The land-sea coastal border: A quantitative definition

Abstract. A quantitative definition for the land-sea (coastal) transitional area is here proposed, based on variability and isotropy of met-ocean processes.Wind velocity and significant wave height fields are examined for anisotropy along four perpendicular transects on the Catalan coast (northwestern Mediterranean) illustrating a case of significant changes along shelf. The variation of anisotropy as a function of distance from the coast and water depth has been analyzed through heatmaps and scatter plots. The results show how the anisotropy of wind velocity and significant wave height decrease towards the offshore, suggesting an objective definition for the coastal fringe width. The more robust estimator turns out to be the distance at which the significant wave height anisotropy is equal to the 90th quantile of variance within a 100 km distance from the coast. Such a definition, when applied to the Spanish Mediterranean coast, determines a fringe of width of 2–4 km. Regarding the probabilistic characterization, the inverse of wind velocity anisotropy can be fitted to a lognormal distribution function, while the significant wave height anisotropy can be fitted to a log-logistic distribution function. The joint probability structure of the two anisotropies can be best described by a Gaussian copula, where the dependence parameter denotes mild to moderate dependence between both anisotropies, reflecting a certain decoupling between wind velocity and significant wave height near the coast. This wind-wave dependence remains stronger in the central, bay-like part of the study area, where the wave field is being more actively generated by the overlaying wind. Such a pattern controls the spatial variation of the coastal fringe width.