The beat of the ocean

People have always been fascinated and scared by the unending motion of the sea. Although the sea motion can be generated by several mechanisms (gravitational forces, seismic activities, meteorological disturbances,…), the wind is the principal source of sea energy. The wind and the related pressure fluctuations represent a disturbance that acts on the sea surface causing the appearance of ripples that increase the sea surface roughness. This gives rise to a self-sustaining process since the increased roughness makes even more efficient the energy transfer from the atmosphere to the ocean. Small and short waves appear and start to interact under the wind forcing. Energy is transferred from short capillary waves to progressively long gravity waves that are able to propagate even in the absence of the generating winds. The waves that leave the generation area are longer than the wind-forced waves and are usually called swell waves. They relentlessly propagate over long distances across the ocean with virtually no energy loss. This journey ends as swell waves approach the nearshore where they steepen, pitch-forward and eventually break in shallow waters.

It is common to see people gathering on coastal areas when a massive swell approaches the shore allowing the contemplation of roaring breaking waves. During the breaking process, the fluid velocity at the top of the wave exceeds the velocity at which the wave is traveling resulting in the crest curling. Fluid is injected at the surface by the overturning wave that develops a turbulent front, called the roller, in which water is tumbling down towards the trough. The wave height decreases within the surf zone as a large amount of energy is dissipated and turned into turbulence. Moreover, wave breaking is accompanied by the conversion of a significant part of wave energy into motions with periods much longer than the peak period of the incoming swell. These long-period motions, called surf beat, give rise to oscillations of the mean water level and slowly-varying currents that are particularly strong in very shallow waters. In the inner part of the surf zone, broken waves attains a stable form and finally oscillate on the beach face where the residual wave energy is either dissipated or reflected back to the open sea.

As we have seen, the processes of wave energy generation, propagation and dissipation encompass a wide range of time and spatial scales: from the small scales of wind-generated capillary waves and breaking-related turbulence to the large scales of surf beat. Sea waves are an oscillatory phenomenon by definition and they can be thought as part of longer fluctuating cycles such as the atmospheric pressure systems, seasons and interannual ocean oscillations. Sea waves are the expression of the energy that is gained by the sea from the atmosphere. The sea delivers this energy in the surf zone where swells generate a rhythmic beat with periods that correspond to those of natural phenomena. We can argue that the reason why we are so magnetized by swell waves approaching a shore is that we feel they are part of a bigger natural phenomenon: waves that break in shallow waters and oscillate on the beach face give us the opportunity to contemplate the pulse of the ocean.

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