Atmospheric variability and the Atlantic multidecadal oscillation. Mathematical analysis of Low-order models

PhD ceremony: Mr. A.E. Sterk, 16.15 uur, Academiegebouw, Broerstraat 5, Groningen

Thesis: Atmospheric variability and the Atlantic multidecadal oscillation. Mathematical analysis of Low-order models

Promotor(s): prof. H.W. Broer, prof. C. Simó, prof. H.A. Dijkstra

Faculty: Mathematics and Natural Sciences

Observations of the surface temperature of the North Atlantic Ocean indicate the presence of variability with a time scale of several decades. This variability is referred to as the Atlantic Multidecadal Oscillation (AMO). Models for the large scale ocean circulation suggest that the pattern and time scale of the AMO are explained by variations in the thermohaline circulation. However, the amplitude of the AMO is presumably determined by the atmosphere above the North Atlantic Ocean.

In this thesis we investigate (1) the dynamics of low-frequency variability of the atmosphere at midlatitudes and (2) its effect on the AMO. To that end, we study so-called low-order models, which are derived by projecting the (unknown) solution of a system of partial differential equations onto a suitably chosen finite-dimensional function space. A systematic investigation of attractors and their bifurcations gives a coherent view of the model's dynamics. Of particular importance are the transitions from orderly to more complex, or even chaotic, dynamics.

The three main results of this thesis are (1) a new dynamical scenario or atmospheric low-frequency variability in which strange attractors represent irregular planetary waves, (2) a strongly simplified model that captures the qualitative aspects of the AMO, and (3) a deterministic mechanism in which the AMO is excited by a chaotic atmosphere.