[A-OS18] Physical Oceanography (General)

[AOS18-02] The Langumuir number dependence of the horizontal transport of microplastics simulated with ocean surface layer LES

*Yoshifumi Fukumoto ¹, Yasushi Fujiwara¹ (1.Kobe University )

Keywords:microplastics, drift simulation, large eddy simulation, Stokes dirft

Microplastics (MP) are abundant in the ocean, but there are many unanswered questions about their transport processes. Therefore, previous studies have pointed out the need to improve the accuracy of MP drift predictions using drift models. One of the factors causing errors in the drift model is the flow on a scale finer than the computational grid size (SGS) . One type of SGS flow is mixed-layer turbulence. In our research , focusing on the initial mixed-layer depth which is one of the parameters that characterize mixed-layer turbulence, we showed that the vertical structure of microplastics and the flow contribute significantly to the horizontal transport of MP. From this result, it is expected that similar results can be obtained for other parameters that characterize mixed layer turbulence, and clarifying the dependence on these parameters will be the first step toward improving the parameterization of drifting models. Therefore, this study aims to clarify the effects of mixed-layer and floating characteristics on the horizontal transport of MP using the ocean surface LES. In the present experiment, we focused on the Langmuir number, one of the parameters that characterize mixed layer turbulence. The Langmuir number is expressed as the ratio of Stokes velocity to wind speed at the surface layer , and is a dimensionless number that expresses the strength of the Stokes drift .
Using the nonhydrostatic ocean model KINACO, mixed layer simulations with seven different Langumiur number are conducted in a rectangular domain with periodic horizontal boundaries. There, Lagrangian particles with eight different rising speeds, assuming microplastics are released, and each particle’s total horizontal displacement is evaluated by counting the number of times that each particle crosses the horizontal boundaries.
The results show that the horizontal transport of MP is Langmuir number dependent. In addition, the Langmuir number dependence was not monotonic in the transport in the direction perpendicular to the wind. This suggests that the vertical distribution of particles is important in determining the horizontal transport of MP, and that the vertical distribution is determined by the strength of vertical mixing in addition to the MP rising velocity. In addition, it was suggested that Stokes drift does not merely enhance the downwindward transport of MP, but may in some cases weaken it.