Straits are important passages for global shipping. They are also places where little-known ocean phenomena occur.

In many of them, a pattern of concentric circles can be seen where the strait widens, much like what happens when you throw a stone into water. Such so-called internal waves are generated by water flowing through the narrow passage formed by a strait, and by friction with the seafloor topography.

In the ocean, these waves usually form where water flows over a seafloor feature like an underwater mountain chain, a seamount or between ocean basins of different depths. They play an important role in the mixing of ocean water layers, a mechanism studied by scientists that mixes surface and deeper waters, transporting heat, nutrients and CO₂ from the ocean depths.

In the above image, the SWOT mission provides a detailed picture of internal waves near the Strait of Gibraltar, generated by the shape of the coastline and the seafloor topography. The blue and red hues in this image represent sea surface heights with respect to the mean level (“0”, in white), showing waves flowing into the Mediterranean Sea.

Such waves have previously been observed by radar and optical satellites like Sentinel-1 and Sentinel-2, but they remained hard to detect by conventional altimetry missions that can only acquire measurements along a straight ground track. The KaRIn instrument aboard CNES and NASA’s SWOT satellite, developed with contributions from the Canadian (CSA) and United Kingdom (UKSA) space agencies, has the capability not only to detect these waves in two dimensions, but also to measure wave heights all over the globe.

Scientists are sifting through these data to gain new insights into these phenomena and how they affect the ocean component of the Earth system.

Oceanography data from the SWOT mission can be accessed via the AVISO platform and hydrology data at hydroweb.next.

To learn more, see our dedicated SWOT mission web page.