The Role of Ekman Currents, Geostrophy, and Stokes Drift in the Accumulation of Floating Microplastic

• Published in: Journal of geophysical research. Oceans Vol. 124; no. 3; pp. 1474 - 1490
• Main Authors: Onink, Victor, Wichmann, David, Delandmeter, Philippe, Sebille, Erik
• Format: Journal Article
• Language: English
• Published: United States 01.03.2019
• Subjects: global ocean circulation; Lagrangian modeling; microplastic accumulation; stokes drift; stokes drift; global ocean circulation; microplastic accumulation; Lagrangian modeling
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2018JC014547

Floating microplastic in the oceans is known to accumulate in the subtropical ocean gyres, but unclear is still what causes that accumulation. We investigate the role of various physical processes, such as surface Ekman and geostrophic currents, surface Stokes drift, and mesoscale eddy activity, on the global surface distribution of floating microplastic with Lagrangian particle tracking using GlobCurrent and WaveWatch III reanalysis products. Globally, the locations of microplastic accumulation (accumulation zones) are largely determined by the Ekman currents. Simulations of the North Pacific and North Atlantic show that the locations of the modeled accumulation zones using GlobCurrent Total (Ekman+Geostrophic) currents generally agree with observed microplastic distributions in the North Pacific and with the zonal distribution in the North Atlantic. Geostrophic currents and Stokes drift do not contribute to large‐scale microplastic accumulation in the subtropics, but Stokes drift leads to increased microplastic transport to Arctic regions. Since the WaveWatch III Stokes drift and GlobCurrent Ekman current data sets are not independent, combining Stokes drift with the other current components leads to an overestimation of Stokes drift effects and there is therefore a need for independent measurements of the different ocean circulation components. We investigate whether windage would be appropriate as a proxy for Stokes drift but find discrepancies in the modeled direction and magnitude. In the North Pacific, we find that microplastic tends to accumulate in regions of relatively low eddy kinetic energy, indicating low mesoscale eddy activity, but we do not see similar trends in the North Atlantic. Plain Language Summary Microplastic is a common form of pollution in the oceans, and high floating microplastic concentrations tend to be observed at the surface in the subtropical ocean gyres. These regions are commonly referred to as garbage patches. However, the physical processes that control the buildup in these regions are not yet fully understood. Therefore, we model microplastic transport with various surface current components that correspond to different physical processes. We do this with Lagrangian modeling, where microplastic is represented by virtual particles that are transported by ocean currents. We find good agreement between the modeled distribution with the full surface currents with observations in the North Pacific and North Atlantic and find that the microplastic accumulation is mainly due to the wind‐driven Ekman currents. Meanwhile, wave‐driven Stokes drift results in microplastic transport to Arctic regions. Since Stokes drift has not consistently been included in microplastic transport modeling, microplastic contamination of Arctic regions might be more severe than currently expected. Key Points Ekman currents are the main process behind microplastic accumulation in the subtropical ocean gyres Stokes drift contributes to microplastic transport to Arctic regions Windage is, on a global scale, not an accurate proxy to model Stokes drift dynamics