Time scale interaction in low-order climate models

• Main Author: Veen, Lennaert van
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2002
• Subjects: Wiskunde en Informatica; climate dynamics; low-order models; bifurcation analysis; singular perturbations; intermittency; North Atlantic
• Online Access: https://dspace.library.uu.nl/handle/1874/870

Over the last decades, the study of climate variability has attracted ample attention. The observation of structural climatic change has led to questions about the causes and the mechanisms involved. The task to understand interactions in the complex climate system is particularly di±cult because of the lack of observational data, spanning a period of time typical for natural climate variability. One way around this problem is to represent the earth s climate in a computer model, as a set of prognostic equations. A disadvantage of this approach is that, if the model under consideration is to faithfully represent the climate system, it has to be large in terms of the number of degrees of freedom. This puts it out of reach of the ordinary analysis of dynamical systems theory. Alternatively, we can impose symmetries, consider limits of physical parameters, exploit perturbation theory and use Galerkin approximation to obtain simplified models of the earth s climate. Such models should highlight some isolated aspects of climate dynamics. A feature these simplified models have in common is the presence of widely different time scales. Throughout this thesis the emphasis is on the question to what extent the slow time scales play a role in the model s dynamics. The slow time scales are related to ocean dynamics and the fast time scales to atmospheric dynamics. The atmosphere model, studied here, was introduced by Edward Lorenz (1984). In chapter 2 a derivation of this model is given and it is shown that the Lorenz-84 model describes the jet stream in the mid-latitude atmosphere, and planetary waves, which can grow if the jet stream becomes dynamically unstable. The Lorenz-84 model is coupled to two different low-order ocean models. In chapter 3, it is coupled to Stommel s two box model. Stommel s model mimics the thermohaline circulation in the North Atlantic ocean. The typical time scale of variability of this circulation is of the order of centuries. This will be the longest time scale in the coupled models. In ...