Strait Science: High frequency radar maps ocean currents
As part of UAF Northwest Campus’s Strait Science series UAF associate professor Seth Danielson presented a talk on how high frequency radar systems are mapping surface ocean currents and providing data useful to the scientific community and to the general community as well. Danielson is the project leader and gave the talk last Thursday evening via Zoom.
HFR systems, short for high frequency radar, have been installed at Wales and Shishmaref but are not yet fully operational. The pandemic has slowed down the process of getting them online. “HFR is what we use to map ocean currents from land,” began Danielson. It’s actually a 5-megaherz radio signal that is transmitted out over the ocean from a land-based station. They can measure the radio wave as it bounces off the ocean wave. They compute the doppler shift that is part of the radio wave when it comes back to their receive antenna. From that they extract the speed of the ocean flow. You have to have at least two systems to make this an operational unit. “You have HFR systems distributed along the coast such as at Pt. Barrow, Wainwright, and at Point Lay. Where the systems overlap is where you get a complete data set.“The first installations were at Pt. Barrow and at Wainwright in 2009. Those have been operational every year up until this year, 2020. In 2019 they installed systems in Shismaref and Wales.
The data collected by the HFR units allows the scientists to make maps showing the speed and direction of ocean flows. Measurements are taken every hour and displayed on a website. There are a lot of uses for the data. The Coast Guard uses it when they undertake search and rescue missions. Knowing the speed and direction of currents can be very helpful when searching for a lost vessel. Operators of fishing vessels are aided by the information, enabling them to save money of fuel. And the data is used to track the distribution of harmful algal blooms.
The entire East and West coasts of the USA are covered by HFR systems. The biggest gaps in the nation are in Alaska. “There’s good reason for that,” said Danielson. “We don’t have a power outlet every 50 miles down the coast. There are places in Alaska where having these measurements would be useful, and the Bering Strait region is one of those.”
“Getting measurements of these ocean currents also allows us to validate and even improve on the long time scale our sea ice and circulation models,” said Danielson. “There are a lot of scientific research applications as well. In particular we’re interested in the heat, the nutrients, and the plankton where they are entering the Arctic Ocean from the Bering Sea.”
The systems do not function when faced with ice. When the ocean freezes there is no data.
In Shishmaref power to the unit comes from the school. The flows going through the Bering Strait can vary a lot from day to day. The wind is a big influence on ocean currents. “We’ve got data clear over to the Russian coast. That’s fantastic because when we go out on our ships and put our moorings out there we don’t have the ability to make measurements on the other side of the convention line.” The HFR allows them to get measurements where they can’t normally access.
They are hoping to relocate the Wales system to expand coverage on the north side of the Bering Strait. To power it they’ll use their remote power module, the RMP, the same as has been successfully tried at Pt. Barrow and Cape Simpson. They build them at UAF. The modules can be disassembled and erected at a remote location. They include five small windmills and solar panels. And there’s a small hut for the instruments such as a satellite feed back to Fairbanks. “These things are fantastic work horses,” said Danielson. “Originally designed with diesel generators as backups but the power density of the renewable energy sources along the Alaska coast are so great that the backup generators where never coming on. So they are able to power HF radar systems through the entire open water season completely on the solar and the wind power.” One RPM is built and waiting at UAF. They want to verify a good site and then do the land permitting and get the Wales system relocated. There is more information about the modules on the web at www.straitcurrents.com.
Next, Danielson spoke about sea ice, prevailing currents, and warming temperatures in the Bering Sea and Arctic Ocean. “In recent winters we’ve had extremely low amounts of sea ice. Looking at a map of sea ice in 2018 it’s apparent that the ice edge was a few hundred kilometers north of where it normally was at that time. Those ice extent anomalies were often associated with features like low sea ice in the Anadyr Strait.” The prevailing currents flow in the direction of the Anadyr Strait so the ice is blown northward. Also, that water flowing northward can carry heat, which melts out the ice that happens to be there. The constriction of the Strait generates a lot of turbulence in the flowing waters. Subsurface heat is brought up to the surface and melts
the sea ice as it flows through Anadyr Strait into Chirikoff Basin.
In the springtime the low sea ice enables the ocean to absorb a lot more of the incoming radiation because the sea ice is either not there at all or else it’s there in lower concentrations. The season begins early and over the summer the sea absorbs additional heat. By fall the ocean is very warm. “Before you can get back down to making ice again you’ve got extra heat that you need to lose from the water column to get back down to the freezing point,” said Danielson. “So you have a large heat loss from the ocean back up into the atmosphere.” In recent years this has brought about a new thermal regime for the Bering and Chukchi Seas. There’s more heat going in in the spring and the summer, and more heat going out into the atmosphere in the fall. The average temperature has
gone up. The change in sea ice conditions is a consequence of these interactions between the ocean and the atmosphere. “So we have this annual cycle of heat. In spring you have less sea ice and that leads to enhanced absorption of radiation by the ocean, which leads to elevated summer and fall temperatures. Temperatures in summer to fall have increased by about 1.4 degrees C over the last 100 years.”
“There are many consequences of this altered heat cycle that have implications for the timing of the sea ice retreat, the timing of the oceanic warming, the amount of heat the ocean is sending out into the high Arctic basin, the air temperature across all of the Arctic. Even the salinity over the Bering Sea shelf is affected.”