10 pages, 8 figures, 1 table.We demonstrate the utility of using the equivalent bottom roughness for calculating the friction factor and the drag coefficient of a seagrass meadow for conditions in which the meadow height is small compared to the water depth. Wave attenuation induced by the seagrass Posidonia oceanica is evaluated using field data from bottom-mounted acoustic doppler velocimeters (ADVs). Using the data from one storm event, the equivalent bottom roughness is calculated for the meadow as ks ~ 0.40 m. This equivalent roughness is used to predict the wave friction factor ƒw, the drag coefficient on the plant, CD, and ultimately the wave attenuation for other storms. Root mean squared wave height (Hrms) is reduced by around 50% for incident waves of 1.1 m propagating over ~1000 m of a meadow of P. oceanica with shoot density of ~600 shoots m−2.E. Infantes acknowledges the financial support received from the Spanish Ministerio de Educacion y Ciencia, FPI scholarship program (BES-2006-12850). G. Simarro is supported by the Spanish government through\ud the Ramón y Cajal program. A. Orfila and G. Simarro are grateful for financial support from Spanish MICINN through project CTM2010-12072. M. Luhar was supported on grant number 0751358 from the U.S. National Science Foundation Ocean Sciences Division.Peer reviewe

Medicanes, for Mediterranean hurricanes, are mesoscale cyclones with morphological and physical characteristics similar to tropical cyclones. Although less intense, smaller and rarer than their Atlantic counterparts, medicanes are very hazardous events threatening islands and continental coasts within the Mediterranean Sea. The latest strong episode, Medicane Ianos (September 2020), resulted in severe damages in Greece and several casualties. This work investigates the oceanic response to these extreme events along the Mediterranean coasts under present-day and future (late 21^st century) climate conditions. To this end, a coupled hydrodynamic-wave model is used to simulate both storm surges and wind-waves generation and propagation in the Mediterranean Sea at high resolution ( ∼ 2 km) along the coastlines. A dataset of thousands of medicanes synthetically generated from twenty global climate models and two atmospheric reanalyses is used to derive the atmospheric forcing fields. Regional coastal hazards assessment is performed for the present and future climates. For the first period, highest medicane-induced waves are found in the central and the southwest part of the western Mediterranean, while greatest storm surges are found in the Adriatic Sea and regions characterized by wide and gently sloping continental shelves. Results obtained for future changes show amplitudes generally smaller than the associated uncertainty due to limited agreement among models (especially for coastal elevation). Though, model consensus is reached (60-75%) and relative intensity change is significant (10-20%) at some locations (e.g. 1m increase of medicane-induced significant wave height on average for south coasts of Sicilia).

A combined space and time domain Self‐Organizing Maps analysis of Altimetry‐derived finite‐size Lyapunov exponents is used to extract characteristic spatial patterns of mesoscale features and to classify the western Mediterranean in regions of different variability of mixing activity. In particular, a recurrent and intense front is unveiled between Cartagena, Spain, and Tenes, Algeria, in the eastern boundary of the Alboran Sea, the here called Cartagena‐Tenes front (CTF). The temporal evolution and the frequency of occurrence of the spatial patterns and the regional variability of the mixing structures together with additional complementary Lagrangian diagnosis have been used to investigate the effect of the CFT on the regional flow connectivity. The seasonal behavior in the flux of particles through the domain associated with the degree of activation of the CTF shows that surface connectivity between the Alboran Sea and the western Mediterranean is stronger during winter and spring when the CTF is not active. Furthermore, the analysis shows that CTF becomes stronger than the Almeria‐Oran front over particular time periods, controlling the transport processes in this area. The impact of these fronts on transported substances has been analyzed based on Chlorophyll‐a satellite distribution, showing how the Lagrangian Coherent Structures, separate regions of different connectivity properties, affecting on the dynamical and biological behavior.

A new approach to infer the bathymetry from coastal video monitoring systems is presented. The methodology uses principal component analysis of the Hilbert transform of video images to obtain the components of the wave propagation field and their corresponding frequency and wavenumber. Incident and reflected constituents and subharmonics components are also found. Local water depth is then successfully estimated through wave dispersion relationship. The method is first applied to monochromatic and polychromatic synthetic wave trains propagated using linear wave theory over an alongshore uniform bathymetry in order to analyze the influence of different parameters on the results. To assess the ability of the approach to infer the bathymetry under more realistic conditions and to explore the influence of other parameters, nonlinear wave propagation is also performed using a fully nonlinear Boussinesq-type model over a complex bathymetry. In the synthetic cases, the relative root mean square error obtained in bathymetry recovery (for water depths $0.75\mathrm{m}⩽h⩽8.0\mathrm{m}$ ) ranges from ∼1% to ∼3% for infinitesimal-amplitude wave cases (monochromatic or polychromatic) to ∼15% in the most complex case (nonlinear polychromatic waves). Finally, the new methodology is satisfactorily validated through a real field site video.

The spatial and temporal variability of extreme wave climate in the North Atlantic Ocean and the Mediterranean Sea is assessed using a 31-year wave model hindcast. Seasonality accounts for 50 % of the extreme wave height variability in the North Atlantic Ocean and up to 70 % in some areas of the Mediterranean Sea. Once seasonality is filtered out, the North Atlantic Oscillation and the Scandinavian index are the dominant large-scale atmospheric patterns that control the interannual variability of extreme waves during winters in the North Atlantic Ocean; to a lesser extent, the East Atlantic Oscillation also modulates extreme waves in the central part of the basin. In the Mediterranean Sea, the dominant modes are the East Atlantic and East Atlantic–Western Russia modes, which act strongly during their negative phases. A new methodology for analyzing the atmospheric signature associated with extreme waves is proposed. The method obtains the composites of significant wave height (SWH), mean sea level pressure (MSLP), and 10 m height wind velocity (U₁₀) using the instant when specific climatic indices have a stronger correlation with extreme waves.

The variability of La Guajira upwelling system, in the south-central Caribbean Sea, is strongly influenced by the intensity and location of the atmospheric Caribbean Low-Level Jet (CLLJ), a near-surface branch of the easterlies, as well as by the regional ocean circulation. During favorable conditions (i.e., strong easterlies blowing almost parallel to the coast), upwelling is enhanced and a large amount of primary productivity occurs in La Guajira area. In contrast, during relatively mild wind conditions, the CLLJ is misaligned to the coast and the Caribbean Counter Current (CCC, locally also known as the Darien Current), which forms as a branch from the Panama-Colombia Gyre, flows northeastward over the continental shelf advecting waters from the southwestern Caribbean basin toward La Guajira. The CCC has a clear signature at the surface layer that extends from the Darien Gulf toward La Guajira peninsula during mild wind periods, while disappears during the months of strong winds. The direction and the magnitude of the easterlies, and more specifically of the CLLJ, control the position and pathway of the CCC, which extends more than 900 km in the southern Caribbean Sea during May, June, August, September, and October. The high concentration of chlorophyll-a at the sea surface evidenced by satellite-based color images is semi-seasonally modulated by the CLLJ, which during its relaxation phase allows the irruption of the CCC toward the east up to La Guajira.