Publication Date:
2023-07-27
Description:
This PhD thesis consists of three research papers. The first research paper addresses crucial issues of the present climate change debate, namely the response of the meridional overturning circulation (MOC) of the Southern Ocean (SO) to decadal-scale trends in wind stress forcing, and the ability of up-to-date meso-scale eddy parameterisations to represent the corresponding changes in the eddy field in climate models. Results from an idealised SO model in both eddy-permitting and parameterised configurations show that the MOC is characterised by an eddy-driven part which generally opposes the wind-driven part and that the increase of the MOC diminishes with amplifying winds, with the possibility that the MOC in the SO may become completely insensitive to wind stress changes. However, for moderate wind stress, the MOC is still significantly increasing in the eddy-permitting model configuration. The parameterisations are able to reproduce the MOC for certain wind stresses, but all parameterisations overestimate the sensitivity of the MOC on wind stress. The results show that it is indispensable to incorporate the correct sensitivity of eddy field into climate models in order to reproduce the correct sensitivity of the MOC to wind stress and that up-to-date meso-scale eddy parameterisations are only partially successful. The second and third research papers are guided by a more conceptual perspective and focus on one of the most common diagnostics of the MOC: the concept of the MOC streamfunction. The second research paper clarifies the question: Is it possible to define a MOC streamfunction completely void of standing eddies? It is shown that the construction of a MOC streamfunction with an exactly vanishing standing eddy part has to be performed by zonal integration along depth-dependent horizontal isolines of time-mean density. In contrast, zonal integration along time-mean geostrophic streamlines, typically applied to neutralise the impact of standing eddies, generally only leads to a MOC streamfunction with a reduced standing eddy part. Finally, the third research paper considers the two most common approaches to calculating MOC streamfunctions directly in Eulerian space: the series expansion of the residual - mean eddy streamfunction and the series expansion of the quasi-Stokes streamfunction. Using several idealised eddy-permitting zonal channel model experiments, the two series are compared up to third order in buoyancy perturbation. In model configurations with at bottom, both streamfunctions may be well approximated by the first one or two leading order terms in the ocean interior, although terms up to third order still significantly impact the implied interior circulations. Further, differences in both series expansions up to third order remain small here. Near surface and bottom boundaries, on the other hand, the leading order terms differ and are initially of alternating sign and of increasing magnitude such that the low order approximate expressions break down there. In more realistic model configurations with significant topographic features, physically inconsistent recirculation cells also appear in the ocean interior and are not effectively reduced by the next higher order terms. Therefore, the diagnosis of the MOC from empirical data or realistic model results via approximations of the series expansion of the residual-mean eddy streamfunction or the series expansion of the quasi-Stokes streamfunction must be treated with care or even completely ruled out.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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