Noack, Julia (2022) How do mesoscale ocean dynamics affect Antarctic Circumpolar Current strength in climate models?. (Bachelor thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 39 pp.

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Abstract

Given the importance of the Southern Ocean for the global climate, a realistic representation of the dynamics and circulation in this region in climate models is essential for enhancing confidence in model projections. In this thesis, we investigate the effect of increased horizontal
ocean resolution and associated more explicitly resolved mesoscale ocean dynamics on the Antarctic Circumpolar Current (ACC) strength in two coupled climate models. We consider a medium and high-resolution version of FOCI (1/2° and 1/10°) and two versions of HadGEM with a 1/4° and 1/12° ocean resolution in our analysis.
With a mean Drake Passage transport of 151 Sv and 115 Sv both high-resolution versions show a significantly better representation of the ACC strength that is closer to observational estimates than their respective coarser-resolution version with lower mean transports of 78 Sv and 80 Sv.
The bias between the different resolution models is found to be related to the horizontal structure of the ACC displayed in each model. The medium-resolution versions exhibit several strong countercurrents causing an overall weaker ACC strength. These westward currents are not simulated to this extent in the high-resolution versions and are not proven by observations.
Variations in the meridional density gradient found between the models partly explain the difference in the flow as implied by the thermal wind relation. The density differences in turn can be traced back to a stronger salinity gradient in the high-resolution models. Furthermore, we also show that the transport bias between the different resolution simulations in FOCI only starts to build up after several decades of model run length while initially both resolution versions have very similar and high transports. This can be linked to an increasingly different density stratification due to a stronger temperature gradient and a more saline Antarctic continental shelf in the high-resolution simulation after several decades of the model run.
Furthermore, bathymetry is also found to have an impact on the representation of the ACC in the different resolution versions since topography is shown with more detail in high-resolution models which causes the flow to be steered and additionally impacts local density gradients.

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