October 17, 2006

U of C scientist shows hurricanes have far-reaching effects

A butterfly flaps its wings in Australia and sparks a Category 3 hurricane off the coast of West Africa. Sound implausible? Perhaps, but one University of Chicago researcher and his partner say they have discovered evidence of the massive effect storms can have thousands of miles from where they take place, according to a new study in the October issue of the journal Geophysical Research Letters.

Douglas MacAyeal, a professor in geophysical sciences at the University, and Emile Okal, a professor in geological sciences at Northwestern University, determined that a storm off the coast of Alaska last year caused the breakup of a gigantic iceberg off the coast of Antarctica, thousands of miles away.

The iceberg, known as B15A, was approximately 60 miles long and 18.5 miles wide. On October 27, 2005, it suddenly shattered into several pieces.

Following the breakup, the researchers arranged for a team to retrieve some seismometers from the iceberg. The instruments had been planted there years before to study iceberg movement.

The seismometers showed that the iceberg, which was about twice the size of Delaware, began moving half an inch up and down and four inches side-to-side hours before it broke up. The researchers first attributed this to ocean swelling, but there were no weather disturbances in the area that day. The team began looking for faraway events that could have generated waves to cause the swell.

In deep water, lower frequency waves move faster than higher frequency waves. After inspecting the seismometer data, the researchers determined that low-frequency waves were arriving days before high-frequency ones, indicating a tremendous distance between the iceberg and the storm that caused these waves.

“The distance to the storm was about 13,500 km (8,370 miles). This meant that...we had to find a storm that was at that great distance...but still over an ocean...and with a reasonable travel path to Antarctica,” MacAyeal said.

The researchers decided the best place to look was the Gulf of Alaska. After checking meteorological records, they spotted a storm at just the right distance in the Gulf six days earlier.

Scientists have known for decades that similar waves can travel thousands of miles around the world and trigger ocean swells, but this is the first known instance of a storm so far away actually causing an iceberg to split.

“So far, this is an isolated incident; however, we are looking back at seismic records from other break-up see if there are other instances,” MacAyeal said.

If this turns out not to be an isolated event, it could raise fears about global warming, which may be increasing the frequency and intensity of ocean storms with far-flung effects. For now, MacAyeal urges people not to get too excited.

“What we have demonstrated is that there is a link,” MacAyeal said. “However, we don’t know as much about this link as we would need to say that global warming would directly lead to a reduction of ice cover in Antarctica.”