Tides

1. Meaning and physical basis

Tides are long-period oscillations of sea level accompanied by horizontal movements called tidal currents. The rising stage is associated with flood currents and the falling stage with ebb currents, although current reversals need not coincide exactly with high or low water. Tides differ from ordinary wind-generated waves, persistent ocean currents and tsunamis. They are astronomically forced and therefore largely predictable.

Tides arise from differences in gravitational attraction across the Earth rather than simply from the total pull exerted on it. The Moon attracts the near side of the Earth more strongly than the Earth's centre, and the far side less strongly. Relative to the centre, this differential acceleration produces an idealised pair of tidal bulges: one facing the Moon and another on the opposite side.

In a reference frame rotating with the Earth–Moon system, the same result can be explained using lunar attraction and the centrifugal effect associated with their revolution about a common centre of mass, or barycentre. The far-side bulge should not be attributed simply to the Earth's daily rotation. Daily rotation instead carries locations through the changing tidal pattern.

Tide-generating influence varies approximately as mass divided by the cube of distance. Although the Sun exerts a much larger total gravitational pull on the Earth than the Moon does, its much greater distance makes its tidal influence weaker. Thus, greater mass alone does not determine which celestial body produces stronger tides.

2. Spring–neap cycle and astronomical variations

At new moon and full moon, the Sun, Earth and Moon are approximately aligned, a configuration called syzygy. Solar and lunar tidal effects reinforce each other, producing spring tides with a relatively large range. High waters tend to be higher and low waters lower than during neap tides. The word spring refers to the tide's increased rise and has no connection with the spring season.

At the first and third lunar quarters, the directions from the Earth to the Sun and Moon are approximately at right angles, or quadrature. Their tidal effects partly counteract each other, producing neap tides with a smaller range. Both spring and neap tides occur approximately twice during a synodic month of about 29.53 days; successive spring-tide periods are about 14.77 days apart.

The Moon's orbit is elliptical. Its tide-generating effect strengthens near perigee, when it is closest to the Earth, and weakens near apogee. A spring tide close to perigee may produce an unusually large astronomical range. Earth's changing distance from the Sun also matters, but less strongly.

The declination of the Moon and Sun—their angular distance north or south of the equatorial plane—contributes to unequal daily high and low waters. Actual maximum spring ranges may occur after syzygy because local basins respond with a time lag. Astronomical alignment therefore indicates the broad cycle, not the exact time or height at every port.

3. Daily tidal patterns and the dynamic ocean

A semidiurnal tide has two high waters and two low waters of approximately equal height during a lunar day. A diurnal tide has one high and one low water. A mixed tide, usually predominantly semidiurnal, has two high and two low waters with marked inequality between successive highs or lows. Local tidal records, rather than latitude alone, determine the classification.

The lunar day averages about 24 hours 50 minutes because the Moon moves eastward in its orbit while the Earth rotates. The Earth must rotate a little farther for a meridian to face the Moon again. This explains why corresponding lunar tides generally occur later on successive days, although actual daily delays vary.

The two-bulge equilibrium model is a useful starting point, not a literal map of ocean tides. Continents obstruct movement; water depth controls wave propagation; and friction, basin shape and the Coriolis effect modify the response. Real tides behave as interacting long waves that may form rotating systems around amphidromic points, where tidal amplitude is very small.

Funnel-shaped inlets can concentrate tidal movement, while resonance amplifies tides when a basin's natural oscillation period approaches that of the forcing. Consequently, neighbouring coasts can have different ranges and high-water times. Operational predictions combine several astronomical tidal constituents with observations from tide gauges.

4. Coastal landforms, Indian distribution and hazards

Tidal currents transport sediment, maintain estuarine channels and help shape mudflats, salt marshes and mangrove environments. Repeated exposure and submergence create the intertidal zone, where organisms must tolerate changing moisture, salinity and temperature. Tidal exchange supports nutrient movement and fisheries, but can also carry pollutants and saline water into estuaries.

In India, the funnel-shaped gulfs of Khambhat and Kachchh in Gujarat have comparatively large tidal ranges and strong currents. The Hooghly estuary and the Sundarbans are also strongly influenced by tides. These variations reflect coastal and shelf geometry rather than a simple distinction between the Arabian Sea and the Bay of Bengal.

A tidal bore is a steep advancing surge, sometimes followed by a wave train, generated when an incoming tide travels upstream into a sufficiently shallow, narrowing river or estuary. Bores occur in suitable reaches of the Hooghly and, on a spectacular scale, China's Qiantang River. Not every tidal estuary develops one.

A storm surge is a meteorological rise in sea level driven mainly by strong winds and low atmospheric pressure. Its overlap with high astronomical tide can greatly increase coastal inundation. Tsunamis, by contrast, result from sudden displacement of water, commonly by submarine earthquakes; calling them tidal waves is scientifically misleading.

5. Applications, energy and exam relevance

Ports use tide predictions to manage under-keel clearance, ship movements and dredging. Hydrographic charts commonly express depths relative to a chart datum, often a low-water reference. Survey of India maintains tidal observations and publishes predictions for Indian ports. Safe decisions must also account for weather-driven departures from predicted water levels.

Tidal-range power uses the water-level difference across a barrage or lagoon, whereas tidal-stream turbines extract energy from fast tidal currents. Both are predictable, but generation remains cyclic rather than continuous. Barrages may alter sediment transport, fish migration, salinity and intertidal habitats; stream projects also require ecological and navigation assessments.

India recognised ocean energy, including tidal energy, as renewable energy in 2019. However, a large theoretical resource does not imply equally large commercially viable capacity. For examination questions, connect astronomical forcing with local basin response and distinguish tidal range from tidal-current speed, storm surge height and mean sea-level rise.

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