Tetjana Ross

Western Mediterranean sea‐level rise: Changing exchange flow through the Strait of Gibraltar

Sea-level data from tide gauges and satellite altimetry show a decrease of nearly 40% in the sea-level drop between the Atlantic and the Mediterranean from 1994 to 1997, coming mainly from a rapid rise of western Mediterranean sea-level by more than 10 mm/year. A decrease in the sea-level difference across the Strait, coincident with this Mediterranean rise, indicates that the surface inflow is reduced by an amount dynamically consistent with the change along the strait. These secular changes are accompanied by a seasonal cycle in the sea-level drop between the Atlantic and the Mediterranean, which indicates a seasonal flipping of hydraulic exchange states in the Strait of Gibraltar. Thus, we suggest the sea-level rise in the Mediterranean is a consequence of a changing exchange flow through the Strait of Gibraltar, driven indirectly by changing conditions in the Mediterranean.

Acoustic scattering from double-diffusive microstructure

Laboratory measurements of high-frequency broadband acoustic backscattering (200-600 kHz) from the diffusive regime of double-diffusive microstructure have been performed. This type of microstructure, which was characterized using direct microstructure and optical shadowgraph techniques, is identified by sharp density and sound speed interfaces separating well-mixed layers. Vertical acoustic backscattering measurements were performed for a range of physical parameters controlling the double-diffusive microstructure. The echoes have been analyzed in both the frequency domain, providing information on the spectral response of the scattering, and in the time domain, using pulse compression techniques. High levels of variability were observed, associated with interface oscillations and turbulent plumes, with many echoes showing significant spectral structure. Acoustic estimates of interface thickness (1-3 cm), obtained for the echoes with exactly two peaks in the compressed pulse output, were in good agreement with estimates based on direct microstructure and optical shadowgraph measurements. Predictions based on a one-dimensional weak-scattering model that includes the actual density and sound speed profiles agree reasonably with the measured scattering. A remote-sensing tool for mapping oceanic microstructure, such as high-frequency broadband acoustic scattering, could lead to a better understanding of the extent and evolution of double-diffusive layering, and to the importance of double diffusion to oceanic mixing.

On the turbulent co-spectrum of two scalars and its effect on acoustic scattering from oceanic turbulence

Author Posting.

Long‐term broadband acoustic observations of zooplankton scattering layers in Saanich Inlet, British Columbia.

The application of broadband techniques to fish and zooplankton bioacoustics is showing potential to transform the field into one that is much more quantitative. This is because broadband techniques allow the use of the known spectra of organisms or nonbiological sources of scattering to distinguish between scatterers, allowing discrimination without the need for extensive groundtruthing. This makes it ideal for remote monitoring of fish or zooplankton assemblages, since continuous net‐sampling is often not possible. An upward‐looking 85–155 kHz broadband sonar has been collecting data nearly continuously on the Victoria Experimental Network Under the Sea (VENUS) mooring in Saanich Inlet, British Columbia since March 2008. Saanich Inlet is known to have large populations of euphausiids, which create a strong acoustic scattering layer that migrates from depth to the surface and back each day. The thickness, timing, strength and spectral response of this layer is examined throughout the annual cycle and the...

Acoustic Detection of Oceanic Double-Diffusive Convection: A Feasibility Study

Abstract The feasibility of using high-frequency acoustic scattering techniques to map the extent and evolution of the diffusive regime of double-diffusive convection in the ocean is explored. A scattering model developed to describe acoustic scattering from double-diffusive interfaces in the laboratory, which accounted for much of the measured scattering in the frequency range from 200 to 600 kHz, is used in conjunction with published in situ observations of diffusive-convection interfaces to make predictions of acoustic scattering from oceanic double-diffusive interfaces. Detectable levels of acoustic scattering are predicted for a range of different locations in the world’s oceans. To corroborate these results, thin acoustic layers detected near the western Antarctic Peninsula using a multifrequency acoustic backscattering system are shown to be consistent with scattering from diffusive-convection interfaces.

