By Edwin Schiele
With advances in buoys and satellite-based instruments, scientists are learning more and more about surface currents in the remotest parts of the ocean. Yet close to shore and in bays and estuaries, currents have remained difficult to pin down. Winds, tides, the outflow of rivers, and the shape of the seafloor and land all play a role in shaping coastal currents. Throw in shifting sandbars, random storms, and the influx of eddies spinning off of larger currents, and researchers have their work cut for them.
Until recently, scientists relied on current meters to measure coastal currents. While accurate, these instruments are expensive to deploy and maintain and only measure currents in the spots where they are tethered. Satellite based instruments such as altimeters, which track currents by measuring ocean topography, do not pick up currents close to shore.
Then starting in the late 90s, scientists got their hands on a new tool called high frequency (HF) radar. These instruments map surface currents in wide swaths of coastal waters up to 200 km off shore, 24 hours a day, and in all weather conditions. Researchers began installing HF radar stations along the East, West, and Gulf coasts, and today the network continues to expand. Developers of these systems hope that someday soon, anybody who is interested will be able to look up information on currents and waves as easily as they can look up information on the weather.
Even though HF radar installations are new, the technology is not. The idea that HF radar could be used to measure surface currents dates back to the 1950s, and the technology was first demonstrated in the 1970s.
The physics behind HF radar is fairly straightforward. A transmitter broadcasts electromagnetic waves usually between 5 and 25 megahertz. These signals scatter off waves on the ocean surface. If a signal strikes an ocean wave that is exactly half of the broadcast signal’s wavelength and if the ocean wave is traveling towards or away from the transmitter, the signal reflects back. This phenomenon is known as Bragg scattering. Since there are abundant waves of all wavelengths present in the ocean, there are always plenty of waves that fit this criterion.
In ideal conditions with ideal ocean waves, scientists can predict the frequency of the returning signal based on the ocean wave’s size (it’s half the wavelength of the transmitted signal) and its calculated phase speed (how quickly it moves). Of course in the real world, ocean waves are never ideal, but for these purposes they are close enough.
Surface currents, however, cause these waves to move. This movement of the ocean waves shifts the frequency of the returning signal due to the Doppler effect. The frequency increases if the current pulls the waves towards the transmitter and decreases if it pushes them away. By measuring this Doppler shift, scientists can determine the speed of the currents towards or away from the transmitter.
To calculate the directions of the currents, scientists need a second HF radar installation measuring the same currents from a different angle. They can then calculate the vectors.
Designers of these systems still had to overcome some technical challenges before these radar systems could be deployed. In most systems, the transmitter broadcasts signals in all directions at once. Signals therefore reflect back off the ocean from all angles at once, bombarding the antennas. The receivers are now able to sort through all these signals to determine both the direction and the distance each signal is coming from so that scientists can locate the currents.
As more and more HF radar installations come on line, scientists are gaining greater confidence in their ability to accurately map currents. In one study, researchers deployed drifters within a 2 km by 2 km square, returning drifters that floated out of the square, then compared the data from this drifter with data from HF radar. The two datasets closely agreed. Still there are minor bugs that need to be worked out, and researchers are still determining the range of errors that can occur.
Currently NOAA and the Scripps Institution of Oceanography are developing a system that links all of the HF radar installations to a central database that is available to the general public in formats that are easy to understand.
Data on coastal currents will be useful to many people. They will help physical oceanographers model circulation along the coast and in bays and estuaries and observe how currents change during storms or when eddies intrude. Physical oceanographers will also be able to incorporate these data into their larger ocean circulation models, filling in important gaps. Scientists studying coastal ecosystems will be able to use the data to study the movement of juvenile fish and invertebrates, the influx of nutrients from the deep ocean, and the outflow of pollutants from land. The data will help the Coast Guard improve its search-and-rescue operations and track oil spills. The data will also help ships navigating into ports and commercial and recreational fishermen planning their expeditions.
As the network of HF radar installations continues to expand and as data becomes more accessible, the number of people taking advantage of these data will expand as well.