow creates a steep oceanic thermal front at the midlatitude intergyre boundary. This frontogenesis sharpens the atmospheric temperature gradients and locally increases the atmospheric eddy heat transport. The result is a well-defined storm track that, because of the delayed adjustment of the gyres to the wind stress, oscillates in time with a period of about 18 yr.

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Raffaele Ferrari
and
Paola Cessi

Abstract

The signatures of feedback between the atmosphere and the ocean are studied with a simple coupled model. The atmospheric component, based on Lorenz's 1984 model is chaotic and has intrinsic variability at all timescales. The oceanic component models the wind-driven circulation, and has intrinsic variability only in the decadal band. The phase of the cospectrum of atmospheric and oceanic temperatures is examined and it is found that in the decadal band, the oceanic signal leads the atmospheric one, while the opposite is true at shorter and longer timescales. The associated atmosphere-only model, driven by the oceanic temperature derived from a coupled run, synchronizes to the coupled run for arbitrary initial conditions. When noise is introduced in the time series of oceanic driving, episodic synchronization still occurs, but only in summer, indicating that control of the atmosphere by the oceanic variables is prevalent in this season.

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François Primeau
and
Paola Cessi

Abstract

A model for the interaction between the midlatitude ocean gyres and the wind stress is formulated for a shallow-water, spherical hemisphere with finite thermocline displacement and the latitudinal dependence of the long Rossby wave speed. The oceanic currents create a temperature front at the midlatitude intergyre boundary that is strongest near the western part of the basin. The intergyre temperature front affects the atmospheric temperature gradient in the storm track region, increasing the eddy transport of heat and the surface westerlies. The delayed adjustment of the gyres to the wind stress causes the westerly maximum to migrate periodically in time with a decadal period. The behavior of the model in a spherical geometry is qualitatively similar to that in a quasigeostrophic setting except that here the coupled oscillation involves oceanic temperature anomalies that circulate around the subpolar gyre, whereas the quasigeostrophic calculations favor the subtropical gyre. Another difference is that here there is a linear relationship between the period of the coupled oscillation and the delay time for the adjustment of ocean gyres to changes in the wind stress. This result departs from the quasigeostrophic result, in which the advection timescale also influences the period of the decadal oscillation.

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Blanca Gallego
and
Paola Cessi

Abstract

A model of the midlatitude, large-scale interaction between the upper ocean and the troposphere is used to illustrate possible mechanisms of connection between the decadal variability in the North Atlantic and in the North Pacific. The two ocean basins are connected to each other only through their coupling to the common, zonally averaged atmosphere. The ocean–atmosphere coupling takes place via wind-driven torques and heat fluxes at the air–sea interface. In this formulation, the decadal variability in each ocean basin consists of ocean–atmosphere modes and arises from a delayed feedback of the upper-ocean heat content onto the wind-driven flow mediated by the atmosphere through the requirements of global heat and momentum balances. The presence of two ocean basins leads to three basic kinds of coupling-induced behavior: phase locking, oscillation death, and chaos. In the phase-locked regime, the western boundary currents of the two basins oscillate in synchrony, with the narrower basin following the wider basin by a small time lag. In the oscillation death solutions, a steady solution is reached, even though each ocean basin, when uncoupled, would have sustained oscillations. In the chaotic regime, the interbasin coupling induces aperiodic fluctuations in both basins characterized by variability at centennial, as well as decadal, timescales.

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