A Note on the Role of the Initial Error Structure in the Tropics on the Seasonal-to-Decadal Forecasting Skill in the Extratropics

The predictability of a coupled system composed of a coupled reduced-order extratropical ocean–atmosphere model forced by a low-order three-variable tropical recharge–discharge model is explored with emphasis on its long-term forecasting capabilities. Highly idealized ensemble forecasts are produced taking into account the uncertainties in the initial states of the system, with specific attention to the structure of the initial errors in the tropical model. Three main types of experiments are explored: with random perturbations along the three Lyapunov vectors of the tropical model; along the two dominant Lyapunov vectors; and along the first Lyapunov vector only. When perturbations are introduced along all vectors, forecasting biases develop even if in a perfect model framework and with known initial uncertainty properties. Theses biases are considerably reduced only when the perturbations are introduced along the dominant Lyapunov vector. Furthermore, this perturbation strategy allows a reduced mean square error to be obtained at long lead times of a few years, as well as reliable ensemble forecasts across the whole time range. These very counterintuitive findings further underline the importance of appropriately controlling the initial error structure in the tropics through data assimilation.