Role of Indian Ocean Dynamics on Accumulation of Buoyant Debris

Buoyant marine plastic debris has become a serious problem affecting the marine environment. To fully understand the impact of this problem, it is important to understand the dynamics of buoyant debris in the ocean. Buoyant debris accumulates in garbage patches in each of the subtropical ocean basins because of Ekman convergence and associated downwelling at subtropical latitudes. However, the precise dynamics of the garbage patches are not well understood. This is especially true in the southern Indian Ocean (SIO), where observations are inconclusive about the existence and numerical models predict inconsistent locations of the SIO garbage patch. In addition, the oceanic and atmospheric dynamics in the SIO are very different from those in the other oceans. The aim of this paper is to determine the dynamics of the SIO garbage patch at different depths and under different transport mechanisms such as ocean surface currents, Stokes drift, and direct wind forcing. To achieve this, we use two types of ocean surface drifters as a proxy for buoyant debris. We derive transport matrices from observed drifter locations and simulate the global accumulation of buoyant debris. Our results indicate that the accumulation of buoyant debris in the SIO is much more sensitive to different transport mechanisms than in the other ocean basins. We relate this sensitivity to the unique oceanic and atmospheric dynamics of the SIO. Plain Language Summary Plastic pollution of the oceans is a serious problem. To better understand how big the problem is, it is important to know how plastics move in the oceans and where they end up. Floating plastics move with ocean currents, waves, and wind. Because these interact with each other in complex ways, the exact movement of plastic in the ocean is hard to predict. From observations and computer simulations, it is known that plastic collects in subtropical garbage patches in the Pacific and Atlantic oceans, but it is not clear if this also happens in the Indian Ocean. The aim of this paper is to determine the different influences of ocean currents, waves, and wind on the development of a garbage patch in the Indian Ocean. Because we cannot track plastics, we use data from over 22,000 GPS-tracked drifting buoys that have been released worldwide in the oceans since 1979 to simulate the movement of plastic. Our results show that waves and wind prevent a garbage patch forming in the Indian Ocean. Because large amounts of plastic waste probably enter the Indian Ocean, an important question for follow-up research is what happens to these plastics if they do not collect in a garbage patch.