Identifying pathways of plastic in the Southern Ocean using Lagrangian particle tracking

Abstract

The Southern Ocean was thought to be relatively free of plastic pollution, but multiple observations of plastic in the Southern Ocean surface waters have been reported in recent years. In this thesis, sources of plastic from outside the Southern Ocean are investigated using Lagrangian particle tracking. Plastic particles are simulated by releasing virtual particles of which the dynamics are integrated in time based on high resolution eddy resolving ocean model data. In this study, it is investigated where and at what time of the year particles enter the Southern Ocean. Simulations are performed for floating plastic and for neutrally buoyant plastic in the interior of the ocean. Particles are also simulated along isopycnals because simulations may be very diapycnal which is caused by the steeply tilted isopycnals in the Southern Ocean. Results for floating plastic show that particles are able to cross the Antarctic Circumpolar Current and enter the Southern Ocean in two identi?ed areas because of a combination of the ocean currents and Stokes drift. A clear peak in the winter months suggests that this is a consequence of storm events. Simulations with three different initial depths are performed for plastic in the interior of the ocean. Results show areas where particles are travelling southwards and are upwelling. Particles that enter the Southern Ocean have generally been upwelled. Also, a clear correlation between the change in depth and the change in latitude is found. Particles are entering the Southern Ocean uniformly throughout the year suggesting it is driven by the topography of the ocean floor. Two methods are described to simulate particles along isopycnals. It is shown that, besides some small differences, the general results of the isopycnal simulations are the same as for the simulations in the interior of the Southern Ocean where the particles are advected with the three dimensional ocean model data. This suggests that particles do not have to be simulated along isopycnals to be accurately described.