What Are Tides?

Tides are the periodic rise and fall of sea levels caused by the gravitational interaction of the Earth with the Moon and, to a lesser extent, the Sun. As a scientific and engineering topic, tides encompass the physical mechanisms generating these oscillations, the mathematical methods used to predict them, and the instrumentation systems deployed to measure them. Tidal science draws from oceanography, geodesy, fluid dynamics, and remote sensing, and connects directly to ocean circulation because tidal currents redistribute heat, nutrients, and momentum across ocean basins.

The Moon is the dominant tide-generating body because tidal forces decrease with the cube of distance rather than the square, meaning proximity outweighs the Sun's greater mass. The Moon's gravitational pull creates a tidal bulge on the near side of Earth and, through inertial effects, a corresponding bulge on the far side, producing two high tides per lunar day at most coastal locations. The Sun generates a tide-raising force roughly half that of the Moon; when the two bodies align at new or full moon, their combined effect produces the enlarged spring tides, while quadrature alignment at quarter phases yields the reduced neap tides.

Tidal Forces and Dynamics

The equilibrium theory of tides, formulated by Newton in the seventeenth century, treats the ocean as a thin water layer that deforms instantly to the gravitational potential. The dynamic theory of tides, developed by Laplace and refined throughout the nineteenth and twentieth centuries, accounts for the ocean's finite depth, basin geometry, Coriolis deflection from Earth's rotation, and the resonance properties of individual ocean basins. These factors cause actual tidal patterns to deviate substantially from equilibrium predictions. The tidal forcing is decomposed into harmonic constituents, each identified by a period and an amplitude; the principal lunar semidiurnal constituent M2 at 12.42 hours dominates at most Atlantic and Pacific locations, while diurnal constituents K1 and O1 dominate in parts of the Gulf of Mexico and Pacific.

Tidal Prediction

Harmonic analysis is the standard method for predicting tides at a given station. A sufficiently long time series of water level observations is decomposed by least-squares fitting to extract the amplitude and phase of each tidal constituent. These harmonic constants are then summed with their future arguments to produce predictions extending years into the future. NOAA's Center for Operational Oceanographic Products and Services maintains harmonic constants and publishes predictions for more than 3,000 U.S. stations, generating tide tables used by commercial mariners, port operators, and coastal engineers. Where observational records are short, numerical hydrodynamic models forced by the known tidal potential provide predictions for ungauged locations through data assimilation and basin-scale simulation.

Measurement and Monitoring

Water level measurements are obtained using acoustic sensors, radar gauges, and pressure transducers referenced to geodetic benchmarks through satellite-surveyed tide gauge networks. Satellite altimetry, pioneered with TOPEX/Poseidon in 1992 and continued with the Jason series missions, provides global tidal information independent of the coastal gauge network, enabling scientists to resolve open-ocean tidal amplitudes at centimeter accuracy as described in NASA's ocean surface topography mission documentation.

Applications

Tidal science and engineering have applications across a wide range of fields, including:

• Maritime navigation and port operations planning using tide table predictions
• Tidal energy resource assessment and turbine array design in straits and estuaries
• Coastal engineering for sea wall, breakwater, and dredging projects
• Flooding and storm surge forecasting combined with meteorological models
• Ocean circulation research, as tidal mixing drives deep ocean overturning
• Geodetic sea level monitoring for climate change assessment per NOAA oceanographic data products