Impact Of The Eddy-Low Frequency Flow Interaction On Annular Mode In A Simplified AGCM

As well known that the low frequency patterns such as the PNA, NAO, AO and AAO dominate the short term climate change in the middle latitudes, but its dynamic mechanism is still not fully understood. Recently, a number of studies show that the eddy plays very important role in maintaining the low frequency pattern (LF), the associated eddy momentum fluxes positively feed back onto the low frequency flows, and the evidences can be also found both in observational and the numerical results. However, since the nonlinear character of the interaction between the eddy and the low frequency flow (ELF), the dynamical mechanism of ELF is still not clear.Our study employed a numerical method of the twin-experiment by superposing a small planetary perturbation on the control case. The main goal is studying the dynamic mechanism of the ELF. By considering the backgrounds with the baroclinicity of basic flow is either maintained (forced) or unmaintained (unforced), our results show that the eddy forcing positively feedback onto the LF, ELF interaction plays an essential role in maintaining the low frequency flow. The foundmental reasons of the vertical barotropical and the meriodinal dipole structure of the LF are also discussed in our study. Furthermore, the eddy induced instability theory (EⅡ) is applied to understand the dynamical mechanism of the annular modes. The main conclusions include:(1) Based on the unforced background, the eddy with wave number6(the most unstable mode) is used to disturb the baroclinicity instability. The irreversible eddy lifecycle process, which means the baroclinic basic flow becomes barotropical equilibrium during the barotropic decaying phase is considered. The results show that:eddy forcing positively feedback to the LF, eddy forcing likes the eddy momentum fluxes and the eddy vorticity fluxes accelerate the planetary perturbed flow aloft, which contributes the baroclinicity tendency of the perturbed flow growth. However, since the eddy heat fluxes reduces the meridional gradient of the temperature, suppresses the baroclinicity development and eventually causes to the brotropical distribution of the perturbed flow.By applying the EⅡ theory and changing the strength and the latitudes of the basic flow, as well as the seeding wave number, we found that the eddy activity is essential to the growth of the perturbed flow, the energy of the perturbed flow comes from the background eddy activity. Moreover, this instability is scale selective, meridional dipole or the triple pattern has a fast growth rate, which is the essential rule of the meridional structure of the low frequency pattern. (2) Under the background with unforced condition, we further consider more realistic atmosphere circulation with the turbulence eddy. By using the observed climatelogical winter mean flow and the high frequency eddy data, the ELF interaction is also discussed. The results show that the eddy positive feedback only occurs when basic flow is baroclinicity instability with elicpose eddy structure.(3) In order to study the long term influence of the ELF interaction, Newtonin adjustment and the surface friction terms are added to the momentum equation and the temperature equation. It is found that, under the forced background, eddy forcing positive feedback to both of the initial and the steady planetary perturbed flow; meanwhile, surface friction play an important role in ELF interaction through modifying the background baroclincity and the eddy decay time scale.In all, the essential mechanism of the maintenance of the low frequency flow is the interaction between the eddy and the low frequency flow interaction. Eddy activity and the eddy decaying time scale mainly contribute to the eddy induced growth rate. Moreover, this instability is scale selective, the initial meridional dipole or triple pattern grows fast. The surface friction can also influence the eddy feedback, it works to decrease the strength of low level flow and enhance the vertical shear of the flow. Therefore, the strong surface friction contributes to the enhanced eddy feedback.