Large amounts of plastic waste enter the ocean every year , potentially harming marine species and ecosystems . A large percentage of global plastic waste is estimated to enter the Indian Ocean. Despite this, buoyant marine plastic debris (“plastics”) is relatively under-sampled and under-studied in the Indian Ocean . The Indian Ocean atmospheric and oceanic dynamics are unique , so the dynamics of plastics in the Indian Ocean differ from those in the other oceans .

In the Pacific and Atlantic oceans, plastics accumulate in so-called “garbage patches” in the subtropical ocean gyres e.g.. Plastics also accumulate in a subtropical garbage patch in the Southern Hemisphere Indian Ocean, but this patch is much more dispersive and sensitive to different transport mechanisms (currents, wind, waves) than the garbage patches in the other oceans . In contrast, the subtropical Northern Hemisphere Indian Ocean is blocked by landmass, so there is no subtropical gyre and associated garbage patch. In addition, it is unclear if plastics entering the Northern Hemisphere Indian Ocean cross the Equator into the subtropical garbage patch in the Southern Hemisphere, as we explain further in the following paragraphs.

Strong currents are known to act as transport barriers for buoyant objects . For example, most fluid parcels in the Gulf Stream flow downstream; cross-stream transport only occurs at depth . As a result, there is almost no surface transport between the subtropics and the subpolar region in the North Atlantic Ocean: in 30 years only one ocean surface drifter crossed this boundary . In the equatorial region, the easterly trade winds drive strong equatorial currents and counter-currents . As a result, ocean surface drifters do not tend to cross the Equator and ultimately return to their original hemisphere . It has therefore been suggested that plastics do not generally cross the Equator but remain in the hemisphere where they entered the ocean .

However, in contrast to the other oceans, the easterly trade winds in the Northern Hemisphere Indian Ocean are not steady. Instead, they generally only have an easterly component during December, January, and February and have a westerly component during the remainder of the year . As a result, the North Equatorial Current and the South Equatorial Countercurrent in the Indian Ocean are not steady either. In addition, although the surface connectivity is split into two hemispheres in both the Pacific and Atlantic oceans, the surface of the Indian Ocean appears connected between hemispheres . Because of this, it is unclear if plastics tend to remain in their original hemisphere in the Indian Ocean. The question is therefore what happens to plastics entering the Northern Hemisphere Indian Ocean (NIO).

Measurements of open-ocean plastic concentrations in the Indian Ocean are scarce Fig. ; and insufficient to determine the fate of plastics entering the NIO. However, numerical modelling studies show a garbage patch forming in the Bay of Bengal . Sampling studies confirm that there are high concentrations of plastics in the Bay of Bengal , but it is not clear whether this is a result of plastics accumulating here or due to large nearby sources.

Another hypothesis is that plastics end up on coastlines in the NIO. Multiple studies sampled plastics on beaches in the Indian Ocean Fig. ;, but because they used very different sampling methods on different timescales (Table ), their results are difficult to compare. However, they do provide qualitative evidence that plastic is found on coastlines throughout the Indian Ocean, both on populated beaches close to plastic sources and on remote uninhabited coastlines and islands . Which coastlines are most heavily affected by stranding plastics depends both on the location of plastic sources and the ocean dynamics in the region.

In the NIO, both the atmospheric and oceanic dynamics are dominated by the monsoon system, which is driven by differences in air temperature above the Asian continent and above the NIO . During the southwest monsoon season (boreal summer: June, July, August) the air above the Asian continent is warmer than above the ocean, leading to predominantly southwesterly winds. In contrast, during the northeast monsoon season (boreal winter: December, January, February) the air above the ocean is warmer than above the Asian continent, resulting in predominantly northeasterly winds. These monsoonal winds result in strong seasonal variations in ocean surface currents in the NIO and Indian Ocean equatorial region.

During the northeast monsoon season, the Northeast Monsoon Current (NMC) flows from the Bay of Bengal westwards past Sri Lanka and into the Arabian Sea Fig. a;. The North Equatorial Current (NEC) also flows westwards during this season, feeding into the southwestward-flowing Somali Current (SC), which in turn feeds into the eastward-flowing South Equatorial Countercurrent (SECC). The South Java Current (SJC) flows southeastwards along Sumatra and Java, but is relatively weak during the northeast monsoon season .

