Tides are a complex phenomenon driven primarily by the gravitational forces of the Moon and, to a lesser extent, the Sun. This celestial pull generates massive bulges of water on opposite sides of the Earth, which the planet rotates through daily. While the fundamental mechanism is universal, the resulting tidal pattern observed at any given coastline varies significantly around the globe. Mixed tides represent one of the three primary patterns, characterized by a unique daily fluctuation.
Defining the Three Global Tidal Cycles
Oceanographers categorize the world’s tidal patterns into three main types based on the number of high and low tides that occur within a lunar day, which lasts approximately 24 hours and 50 minutes.
Diurnal Tides
The simplest of these are diurnal tides, which feature only a single high tide and one low tide during this period. These are typically found in localized areas, such as parts of the Gulf of Mexico and the Java Sea.
Semidiurnal Tides
Semidiurnal tides are the most common pattern globally, characterized by two high tides and two low tides of nearly equal height within the lunar day. This symmetrical pattern is typical along the Atlantic coast of North America.
Mixed Tides
Mixed tides, or mixed semidiurnal tides, exhibit two high tides and two low tides each lunar day. However, the successive high tides, and often the successive low tides, exhibit significant differences in height.
Key Features of Mixed Semidiurnal Tides
The defining characteristic of a mixed tidal pattern is the presence of “diurnal inequality,” which refers to the measurable difference in height between the two daily high waters or two daily low waters.
To accurately describe the water level during a mixed tide, specific terminology is used to differentiate the four daily extremes. The two daily high waters are designated as the Higher High Water (HHW) and the Lower High Water (LHW). Similarly, the two daily low waters are labeled the Higher Low Water (HLW) and the Lower Low Water (LLW).
The degree of diurnal inequality changes over the course of a month. Sometimes the inequality becomes so small that the pattern temporarily resembles a pure semidiurnal tide. At other times, the inequality can become so great that the pattern briefly approaches a diurnal cycle. This continuous fluctuation is why the pattern is so dynamic and requires detailed prediction tables.
Astronomical and Local Factors Influencing Mixed Tides
The primary astronomical driver behind the diurnal inequality and the resulting mixed tide is the Moon’s declination, which is its angular distance north or south of the Earth’s equator. The Moon’s orbit is tilted relative to the Earth’s equator, causing its position to shift over the course of its monthly orbit.
When the Moon is directly above the equator, its two associated tidal bulges are centered on the equator, and a location rotating through them experiences two nearly equal high tides. As the Moon’s declination increases and it moves away from the equator, the tidal bulges also shift north and south. This shift causes an observer at a mid-latitude location to pass through one bulge more directly than the other, resulting in the two unequal high waters that define the mixed tide.
While the Moon’s declination is the fundamental cause, local geographical features determine where this pattern is strongly observed. The shape of ocean basins, the depth of the water, and the configuration of the coastline all modify the celestial forces. For instance, the Pacific coast of North America and many Pacific islands are well-known regions that experience pronounced mixed tides.