The Wisconsin scientist who first discovered mini tsunamis on the Great Lakes now says those "meteotsunamis" are a major cause of rip currents, the underwater backwash of waves known to pull swimmers away from shore.
Chin Wu, professor of civil and environmental engineering at the University of Wisconsin-Madison, headed a team that studied what caused the July 4, 2003, events that spurred seven drownings on Lake Michigan.
Unlike ocean tsunamis caused by geological action, namely earthquakes, meteotsunamis are caused by weather, namely wind. Meteotsunamis are a single wave as opposed to a wind-whipped series of waves. They are generally fairly small, nothing like ocean tsunamis.
The study's findings, published Feb. 14 in the journal Scientific Reports, said the 15-minute windstorm on July 4, 2003, formed a moderate-height - less than 4 inches - meteotsunami, which is what eventually caused the unexpected rip currents that took seven lives near Warren Dunes State Park several hours after the storm passed.
Wu said rip currents formed by the one-time wave of a meteotsunami can last for hours afterward, so beachgoers need to be cautious even after a storm passes.
"Please do not go back to the beach because that's the most dangerous time. The water might look calm, but underneath, there's a hidden danger," Wu said.
Although these conditions may be more common than previously known, this study is the first time researchers have verified a meteotsunami-generated rip current, now dubbed a meteo-rip.
Eric Anderson, physical oceanographer with the National Oceanic and Atmospheric Administration's Great Lakes Environment Research Laboratory and a member of the research team, said the meteotsunami effect is the first time the seven deaths were linked.
"It wasn't explained before exactly what happened that day to connect all these different swimmers together into one event," Anderson said. "People maybe weren't even aware that a storm had passed a couple of hours earlier, and they decided to go to the beach because things had cleared up. They went in the water and this caught them off guard. When you see clear skies and calm conditions, you don't expect dangerous conditions on the shoreline."
The research found how a relatively small wave height of the meteotsunami translates into dangerous currents.
"It comes from the wavelength being so long. The wave may be short, but when it's a kilometer long, that's a lot of water that comes to the shore and then retreats back over a few minutes," Anderson said. "When that much water moves that fast, you get some dangerous currents," Anderson said. "The water pushed (up towards shore) by the wave escaped (back out to the lake) by forming rip current channels."
When swimmers entered those channels they were pulled out to their deaths, unable to swim against the rapid current.
The National Weather Service currently issues beach hazard statements, which include conditions for rip currents but not for meteotsunamis. But this new study has forecasters considering how to include meteotsunami potential in rip current alerts. So far that's not possible because there's no good indicator when meteotsunamis hit.
"Meteotsunami-induced rip currents are something that will require an education effort. And then we're going to need data in real-time to try to be able to forecast them while they're going on,'' said Robert Dukesherer, senior forecaster for the Weather Service in Grand Rapids, Mich.
Wu said more people need to be watchful for rip currents.
"After a storm is the time we should pay great attention for rip currents. The highest risk occurs when people pay no attention,'' he noted.
There were 117 drownings last summer across the Great Lakes - the highest number ever reported, according to the Great Lakes Surf Rescue Project.
A father and his 10-year-old daughter died in August 2017 after being pulled out into Lake Superior off Park Point's beach by what was believed to be rip currents. They did not see or ignored warnings that the beach was unsafe because of rip current potential.
Experts say the best way to escape the pull of a rip current, no matter what causes it, is to swim parallel to the shore until you feel the current subside, then swim back to shore.
This research was funded by the NOAA Coastal Storms Program, Wisconsin Sea Grant, the Cooperative Institute for Great Lakes Research, the National Science Foundation's Graduate Research Fellowship Program, the Wisconsin Coastal Management Program, NOAA Great Lakes Environmental Research Lab and the Illinois State Water Survey-University of Illinois.