10.5: Tides in the Open Ocean

Having examined the many factors that control tides, we can now look at some characteristics of real tides in the ocean basins. Much of what we know about the behavior of the tides is based on tidal-height measurements made along the coasts and on islands. The details of tidal movements in the deep oceans generally are derived from these measurements and dynamic-tide theory, although a number of deep-sea tide gauges have been deployed to make direct observations during the past several decades. Satellites, moored current meter arrays, and AUVs now also provide data that is helping to steadily refine our understanding of deep-ocean tidal motions.

Figure 10-14 shows the progression and height of the principal lunar semidiurnal component of Atlantic Ocean tides. This component is only one of the many different partial tides that must be added to determine tidal height at any time and location.

Figure 10-14. The principal lunar tidal component wave enters the Atlantic Ocean from the Indian Ocean around Antarctica and travels north into the North Atlantic, where it forms an amphidromic wave. Red lines show the location of the tidal maximum. Dashed blue lines are contours of tidal range. Note the higher tidal range on the west side of the South Atlantic compared to the east side. Also note that the tidal range increases in all directions from the center of the amphidromic system.

The most striking feature of Figure 10-14 is that the tide wave does not move from east to west with the moon, as equilibrium tide theory predicts. This is because the Atlantic Ocean is a relatively narrow basin in which only a small tide wave can be generated during a single pass of the moon. The small east–west tide wave soon encounters the American continents, where it is partially reflected and much of its energy is dissipated.

The only segment of the Earth where the tide wave can travel east to west around the world without encountering a landmass is near Antarctica. At this high latitude, the orbital velocity of the Earth’s surface is low enough (Fig. 10-11) that the shallow-water tide wave can travel fast enough to keep up with the moon’s orbital movement. The tide wave around Antarctica is therefore well developed (Fig. 10-13). It enters the Atlantic Ocean between the tip of South Africa and Antarctica and is partially deflected and dispersed into the South Atlantic Ocean. The tide wave moves northward through the South Atlantic Ocean as a progressive wave. As it travels north, it interacts with the weaker east–west wave formed in the Atlantic and is reflected and refracted in complex ways. It is also deflected by the Coriolis effect. Although the deflection is obscured by other factors in the southern part of the South Atlantic, it causes tides to be slightly higher on the South American coast north of Rio de Janeiro than on the opposite African coast (deflection to the left in the Southern Hemisphere). The Coriolis deflection is also partially responsible for tides being slightly higher on the coasts of Europe and North Africa than on the North American coast (deflection to the right in the Northern Hemisphere).

In the North Atlantic, the progressive wave traveling north from Antarctica is converted into the standing wave of an amphidromic system (Fig. 10-14). The high tide moves around the North Atlantic basin counterclockwise and arrives back where the next crest of the Antarctic progressive wave arrives almost exactly 12 hr and 24½ min later. The North Atlantic Ocean basin is therefore well tuned to the semidiurnal tidal component, and this component dominates and produces semidiurnal tides in this region (Fig. 10-7).

Amphidromic systems similar to the system in the North Atlantic are also present in the North Sea and the English Channel. However, the Gulf of America (Golfo de México) has a natural period of about 24 h. Therefore, the semidiurnal tidal component in the Gulf is poorly developed, and the diurnal component is stronger, so diurnal tides dominate in some parts of the Gulf (Fig. 10-7). The Pacific Ocean basin is wider than the Atlantic Ocean basin. Therefore, the Pacific has a more developed east–west tide wave and greater complexity than the Atlantic Ocean. In both the Pacific Ocean and the Caribbean Sea, diurnal and semidiurnal tide waves are relatively well tuned, and tides are generally mixed tides.