Mr. Tom's Blog

“Acoustic backscatter in cold core rings."

Cold core rings shed from Gulf Stream meanders dominate much of the eddy activity to its south. Rings can break away almost anywhere along the stream east of roughly 70°W. Once formed they will drift west toward Cape Hatteras near where they will eventually, in weakened form, rejoin the stream. Thanks to the Oleander’s weekly roundtrips between Bermuda and New Jersey we have an extensive archive of ADCP velocity transects of numerous rings, some recently formed, others older and from farther east. These enable us to see how they form and age. It’s a bit like stroboscopic sampling.

The acoustic Doppler current profiler depends upon the presence of particulate matter in the water column to backscatter acoustic energy from which velocity can be determined. In the process we also obtain information on backscatter density, an interesting combination. When a ring breaks off from a meander, a pool of trapped Slope Sea water with high backscatter levels in the center remains at rest. But within a couple of weeks the swirling ring of Gulf Stream water will have accelerated the center into something like solid body rotation. At the same time, measured in a month or so, the backscatter in the center will decrease from its Slope Sea excess into a striking void, a pattern we’ve seen many times (see the first reference below for a beautiful example). I don’t know what causes the void, but suspect Sargasso Sea nekton come in and feed on the Slope Sea zooplankton. The fact that backscatter density decreases from a striking maximum to a conspicuous minimum in the center suggests a local process rather than exchange with the surrounding waters. If I were younger I’d love to deploy isopycnal RAFOS floats in a nascent cold core ring to see how effectively the water in the central core remains trapped. The persistent lower temperature in the center of rings relative to the surrounding waters suggest effective trapping.

An analysis of the first decade of transects (Luce and Rossby, 2008) identified several cold core rings for closer study. With help from an undergraduate student, I compiled a set of figures showing the velocity field and backscatter intensity for a) the genesis of a cold core ring, and b) for seven others at various ages. I can’t post the figures here, but happy to send them if you wish. A teaser for some potentially very exciting research with all the materials we have collected since then!


Rossby, T., C. Flagg, P. Ortner, and C. Hu. A Tale of Two Eddies: Diagnosing coherent eddies through acoustic remote sensing. Journal of Geophysical Research. 116, C12017, doi:10.1029/2011JC007307, 2011

Luce, D. and T. Rossby, 2008. On the size and distribution of rings and coherent vortices in the Sargasso Sea, J. Geophys. Res., 113, C05011, doi:10.1029/2007JC004171.