Decadal extra-tropical Pacific variability

The climate in the Pacific basin exhibits robust variability on decadal time scales. One theory for decadal variability suggest that the memory within the climate system lies in the ocean, and variability on decadal time scale is a result of the ocean response to stochastic atmospheric forcing. A number of recent studies show that the ocean response to stochastic forcing is not just a red noise response, but has a distinct decadal time scale associated with planetary baroclinic basin modes. We developed a method for finding multiple basin modes in linear ocean models without the necessity of solving the computationally expensive eigenvalue problem. The method is based on the "breeding technique". We applied the method to a linear shallow water model (SWM) to analyze the sensitivity of planetary basin modes to model formulation.; Though the decadal planetary basin modes are the dominant modes in the SWM, it remains an open question if similar decadal modes have any significant amplitude in a nonlinear coupled general circulation model (CGCM). In contrast to the SWM, CGCMs have rapidly growing modes in addition to the baroclinic planetary and Kelvin basin modes. These modes are related to atmospheric, oceanic and coupled instabilities.; The slowly growing modes can be effectively isolated using the "breeding method", if they are well separated in time scale and nonlinear saturation level. The breeding method consists of two integrations: a control and a perturbed integration. The difference between these integrations is a random initial disturbance. This random disturbance is expected to project onto all the eigen-modes of the system. During integration, the most unstable modes will grow, becoming the dominant components of the difference between the control and perturbed run. Periodic rescaling of perturbation is necessary to avoid the nonlinear saturation of the modes of interest. We applied the breeding method to the COLA anomaly coupled GCM and found, that in the ocean subsurface in central and eastern North Pacific, the decadal modes are the dominant modes in the model, and can be isolated by choosing a sufficiently long rescaling interval.