Spatial structure of thermocline and abyssal internal waves in the Sargasso Sea Thomas B. Sanford n Applied Physics Laboratory and School of Oceanography, University of Washington, 1013 NE 40th Street, Seattle, WA 98105-6698, USA article info Available online 27 July 2012 Keywords: Velocity profilers Internal waves Vertical profiling Vertical shear abstract Vertical and horizontal spatial structures are analyzed for the steady and internal wave velocity contributions in one hundred full-water-depth velocity profiles collected in the Sargasso Sea in water depths between 4500 and 5500 m. Temporal decompositions into subinertial, near-inertial, and high- frequency velocity contributions are obtained from multiple, but brief time series at several locations. Horizontal spatial variability is evaluated from two simultaneous velocity profiles at separations ranging from 15 m to 12.5 km. The total internal wave field exhibits equipartition between east and north velocity components, a decrease in energy density at the smallest vertical wavenumbers, and an overall dependence for kinetic energy (KE) on vertical wavenumber as m 2.5 . Most of the internal wave energy is in near-inertial motions and, of this, most occurs at Wentzel–Kramers–Brillouin (WKB) normalized vertical wavelengths of 150–800 stretched-m (i.e., sm) with a spectral peak at 500-sm wavelength (for N o ¼3 cph) and average surrounding the peak of 3 c/skm (330 sm). Near-inertial contributions exhibit a power law of m 3 , while higher-frequency internal waves (o 42f) a slope of m 2 . There is strong vertical polarization (clockwise 4anticlockwise) (CW4ACW) of the near-inertial contribution but little or none for higher-frequency motions. There is more WKB normalized near- inertial KE in the lower than in the upper half of the WKB-scaled water column while high-frequency internal waves have comparable upper and lower halves energies. The upper half shows a deficit compared to the Garrett and Munk model spectrum at vertical wavelengths shorter than 100 sm. Time- mean shear is largest in the upper half, so critical-layer processes may play a role. The internal wave KE of simultaneous but spatially separated profiles has a zero-correlation scale of 15–20 km, dominantly due to near-inertial waves. Thus, deep near-inertial motions exhibit wavelengths of 60–80 km in contrast to longer scales reported in the surface mixed layer and upper pycnocline. The aspect ratio k/m (330 sm/70 km) corresponds to a wave frequency of 1.05f. The downward group velocity is 0.6 mm s 1 , with a vertical energy flux for the near-inertial motions of 0.6 mW m 2 . & 2012 Elsevier Ltd. All rights reserved. 1. Introduction The least measured aspect of the oceanic internal wave field is its spatial structure. This is especially true in the abyssal oceans where few measurements have been made. This is unfortunate, because there are numerous reasons to study the spatial structure of internal wave velocities. Spatial characteristics must be observed to improve our understanding of deep flows and inter- nal waves. Not only that, volume transport for deep currents and determinations of abyssal mixing depend on obtaining represen- tative vertical and lateral velocity measurements, often with vital density and turbulence observations. Fortunately, methods for profiling below the pycnocline have advanced to a refined state today in which high-resolution measurements are made through- out the water column. For example, the kinematic and dynamic structure of the Deep Western Boundary Current at 4000 m off Blake Outer Ridge required such capability (Stahr and Sanford, 1999). Determinations of internal tidal energy fluxes, such as Althaus et al. (2003) and Lee et al. (2006), depend critically on observations throughout the water column to observe the near- surface and abyssal structure to obtain the total energy flux. Tom Rossby has contributed much to our understanding of the spatial structure of ocean currents and waves. In addition to his invention of the SOFAR/RAFOS floats that have been used exten- sively, he pioneered the measurement of the vertical profiles of ocean currents by acoustically tracking a falling sound source from bottom-mounted receivers (Rossby, 1969, 1974). Doubts were expressed about the value of single or brief observations of velocity structures. This was an era of emphasis on long-duration, moored current meter observations and the importance of long Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dsr2.2012.07.021 n Tel.: þ1 206 543 1365. E-mail address: sanford@apl.washington.edu Deep-Sea Research II 85 (2013) 195–209