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Atmospheric fronts the driving force behind meteo-tsunamis

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Tsunamis are well known for their inundation of coastal regions in the South Pacific, but most Floridian boaters have little concern with this natural process. Guess again.

Tsunamis of a different type, called mateo-tsunamis, can devastate marinas and damage vessels, as Daytona Beach boaters learned in 1992. Unfortunately, tsunamis are easily forgotten in the United States.

Normal tsunamis are caused by subsea seismic events such as a major vertical shift in the seafloor that sends a sea wave toward shore. These waves heighten as they encounter shallow areas. Devastation results from the shoreward movement and pile-up of water, versus a normal wave that causes only oscillatory motion at the sea surface and then breaks on the beach.

In contrast, meteo-tsunamis (MTs) are caused by long, intense atmospheric fronts (thus the meteo prefix) that move rapidly from west to east across the continental United States. Meteorologists call these fronts derechos from the Spanish adjective for straight, a long, straight front. They develop in the Great Lakes region as well as across the East Coast.

The intensity of an MT is governed by the magnitude of the atmospheric pressure fluctuation observed during the passage of the front, such as a 5-millibar pressure drop in 10 minutes, and by the speed at which the front passes over lake or coastal waters.

Here are a few basics for captains and crew to know:

The strong atmospheric front can move in any direction as it passes over the impacted coastal waters.

The MT wave originates offshore at a specific water depth and propagates perpendicular to the depth contours in two directions: onshore and offshore.

A harbor can be impacted by an MT generated from a strong front that passed well north of the area and many hours earlier. Thus, MTs can affect harbors when local skies are clear and winds are calm, a big surprise to the unwary boater.

Conversely, MTs are not generated in a harbor during the peak of a squall nor when a vessel’s barometer drops as it’s tied to the dock. But they can impact vessels and marinas after a storm. Monitor meteorological forecasts if a large, intense front approaches.

The National Geophysical Data Center (NGDC) tsunami database contains global observations of 1,680 tsunamis between 2,000 B.C. and 2014, with about 3 percent (32 events) associated with meteorological processes, thus MTs. Additionally, a large number had unknown sources and many could have been MTs.

In Japan, where MTs are called abiki, a major event occurred in 1979 with a wave height of nearly 15 feet. The Spanish coastline in the western Mediterranean also experiences major MTs, locally called rissaga. One such event occurred in 2006 when a storm crossed the region and generated waves up to 15 feet high within a harbor whose geometry was resonance-favorable.

The eastern shore of the Adriatic Sea also encounters numerous MTs as occurred in 1978 and 2003, having wave heights up to 19 feet.

In the past century, significant MTs have been reported in the U.S., including in Lake Michigan, which encountered a 10-foot wave in 1954. Lakes Michigan and Erie are hot spots because of the frequency of intense meteorological fronts that move eastward during summer months.

Daytona Beach was impacted by a 10-foot wave in the summer of 1992, caused by a squall line that passed north and offshore. There was no local meteorological disturbance to warn the public that such a wave would occur.

The coast of Maine experienced an MT event in 2008 with surge heights up to 12 feet caused by a meteorological storm that passed over the continental shelf in the Gulf of Maine.

Southern New England was impacted by an MT in June 2013, caused by an intense front that moved offshore. Rapid sea level rise was experienced at numerous locations with a maximum height of 2 feet in Narragansett Bay.

In 1984, the Spanish Meteorological Agency established a Rigassa Warning System that notifies the coastal public if a forecasted storm is likely to generate an MT. Because of the large area and metropolitan population along the U.S. East Coast, an MT warning system would be difficult and costly to implement. Regardless, there needs to be better education of the coastal populace and boaters in the United States so they understand the MT phenomenon, atmospheric warning signs and potential risk of impact.

Scott E. McDowell has a doctorate degree in ocean physics, and is a captain and author. Contact him through www.scottemcdowell.com.

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