Sea ice–wave interactions and ADCPs
Thomson has been studying wave–sea ice interactions in the Arctic for almost a decade. His instruments of choice are Nortek’s Acoustic Doppler Current Profilers (ADCPs). “These ADCPs have let us measure waves, currents and turbulence simultaneously so that we can understand the fully coupled system of the ocean surface layer,” says Thomson.
Over the years, Thomson has used several Nortek ADCPs in his sea ice–wave research – the Acoustic Wave and Current Profiler (AWACs), the Signature1000 and the Signature500. The instruments are deployed in two ways: (1) to monitor wave and sea ice in a fixed location over a long period and (2) to follow a particular body of water as it moves and changes over time. In the former, ADCPs are mounted on a tripod or subsurface buoy in an upward-looking configuration to measure waves, currents and sea ice. In the latter, ADCPs are mounted onto drifters, placed in a downward-looking configuration to measure turbulence – the primary process for mixing in the ocean. These turbulence measurements help Thomson understand how sea ice inhibits ocean mixing and dampens wave activity.
Understanding how wave–ice conditions change over time
One crucial element of Thomson’s research is understanding how wave–ice conditions change over time. Since the ADCPs can be duty-cycled to record data at pre-set intervals, Thomson has successfully left the instruments collecting data for an entire year at a time. “Getting a whole year of data lets us see the ice retreating in the summer, coming back in the autumn, then getting thick and growing throughout the winter, and then melting again in spring. That seasonal cycle is key to understanding the Arctic,” Thomson explains.
Long-term data collection isn’t the only advantage provided by ADCPs. “We have been able to measure the full spectrum of ocean surface waves beneath sea ice,” Thomson says, explaining that other instruments such as pressure sensors truncate the wave spectrum and lack wave directional information. “The fact that the instrument is capable of producing raw data that lets us build up a wave spectrum, not just individual waves, is really useful.”
ADCPs have another key advantage over pressure sensors. Some of the sites Thomson studies are deep-water locations – much deeper than the ADCPs’ range. To overcome this, Thomson attaches the ADCPs to subsurface buoys, tethered to a tripod on the seafloor. While pressure sensor measurements are sensitive to the twists and turns of a subsurface buoy as water currents change, the ADCP measurements remain accurate.
Improving the accuracy of climate science
Climate models are crucial tools for understanding our changing climate. “It’s becoming obvious [for climate modeling] that we have to get the Arctic right to get the global climate right,” says Thomson. However, “we don’t have the in-situ data – the real field observations – which are critical to verify, tune and adjust the models.”
As Dr Lucia Hošeková (University of Washington) explains, ice poses a particular problem for models, such as the European Center for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis product: “Whenever these models recognize there is sea ice present, they shut down the waves completely, but in reality, sea ice attenuates waves gradually due to a number of scattering and dissipative processes.” Hošeková also notes that the resolution of models can be too coarse to pick up features like landfast ice.
Hošeková, Thomson and colleagues studied the interaction between waves and landfast ice using a series of Nortek’s Signature500 ADCPs placed 12 nautical miles off the Chukchi Sea coast. “The ADCPs told us what the offshore waves were doing, about the presence of offshore ice, and about ice drift,” Hošeková explains. Other instruments placed in the landfast ice zone monitored changes at the coast.
The scientists demonstrated how both mobile and landfast ice control the seasonal wave climate, and that landfast ice delays coastal wave action by up to two months. They also highlighted that ERA5’s inability to resolve landfast ice means the product overestimates cumulative spring coastal wave exposure by up to 47 percent. Using data from their study, the team has provided a tool to reduce this overestimation.
This study isn’t the first time Thomson has used Signature500 ADCPs to identify where models can be improved. In 2018 and 2019, Thomson and his PhD student Samuel Brenner used ADCPs to measure the rugosity, draft level and drift of sea ice in the Beaufort Sea. They found that models tend to overpredict drag efficiency while the ice is melting.
ADCPs are an essential part of research in an autonomous future
For Thomson, ADCPs will continue to be an essential tool for Arctic Ocean research. What he is particularly excited about is the growing opportunity to integrate ADCPs with autonomous platforms; “The robots are here – this is how the next era of ocean data will be collected,” he says. “The sparsity of wave data in the open ocean, and the polar regions in particular, can be addressed by adding ADCPs on more surface and underwater vehicles (as well as on conventional moorings and drifters).”
Learn more about this research in the open-access paper “Landfast Ice and Coastal Wave Exposure in Northern Alaska at agupubs.onlinelibrary.wiley.com.