Special CASPO Seminar before Ocean Sciences!
Yasushi Fujiwara (Kyoto University) will be presenting a talk titled "Explicit numerical simulations of waves and Langmuir circulations"
Langmuir circulations (LCs) are roll circulations in the mixed layer, and their mixing effect is considered to be a large uncertainty in the present climate modeling. Our present understanding of LCs heavily depends on the model based on the Craik-Leibovich (CL) equation, which describes the residual wave effect on currents as the so-called vortex force term using the prescribed Stokes drift profile. However, the validity of such an understanding is not directly supported by field and laboratory observations because of the difficulty in measurement and quantification of wave-current coupling.
We numerically investigated the wave-current interaction using explicit simulations of surface wave motions and underlying currents (called wave-resolving simulations or WRS). For this purpose, a free-surface numerical fluid model was developed with special care to the conservation properties of momentum and energy. The results of the simplified simulations show that in cases with strong wind-driven shear currents (produced by surface wind stress), waves become amplitude-modulated due to a refraction-like response to the downwind current. The Stokes drift varies corresponding to the current field, and its horizontal shear serves to intensify the LCs. Such an effect cannot be reproduced by the CL equation only because it does not account for the current effect on waves. The implications of the simulation results will also be discussed based on parameter dependences of the process.
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