Bouncing icebergs ride Alaskan swells

Alaskan storms are literally rocking the South Polar ice – jarring it about 5 mm vertically and sometimes bouncing icebergs along the sea floor.

The image of house-sized waves detonating on a reef is a standard one in any surfing tale, but when the waves hit a “nascent iceberg” carrying a $7,000 seismometer there’s actually proof – in the form of a pretty blue-and-orange picture like the one above. No, it’s not an image swiped from a natural-gas ad, it’s actual data as it appears on Mac Cathles‘s poster (and recorded on my cell phone).

The bands of orange and red come from waves smacking into the iceberg. Time (calendar months) is on the horizontal axis and wave frequency is on the vertical axis. Frequency is better known to surfers as period, their single most coveted wave statistic. Low frequency waves (low on the graph), are the beautiful, widely spaced, high-energy lines that sneak up on point breaks and explode out of the deep water. Higher on the axis is the depressing, confused, beach-break windslop that knocks down unwary tourists and fills their ears with sand.

As storm winds sweep over the ocean, they stir up waves at many frequencies. But lower-frequency waves move faster than higher frequencies. So when storms are far away, low frequencies arrive ahead of high frequencies. On the graph, that means the scatter of datapoints leans to the right. It’s such a reliable relationship, in fact, that the degree of lean quite clearly indicates the distance to the storm.

In the record from 2006 on this picture, Cathles said, at least eight of the “flames” are leaning far enough to have come all the way from the Gulf of Alaska, more than 8,000 miles away.

Of course, Cathles and his coauthors didn’t do all this just so surfers could daydream about sharing epic waves with penguins. They’re interested in the cumulative effect of all those incessant waves (“like wiggling a tooth”) on the composition of the Antarctic ice sheets. If global warming causes fiercer storms, it could batter Antarctica with more wave energy, perhaps leading to earlier or more frequent ice breakup. With their iceberg-seismometer contraption, Cathles and colleagues Douglas MacAyeal and Emile Okal hope to keep track of that wave energy as it arrives.