Variations in the ocean circulation can strongly influence climate due to the large heat transport by the ocean currents. Variability of the thermohaline ocean circulation, the part of the ocean circulation driven by density gradients, occurs typically on (inter)decadal and longer time scales and is an important issue in present-day climate research. Although there are many indications from observations and numerical modeling studies that internal variability of the thermohaline circulation might play a role in interdecadal and centennial climate variability, the dominant periods and spatial patterns are still uncertain. Moreover, there is still no satisfactory theory explaining these periods and patterns and it is unclear how results from numerical simulations are linked to each other and to the observations. In this thesis, internal thermohaline variability is studied systematically, using techniques from dynamical systems theory. Steady-state solutions of the three-dimensional thermohaline flows are computed, and their linear stability is determined. Focus is on understanding the physical mechanisms of the oscillatory eigenmodes in an idealized limiting case and identifying characteristics that belong to these mechanisms. These characteristics are used to relate the variability in a hierarchy of single-hemispheric basin models to the eigenmodes of the limiting case. Within the idealized context of viscous thermally driven flows in a single-hemispheric ocean basin, the least damped oscillatory mode has an interdecadal period. Under prescribed heat-flux forcing, the flow is unstable to this interdecadal mode. Its physical mechanism is associated with westward propagating temperature anomalies, which cause a phase di.erence between the zonal and meridional overturning anomalies. These, in turn, lead to new temperature anomalies, consistent with the propagating anomaly patterns. The period is determined by the propagation speed of the temperature anomalies. Numerical time integrations with the same model con.guration show that the variability at interdecadal time scales is caused by the interdecadal mode. Also oscillatory modes with centennial periods are shown to exist in the idealized context. They are characterized by advection of temperature anomalies around the overturning loop, so that the oscillation period is mainly determined by the overturning time scale of the steady-state flow. For all parameter settings investigated, the centennial modes are damped. The interdecadal mode is followed along two paths in the model hierarchy towards more realistic situations. The growth rate of this mode is rather sensitive to the shape of the applied freshwater-flux forcing, but the physical mechanism of the mode is not a.ected by the inclusion of salinity. Using the westward propagation of temperature anomalies and the phase di.erence between meridional and zonal overturning as characteristics, also the variability in a less viscous ocean model in which continental geometry and bottom topography are included is shown to be caused by the interdecadal mode. The studies presented in this thesis provide a framework within which many other results from numerical modeling studies can be interpreted. Such a framework may contribute to our understanding of the observed climate variability on interdecadal and longer time scales.
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