Oceans: Structure & Motion
Layers, currents, waves & tides
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Reading: Oceans: Structure & Motion
Oceans: Structure & Motion
The ocean covers about 71% of Earth's surface. Its deepest point, the Challenger Deep in the Mariana Trench, reaches about 10,935 m, deeper than Mount Everest is tall. The ocean is not a still pool. It is in constant motion: surface currents driven by wind cross entire ocean basins, deep currents driven by density differences connect the poles to the tropics, waves transfer wind energy across thousands of kilometers, and tides rise and fall under the pull of the Moon.
Ocean Depth Zones
Surface Currents
Surface currents are driven primarily by prevailing winds. Wind friction drags the surface water into motion, and the Coriolis effect deflects currents to the right in the Northern Hemisphere and left in the Southern Hemisphere, creating large circular current systems called gyres.
There are five major gyres: North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean. The Gulf Stream carries warm water from the Gulf of Mexico northeast toward Europe at speeds up to 2.5 m/s, moderating Western European climate. Currents redistribute heat globally: warm currents carry tropical heat poleward, cold currents carry polar water toward the equator.
Deep-Ocean Currents: Thermohaline Circulation
Deep currents are driven by density differences caused by temperature and salinity. Cold, salty water is denser and sinks; warm, fresh water is less dense and stays near the surface.
In the North Atlantic, cold surface water that has become saltier through evaporation sinks near Greenland and Iceland, forming North Atlantic Deep Water, which flows southward along the ocean floor. This global thermohaline circulation: sometimes called the "global conveyor belt", connects surface and deep currents in a slow, continuous loop that takes roughly 1,000 years for a full circuit. It distributes heat and nutrients globally and plays a major role in regulating climate.
Waves & Tides
Waves form when wind transfers energy to the water surface. The water itself does not travel with the wave, individual water particles move in roughly circular orbits, returning nearly to their starting position as each wave passes. The wave transmits energy, not matter.
Wave parts: crest (highest point), trough (lowest point), wavelength (crest to crest). Waves break when they enter water shallower than about half their wavelength, because friction with the seabed slows the base while the top continues forward.
Tides are caused primarily by the gravitational pull of the Moon on Earth's ocean water, with the Sun contributing about 46% as much. Most coastlines experience two high tides and two low tides roughly every 24 hours and 50 minutes.