The periodic rise and fall of sea level, known as the tide, is a complex phenomenon driven primarily by the gravitational forces of the Moon and the Sun. These celestial influences create predictable, repeating patterns in the ocean, with Earth’s rotation determining how frequently a location passes through the resulting tidal bulges. The specific shape of coastlines and ocean basins further modifies these forces, leading to three distinct global tidal patterns. Understanding these patterns is important for navigation, coastal planning, and studying marine ecosystems.
Characteristics of the Diurnal Tidal Pattern
The diurnal tidal pattern is characterized by a single complete cycle within a lunar day, featuring precisely one high tide and one low tide. The lunar day lasts approximately 24 hours and 50 minutes, which is the time it takes for a point on Earth to rotate once beneath the Moon. Consequently, a location with a diurnal pattern experiences a full tidal cycle only once every 24 hours and 50 minutes.
This single-cycle pattern results from a combination of celestial mechanics and local geography. A primary factor is the Moon’s declination, its angular distance north or south of the Earth’s equator. When the Moon is positioned far from the equator, the two main tidal bulges are significantly shifted away from the equator. As Earth rotates, a location may only pass through one of these bulges, experiencing only one high tide and one low tide.
A diurnal pattern is also common in semi-enclosed bodies of water, such as the Gulf of Mexico. In these areas, the restricted basin size and shallow depth filter out the semidiurnal components of the tidal forces. This geographic restriction prevents the full development of the two-tide cycles seen in open ocean areas.
Variations in Global Tidal Patterns
The diurnal pattern is distinct from the two other main types of tidal cycles found globally, the semidiurnal and the mixed semidiurnal patterns. The semidiurnal pattern is the most common and features two high tides and two low tides per lunar day. In this pattern, the two high tides are of nearly equal height, and the two low tides are also of nearly equal height, resulting in a symmetrical, twice-daily cycle. This symmetrical pattern is typical along the eastern coast of North America and in much of the Atlantic Ocean. The cycle length between successive high tides is roughly 12 hours and 25 minutes.
The third pattern, the mixed semidiurnal tide, also has two high tides and two low tides within a lunar day. However, it is defined by a significant difference in height between the two successive high tides and/or the two low tides. This inequality means that one high tide is noticeably higher than the other, and similarly for the low tides. This mixed pattern is often observed along the Pacific coast of North America. The varying dominance of the different tidal forces, particularly the Moon’s declination and the influence of ocean basin shape, causes this complex, unequal pattern.