Low Frequency Variability in a Stochastic Atmosphere - Ocean Mixed Layer Model

Abstract:

The climate system exhibits low-frequency variability in characteristic spatial structures, but the mechanisms for this variability have remained unclear partly due to observational limitations and partly due to difficulties in analyzing simulations from nonlinear, chaotic models. In addition, recent studies have questioned the necessity of ocean circulations to generate such low-frequency variability. Our research is intended to clarify mechanisms of low-frequency climate variability that can occur purely from atmospheric dynamics coupled to an ocean mixed-layer model. For this purpose, we have built a new stochastic model based on the linearized primitive equations for the atmosphere, a slab mixed-layer model for the ocean, a gray radiation scheme for radiative effects, and a diffusive scheme for vertical turbulent eddy fluxes. Temperature is randomly excited in midlatitudes, and all variables except surface pressure are damped artificially with a 1-day time scale. The atmospheric model alone is shown to produce realistic seasonal mean eddy variances and fluxes in midlatitudes, despite the absence of moisture, clouds, moist convection, topography, and zonal asymmetries in the back- ground state. Because the atmospheric eddy statistics are realistic, it is argued that coupling these eddies to a mixed-layer model will produce more realistic low-frequency variability than the traditional Hasselmann model in which the atmospheric stochastic forcing is imposed by fiat. We have shown that such coupling does indeed generate peaks in the low-frequency power spectrum that otherwise would not occur in the absence of coupling. Now, we are trying to comprehensively analyze the mechanism for these low-frequency peaks, exploiting the fact that the model is purely linear. We further aim to analyze simulations from a comprehensive nonlinear aquaplanet GCM. The results from nonlinear simulations will serve as a baseline for theoretical statistical studies in low frequency variability.