Role of Indian Ocean Dynamics on Accumulation of Buoyant Debris

Abstract

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.

The physical oceanography of the transport of floating marine debris

“…There is also much experience in prediction of transport for oil spills (e.g. Reed et al 1994, Fingas 2016, D'Asaro et al 2018 and in search and rescue (Breivik et al 2013, as well as theoretical work on the transport of material by oceanic Lagrangian Coherent Structures (e.g. Haller 2015).…”

The Surface Pathways of the South Atlantic: Revisiting the Cold and Warm Water Routes Using Observational Data

“…This yields an average of 860 ± 686 observations per grid cell, with 237 grid cells (5%) containing less than 50 observations. Note that approximately 1% of the grid cells are absorbing, that is, once a drifter enters an absorbing grid cell it remains there indefinitely, since there is a zero probability of leaving an absorbing grid cell (Miron et al, ; van der Mheen et al, ). Accordingly, we exclude the absorbing grid cell from our analysis (van der Mheen et al, ).…”

Mixing of passive tracers at the ocean surface and its implications for plastic transport modelling

“…Each of these five Markov chains has one unique stationary density corresponding to the 'garbage patch' of the respective basin. It is important to note that the projection is an approximation, as some of the garbage patches are only approximately stationary [8,30] in finite time (a few decades), see section 2.3.…”

“…the North Atlantic patch has its region of attraction mostly in the North Atlantic, etc.). In our case however, the basins of attraction will only be approximations, as the 'garbage patches' themselves are only approximately stationary [8,30]. We call our basins of attraction simply basins and identify them by making use of the particle dynamics instead of drawing arbitrary geographic boundaries.…”

“…For the Pacific and Atlantic basins, more than 80% of the particles remain in their respective basins during that time. Only 65% stay in the Indian Ocean, reflecting the fact that the Indian Ocean itself is relatively leaky in the long term (see also [30] and figure S2). This is also a consequence of the definition of the Indian Ocean region, which extends substantially into the Southern Ocean.…”