Research

What sorts of ocean processes do we study?

All sorts! The acoustic pulse we send into the ocean is scattered by changes in impedance (combination of density and sound speed). Impedance changes can come from boundaries - like the seafloor, sea surface, ice (glacier or sea ice) - and from different phenomena in the water column - like biology (fish, plankton), energetic mixing processes, bubbles, sediment, even the gradients between different water masses!

We use the information contained in the scatter acoustic signal to characterize the coastal polar seas. Below are overviews of some of our research projects. Interested in getting involved in this kind of research? Check out our opportunities page for current positions in the lab!

Ice-ocean dynamics at the glacier terminus

Marine-terminating (tidewater) glaciers directly connect the land, ice, and ocean in polar systems. These glaciers are crucial not to polar regions but the entire world's oceans. They contribute to sea level rise as they melt, alter ocean properties from fresh water release, influence ocean circulation patterns, and support unique marine ecosystems. These glaciers have been retreating (losing ice mass) at an accelerated rate over the past twenty years due to their direct exposure to atmospheric and oceanic changes, combined with rapid warming trends in high-latitude regions.

Despite their critical role, we still have many questions about how climate change affects these glacier systems. Why? Because getting near to the ice face is dangerous! Calving events (where ice breaks off the glacier and falls into the ocean) put people, ships and equipment at risk and this means we have very few measurements of this region!

Acoustic systems can make measurements from a safe distance to help us better understand these ice-ocean systems. Pointing broadband active acoustic systems are the ice-ocean interface is a critical part of ongoing research in the Polar Acoustics Lab.

Our current work studying marine-terminating glaciers is focused on three major topics:

Understanding morphology of the submerged terminus and its connection of submarine melt rates.
Observing and characterizing subglacial discharge plumes - freshwater that is release at the base of a marine-terminating glacier and rises along its face towards the surface (see echogram below showing scattering from a plume).
Describing the changes in water column structure (thermohaline structure) brought about by freshwater input from the glacial system - see next section!

Relevant publications:

Weidner, E., G. Deane, A. LeBoyer, M. Alford, H. Vishnu, M. Chitre, D. Stokes, O. Glowacki, and H. Johnson, “High frequency broadband acoustic systems as a tool for high latitude glacial fjord research”, submitted to the Cryosphere.

Saeed, Z., E. Weidner, B.A. Johnson, and T.L. Mandel (2022), Buoyancy-modified entrainment in plumes: Theoretical predictions, Physics of Fluids, 34, 015112. DOI: 10.1063/5.0065265.

Thermohaline structure and mixing dynamics

The ocean is made up of different water masses with unique properties (think temperature and salinity). The majority of the ocean is stably stratified, meaning the structure of the water column is defined by a series of increasingly dense fluid layers. At the boundary between these layers there are (often sharp) gradients in temperature and salinity. Vertical transport of parameters like oxygen, heat, and salt is strongly influenced by the intensity of those gradients (the degree of stratification) and external forcing, like wind, tides, and currents.

Active acoustic systems can observe boundaries between water masses from scattering off of sharp gradients in temperature and salinity and regions of intense mixing along boundaries. Our research in the Polar Acoustic Lab is focused on understanding how the structure of the water column in coastal seas is influenced by the input of freshwater from glaciers. We want to know how that structure might change in the future from increased glacial retreat and how those changes will alter the transport of heat and salt in polar seas.

An echogram showing strong scattering from very sharp gradients in temperature and salinity in the Baltic Sea. The bottom water of this region is anoxic (has no dissolved oxygen) and the onset of those conditions is coincident with boundary between water masses. We used this information to track the position of the anoxic zone across this part of the Baltic - creating a high resolution map. This information can be used to better understand the impact of low oxygen conditions on important Baltic fisheries, such as Baltic cod.

Relevant publications:

Weidner, E. and T.C. Weber (2023), “Broadband acoustic characterization of backscattering from a rough stratification surface”, Journal of the Acoustical Society of America, 155: 114–127, DOI:10.1121/10.0024148.

Muchowski, J., L. Umlauf, L. Arneborg, P.L. Holtermann, E. Weidner, C. Humborg, and C. Stranne (2023), “Potential and Limitations of a Commercial Broadband Echosounder for Remote Observations of Turbulent Mixing”, Journal of Atmospheric and Oceanic Technology, 39: 1985-2003, DOI:10.1175/JTECH-D-21-0169.1.

Weidner, E. and T.C. Weber (2021), An acoustic scattering model for stratification interfaces, Journal of the Acoustical Society America, 150(6): 4353-4361. DOI:10.1121/10.0009011.

Weidner, E., C. Stranne, J. H. Sundberg, T.C. Weber, L. Mayer, and M. Jakobsson (2020), Tracking the spatiotemporal variability of the oxic-anoxic interface in the Baltic Sea with broadband acoustics, ICES Journal of Marine Science. DOI:10.1093/icesjms/fsaa153

Carbon flux from bubble seeps

In the world's oceans gas bubbles escape the seabed to form trains of bubbles that rise toward the surface. We refer call these gas seeps. The majority of these bubbles contain methane gas, a powerful greenhouse gas. In some regions gas seeps reach the sea surface and injecting methane into the atmosphere where it directly influences climate.

One region where this is of particular concern is in the climatically sensitive Arctic Ocean. It is important for us to quantify how much gas is being added to the atmosphere and how that amount might change due to changing climate. However, gas seeps are difficult to study! The bubbles are small, typically <5 mm in radius, and seeps can turn on and off. Finding them with optical methods, like underwater vehicles or cameras, is expensive and time consuming.

Fortunately, active acoustic systems can be used precisely locate the emission location of marine seeps, as bubbles are excellent acoustic scatterers!

The polar acoustics lab is interested using broadband echosounders to understand bubble fate in the polar oceans. We identify scattering from individual bubbles using our echosounders and then comparing scattering levels measured in the field to predicted scattering from theoretical scattering models. This way we can actually determine the bubble size (radius) remotely from acoustic data! Our specific research goals include:

Identifying locations of gas seeps and external processes linked to seep sites,
Quantifying the amount of gas escaping the seafloor, and
Estimating the magnitude of methane transported to the atmosphere.

Relevant publications:

Jakobsson, M., M. O’Regan, C. Mörth, C. Stranne, E. Weidner, J. Hansson, R. Gyllencreutz, C. Humborg, T. Elfwing, A. Norkko, J. Norkko, B. Nilsson, and A. Sjöström, Links between Baltic Sea submarine terraces and groundwater sapping, Earth Surf. Dynam., 8, 1–15. DOI:10.5194/esurf-8-1-2020.

Weidner, E., T.C. Weber, L. Mayer, M. Jakobsson, D. Chernykh, and I. Semiletov, A wideband acoustic method for direct assessment of bubble-mediated methane flux, Continental Shelf Research, 173: 104-115. DOI:10.1016/j.csr.2018.12.005.

Stranne, C., L. Mayer, M. Jakobsson, E. Weidner, K. Jerram, T. C. Weber, L. G. Anderson, J. Nilsson, G. Björk, and K. Gårdfeldt (2018), Acoustic mapping of mixed layer depth, Ocean Sciences, 14(3): 503-514. DOI:10.5194/os-14-503-2018.

Stranne, C., L. Mayer, T. C. Weber, B. R. Ruddick, M. Jakobsson, K. Jerram, E. Weidner, J. Nilsson, and K. Gårdfeldt (2017), Acoustic mapping of thermohaline staircases in the Arctic Ocean, Scientific Reports, 7: 15192. DOI:10.1038/s41598-017-15486-3.

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