Why the Sea Rises Twice a Day in Most Places and Only Once in Some
The Moon Pulls, But Not Evenly
The Moon's gravity is stronger on the side of Earth facing it than on the side facing away. That difference, not the pull itself, but the difference across the planet's diameter, stretches the ocean into a slight bulge on both sides simultaneously. One bulge faces the Moon, one faces away from it, and Earth rotates beneath both. In theory, every coastline should pass under two bulges every twenty-four hours and fifty minutes, producing two high tides and two low tides each day. Most coastlines do exactly that. But the ocean is not a smooth, unobstructed sphere of water. It is a collection of basins.
Basins Have a Natural Rhythm
Every enclosed or semi-enclosed body of water has a natural period, the time it takes water to slosh from one end to the other and back, the way water rocks in a bathtub. The Atlantic Ocean's natural period is close to twelve hours. When the tidal forcing from the Moon arrives at roughly the same twelve-hour rhythm, the Atlantic resonates with it, amplifying the twice-daily signal. The result is that the US East Coast, the UK, and most of the Atlantic-facing world experience two strong, clearly separated tides each day, a pattern called semidiurnal.
Where the Once-Daily Tide Lives
The Gulf of Mexico, parts of the Gulf of Tonkin in Vietnam, and stretches of the coast around the South China Sea see predominantly one tide per day, a diurnal pattern. The Gulf of Mexico is the clearest example. Its basin is roughly the right size to resonate with a twenty-four-hour forcing period rather than a twelve-hour one. The twice-daily signal gets suppressed; the once-daily signal gets amplified. A visitor to Pensacola or Galveston watches the water rise once, fall once, and that is the day's tide. Some locations sit in between, two tides a day, but one noticeably higher than the other, a mixed pattern that reflects a basin catching both signals at partial strength.
The Straits, the Shelves, and the Geometry That Decides
Continental shelves, the shallow underwater ramps extending from coastlines, slow tidal waves and concentrate their energy. The Bay of Fundy in Nova Scotia, where tides reach fifteen metres, the highest recorded anywhere on Earth, owes that extreme to a funnel-shaped bay whose length matches the tidal period almost perfectly. The bay resonates like an organ pipe. Tidal energy piles in from the Atlantic and has nowhere to go but up. The Indian Ocean, by contrast, is bounded to the north by the Asian continent, which cuts off the resonance pathways that would amplify a twice-daily signal. Much of the Indian coastline sees a mixed pattern, with the two daily highs differing enough in height that one can seem almost absent.
What the Moon Starts, the Earth Finishes
The Moon provides the energy and the timing. The ocean basins decide what to do with it. A tidal forcing arrives at every coastline in the world at the same fundamental frequencies, once a day, twice a day, and several smaller harmonics. What a particular coast experiences depends on which of those frequencies its local basin amplifies and which it ignores. This is why tidal prediction requires a different set of constants for every port in the world. Tide tables for Mumbai cannot be derived from tide tables for Chennai by a simple calculation. Each location has been tuned by its own geography over geological time, and the sea rises on its own schedule.