At last: An explanation for high June/July tides
Lots of Marylanders noticed it. A few sent me emails asking why the tides in Maryland during June and early July seemed so persistently high - from a few inches to a few feet at times, with some minor coastal flooding. I said it was likely a combination of astronomical effects, and persistent wind and weather patterns.
Well, I was mostly right.
Scientists with the National Oceanic and Atmospheric Administration noticed the persistently high sea levels, too, and set out to find the explanation. On Monday, they issued a 40-page report on the phenomenon, which blames it on a combination of (ta-da!) "steady and persistent Northeast winds," and a weakening of something called the Florida Current Transport.
"The ocean is dynamic and it's not uncommon to have anomalies," said Mike Szabados, director of NOAA's Center for Operational Oceanographic Products and Services. "What made this event unique was its breadth, intensity and duration."
The report's executive summary notes, as I did, that high tides in the latter part of June coincided with a "perigean spring tide." That's when the moon is at perigee (nearest to Earth), and aligned opposite the sun in a "new moon" phase (June 22), which causes higher-than-average "spring tides." Those factors amplified the tides, the report said.
But such astronomical factors are included in the forecast tide levels. What occurred was well beyond those predictions. And it was wind and current that really made the high tides notable.
In June, winds from Cape Hatteras to the Gulf of Maine had a persistent northeast component, the report said. That drove ocean and bay waters to the southwest, piling it up against the east-facing shorelines, raising sea levels and holding them higher than predicted levels, even during the times of low tides.
South of Hatteras, winds were mostly out of the southwest. The high sea levels observed there, the report found, were not the result of winds, but of a slackening of the Florida Current, which flows through the Florida Straits and feeds into the Gulf Stream. And when the Florida Current relaxes, the coastal sea levels along the Southeast Atlantic coast rises. When the Florida Current picked up again in mid-July, sea levels returned to normal.
"The June-July 2009 anomaly is unique," the NOAA scientists concluded, not because the Northeast winds and the Florida Current were at remarkable extremes, but because the two in combination created conditions that affected the entire U.S. East Coast, from Maine to Florida, simultaneously.
And the stretch from the Carolinas to New Jersey -including Maryland - were where the two forces overlapped to create the most extreme effects.
There has been nothing like it, over such a broad geography, the report says, in any spring/summer period since at least as far back as 1980.