Time-Lapse Volumetric Seismic Imaging of a Major Oceanic Front and Submesoscale Coherent Vortices at South Atlantic Ocean
Seismic oceanography (SO) involves deploying a ship that tows an acoustic energy source and one or more receiver cables just beneath the sea surface. Acoustic waves reflected from temperature and salinity gradients are recorded by arrays of hydrophones located along the receiver cables. Compared with standard hydrographic measurements, seismic reflection imagery provides much improved spatial resolution and extensive coverage over a whole range of length scales, enabling detailed spatial and temporal quantification of thermohaline structures and ocean mixing. Between December 2012 and April 2013, a three-dimensional (3-D) seismic survey was carried out at the Brazil-Malvinas Confluence of the South Atlantic Ocean. Each individual seismic swath is approximately 140 km long and 600 m wide. The complete survey consists of 28 seismic profiles that were analyzed using an industry-standard signal processing package. Together, these profiles image a dynamic frontal system with intense mesoscale eddy activity. During February 2013, a mesoscale front occurs as a planar surface dipping at less than two degrees. This front is traceable to a depth of nearly 2 km. Within the top 1 km, where cold fresh water subducts beneath warm salty water, several tilted lenses are observed. The largest lens, identified as a submesoscale coherent vortex (SCV), had a maximum diameter of $\sim$ 40 km and a height of $\sim$ 750 m. This SCV is associated with a negative acoustic velocity anomaly that is consistent with a cold fresh water core. Time-lapse imagery reveals its growth and decay over an 11-day period. Surface measurements confirm that the front and SCV translate at $\sim$ 0.1 m/s from northeast to southwest. A second SCV with similar scaling is observed during April 2013. It moves at a slower speed of $\sim$ 0.05 m/s. Sub-surface hydrographic measurements confirm that these imaged SCVs consist of cold and fresh water that is also characterized by low stratification and minimal horizontal velocity. The estimated potential density range is 26.5–27.2 \(\text{kg m}^{-3}\), which is consistent with seismically derived observations. High-resolution numerical simulations of the Brazil-Malvinas Confluence show that SCVs are generated along the continental shelf south of the confluence. These SCVs translate northwards where they are entrained and deformed into elongated filaments. Typically, a simulated SCV of appropriate dimensions occurs at a depth of 750 m where it is bounded by isopycnal surfaces of 26.5 and 27.2 \(\text{kg m}^{-3}\). The integral of potential vorticity within this isopycnal range demonstrates that the extremely low potential vorticity associated with the simulated SCV can be traced back to the continental boundary at depths of 500 to 1000 m. Finally, spectra of the horizontal gradient of vertical displacement are constructed from automatically tracked seismic reflections. These spectra have clear transitions across internal wave, turbulent, and ambient noise sub-ranges. The spatial variation of diapycnal diffusivity is estimated by fitting the turbulent sub-range. Results show that diapycnal diffusivity varies between \(O(10^{-5})\) and \(O(10^{-3}) \text{ m}^2 \text{ s}^{-1}\) across the frontal region with anomalously low diffusivities at depths of 500--1000 m. At the same depths, there is an association between the presence of SCVs and significantly enhanced diapycnal mixing.
Diapycnal Mixing, Seismic Oceanography, Submesoscale Coherent Vortices
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