From Maine to Florida, the Atlantic seaboard has experienced higher tides than expected this summer. At their peak in mid-June, the tides at some locations outstripped predictions by two feet.

The change has come too fast to be attributed to melting ice sheets or anything quite that dramatic, and it's a puzzle for scientists who've never seen anything quite like it.

"The ocean is dynamic. It's not uncommon to have anomalies like this but the breadth and the intensity and duration were unique," said Mike Szabados, director of the National Ocean and Atmospheric Administration's tide and current program.

The unexpected tidal surge is subsiding, has reduced its reach from the entire coast, and is now concentrated just in the mid-Atlantic states.

NOAA is rushing to study the data in an effort to understand what happened. Szabados' office is already putting the finishing touches on a report that will be released next month on the wind and current patterns that appear to be correlated with the tidal surge.

Szabados said that two main factors appear to have contributed to the extra high tides. First, there were steady winds out of the northeast throughout this anomaly. Second, the ocean current running from Florida up along the coast weakened. While the associations between these phenomena and the tides are provocactive, it's too early to tell how fully they explain this unexpected tidal event.

"I'm quite sure that there will be more intensive analysis of this event. By no means will this report be the definitive answer to anything," Szabados said. "Further assessment of this event should be encouraged to better understand the phenomena."

One thing is for sure: The tidal rise is strange. Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research, looked over June climate data on wind, atmospheric pressure and the ocean. June was — high tides aside — "nothing to write home about" in Trenberth's estimation.

"It's a bit of a mystery," he said.

Szabados' team was initially puzzled that there were no major wind anomalies accompanying the tides, but his team's wind expert figured out it wasn't the magnitude that was anomalous.

"He said there's no significant anomalies in the magnitude of the wind, it's the persistence of the winds," Szabados said.

An even bigger mystery is why such winds would suddenly appear and why the current running up the Atlantic coast would weaken. Was it a freak coincidence, some jitter in the data, or part of a long-term trend or cycle?

John Boon, professor emeritus of oceanography at Virginia Institute for Marine Studies, thinks it could be part of a long-term global trend that's tied in with the Pacific region's El Niño weather pattern.

"When I'm comparing these decadal cycles, I see that some of the highs in these decadal cycles coincide with El Niño events," He said. "It's not to say that one is caused by another, but the degree of association is somewhat surprising."

But long-term tidal patterns can be hard to spot, Boon said.

"It's such a long time scale that's working in this process that we don’t sense it going on like we sense a hurricane coming and going," he said. "A subtle but persistent pattern that affects the whole North Atlantic ocean: It's acting without giving any immediate clues that it's going on until we see, 'Whoops, the sea levels are higher than normal.'"

Boon and a team of other researchers are crunching data now on the phenomenon and hope to put out a paper later this year. Szabados wouldn't rule out the strange tide as an indication of a larger trend, but he wasn't ready to make that leap just yet.

"Is this part of a long decadal variation? Potentially, yes," Szabados said. "But it's premature to make that linkage."

See Also:

Image: Flickr/djwhelan

WiSci 2.0: Alexis Madrigal's Twitter, Google Reader feed, and green tech history research site; Wired Science on Twitter and Facebook.**