Surface drift in the South Pacific Gyre: Observations and simulations of drifters and plastic
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- Geophysical Institute 
The objective of this thesis is to investigate the residence times and behavior of microplastics and macroplastics in the southern Pacific Ocean. I use the open- source Lagrangian particle tracking framework OpenDrift to reproduce drifter trajectories and simulate the large-scale drift of marine plastic in the south pacific subtropical gyre. I conducted three different simulations. First, I conducted simulations to assess of how well the ORAS5, ERA5, and OpenDrift reproduced the observed drifter trajectories from the Kon-Tiki 2 expedition before and after they lost their drogue and until they stopped transmitting. The results showed that ORAS5 did a varying job in reproducing the undrogued drifters but a better job reproducing the drogued. This result was as expected because no wind effects were included in those simulations as the leeway coefficient was estimated to be less than 1%. From the simulation of the drift of micro- and macroplastics using Lagrangian particle tracking with ocean and atmospheric forcing from reanalysis products, I find that microplastic and macroplastic accumulate in the eastern part of the South Pacific and forms a well-defined plastic gyre within 12 years. After the plastic reaches the accumulation zone, the zone and borders are not stationary in time but vary in shape and location. However, the minimal number of plastic particles within the zones are relatively steady, indicating that there is no, or only minimal, leakage of plastic over the final 12 years of simulation. The seasonal variation of the particles crossing the 30°S line is more prominent in the micro than in the macro simulation. On top of the seasonal variability, there seems to be a signal on a longer time scale, which might be an ENSO signal, but this would need further investigation.
Postponed access: the file will be accessible after 2023-08-12