During the southwest monsoon season the NMC dissolves, and instead the Southwest Monsoon Current (SMC) flows from the Arabian Sea eastwards past Sri Lanka and into the Bay of Bengal Fig. b;. There is no NEC during this season, and as a result the SC reverses direction as it is supplied by the westward-flowing South Equatorial Current (SEC) and the East African Coastal Current (EACC). The SJC continues to flow southeastwards along Sumatra but flows northwestwards along Java , as it is supplied by the strengthening Indonesian Throughflow (ITF) during the southwest monsoon season . At the convergence of the two opposing flows, a current flows southwestwards and feeds into the SEC.

During the intermonsoon periods, strong eastward-flowing surface Wyrtki jets develop along the Equator ; these are unique to the Indian Ocean. The Wyrtki jets are strongest during the intermonsoon period following the southwest monsoon season . They strengthen the SJC, which flows southeastwards during the intermonsoon periods.

The aim of this paper is to determine how these seasonally reversing ocean surface currents transport plastics that enter the NIO. Specifically, we focus on which coastlines are most heavily affected by stranding plastics. For convenience, we refer to plastics that are stranded on coastlines as “beaching” or “beached plastics”, where beaching can occur on any type of coastline, not just beaches. In addition to surface currents, wind and waves have a significant impact on the dynamics of buoyant objects in the Southern Hemisphere Indian Ocean SIO;. However, we only consider the influence of surface currents on the transport of plastics in this study; including dynamics due to wind and waves is beyond the scope of this paper. We discuss the reasons behind this, as well as the possible implications, in more detail in Sect. .

Our results show that plastics in the NIO move back and forth between the Bay of Bengal and the Arabian Sea, following monsoonal winds and currents. Plastics beach on coastlines throughout the NIO. Countries bordering the Bay of Bengal are most heavily and consistently affected. We also show that plastics from the NIO can cross the Equator into the SIO. In our simulations, this mainly occurs during the intermonsoon period following the southwest monsoon season (September, October, November), and we suggest a mechanism for the “escape” of plastics from the NIO into the SIO. Plastics that cross into the SIO beach along the entire eastern African coastline as well as on remote islands.

The aim of this paper is to determine what happens to plastics entering the NIO from rivers and which countries and islands are most heavily affected by beaching plastics. Our particle-tracking simulation results illustrate that particles move between the Arabian Sea and the Bay of Bengal depending on the monsoon season. During the northeast monsoon season large amounts of particles are present in the Arabian Sea as they are transported from the Bay of Bengal by the Northeast Monsoon Current (NMC). In contrast, during the southwest monsoon season particles are largely depleted from the Arabian Sea by the Southwest Monsoon Current (SMC) and move into the Bay of Bengal. Despite the annual back-and-forth movement, particles remain present year-round in the Bay of Bengal. This is possibly a result of the annual mean eastward flow in the equatorial region , as well as anti-cyclonic and cyclonic gyres that develop in the Bay of Bengal throughout the year , which may trap plastics.

Countries bordering the Bay of Bengal are consistently and heavily affected by beaching plastics. Specifically, Bangladesh, Myanmar, India, Malaysia, Indonesia, Sri Lanka, Thailand, Pakistan, the Maldives, and Somalia are in the top 15 most affected countries in all our simulations. For high beaching probabilities, all particles beach in the NIO within 3 years. In this case, the locations where particles beach is mainly a result of large plastic sources in the region, and plastics mainly beach in their country of origin. However, for low beaching probabilities, this is more likely a result of ocean dynamics, and beached plastics originate from multiple different countries. Because the NIO dynamics concentrate plastics in the Bay of Bengal, bordering countries are affected by beaching even on long timescales of O(10) years.

