Resolving eddies in the Gulf Stream
Ocean eddies are turbulent vortices that transport heat, salt, carbon, and other biochemical nutrients. At the surface, they can be large and energetic, with radii extending hundreds of kilometers. Below the surface, they are smaller, typically with radii of 1–10 km. Interior eddies can remain coherent for more than a year and travel thousands of kilometers. But unlike surface eddies, they are usually invisible to satellites.
A team led by oceanographer Jonathan Gula used seismic oceanography to find several small-scale vortices off the coast of Cape Hatteras, North Carolina. In their method, small variations in the water’s temperature, and thus its acoustic impedance, allow the imaging of acoustic reflections. The method uses the same airgun source and multichannel hydrophone streamer as are used to image structures below the seafloor. Most of the vortices were generated where the Gulf Stream meets the Charleston Bump—a deep topographic mountain range (the black track in the figure inset) that deflects the stream. The bump’s frictional interactions with the flow produce both negative (clockwise) and positive (counter-clockwise) vorticity in the bump’s wake.
The water on the shoreward side of the bump is cooler and fresher than the water offshore. The seismic images are essentially two-dimensional maps of that temperature variation—the vertical derivative of temperature as a function of depth. The researchers also made numerical simulations that reproduced the lens-shaped vortices of well-mixed fluids and confirmed the vortices as robust features of the ocean. The map shown in the figure came from the team’s high-resolution model of Gulf Stream flow east of Cape Hatteras. Two coherent vortices (circled in red) are identified by the low-gradient region in the thermocline. The new observations fill an important gap: Although the interior vortices strongly influence the mean circulation of the oceans, they are currently missing from global climate models. A possible consequence of that omission is an underestimation of the contribution of ocean mixing to climate–ocean interactions. (J. Gula, T. M. Blacic, R. E. Todd, Geophys. Res. Lett. 46, 2704, 2019 ; Gulf Stream thumbnail image courtesy Norman Kuring, NASA MODIS Ocean Team.)
