Since the mean tidal range in PWS is about 3 meters, all mariners need to consider the currents created by the ebb and flood of the tides. When winds and waves are also factors, the velocity of the currents can magnify waves to dangerous heights. Currents are also important in the set and drift of vessels in the tanker traffic corridor leading to the Port of Valdez, as well as the trajectory of drifting debris, icebergs and oil spills. But how can we measure these currents and provide this information directly to mariners' Surface Current Mappers (SCM) are one tool that can be used. Every hour SCMs measure the direction and speed of surface currents in the central basin of PWS and transmits this information to the internet.
A SCM is, essentially, a combination of a powerful radio wave transmitter and receiver that use the Doppler shift to determine how fast surface currents are traveling. In the same way that most people can tell where an ambulance is by the sound of its siren, the scientists who operate the SCMs in PWS can tell where a wave is traveling and how fast it will get there. "[SCM stations] transmit radio waves, and the radio waves are reflected off the waves on the ocean," said Dr. Hank Statscewich, a researcher from the University of Alaska Fairbanks involved in the project. "From that information we can calculate the exact (surface) current velocity and direction."
Two SCM stations were set up in Prince William Sound in 2004 at Knowles Bay and Shelter Bay. The stations transmit and receive radio waves that travel as far as 60 km across the Sound in all directions. As each radio wave reflects off the ocean and returns to the SCM station, the changes in the wave's amplitude and frequency are entered into Doppler equations to determine the direction and speed of the waves. Each SCM site generates very clear information on the behavior of the currents in PWS. In mid-summer of 2004 the stations began to reveal something that had never been seen before: a strong current that traveled counter-clockwise, coming into the Sound at Hinchinbrook Entrance and leaving through Montague Strait.
"We started seeing these currents, and people who have been working in Prince William Sound for a long time doing traditional measurements with moorings or CDT surveys said, 'Your equipment's not working right, the currents don't do that here in PWS", Statscewich said. But there was nothing wrong with the SCM instruments, and further studies proved that this strong mid-summer current does exist. The pattern starts as the water heats up in the summer, when a well-known upwelling of bottom water creates a cold spot in the center of the Sound. Warmer waters coming in through Hinchinbrook entrance are deflected around the cold spot toward the East, and create a cyclonic gyre that can travel as quickly as 1 knot on calm days.
Since the SCM stations are now operating in PWS as part of the PWSOS, the public will have access to the information on a daily basis. Scientists will also use the data to develop hypotheses about how nutrients, plants and animals follow currents around PWS. Data transmitted by the weather stations will be accessible through the AOOS and PWSOS web pages and archived at the University of Alaska Fairbanks.
Data Access
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