I don't know
if many people know about this. But apparently a small tsunami hit Narragansett Bay (on the
north side of Rhode Island Sound) on June 13 last month.
I've known
about this for a few weeks. However I wanted to talk to NOAA and others about
it first. I haven't heard back from Brian McNoldy from the University of Miami's
Rosenstiel School of Marine and Atmospheric Science; if I do, I will amend this
post to include what he has to say.
Anyway on to the post.
Instrument
readings provide evidence that on June 13 at 4 p.m. a tsunami hit Rhode
Island. The uncharacteristic tidal flow stretched from South Carolina to Maine,
affecting sea levels and causing some minor damage along parts of the eastern seaboard. In fact the National
Oceanic and Atmospheric Administration (NOAA) says tsunami-like conditions were
observed June 13 at more than 30 tide gauges and one DART buoy, monitored
by the West Coast/Alaska Tsunami Warning Center (WCATWC) along the East Coast,
Bermuda and Puerto Rico.
If you
remember, we had a low-end derecho on Wednesday
evening June 12th-Thursday June 13th.
There were widespread
severe thunderstorms over the Midwest on the late evening of the 12th. Just before
Midnight. The squall line that developed caused a few significant wind damage reports in Indiana. The
line became more organized and moved east across Ohio, Pennsylvania and New Jersey,
reaching the Jersey Shore about 11am Thursday morning.
You might be
wondering what does a derecho and a tsunami have to do with each other? Well to
answer that, "a straight line of thunderstorms - called a derecho -
apparently triggered a tidal wave on June 13", a University of Rhode
Island scientist told The Providence Journal.
The idea
being, Once the derecho was out of the range of East Coast radar it may have developed
into something much more organized, with much stronger winds. Due to the effect
of the storm being over the warmer ocean waters; the storm could have developed
downdraft winds over the Atlantic ocean, causing some kind of tsunami like wave.
The wave the storm supposedly pushed would have taken several hours to move
back to the coast. This is supported by tidal gauges which showed the wave moved a lot slower than a traditional earthquake
based tsunami wave. Wind will not push a wave as fast as a jolt from a rock fault under the
surface.
WCATWC officials say the source of the tsunami is very
“complex” and “still under a review.” However, they also point out that "the earlier storm system that struck the
area was a possible cause."
WCATWC wrote
in their report, "The event occurred in close conjunction with a weather
system labeled by the National Weather Service as a low-end derecho which
propagated from west to east over the New Jersey shore just before the tsunami."
I will come back to the subject of storm caused tsunami like waves later.
After hearing
several reports from witnesses, NOAA officials confirmed that a tsunami had indeed
struck the area that day. Here are a few of the reports, I've seen.
In Wickford
Harbor, the swift ebb and flow of the tides pushed boats around with enough
pressure to pull a cleat from a dock and rip out three stanchions, a Wickford
Yacht Club employee told the Providence Journal.
In another incident,
officials say a man named Brian Coen was on his boat fishing near the mouth of
the Barnegat Inlet. Coen noticed that there was
an outgoing tide with strong currents coming towards him. According to
Coen, the heavy waves continued for about two minutes until the rocks in the
submerged breakwater were exposed, forcing Coen to back his boat out of the
area.
A few minutes later Coen
spotted a large wave, approximately 6-feet from crest to trough, coming across
the inlet. The wave was so powerful that it swept two people off a rock jetty
and into the water on Long Beach Island. They were both rescued from the water.
Coen also said, he had two friends who
were scuba diving off the boat. He says these people were pulled a few hundred
yards across the inlet by the wave. Fortunately
everyone involved in the incident only had non-life threatening injuries.
How
Traditional Tsunamis Form:
First, I want
to clear up a common misinterpretation most
people have about water waves of all kinds. A waves is not the movement of
water. Instead, they are the movement of energy through water.
This concept of energy is very important because what distinguishes tsunamis
from ordinary waves is the vast difference in the amount of energy that is
carried. Ordinary waves are made by the constant force of wind friction
along the surface of the water. The friction takes energy from the wind
and transfers it into the water, because of this we see waves. The longer
and faster the wind blows impacts the amount of
energy going into the waves. So the stronger the wind the higher the
waves.
In contrast, a tsunami gets its
energy by way of an intense direct impulse of force.
The Indonesian tsunami of December 2004 was caused by an earthquake. The
earthquake lifted the seafloor up quite a few meters, which lifted the water
above it the same amount. The energy moved from its epicenter outward at
hundreds of miles per hour. Clearly the water wasn't moving at that speed,
instead the energy moving through the
water at that speed.
Most tsunamis are caused by earthquakes. However, they can be formed
by anything that causes an intense direct impulse of force; such as a landslide,
underwater volcanic eruption, or even a meteorite.
The NOAA tsunami website can be found here.
Hurricane caused waves:
As I just stated, wind energy is transferred to water, becoming water
energy. The stronger the wind and the longer it blows, the higher the waves. So
it should be no surprise that hurricanes can cause huge waves.
