Oceanography

Controls on tidal range

In addition to the alignment of the Sun and the Moon, tidal range is also controlled by variation in gravitational attraction, declination of the Earth relative to the orbits of the Sun and Moon, and distribution of land. The orbit of the earth about the Sun, and the Moon and Earth about each other, are elliptical not circular. At times the Earth and Sun, and Earth and Moon are closer together,because gravity decreases as objects move apart and increases as they move together, the tidal bulges respond to changes in the orbits. When in perigee, the Sun and Earth or Moon and Earth are closesr together in their orbit the tidal bulges are higher. In contrast, when in apogee, the Earth and Sun or Moon and Earth are farthest apart in their orbit the tidal bulges are smaller. It is estimated that lunar tides at perigee have about a 20% greater range than at apogee.

Declination is the inclination of the earth’s axis relative to the orbital plane of the Earth around the Sun or of the Moon. It is a measure of the relative angular position north or south of the equator that the Sun or Moon appears in the sky. Each month the lunar declination varies from 28.5°N to 28.5°S. The more similar the declination of the Sun and Moon, the greater the overlap of the tidal bulges and the greater the tidal range.

Distribution of land greatly influences the expression of tides. If earth was an ocean-covered planet, tides would sweep westward unimpeded. All points along the same line of longitude would experience tides at the same time. Except around Antarctica and in the Arctic Sea, continents interrupt tidal flow by dividing the ocean into a series of interconnected basins. Basin shape and depth determine the period of the basin and how the body of water responds to tides. Period of a basin is the time required for a standing wave in the basin to complete one oscillation. If the period of the basin is in phase with the tidal period, the tides will reinforce the standing wave in the basin. For a large basin, such as the North Atlantic, the standing wave migrates around the basin as a rotary seiche and is called an amphidromic system. In this system, the ocean surface displays the same motion as would the water surface in a round bowl as the bowl is swirled in a circular motion.

The center of the amphidromic system is called the amphidromic point. It is a node and experiences no tide. Tidal range increases outward and lines connecting points of equal tidal range from concentric circles, called corange circles, about the amphidromic point. Lines connecting points that experience the tide at the same time are called cotidal lines and these radiate out from the amphidromic point.

In a basin with a period of about 12 hours (e.g. Atlantic Basin) semidiurnal (or semidaily) tides will develop. Each day two tides of about equal range will affect the coast. Diurnal(or daily) tides are produced in basins with a period of about 24 hours. These basins  experience only one tide a day. Basins with periods other than 12 or 24 hours will display mixed tides and have two tides of unequal range. The difference between the height of successive high tides or successive low tides within a day is the daily inequality of a mixed tide. For mixed tides, the two crests are referred to as high high-water and low high-water; the troughs are high low-water and low low-water. Mixed tides are developed in many areas, for example the Caribbean Sea and Pacific Ocean.

Distribution of land may alter the tidal range. Where land funnels the tide into a narrowing valley or long arm of the sea, tidal range is greatly increased at the distal end. In contrast, tidal range will be considerably decreased where an opening restricts the inflow of the tide.