Seismic Imaging of a Thermohaline Staircase in the Western Tropical North Atlantic

Multichannel seismic data acquired in the Lesser Antilles in the western tropical North Atlantic indicate that the seismic reflection method has imaged an oceanic ther- mohaline staircase. Synthetic acoustic modeling using mea- sured density and sound speed profiles corroborates infer- ences from the seismic data. In a small portion of the seis- mic image, laterally coherent, uniform layers are present at depths ranging from 550-700 m and have a separation of 20 m, with thicknesses increasing with depth. The reflec- tion coefficient, a measure of the acoustic impedance con- trasts across these reflective interfaces, is one order of mag- nitude greater than background noise. Hydrography sampled in previous surveys suggests that the layers are a permanent feature of the region. Spectral analysis of layer horizons in the thermohaline staircase indicates that internal wave ac- tivity is anomalously low, suggesting weak internal wave- induced turbulence. Results from two independent measure- ments, the application of a finescale parameterization to ob- served high-resolution velocity profiles and direct measure- ments of turbulent dissipation rate, confirm these low levels of turbulence. The lack of internal wave-induced turbulence may allow for the maintenance of the staircase or may be due to suppression by the double-diffusive convection within the staircase. Our observations show the potential for seismic oceanography to contribute to an improved understanding of occurrence rates and the geographical distribution of thermo- haline staircases, and should thereby improve estimates of vertical mixing rates ascribable to salt fingering in the global ocean.

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