Giudici, A.; Kalda, J., and Soomere, T., 2018. Joint Impact of Currents and Winds on the Patch Formation Near the Coasts of the Gulf of Finland. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1156–1160. Coconut Creek (Florida), ISSN 0749-0208.

Both coastal and offshore areas worldwide are greatly affected by the impact of large patches of floating pollution. The main mechanism of clustering of floating litter into patches is the ability of water masses to dive (e.g. during a downwelling event) whereas the litter remains on the surface. This property is quantified in terms of compressibility of the surface velocity field. When this quantity increases over a certain threshold, patchiness of floating litter may increase explosively. However, for quasi-two-dimensional flows such as typical fields of currents over shallow areas, the compressibility is typically reduced well below this threshold. We adress another factor that can increase the patchiness growth, namely, the effect of wind drift. More specifically, we explore the effect of the dependence of the wind-driven drift speed on the size of the floating objects. We present the results of simulations of surface clustering of various types of floating items. The differences in the drift speed are taken into account within a novel Eulerian tracking model. To speed up calculations, we make use of so-called kd-tree spatial subdivision. The model is run for the Gulf of Finland, shows a consistent behavior under several of the discussed conditions, and demonstrates that this mechanism, when coupled with the turbulent advection, can give rise to a dramatic increase in the patchiness growth rate.

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