Finestructure and turbulence in the deep ocean

Abstract

Millimeter scale fluctuations in refractive index recorded with a freely sinking shadowgraph system are correlated with finestructure profiles of temperature, salinity and density and compared to models of ocean turbulence. Images with vertically aligned periodic structure, called bands, are identified as salt fingers, while others with chaotic structure are turbulent. Images are found on interfaces that are 1-10m thick and have gradients at least several times the mean. From 6 profiles in the Mediterranean Outflow region of the eastern North Atlantic between 1.0 and 1.9 km depth, 398 interfaces have been identified and a significant fraction (about 1/3) of these have detectable images. High contrast images, including bands, are most often found below warm, saline intrusions and within stepped structure where there is a regular sequence of homogeneous mixed layers separated by interfaces. As the interfacial salinity gradient increases in the sense that allows salt finger convection, the fraction of interfaces with images increases. The horizontal spacing of bands (~5 mm) is consistent with calculated salt finger diameter. The calculated and observed length of ocean salt fingers (10-20 cm) is a small fraction of the interface thickness. High levels of small scale variability in the shadowgraphs is reflected in high levels of variance in the finestructure band of the temperature spectra. The temperature gradient spectra have a slope of -1, indicative of turbulence affected by buoyancy forces, and there is a relative peak at a wavelength near the observed salt finger length. The high contrast images are found at interfaces within the enhanced mean salinity gradient below saline intrusions. For very strong salinity gradients there is a solitary interface with intense images, but for weaker mean gradients the convection takes the form of stepped structure. The steps may evolve from the solitary interface as the salinity gradient is run down by salt finger convection. This study identifies parts of the ocean where salt finger convection is prevalent and includes the first comprehensive description of salt fingers in the ocean. Existing models of salt fingers are evaluated in light of ocean observations, and models of ocean turbulence are compared to measurements.