Research interests:

My primary research interest is ocean observation. I’m keen to develop new tools and apply novel techniques to see what’s happening under the ocean surface. This ranges from developing new acoustic techniques to “see” more accurately underwater, to using actual underwater cameras to observe marine organisms in different fluid dynamic environments, to equipping ocean gliders (the drones of the sea) to remotely observe large swaths of the ocean. The foci of my laboratory’s research, past and present, can be broadly split into three categories: Observing the Ocean using Sound, Biophysical Interactions and Straits and Fjords.

Observing the Ocean using Sound

Acoustics opens a window revealing an underwater world. Light cannot reach the oceans' depths, so the only way to "see" something at a distance is through the observation of reflected sound waves rather than light

waves. For nearly three quarters of a century oceanographers have used sonar technology to learn about the ocean interior. This powerful tool does, however, have some limitations. Using standard narrowband sonar it is difficult to distinguish between different sources of scatter. This makes it tricky to apply quantitative data analysis to determine concentrations of fish or plankton, or the amount of mixing between interacting ocean currents. This is where we come in.

Acoustic colour

One problem is that we have been limiting ourselves by "viewing" the ocean in black and white. Traditional narrowband sonar uses a single acoustic frequency. This is like viewing the world in only one colour, since colour is defined by the frequencies of light that are preferentially reflected by an object. Restricting our eyes to observing only one frequency of light is like observing the world in black and white. Clearly, we have a much easier time identifying objects when we see them in full colour.

My lab works on developing acoustic colour techniques for the ocean. Broadband sonar allows the observation of a wider frequency spectrum of acoustic scattering. We develop and apply broadband acoustic methods, in both the laboratory and the field, to extract information on plankton size and variability of fluid properties from spectral data. Recently we’ve become interested in acoustic quantification of phytoplankton with an eye to the remote detection of harmful algal blooms (HABs).

Moving towards acoustic remote sensing of ocean turbulence
The basic fluid mechanics controlling acoustic scattering from turbulent microstructure is that the turbulent motions act on the ambient gradients in sound speed and density (prescribed by temperature and salinity gradients in the ocean) to create fluctuations in sound speed and density on many scales. When the incident acoustic wave encounters a parcel of water the size of its wavelength, with a different sound speed or density, it is scattered (either through an isotropic compression or a dipole-like inertial oscillation).

While the physics leading to sound scattering from turbulence is relatively easy to understand, we are still a long way from using acoustics to routinely remotely sense turbulence and mixing in the ocean. My work uses a combination of field and laboratory experiments to further our understanding of the relative new field of acoustic scattering from turbulence and turn it into a practical tool for oceanographers.

Mapping double-diffusive convection in polar regions

Polar regions, with their supercooled and relatively fresh surface water, are highly susceptible to the diffusive regime of double-diffusive convection (DDDC). Whether the fluxes associated with the DDDC play a significant role in the heat/buoyancy budgets in polar regions is an open question. We’re working on a high-frequency acoustic technique that can be used to quickly map the extent and evolution of the DDDC. Further, broadband acoustics offers a rapid and remote method to infer fluxes, without the need for time-consuming microstructure measurements, suggesting that this technique will be a boon to field studies of DDDC.

Biophysical Interactions

We explore many types of biophysical interactions --mostly small-scale-- from the behaviour of zooplankton under different turbulent and non-turbulent flow conditions to the accumulation of right whale food by currents into critical habitats to the contribution of zooplankton to mixing the ocean. This is done through a combination of laboratory experiments and --my favourite-- ocean observation.

Does turbulence affect zooplankton behaviour?

To explore this in the field, over the last decade we’ve developed a new type of profiler --a video plankton and microstructure profiler, to be precise-- designed to study plankton in turbulence in the ocean. So far the answer seems to be “no”; other factors are more important. At least in the systems we’ve looked at. But, we have observed an euphausiid wake and we’re exploring new ecosystems.

How does right whale food get accumulated?

In Roseway Basin, North Atlantic right whale food --i.e. diapausing Calanus copepods-- are most often found below 100 m depth along the southern margin of the basin. This is where a population of these highly endangered whales go to feast in the late summer of most years. By deploying bottom-moored ADCP/CTD sensors across this margin, we simultaneously measured variation in copepod concentration, current velocity, and water mass character. It looks like it is tidally-forced cross-isobath convergence in the flow and the copepods maintaining their depth that causes their accumulation.

Straits and Fjords

This is something I haven’t been active in for a while. I studied inter-annual variations in the exchange flow through the Strait of Gibraltar using tide-gauge and satellite altimetry data.