Somalia and the Maldives are specifically noteworthy countries affected by beaching plastics from the NIO in our simulations. Somalia does not border the Bay of Bengal and does not have any large nearby sources of plastic coming from rivers. Nevertheless, large amounts of particles consistently beach here. For low beaching probabilities, beached river plastics in Somalia mainly originate from countries that border the Bay of Bengal. The westward-flowing North Equatorial Current and the southwestward-flowing Somalia Current likely transport plastics to Somalia during the northeast monsoon season. The Maldives is noteworthy because the NMC and the SMC transport particles back and forth past the islands twice a year, which increases the likelihood of plastics beaching here. The same is true for Sri Lanka in our simulations.

For low beaching probabilities, up to 5 % of particles escape from the NIO into the SIO. This mainly occurs on the eastern side of the NIO basin during the intermonsoon period following the southwest monsoon season (September, October, November). We propose the following mechanism for particles crossing from the NIO into the SIO: (1) particles are transported eastwards by equatorial Wyrtki jets during the intermonsoon period, (2) particles are transported southeastwards across the Equator by the South Java Current (SJC), (3) particles are transported southwestwards as the SJC feeds into the South Equatorial Current (SEC), and (4) particles are transported westwards by the SEC into the subtropical SIO.

Simulated particles that cross from the NIO into the SIO mainly beach on eastern African coastlines or accumulate in the subtropical SIO garbage patch. Madagascar and Mozambique are most notably increasingly affected as more particles cross into the SIO.

Countries and islands in the SIO will of course also be affected by beaching plastics entering the ocean from source locations in the SIO (Fig. a). In this case, simulation results show that the most affected countries in the SIO are similar to those affected by plastics escaping from the NIO into the SIO (Fig. b, c, and d). Notable exceptions to this are the Cocos (Keeling) Islands and Christmas Island, both of which are more severely affected by beaching particles originating from the SIO (especially with high beaching probability, Fig. b). Connectivity matrices indicate that particles mostly beach in their country of origin, or come from Indonesia (Fig. e, f, and g). Besides beaching in the SIO, plastics entering the SIO also accumulate in the subtropical garbage patch (up to 5 % for high beaching probability versus 36 % for low beaching probability). Particles can also cross the Equator and beach in NIO countries, although this occurs less frequently than plastics crossing from the NIO into the SIO (around 2 % crossing from the SIO into the NIO, compared to up to 5 % crossing from the NIO into the SIO for low beaching probabilities). Finally, particles entering the SIO also escape the Indian Ocean entirely: up to 2 % for high beaching probability and up to 7 % for low beaching probability.

Our results indicate that a large percentage of plastics end up on coastlines in the Indian Ocean. In our simulations with a high beaching probability, 100 % of particles beach in the NIO within 3 years. Up to 90 % of particles beach in either the NIO or SIO within 10 years in our simulations with a low beaching probability. These results are in good general agreement with those of , who showed that roughly 67 % of all global plastic waste ended up on coastlines. therefore suggested that the large mismatch between the estimated amount of plastic entering the ocean globally and the total estimated amount of plastic floating on the ocean surface the “missing plastic”, can be explained by plastics stored on coastlines. However, our simulations illustrate that results are sensitive to different beaching conditions, specifically the beaching probability. To determine if beached plastics can indeed explain the whereabouts of the missing plastic, it is therefore important to improve the simulation of beaching in numerical models and apply reliable beaching conditions.

The importance of coastal dynamics in the transport of plastics to the open ocean was recently demonstrated by , who found that as a result of tidal dynamics only roughly 20 % of simulated particles released around the East China Sea were transported to the open ocean. showed that ocean surface drifters in an estuary ran aground on timescales much shorter than the transport time to the open ocean. Both of these studies illustrate the importance of local dynamics in transporting plastics to the ocean. A better understanding of the overall effect of these dynamics, as well as a method to apply them on large scales (for example using a realistic beaching probability), is therefore needed to improve global- and basin-scale models of beaching plastics.

In addition, we applied a single beaching probability throughout the Indian Ocean to our simulation results. Because beaching mechanisms depend on local coastal dynamics and morphology, beaching probabilities likely vary from location to location. A better understanding of the spatial variation of beaching probabilities depending on local conditions will likely improve the numerical simulation of beaching plastics. Finally, we did not take into account that beached plastics can also return to the ocean. Including these dynamics may also improve the simulation of beaching plastics. Recent works by may contribute to this.

