Jackson, J.G.; (2011) Balanced initialisation techniques for coupled ocean-atmosphere models. Doctoral thesis , UCL (University College London).

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Abstract

Interactive dynamical ocean and atmosphere models are commonly used for predictions on seasonal timescales, but initialisation of such systems is problematic. In this thesis, idealised coupled models of the El Ni~no Southern Oscillation phenomenon are used to explore potential new initialisation methods. The basic ENSO model is derived using the two-strip concept for tropical ocean dynamics, together with a simple empirical atmosphere. A hierarchy of models is built, beginning with a basic recharge oscillator type model and culminating in a general n-box model. Each model is treated as a dynamical system. An important step is the 10-box model, in which the seasonal cycle is introduced as an extension of the phase space by two dimensions, which paves the way for more complex and occasionally chaotic behaviour. For the simplest 2-box model, analytic approximate solutions are described and used to investigate the parameter dependence of regimes of behaviour. Model space is explored statistically and parametric instability is found for the 10-box and upward versions: while it is by no means a perfect simulation of the real world phenomena, some regimes are found which have features similar to those observed. Initialisation is performed on a system from the n-box model (with n = 94), using dimensional reduction via two separate methods: a linear singular value decomposition approach and a nonlinear slow manifold (approximate inertial manifold) type reduction. The influence of the initialisation methods on predictive skill is tested using a perfect model approach. Data from a model integration are treated as observation, which are perturbed randomly on large and small spatial scales, and used as initial states for both reduced and full model forecasts. Integration of the reduced models provides a continuous initialisation process, ensuring orbits remain close to the attractor for the duration of the forecasts. From sets of ensemble forecasts, statistical measures of skill are calculated. Results are found to depend on the dimensionality of the reduced models and the type of initial perturbations used, and model reduction is found to result in a slight improvement in skill from the full model in each case, as well as a signifi�cant increase in the maximum timestep.

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