The tide receded Monday and Islanders wondered aloud
Photo by Steve Myrick
High and low tide are facts of life on Martha's Vineyard. Mainland residents may pay little attention to the ocean's rise and fall, but Islanders do, and on Monday, an extremely low tide raised many questions.
Longtime residents said they could never recall seeing the tide so low.
Sunset Lake in Oak Bluffs was completely drained, and in Sengekontacket Pond, an exposed sandbar ran nearly the entire length of the salt pond.
Along the Vineyard Haven harbor seawall, a flat normally covered by water lay exposed. John Best of Vineyard Haven was so surprised as he drove along Beach Road that he called The Times and said he had never seen the tide so low.
"Something unusual happened with the tides yesterday," David Stanwood of West Tisbury emailed The Times Tuesday. "In the late morning, areas of the seabed never before seen were revealed. The outer harbor by the sea wall was dry all the way to the Lagoon channel. There were reports of low tides later in the afternoon as well, which is weird. There has to be a story here, not only in the observations made by locals but the scientific reason for it. It was a quarter moon, normally the weakest time for tidal action."
Mr. Stanwood said he had heard people attribute the low tides to the asteroid that came within 17,000 miles of the Earth, and some even wondered if it was related to a tsunami off the coast.
None of the above, Stephen Gill, NOAA senior scientist for the Center for Operational Oceanographic Products and Services, assured The Times in a telephone conversation Tuesday. It was a strong and sustained northwest wind.
"This is an event that was due to the intense low pressure system that was just offshore the last couple of days," Mr. Gill said. "It formed offshore and intensified."
The gravitational pull of the moon and the sun affect the Earth's tides.
When the sun, moon, and Earth are in alignment (at the time of the new or full moon), the solar tide has an additive effect on the lunar tide, creating especially high high tides, and very low low tides — both commonly called spring tides, according to NOAA.
One week later, when the sun and moon are at right angles to each other, the solar tide partially cancels out the lunar tide and produces moderate tides known as neap tides.
The next spring tide occurs on Monday, February 25. So why the low tide one week earlier?
"It had nothing to do with the astronomical component," Mr. Gill said. "It had to do with the intense, little low pressure offshore system that formed with a strong pressure gradient."
That system produced sustained winds from the north-northwest that affected communities along the coast from Montauk Point at the east end of Long Island to Nantucket. "It affected all of the stations along that part of the coast because the winds were coming and blowing the water offshore," he said. "Very strong, sustained winds from 15 to 20 knots with sustained gusts up to 30, 35 knots. When they are sustained like that and coming from the same direction they will move water. When that happens — the orientation of the coast with the strong sustained winds — it ends up blowing the water offshore and away from the coastline, and that's why you had these low tides."
Because the tides were approximately two feet below the predicted height for that day, it was particularly noticeable in exposed, shallow bays and water bodies, for example Vineyard Haven Harbor and Sengekontacket Pond.
"It is the reverse of a storm surge," Mr. Gill said. "If you are on the other side of a storm, like the blizzard, you had higher than normal tides coming in as that storm came through."
While it was interesting, Monday's tidal event was not historic. It was not all that abnormal, when examined against more than a century of tide data, Mr. Gill said.
"This was not the minimum tide of all time, by any means," Mr. Gill said. "It was not the lowest water levels that we've observed in these places."