Swell: Study reveals how waves from the Southern Ocean reach the Americas

Sea waves are generated by the wind, but not all waves that reach a beach have the same origin. And in some cases, the distance between their point of origin and their destination is 10,000 kilometres, as is the case with the swell that originates on the south coast of Australia and ends on the west coast of the Americas. Based on a new research approach, researchers from the Institute of Environmental Hydraulics at the University of Cantabria (IHCantabria) and the Faculty of Sciences at the University of Lisbon have published an article in the journal Nature Communications revealing a method that distinguishes the origin of groups of waves that break on the west coast of the Americas. The method, which allows the generation of waves to be linked to meteorological phenomena thousands of kilometres away, can be applied to various locations around the globe, the researchers note.

“We already knew that swell waves can propagate freely, but this study has shown us how certain waves reach the west coast of Central America. We have also realised that these events with destructive impacts are increasing in frequency and intensity,” explains Gil Lemos, a researcher at the Dom Luiz Institute Dom Luiz (IDL) of CIÊNCIAS. “Until now, it had not been possible to establish the relationship between the origin and destination of these waves in such a consistent and robust manner,” he adds.

‘Swell’ is a term commonly used by surfers and other enthusiasts of water sports to describe waves that can travel thousands of kilometres, originating from a storm or other weather phenomenon involving wind.

The concept assumes, from the outset, different scenarios for those who have to study it: if, in extreme cases, waves travel 10,000 kilometres in 10 days from Australia to the west coast of the Americas, then it is possible to conclude that there are waves that obey different principles and forces from the weather conditions recorded at a given location. This makes it extremely difficult to predict the occurrence of waves that have a major destructive impact, despite having been generated by distant storms.

“That is why those who study this field distinguish between ‘young’ waves and ‘old’ waves,” emphasises Gil Lemos.

Having existed for ten days and travelled 10,000 kilometres, a wave is a contender for the longevity list, but is no longer affected by the winds and storms encountered during its journey until it reaches the shore. “Young waves” are in the midst of their formation, and it is only at this stage that they are affected by the force of the wind that triggers them. On a beach, both types of waves can occur. And this factor can complicate the work of those trying to study the origin of each swell, as “old waves” can have distant and diverse origins.

Until now, experts and authorities have made projections based on aggregated data, which makes it difficult to pinpoint the origin of components (groups of waves) or even to identify the parameters that allow waves to be described in relation to their place of origin. With the study published in Nature Communications, the researchers have developed a spectral partitioning method that can be used to make projections on the impact of different groups of swells originating at great distances.

“At each location, it is possible to determine partitions of the wave spectrum which, as the name suggests, help to describe the frequency ranges and directions of specific sets of waves. With this method, we are able to isolate wave components based on their direction of propagation or the energy they carry,” explains Gil Lemos.

Researchers believe that the new methodology provides greater detail and reduces the likelihood of erroneous conclusions. “Analysing aggregate parameters may generally indicate that waves are two metres high at a given location, when in fact we know that not all of them are two metres high, nor do they all have the same origin or direction. Aggregated parameters do not allow us to distinguish between waves of different origins and energy contents. We have tried to go beyond conventional monitoring using aggregated parameters, whilst at the same time attempting to reveal characteristics that may have been hidden within different wave components (because they are not evident when the parameters are aggregated),” says Gil Lemos.

The study by the Portuguese-Spanish team involved modelling using 45 years of data on swells originating in the Southern Ocean and reaching the west coast of the Americas. The researchers took into account the so-called “storm belt”, which refers to a band comprising sections of the Atlantic, Indian and Pacific Oceans approximately between 50° and 70° latitude in the Southern Hemisphere, where waves are continuously generated.

This data was combined with news reports of waves that caused damage or even resulted in loss of life on the west coast of the Americas. From the combination of all this data and information processing methods, a new approach to studying swell emerged.

“We studied the evolution of various wave components in the Pacific Ocean over 45 years and found that it is possible to isolate specific components associated with storms in the Southern Ocean that generate swell,” says Gil Lemos. “We concluded that these destructive impacts are becoming increasingly severe and frequent. Just like the storms occurring in the Southern Ocean,” concludes the researcher.