A model-based investigation of transport pathways of thermocline waters to the ocean surface, with a focus on tropical oxygen minimum zones

Numerical models are a tool to better understand oceanic processes and their interaction with the atmosphere and biogeochemical processes on different temporal and spatial scales. Model predictions of future climate under different scenarios are used to formulate international protocols with far-reaching consequences for the well-being of the world’s population. However, models used to produce those predictions are not perfect. This thesis aims to contribute to the understanding of how sensitive ocean models are to the representation of a small scale mixing process and to horizontal grid resolution, and how their performance could be improved. In the first part of this thesis, the influence of the model representation of double-diffusive mixing on physical and biogeochemical upper-ocean properties is investigated. Two model runs in which double-diffusive mixing was parameterized in different ways are compared with a standard run which does not account for this process. Implementing double diffusion leads to changes in simulated physical, and even more in biogeochemical, upper-ocean properties. Changes in for example primary production are of the same order of magnitude as uncertainties in the measurements against which the model results can be compared. CO2 uptake is enhanced by approximately 7% of the anthropogenic CO2 signal. Although no definite conclusion on which representation, if any, is better could be drawn, the importance of further investigating double-diffusive mixing and its implementation in models became evident. In the second and third part, the closeness of the connection between tropical oxygen minimum zones and the surface ocean is investigated. Oceanic oxygen minimum zones are important in the context of climate and the nitrogen cycle, because they allow for very different processes than the rest of the ocean. The time it takes for water to be transported from the oxygen minimum zones to the surface ocean determines the minimum timescale of potential feedback processes. In the area where the oxygen minimum zone waters reach the mixed layer, feedback processes are likely to happen. Using float and tracer release experiments in models with horizontal resolution ranging from 1/12° to 2°, it is shown that oxygen minimum zone waters reach the surface ocean in the equatorial and coastal upwelling regions. In the high resolution model diffusive processes contribute to about half of the total transport, whereas the low resolution results are dominated by advection. Spatial distributions are largely independent of model and method used. The connection between the oxygen minimum zones and the upper ocean is faster in the Pacific than in the Atlantic Ocean, indicating that the intensity of oxygen minimum zones depends mainly on other processes than ventilation. Under a global warming scenario the connection is found to weaken, suggesting a strengthening of the oxygen minima. Findings from this thesis contribute to understanding the impact of specific model features on the resulting simulations and give suggestions on how to improve future simulations.