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Using ADCPs for world-class wave and ice research in the Arctic

Professor Thomson has been researching wave and sea ice conditions in the Arctic for a number of years. Photo ©: Onpoint Outreach.
Major changes are occurring in the ocean. Climate change and subsequent melting sea ice are not necessarily good changes. Why are acoustic Doppler current profilers an invaluable tool to get a complete picture of the Arctic’s changing wave conditions in the context of climate change?

In the Arctic, the end-of-summer sea ice extent in 2020 was the second-lowest in the last 42 years.

“The ice used to melt out in June or July. Now it melts out in May. It used to come back in September or October. Now the ice comes back in November or December,” says Professor Jim Thomson, Senior Principal Oceanographer at the University of Washington’s Applied Physics Lab and a Professor in the Department of Civil & Environmental Engineering.

Research necessary as melting ice exposes coastlines to harsh waves and erosion

The implications of the Arctic’s changing sea ice are many. On a global scale, sea ice influences global climate. Within the region, activities such as commercial shipping and naval operations may find life easier with the decline. For local communities, however, the loss of sea ice means the loss of a protective barrier that shelters their homes from harsh waves driven by storms that would otherwise batter and erode coastlines that are not resilient to their impacts.

In a twist, waves along the coast and further offshore may become a more prominent feature as the sea ice continues to decline.

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The loss of sea ice in places such as the Beaufort Sea means the loss of a protective barrier for waves near coastlines. Photo ©: Onpoint Outreach.

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Professor Thomson has been researching wave and sea ice conditions in the Arctic for a number of years. Photo ©: Onpoint Outreach.

Research on sea ice and the impact of waves

While sea ice lies on the ocean surface, there is less space for waves to form. Any waves that do so find their energy scattered and dissipated by the ice. Historically, winter waves might reach just over half a meter in height, but today they far exceed this. In September 2012 – the year with the lowest recorded end-of-summer sea ice extent – Thomson and colleagues detected waves some 5 m high with a 600 kHz acoustic wave and current profiler (AWAC)mounted on a subsurface mooring in the Beaufort Sea.

AWAC 2012
Wave spectrogram obtained from the AWAC current profiler deployed in 2012 in the Beaufort Sea. The top panel shows wave height, the middle panel peak wave period and the bottom panel frequencies.

Waves can do several things that can expedite ice’s decline. First, waves can erode the ice edge. Second, they can break the ice up.

“Imagine there is a nice big flat sheet of ice, and the waves break it up into lots of bits. Once it’s broken up, it has more surface area exposed to the ocean, and if the oceans warm, the pieces of ice are more likely to melt,” Thomson explains. “There’s a potential feedback mechanism wherein you lose a little bit of ice and make some waves, and those waves eat away at the ice edge or break it up, and so you lose more ice, and then you make bigger waves, and now you’re off and running.”

Thomson’s research in the Arctic has continued. Recently, he and his team paired Nortek Signature500 acoustic Doppler current profilers (ADCPs) mounted on fixed moorings with drifters equipped with Signature1000 ADCPs to get a complete picture of the Arctic’s changing wave conditions.

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Jim Thomson readies a fixed mooring equipped with a Signature500 ADCP for deployment in the Beaufort Sea. The instruments were left in situ for a year. Photo ©: Onpoint Outreach.

Research enabled by long-term collection of data on waves, currents and sea ice

“The moorings [equipped with upward-facing Signature500 ADCPs] provide us with a long time series of data. They sit at one place, and watch the world go by,” Thomson says. The instruments, which collect data on the waves, currents and sea ice when it is present, are duty-cycled to record data every hour. “They have been great for power management. It’s really good to be able to have the instruments reliably collecting data for a whole year and retain robustness in the data collection,” Thomson says.

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The Signature 500 ADCP’s ability to be left to collect data every hour for a year was vital for Thomson’s research. Photo ©: Onpoint Outreach.

Wave Image Thomson
Surface wave heights offshore (red points) and inshore (blue points) measured with the Signature500 mounted on seafloor tripods in the Beaufort Sea. There are no waves in the winter because of sea ice, and in the spring, there is a lag from when the wave activity begins offshore compared with inshore. This lag is the result of “shorefast” ice protecting the coasts.

Sig 500
Full time series from the Signature500 ADCP deployed offshore. The data shows that waves (top panel) were active in the fall, breaking in the winter (except for some brief activity in January) when sea ice covers the sea surface (second panel from the top), and becoming active again in spring and summer. The flow regime (third and fourth panel) is bimodal, with flow that is either southwest or northeast correlated with the water level (given by the average pressures). This suggests wind-driven upwelling/downwelling modulation of the Bering Strait inflow. The echograms of backscatter amplitude (bottom panel) suggest suspended sediment transport is related to phases of this flow, as well as the waves in fall.

Using acoustic Doppler current profilers to assess waves in shallower waters

The Signature1000s have also been attached to moorings to assess waves in shallower waters. In the Chukchi Sea, Thomson and Dr Lucia Hošeková captured a four-day-long wave event near the Alaskan coast, allowing them to explore how sea ice dampens the wave’s energy.

The drifters are, as Thomson puts it, “a totally different animal.” Built by Thomson and his team, the drifters – called SWIFTs (Surface Wave Instrument Float with Tracking) – are designed to operate in harsh conditions. “The SWIFTs are free-floating at the surface, and they just drift, so they’re sort of simple in that regard,” says Thomson. “That puts us in a really neat reference frame. If there’s ice around, we’re moving around the ice, or if it’s just the water, we’re moving with the water. That has some advantages for how we do the analysis because you stay with the same piece of material, whether it’s ice or water, for a while, so you can see how it transforms.”

Along with his former PhD student Dr Samuel Kastner, Thomson previously launched a SWIFT down a river and into a surf zone to assess how breaking waves in the surf zone may enhance mixing between the relative freshwater of the estuary and the saltwater of the ocean. While the ADCPs on the SWIFT measured near‐surface turbulence and velocity profiles, Acoustic Wave and Current Profilers (AWACs) mounted in the outer edge of the surf zone captured wave conditions.

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The AWAC deployed at the outer edge of the surf zone captured nearshore wave height and direction enabling the researchers to get a complete picture of the surf zone system. Offshore tidal elevation and river discharge data came from the USGS, and offshore wind data from the National Data Buoy Center. River mouth velocity is tidally interpolated.

Previous versions of the SWIFT were equipped with a downward-facing Aquadopp Profiler 2 MHz, but are now fitted with a Nortek Signature1000 profiler with an integrated Attitude and Heading Reference System (AHRS) which samples at the same rate as the Signature1000 acoustic measurements (up to 16 Hz), making it even more adept at measuring turbulence and currents when subject to high-frequency motion.

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Jim Thomson deploys a SWIFT equipped with the Nortek Signature1000 ADCP in the Beaufort Sea. Photo ©: Onpoint Outreach.

A next-generation acoustic profiler that really moves things forward

Thomson’s Arctic investigations are continuing, with much of the research being conducted largely made possible by the technical advances being made in the equipment. “The Signature instruments, in general, have been nothing short of a game-changer,” Thomson says, noting the ability of the instrument to capture multiple different types of measurements. “We have a next-generation acoustic profiler that really moves things forward. The data are much cleaner, much higher quality than they used to be.”

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Deploying a fixed mooring equipped with a Signature500 ADCP for deployment in the Beaufort Sea. Photo ©: Onpoint Outreach.

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