Why some seas have almost no tide

Big tides need an open door and the right shape

The moon pulls on every body of water on Earth, from the Pacific to your local lake. So why does the Bay of Fundy swing by more than 15 meters (about 50 feet) while a beach in Barcelona barely moves?

As we explain in what causes tides, the big tides on ocean coasts aren’t made locally. They travel as an enormous wave that sweeps around each ocean basin. To get a large tide at a given beach, two things usually need to happen:

• the ocean’s tide wave has to be able to reach it freely;
• the shape and depth of the coast, bay or basin have to amplify that wave, ideally by resonating with it.

Nearly enclosed seas fail on both counts. Very little of the ocean’s tide gets in, and the sea itself isn’t the right size to build one.

The Mediterranean: a big sea behind a narrow door

The Mediterranean covers close to a million square miles, but its only natural opening to the Atlantic is the Strait of Gibraltar. At its narrowest, the strait is only about 9 miles (14 km) wide, and the ridge on its floor is only about 1,000 feet (300 m) deep.

Think of trying to fill a huge swimming pool through a crack. A rising tide lasts about six hours, and in that time there’s no way enough water can pass through the strait to lift a sea that size by any real amount. Part of the Atlantic tide does get in and is felt just inside the strait, but it fades quickly as you head east.

The contrast in southern Spain is striking:

• in Cádiz, on the Atlantic side, the spring tide range can top 10 feet;
• in Algeciras, inside the strait, even the biggest tides reach only about 4 feet (1.3 m);
• in Málaga it’s noticeably smaller, and it keeps shrinking eastward;
• in Valencia, Barcelona or on the French Riviera, it’s a few inches, rarely more than about a foot.

Why it doesn’t build a tide of its own

The moon also pulls directly on the Mediterranean, with no help from the Atlantic. But in a closed basin that size, that pull only produces a small oscillation.

Every body of water also has its own natural rhythm of sloshing back and forth, like water in a bathtub. When that rhythm matches the tide’s 12-hour, 25-minute beat, the tide grows enormously, which is what happens in the Bay of Fundy. The Mediterranean’s basins don’t have that lucky size, so their tides stay small. In much of the sea, the tide rotates around points where the water hardly rises or falls at all, which oceanographers call amphidromic points.

There are exceptions. In the northern Adriatic, the tide around Venice is somewhat bigger than in the rest of the Mediterranean, and when a high tide coincides with low pressure and a strong southeast wind (the scirocco), the city gets its famous acqua alta floods. The Gulf of Gabès in Tunisia, with its broad, shallow shelf, also gets tides of more than 3 feet.

The Gulf of Mexico: small, and once a day

The Gulf of Mexico is a different case. It’s connected to the Atlantic and the Caribbean through the Florida Straits and the Yucatán Channel, and it does have a real tide. But the Gulf’s shape barely amplifies the twice-daily part of the tide, while the once-daily part comes through. The result, along much of the coast from the Florida Panhandle to Texas:

• often just one high tide and one low tide a day, as in Pensacola and Mobile;
• a range that’s usually only a foot or two;
• days around the moon crossing the equator when the tide almost stalls.

We go deeper into this pattern in our guide to types of tides. For anglers and boaters, the practical upshot is that on the Gulf Coast the wind often matters as much as the tide table. A strong north wind behind a winter cold front can push water out of Texas and Louisiana bays and leave flats and channels far shallower than predicted, the so-called blowout tides. A steady onshore wind does the opposite.

The Baltic, the Black Sea and the Great Lakes

• The Baltic Sea is even more enclosed than the Mediterranean, connected to the North Sea through the narrow, shallow Danish straits. Its tide is tiny, only a few centimeters (an inch or two) on the coast of Finland, and its level follows wind and pressure.
• The Black Sea, linked to the Mediterranean only through the narrow Bosporus and Dardanelles straits, also has barely noticeable tides.
• The Great Lakes have true tides, but NOAA puts the largest at less than 5 centimeters, about 2 inches. Wind and pressure changes swamp them. Strong winds can also set off seiches, where the water sloshes from one end of a lake to the other and can rise several feet at the downwind shore.

When the weather is the real tide

In places with tiny tides, the weather does most of the work. Scientists sometimes call it the meteorological tide:

• Air pressure. The sea rises about 1 centimeter for every millibar the pressure drops, and falls the same amount when it rises. That’s roughly an inch for every 2.5 millibars.
• Wind. Hours of wind blowing toward shore pile water against it, and wind blowing offshore drains it away.
• Sudden pressure jumps. A fast-moving squall line can set off a meteotsunami, a quick surge of water that NOAA has documented on the Great Lakes and the East Coast. In Menorca, in Spain’s Balearic Islands, harbor surges of this kind are called rissaga, and in strong cases they can exceed 3 feet within minutes.

Here’s an example with numbers. Under a strong high-pressure system of 1,035 millibars, the sea sits about 8 to 9 inches lower than it would at a normal 1,013 millibars. Under a 990-millibar storm, it sits about 9 inches higher. That swing of a foot and a half is more than the entire tide in Pensacola on an average day.

Hurricanes take this to the extreme. Their storm surge can raise the water many feet above the predicted tide, and it does the most damage when it arrives on top of a high tide.

What it means for you

• On the Gulf Coast, check the wind forecast and the passage of fronts along with the tide table. A north wind can leave you with a lot less water than the prediction says.
• In the Great Lakes, there’s no tide to plan around, but watch for strong winds that can set off seiches near harbors and river mouths.
• On a Mediterranean trip, don’t expect the beach to change much between morning and evening, except near Gibraltar or in the northern Adriatic.
• When boating in shallow water, remember that a strong high-pressure system or an offshore wind can drop the water below the predicted level, even below the zero on the chart. Our guide to chart datum and MLLW explains where those numbers start.