[1] Waves breaking at the ocean surface are important to the dynamical, chemical and biological processes at the air-sea interface. The traditional view is that the white capping and aero-dynamical surface roughness increase with wind speed up to a limiting value. This view is fundamental to hurricane forecasting and climate research but it has never been verified at extreme winds. Here we show with observations that at high wind speeds white caps remain constant and at still higher wind speeds are joined, and increasingly dominated, by streaks of foam and spray. At surface wind speeds of $40 m/s the streaks merge into a white out, the roughness begins to decrease and a high-velocity surface jet begins to develop. The roughness reduces to virtually zero by $80 m/s wind speed, rendering the surface aero-dynamically extremely smooth in the most intense part of extreme (or major) hurricanes (wind speed > 50 m/s). A preliminary assessment shows that cross swell, dominant in large regions of hurricanes, allows the roughness under high wind conditions to increase considerably before it reduces to the same low values.

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Wave‐supported gravity flows (WSGFs) generate rates of sediment flux far exceeding other cross‐shelf transport processes, contributing disproportionately to shelf morphology and net cross‐shelf fluxes of sediment in many regions worldwide. However, the conditions deemed necessary for the formation of WSGF limit them to a narrow set of shelf conditions; they have been observed exclusively in regions where the seabed consists of very fine‐grained sediment and typically co‐occur with nearby river flood events. Here we document the occurrence of a WSGF event on a predominantly sandy seabed and in the absence of a preceding river flood. Our measurements confirm that the dynamics are governed by the friction‐buoyancy balance observed in other WSGF and that WSGF can form in mixed grain‐size environments and transport high concentrations of sand. The occurrence of WSGF on a predominantly sandy seabed suggests that they may occur under a much wider range of conditions and, given the global prevalence of sandy shelves, they may be a more frequent and more ubiquitous feature of shelf dynamics than previously thought.

Research highlights  Examination of SST and SPM images of the southern North Sea shows pronounced mesoscale variability.  The Rhine ROFI, East-Anglia Plume and a seasonal thermal front dominate SST and SPM signals.  Harmonic analysis highlights pronounced seasonal and spring-neap variability.  Tides and waves control stratification and hence the visibility of near-surface SPM in the images.

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Silva, R.; van Tussenbroek, B.I.; Escudero-Castillo, M.; Mariño-Tapia, I.; Dijkstra, H.A.; van Westen, R.M.; Pietrzak, J.D.; Candy, A.S.; Katsman, C.A.; van der Boog, C.G.; Riva, R.E.M.; Slobbe, C.; Klees, R.; Stapel, J.; van der Heide, T.; van Katwijk, M.M.; Herman, P.M.J. & Bouma, T.J. (2019). Maintaining tropical beaches with seagrass and algae: a promising alternative to engineering solutions. BioScience, 69, 136-142 is available online at: https://dx.

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