Читать книгу Oceans For Dummies - Joseph Kraynak - Страница 22

Ocean water is constantly in motion thanks to a variety of forces, including the rotation of Earth; the gravitational fields of the sun, Earth, and moon; wind; and variations in the temperature and salinity of ocean water.

On a cosmic scale, the interaction of the Earth’s, moon’s, and sun’s gravitational forces cause tides that move water toward and away from coastlines once or, more often, twice daily.

At the ocean’s surface, wind, along with other forces (including Earth’s rotation and variations in water temperature and salinity) drive the formation of large rotating waters called gyres that generally spin clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. (And no, this has no effect on the direction water swirls down your bathtub drain.)

Below the surface, differences in water density drive currents. Near the poles, when water freezes at the surface, it leaves behind its salt. This colder, saltier water near the surface is denser than the warmer, fresher water below it, so it dives down, hits the seafloor, and heads toward the equator. As this cold water moves down and away from the poles, warmer surface water rushes in toward the poles to replace it, resulting in a continuous current that transports water, heat, and nutrients around the globe. This process is called thermohaline circulation, and the resulting system of currents is called the global ocean conveyor belt. It is one of the many reasons the poles are so important. They literally drive our entire ocean system, supporting marine life (and seafood) all around the globe.

See Chapter 16 for more about waves, tides, currents, and gyres and how water, heat, and nutrients are circulated around the world via ocean currents.