Tides

Any movement of ocean water caused by tidal action is known as a tidal current. In open water, tidal currents are relatively weak and tend to change direction slowly and regularly throughout the day. They form a kind of rotary pattern that sweeps around the ocean like the minute hand on a clock. Closer to land, however, tidal currents tend to change direction rather quickly, flowing toward land during high tide and away from land during low tide. In many cases, this onshore and offshore tidal current flows up the mouth of a river or some other narrow opening. The tidal current may then attain velocities as great as 9 mi (15 km) an hour with crests as high as 10 ft (3 m) or more.

All coastal locations (as well as very large lakes) experience some variation in tidal range during each lunar cycle, due to the affects of neap versus spring tides. Most tides attain less than 10 ft in size; 3–10 ft (1–3 m) is common. In some places, however, the tides may be much greater. These locations are characterized by ocean bottoms that act as funnels through which ocean waters rush upward towards or downward away from the shore. In the Bay of Fundy, Canada, for the tidal range may be as great as 46 ft (14 m). At the opposite extreme, the Mediterranean, Baltic, and Caribbean Seas have tides of less than 1 ft (0.3 m).

Deep-ocean tidal currents also occur. In fact, it has recently been discovered that about half of the energy put by Earth-Moon-Sun system into dragging tides around the Earth is dissipated in deep-ocean currents, and the rest in shallow-ocean currents. Some 3 × 10¹² watts of energy are dissipated through friction in deep-ocean circulation alone, with profound long-term effects on Earth's climate.