Thermohaline feedbacks in ocean-climate models of varying complexity

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is considered an important component of the climate system, because of its significant contribution to the heat budget of the Northern Hemisphere. Theoretical models indicate that the AMOC has non-linear dynamics, which result in a strong sensitivity to high latitude freshwater forcing. These models suggest that, as a result of the presence of multiple equilibria, the AMOC may drive large, abrupt shifts of the climate when a certain threshold is exceeded. There is no direct observational evidence that such AMOC related climate variations occur in reality, but the available data are too short and sparse to be conclusive in this case. Therefore, numerical models provide the main source of information regarding the nonlinear behavior of the AMOC. Because numerical models are necessarily incomplete, not in the least because of a lack of computational resources, their results must always be tested for robustness. This thesis presents four studies that examine how the representation of a certain unresolved process affects the behavior of the simulated AMOC The study in chapter 2 deals with the representation of horizontal mixing by mesoscale eddies. It is shown that a simple horizontal tracer mixing scheme is only a reasonable alternative to the more realistic isoneutral / Gent-McWilliams parameterization, provided that no wind forcing is imposed. In chapter 3, it is demonstrated that the use of a stability-dependent tracer diffusivity, which is commonly used to parameterize convection, leads to the occurrence of artificial multiple equilibria. In chapter 4, the representation of ocean-atmosphere interaction is considered. It is found that the sensitivity to anomalous freshwater forcing is only slightly modified if an interactive (sea surface temperature-dependent) atmosphere model is used, instead of a static atmosphere model. In chapter 5, the simulated sensitivity of the AMOC is compared between a model that explicitly resolves eddies, and a model in which eddies are parameterized. It is found that the behavior of an eddy-resolving model is qualitatively different from that of a non-eddying model. What is clear at this point, is that the AMOC is governed by non-linear dynamics. As a result, its simulated behavior depends in a non-trivial way on how unresolved processes are represented in a model. As demonstrated in this thesis, model fidelity can be effectively assessed by examining models of varying complexity.