Have you ever stared into the clouds and seen breaking waves? Those formations are called Kelvin-Helmholtz (KH) billow trains, and they exist both in the sky and within the sea. The enchanting swirls also appear in Saturn’s bands and near Jupiter’s red spot. KH billow trains come to the surface when two fluids of disparate densities slide in opposite directions at different velocities. The shear forces create the distinctive KH pattern of overturning.
Van Haren et al. report the longest train of Kelvin-Helmholtz billows ever recorded on this planet. The train occurred 4000 meters below the surface of the Atlantic Ocean at the entrance of the Romanche Trench. This conduit crosses the Mid-Atlantic Ridge, an underwater mountain range, linking deep ocean basins off the coast of Brazil and western Africa. The trench’s extreme depth allows Antarctic Bottom Water, the densest water mass in the Atlantic, to pass between the basins.
The team found that when water spills from the southwestern sill of the Romanche, the velocity creates KH-like shear forces with the less dense overlying water. They made these observations by deploying two acoustic current meters at depths of 4414 and 4612 meters, respectively, between October 2013 and April 2014. The 198-meter steel cable between those devices was covered in 99 high-resolution temperature sensors that measured ocean mixing every second.
On 1 December, the turbulence at the Romanche sill created KH billows stretching in a chain with more than 250 consecutive swirls that ranged in size from 5 to 100 meters tall. To the authors’ knowledge, this train is the longest “observed in either the ocean or the atmosphere.” (Geophysical Research Letters, doi:10.1002/2014GL062421, 2014)
—Nsikan Akpan, Freelance Writer