David Wang and his colleagues at the Naval Research Laboratory in
Mississippi say, they have evidence that Hurricane Ivan a Cape Verde hurricane;
that moved into the Gulf of Mexico (GOM) in September of 2004, generated waves
greater than 40 meters high from crest to trough, an incredible 131 plus feet
from crest to trough.
William Teague, one of David Wang's colleagues said, "These are the largest
wave heights ever recorded with instruments in US waters" He went on to
say "They're larger than we ever thought they would be." Ivan wasn't
even a particularly large hurricane, he added.
Something else that is interesting. Researchers at the US Naval Research
Laboratory, based at the Stennis Space Center in Mississippi, think that
hurricanes can pile up sediment off shore on the continental shelf.
The researchers setup and secured six sets of instruments for measuring
water depth, pressure and currents on the seabed in Hurricane Ivan's path. At
two of these spots, the storm scoured about 11 inches of sediment from the sea
floor. The before mentioned William Teague said
"All of the moorings were deeper immediately
after the passage of Ivan."
There are eyewitness accounts of similarly enormous waves at sea. In 1995,
the luxury liner
Queen Elizabeth 2 survived
an encounter with one about 30 meters (98 feet) high in the North Atlantic, and
six years later a similar wave smashed windows on the cruise ship Bremen in the
South Atlantic and nearly sank it.
Shuyi Chen, a specialist on computer modeling of hurricanes at the
University of Miami, said the latest hurricane models predict such waves should
occur "very often". The findings are consistent with less accurate
measurements made from ships, she adds.
It is now widely suspected that these rogue waves, generated by wind and
currents, might explain the mysterious, regular disappearance of large ships at
sea. Oceanographers have only recently come to accept that such rogue waves might
not be
extreme rare events. But could be
much more common than they previously thought.
Meteotsunamis caused waves:
As I said I would come back to the subject of storm caused tsunami like
waves.
Meteotsunamis are weather-related
and caused by storms, the storms don't have to be overhead, in fact the storms
can be hundreds of miles away. These storm-driven waves don't effect or inflect
damage on an entire continent's coastline.
They are confided to
narrow harbors and bays The word
Tsunami is a Japanese word that means harbor wave.
So while a Meteotsunami isn't technically a tsunami it is very tsunami like.
In fact, meteotsunamis have similar frequencies to the more classic tsunamis,
except they occur in specific bays and inlets
As a kid and teenager, I was on the Great lakes a lot, (I still am) So the
concept is not really all that new to me..... Meteotsunamis can and do strike the Great Lakes and
along the U.S. coastline.
Meteotsunamis are quite dangerous. It is now known that a meteotsunami
killed seven people on the Chicago shoreline of Lake Michigan on 26 June 1954. There were actually two
recorded in Lake Michigan in June 1954. An 18-foot-tall (5 m) wave took out
cars in Daytona Beach, Fla., in 1992, and a 12-foot-high wave struck Boothbay Harbor, Maine, in 2008.
In order to get a meteotsunami you need three general things to be in place.
1). A meteorological disturbance
These disturbances can be such things as atmospheric gravity waves, passage
of strong cold fronts, very intense squall lines (like the June 13th low-end
derecho ).
2). Resonance.
Resonance is when something oscillates with greater amplitude at some
frequencies than at others. In the case of
meteotsunamis...it's the difference between the speed of the
meteorological disturbance and deep water wave speed.
3). Amplifying characteristics of a harbor, bay or inlet.
Many times, Bays, harbors, and inlets exhibit a quick change in bottom
elevation. If they are especially V shaped the waves are funneled in towards
shore and energy density increases the height of the wave.
If you want more information on meteotsunamis here are two sites.
About.com
Meteotsunamis
So what caused the tsunami along
the East Coast?
Here is what I think caused the June 13 tsunami.
As I said above, I showed how hurricanes can deposit sediment
and dislodge this same sediment from the
seabed.
I also said hurricanes can cause
a big surge and big waves.
But was the
derecho the cause as some think?
I think
the derecho may have been the spark, not the cause. I think Super-storm Sandy
piled up sediment on the continental shelf; When the derecho came along it was just
enough to dislodge the sediment, which provoked a
landslide on the continental shelf. While
Sandy was a Cat 1 hurricane her pressure was
equivalent to a Cat 3, she was also
the largest (in coverage) storm ever recorded in the Atlantic Basin. So there
is no doubt that she was very powerful.
I base the fact that it was a tsunami caused by a landslide on the
NOAA report. The
report stated that tsunami-like
conditions were observed June 13 at more than 30 tide gauges and one
DART buoy, along the East Coast, Bermuda and Puerto Rico.
I pointed out
above that Meteotsunamis occur in bays, harbors, and inlets and don't impact entire. coastlines. I do think places like Narragansett Bay amplified the effects of the tsunami.
So while tsunamis are very rare on the Atlantic East Coast. They do occur and will occur again in the future. The next one might not be so minor.
Rebecca.