The work of showed that different transport mechanisms, due to wind and waves, have a significant influence on the accumulation of buoyant debris in the subtropical SIO garbage patch. In this paper, we only considered the effect of ocean surface currents on the transport of river plastics entering the NIO. It is not straightforward to apply the same beaching methodology when simulations are forced not only by ocean surface currents but also by wind and wave effects. This is because, in contrast to ocean surface currents, the transport due to wind and Stokes drift can be directed perpendicular to coastlines. This means that including wind or wave effects adds a physical mechanism to the beaching of particles. However, in our methodology we assume that there are no physical beaching processes in the particle-tracking simulations, and beaching is included purely as a specified probability acting a certain distance from the coastline. This assumption is reasonable when particles are forced only by ocean surface currents, but it is no longer valid when wind or Stokes drift forcing is included as well. The best method to include wind and wave effects in these beaching simulations needs more careful consideration and extended analysis, which we will do in future work.

Because we have not included wind and wave effects in our simulations, our results are likely applicable only to plastics that are neutrally or slightly positively buoyant and are transported in the upper 2 m of the water column. Wind and waves can have a large influence on local beaching behaviour. However, on a large scale, we do not expect the influence of including either windage or Stokes drift to have such a significant effect as in the SIO. This is because both wind and ocean surface currents in the NIO are driven by the monsoon system. For example, although the timescales on which beaching occurs will likely change by including windage or Stokes drift, the main dynamics of particles moving between the Arabian Sea and the Bay of Bengal depending on the monsoon season should remain the same.

Finally, measurements of plastics on coastlines are needed to confirm and improve numerical modelling results. Although multiple studies sampled plastics on beaches throughout the Indian Ocean , the different sampling methods and timescales mean that their results are difficult to compare quantitatively. In addition, standing stock measurements are of limited use because they provide no information about the time period over which plastics may have accumulated on beaches. Ideally, long-term measurements during different conditions and along different types of coastline are needed.

The aim of this paper is to determine what happens to plastics that enter the NIO from rivers. Our particle-tracking simulation results show that plastics move back and forth between the Bay of Bengal and the Arabian Sea depending on the monsoon season. During the southwest monsoon season, the Arabian Sea is almost completely depleted of particles as they are transported to the Bay of Bengal by the Southwest Monsoon Current. In contrast, there are relatively high concentrations of particles present in the Bay of Bengal year round. This may be due to the annual mean eastward flow in the equatorial region , as well as anti-cyclonic and cyclonic gyres in the Bay of Bengal trapping plastics.

Particles move close to coastlines as they move between the Arabian Sea and the Bay of Bengal. When we allow simulated particles to beach with a high beaching probability (p=0.95 (5 d)-1), all particles beach in the NIO within 3 years, mostly in their country of origin. For low beaching probability (p=0.05 (5 d)-1), 86 % of particles beach in the NIO in 10 years. In most countries, beached river plastics originate from multiple different countries for low beaching probability. Countries bordering the Bay of Bengal are heavily affected by beaching particles in our simulations, likely because ocean dynamics concentrate particles in this region. Somalia and the Maldives are also consistently affected by beaching particles, even though they have no or few river sources of plastics of their own. In the case of the Maldives, this is a result of the Southwest Monsoon Current and the Northeast Monsoon Current transporting particles back and forth past the islands twice a year. In the case of Somalia, the North Equatorial Current and the Somalia Current likely transport particles originating from countries in the Bay of Bengal towards the Somalian coast.

In simulations with low beaching probability, up to 5 % of particles escape from the NIO into the SIO, where they predominantly beach along eastern African coastlines. Particles mostly pass the Equator along the eastern side of the Indian Ocean basin during the intermonsoon period following the southwest monsoon season (September, October, November). We suggest the following mechanism for their escape from the NIO into the SIO: (1) particles are transported eastwards by equatorial Wyrtki jets, (2) particles are transported southeastwards across the Equator by the South Java Current, (3) particles are transported southwestwards where the South Java Current feeds into the South Equatorial Current, and (4) particles are transported westwards into the subtropical SIO by the South Equatorial Current.