Internal variability of the wind-driven ocean circulation

Abstract

The ocean circulation is known to vary on a multitude of time and spatial scales. Due to the large heat capacity of the oceans, variations in its circulation have a profound impact on climate. Therefore, understanding the origin of this variability and its sensitivity to physical parameters is an important issue in climate research. Part of the variability of the earth's climate on intermonthly to interannual time scales is caused by variations of the wind-driven ocean circulation. In particular, the mid-ocean jets associated with the large-scale gyre circulation are highly variable.This type of variability may arise as a result of internal processes in the ocean, external processes like for example changes in the surface wind forcing, and through coupled ocean-atmosphere interactions. In this thesis, focus is on the internal variability of the wind-driven ocean circulation, and on understanding the basic physical processes that give rise to this internal variability. Thereto, the characteristics of an idealized double-gyre circulation, serves as a prototype for major mid-ocean jet systems like the Gulf Stream, are explored in a systematic way. As a consequence of the idealizations in the model configuration, the results of this study have to be discussed qualitatively rather than quantitatively. The approach used in this thesis is based on the notion that the internal variability of the flow is directly linked to its (in-)stability. Therefore, valuable information on the (origin of the) variability can be obtained by determining stationary solutions for the flow, and studying the stability of these flows with respect to small perturbations. This is done here for changing values of a specific model parameter, starting from a parameter regime where the flow is stable. The characteristics of the most unstable perturbations (i.e., those that destabilize the flow when the ratio of forcing to dissipation is still relatively small) can be studied in detail. These modes are expected to dominate the internal variability, both close to the stability boundary as well as far into the unstable regime.

The results of the analyses presented in this thesis allow for a detailed study of the origin, sensitivity, and underlying physics of the internal variability, and provides a valuable interpretation framework for the results of time integrations. In the future , this combined dynamical systems and time series analysis can be used to analyze more realistic models, which for example include the thermohaline component of the circulation as well. Such studies are expected to lead to a better understanding of the observed variability of the ocean circulation.