The regular rise and fall of ocean water, known as the tide, results directly from the gravitational forces exerted primarily by the Moon and, to a lesser extent, the Sun. The Moon’s proximity to Earth makes its pull the dominant factor in creating two tidal bulges on opposite sides of the planet, which Earth rotates through daily. While these gravitational forces are universal, the actual observed tide at any specific location is highly variable. The differences in the number, timing, and height of high and low tides are determined by a complex interplay of astronomical mechanics and local geography.
Global Tidal Patterns and Amphidromic Systems
The type of tide—the pattern of high and low waters experienced over a lunar day—differs across the globe based on how the tidal wave interacts with the ocean basins. If the Earth were uniformly covered in water, most places would experience a semidiurnal tide: two high tides and two low tides of nearly equal height each day. This pattern is common along the East Coast of North America and much of Europe.
The Moon’s gravitational pull does not always align perfectly with the equator, but shifts north and south over the course of its orbit. This changing angle, known as the Moon’s declination, influences the tidal bulges, often creating a mixed semidiurnal pattern. This pattern, seen along the Pacific coast of North America, features two high tides and two low tides each day, but with noticeably different heights. Some areas, like the Gulf of Mexico, experience a diurnal tide, where only one high tide and one low tide occur per day.
On a larger scale, the tidal wave propagates across vast ocean basins in rotating systems called amphidromic systems. These systems form because the tidal wave is blocked by continents and deflected by the Earth’s rotation via the Coriolis effect. The tidal wave rotates around a central point, known as a tidal node, where the tidal range is essentially zero.
The tidal range increases with distance away from this central node. Co-tidal lines, which mark simultaneous high tide, radiate outward from these nodes, showing how the tidal crest rotates around the basin, typically completing a rotation in about 12 hours. This rotary motion and the location of these nodes dictate the timing of high water and the overall tidal range across entire ocean regions.
Coastal and Basin Geography
Once the global tidal wave reaches a continental shelf, local geography dramatically modifies the height and timing of the tide. The bathymetry of the continental shelf plays a significant role in this process. As the tidal wave moves from the deep ocean onto the shallower shelf, the water depth decreases, causing the wave to slow down and its height to increase.
The shape of the coastline itself acts as an amplifier or dampener of the tidal energy. Where the coast is wide and open, the tidal energy disperses, resulting in a gentle, smaller tidal range. Conversely, narrow, funnel-shaped bays and estuaries act like a compression chamber, forcing the incoming water into a continuously smaller volume. This coastal funneling effect drastically increases the height of the tide, as famously demonstrated in the Bay of Fundy, Canada, which experiences the world’s largest tidal range, often exceeding 15 meters.
Another factor influencing tidal height is basin resonance. Every body of water has a natural period of oscillation, similar to the sloshing in a bathtub. If the period of the tidal forcing (about 12 or 24 hours) matches the natural oscillation period of a bay or basin, the tide can be greatly amplified. Shallow water and narrow straits introduce friction, which can dissipate tidal energy and dampen the tide. This friction is why semi-enclosed seas, such as the Mediterranean Sea, exhibit minimal tidal ranges.
Non-Gravitational Factors
While astronomical forces and geography establish the predictable tidal pattern, non-gravitational factors introduce daily, temporary variations. These secondary influences are collectively known as meteorological tides because they are driven by local weather conditions. Tide tables are calculated based on average atmospheric conditions, but any deviation from this average will affect the sea level.
One primary factor is atmospheric pressure, which affects the sea level through the inverted barometer effect. Tidal predictions assume a standard atmospheric pressure (about 1013 millibars). A low-pressure system, such as one associated with a storm, effectively lifts the sea surface because the reduced weight of the atmosphere allows the water to rise. For every one millibar drop in pressure, the sea level rises by approximately one centimeter.
Wind stress also significantly alters the observed tide by pushing water. Strong, sustained onshore winds pile water up against the coast, raising the high tide height and potentially leading to coastal flooding. Conversely, strong offshore winds push water away from the shore, causing the low tide to be lower than predicted. When the effects of low atmospheric pressure and strong onshore winds combine, they can create an extreme rise in sea level known as a storm surge.