Meridionally Propagating Interannual-to-Interdecadal Variability in a Linear Ocean-Atmosphere Model

Abstract

Zonally symmetric sea level pressure perturbations have been observed to oscillate at 3?6-yr periods and travel from one polar region to the opposite polar region in approximately 10 years. Based on previous modeling of linear, meridional (zonally symmetric) modes of uncoupled ocean-atmosphere models, meridional modes of oscillation of a global, primitive-equation, coupled ocean-atmosphere model were analyzed to see if they contained meridionally propagating modes especially surface pressure perturbations, with oscillation periods of years to decades. A two-layer model of the global ocean and a two-level model of the global atmosphere were formulated. The most important ocean-atmosphere interaction processes were parameterized in a manner consistent with the simplicity of the ocean-atmosphere models. Both models and parameterizations were linearized around axially symmetric basic states containing mean meridional circulations. The linearized perturbation system was formulated as an eigensystem. The operator matrix was discretized in the north-south direction with centered finite difference. Meridional modes of oscillation of the coupled ocean-atmosphere model were calculated. In the work reported here, calculations were performed at 2.5° north-south resolution and for basic states based on observations (atmosphere) and simulations with a realistic model (ocean). For ?realistic? parameter values and basic states meridional modes oscillating at periods of several years to several decades are present in the coupled ocean-atmosphere model. Meridional surface pressure and zonal wind perturbations oscillating at 3?20-yr periods and traveling from the North Polar region to the South Polar region in 6 to 70 years appear as natural modes of variability of the coupled ocean-atmosphere model linearized in the presence of basic ocean-atmosphere states containing mean meridional circulations. At tropical latitudes, sea surface temperature perturbations are also associated with surface pressure perturbations. Oscillation periods, travel times, and meridional structures of surface pressure perturbations in some of the coupled ocean-atmosphere modes are comparable to the corresponding characteristics of the observed sea level pressure perturbations. Meridional modes oscillating at periods of a few weeks to a few centuries exist in the coupled ocean-atmosphere model only if the basic ocean-atmosphere states contain mean meridional circulations. Further work is in progress to study the sensitivity of the meridional coupled ocean-atmosphere modes to basic states and parameter values.