Behavior of Midlatitude Decadal Oscillations in a Simple Atmosphere-Ocean System

Abstract

Toward a physical understanding of the decadal oscillation found in midlatitude sea surface temperatures (SSTs), a numerical study has been carried out using a simple atmosphere-ocean coupled model. The ocean model consists of the linearized shallow-water, quasi-geostrophic equation and the mixed-layer temperature equation, while the atmospheric perturbations are expressed by steady equations which employ an empirical relationship between SST and surface wind anomalies. In the model, the empirical relation manifests the positive wind-evaporation feedback with so small magnitude that cannot overcome the thermal damping in the heat fluxes. The coupled solution in this model was sought with the time integration, to explore the dependence on such parameters as the mean and anomalous current strengths (γ_c and γ_t), coupling coefficients (γ_h), and magnitude in the local thermal damping (γ_d).
Numerical solutions with standard parameters yield oscillations with periods 13 and 16 years in the North Atlantic and the North Pacific, respectively. However, the modal structures are hardly identified because the local damping is too strong. When the damping is weakened, temporal evolutions of the temperature anomalies in these oscillations have a similarity to observed decadal changes. As a prototype of the simulated decadal oscillations, two modes were found with different parameter sets: advectioe mode driven by the advection by the mean gyre and Rossby wave mode in which the geostrophic current anomaly induced by the long Rossby wave is responsible for the scillation. The former generally has larger decay than the latter due to direct influence by the thermal damping. An essential feature of these types of oscillation is exemplified by conceptual models.
Stochastic weather noise was introduced into the system to clarify the role of atmosphere-ocean coupling. When the ocean model is forced by the noise alone, the temperature spectrum exhibits the red noise spectra consistent with the stochastic theory. The noise applied to the coupled system increases the low-frequency variance with distinct decadal peaks, which are well separated from the red noise. However, the decadal peaks disappear if the geostrophic advection terms were eliminated. These results indicate that the decadal modes inherent in the coupled system are crucial in generating the decadal spectral peak in the presence of the noise, even if the modes themselves are strongly damped oscillations.