Anisotropy in high-frequency broadband acoustic backscattering in the presence of turbulent microstructure and zooplankton

High-frequency broadband (120-600 kHz) acoustic backscattering measurements have been made in the vicinity of energetic internal waves. The transducers on the backscattering system could be adjusted so as to insonify the water-column either vertically or horizontally. The broadband capabilities of the system allowed spectral classification of the backscattering. The distribution of spectral shapes is significantly different for scattering measurements made with the transducers oriented horizontally versus vertically, indicating that scattering anisotropy is present. However, the scattering anisotropy could not be unequivocally explained by either turbulent microstructure or zooplankton, the two primary sources of scattering expected in internal waves. Daytime net samples indicate a predominance of short-aspect-ratio zooplankton. Using zooplankton acoustic scattering models, a preferential orientation of the observed zooplankton cannot explain the measured anisotropy. Yet model predictions of scattering from anisotropic turbulent microstructure, with inputs from coincident microstructure measurements, were not consistent with the observations. Possible explanations include bandwidth limitations that result in many spectra that cannot be unambiguously attributed to turbulence or zooplankton based on spectral shape. Extending the acoustic bandwidth to cover the range from 50 kHz to 2 MHz could help improve identification of the dominant sources of backscattering anisotropy.

Acoustic scattering from density and sound speed gradients: Modeling of oceanic pycnoclines

A weak-scattering model that allows prediction of acoustic scattering from oceanic pycnoclines (and the accompanying sound speed gradients) based on hydrographic profiles is described. Model predictions, based on profiles from four locations, indicate that scattering from oceanic pycnoclines is measurable using standard scientific sonars operating at frequencies up to 200 kHz but generally only for pycnocline thicknesses less than 10 m. Accurate scattering models are key to assessing whether acoustic remote sensing can be used to map oceanic pycnoclines and for determining whether scattering from pycnoclines needs to be taken into account when estimating, for instance, zooplankton abundance from acoustic data.

High-frequency broadband acoustic backscatter from phytoplankton

Current methods in phytoplankton detection and monitoring are often limited by low temporal and spatial resolution. In principle, the use of a high-frequency broadband acoustic system would be advantageous when used in conjunction with current methods; providing improvements both temporally and spatially. With this motivation, a high-frequency broadband active acoustic system has been developed and used in four separate trials to measure the backscatter from four morphologically-distinct species of phytoplankton. The morphologies studied include (1) a siliceous shelled cylinder, (2) a chain-forming siliceous shell cylinder, (3) a fluid-like spheroid, and (4) a soft-shelled spheroid; and whose sizes range from 10 to 60 µm. Organism cultures were insonified at frequencies between 0.75 MHz and 6.9 MHz giving a ka study range of 0.03–1.73. Volume scattering strength as it varies with ka is presented for each species and compared to potential scattering models drawn from the zooplankton scattering literature. ...

Building a sound and breaking it down.

The sounds we hear everyday are made up of acoustic waves of many different frequencies. When a sound is dominated by waves of a particular frequency, it is referred to as having a certain pitch. However, even single musical notes are often made up of various different frequencies (usually harmonics of the first). This is part of what gives different musical instruments their distinctive sounds. Our ears analyze sounds, with different nerve endings being activated by waves of different frequencies. This breaks sounds down so that we can distinguish different pitches or even chords. This breaking down of sound, showing its loudness as a function of frequency, is called finding its spectrum. In two hands‐on experiments, we will use a computer to visualize the building and breaking down of sounds. First, we will use a keyboard as a wave generator to incrementally build up a sound from waves of different frequencies, all the while using a computer to show us the spectrum of the resultant sound. Second, we will record you singing an “ah” or “oo” sound and examine its spectrum to show all the frequencies involved.

Broadband acoustics on the VENUS observatory in Saanich Inlet.

High‐frequency sonar is by far the most cost‐effective way of “profiling” the water column from an ocean observing system. From a biological oceanographic perspective, long‐term acoustic observations are rich with information on the depths and abundances of fish and zooplankton. The drawback is that it is difficult to conclusively identify which species (or even functional groups) are present at any given time. This can be done, but only with plenty of supporting data, generally acquired non‐autonomously. Broadband acoustics may be the key to making acoustic observations of fish and zooplankton less qualitative. Here we explore this idea. We present nearly two years (Apr. 2008–Feb. 2010) of broadband (85–155 kHz) echosounder data collected on the VENUS observatory in Saanich Inlet. Using historical and contemporaneous (July 30, 2009) zooplankton net‐tow data, we attempt to automate and interpret the resulting classification of scattering layers throughout the long‐term record.