If you say hello to an ocean, it will wave back. Jokes aside, oceans are vast and formidable entities. Approximately 71% of our planet’s surface area consists of ocean, which is divided by continents into five large bodies of saltwater: Arctic, Atlantic, Indian, Pacific, and Southern. In terms of oceanic properties, saltwater is denser than freshwater, comprising nearly 97% of all water on Earth. How does the global distribution and circulation of such plentiful and salty oceanwater work?
Surface Ocean Circulation
Due to blockage by continental land masses, the east-west flow of winds against the surface of the water coupled with the Earth’s rotation forces each ocean to move in a circular motion called a gyre. There are five major gyres: North Pacific, South Pacaific, North Atlantic, South Atlantic, and Indian. Essentially, the friction of the air against the water produces momentum, with wind stress and the Coriolis Effect dragging the water in many overlapping layers. The surface current flows at a 45° angle to the surface wind path, which is a phenomenon known as the Ekman Spiral. This flow is reversed 100 meters below the ocean surface, as the kinetic energy transferred downward dissipates in the form of heat as opposed to movement. The reduction of the flow’s speed creates a 90° angle called the Ekman Transport. The directions of air masses cause gyres to circulate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
In addition, the water moves in a rising and declining manner. In the equatorial region, oceanwater moves in an upwelling motion, bringing nutrient-rich and cooler water to the surface. This effect is known as divergence. When oceanwater spreads toward the poles, it accumulates and sinks in a downwelling motion, supplying the deep ocean with dissolved gases and nutrients from subtropical basins. This effect is known as convergence. Gravity pushes on these domes and depressions of oceanwater, balancing pressure gradients and creating an outward force. The motion of this force is deflected sideways by the Coriolis Effect, causing westward intensification. A circular flow occurs around the gyres, which is called a geostrophic current. This type of current produces narrow, deep, and fast western boundary currents, as well as eastern boundary currents that are cold, shallow, and broad.
Overall, surface ocean circulation helps redistribute energy latitudinally by moving warm water toward the poles and cold water toward the equator. The temperature pattern drives heating, cooling, and moisture exchanges within the atmosphere.
Deep Ocean Circulation
Differences in oceanwater density are determined by variations in temperature and salinity. As oceanwater depth increases, salinity increases. This effect is known as the halocline. The pycnocline refers to the increase in density as oceanwater depth increases. The thermocline refers to the decrease in temperature as oceanwater depth increases. The thermohaline circulation is a global oceanic flow that is driven by such differences.
Salt is transferred into seawater through river weathering, which is the breakdown or alteration of rocks by physical or chemical processes. Salt is removed from oceans through the evaporation of shallow waters, the formation of sea spray, and the chemical reaction between seawater and new volcanic rocks on the sea floor. Dense, cold, and salty waters form at high latitudes due to large differences between the rates of evaporation, precipitation, and sea ice formation. Freezing temperatures at polar regions transform surface water into ice, leaving a dense and salty layer of oceanwater beneath called Antarctic Bottom Water and North Atlantic Deep Water. The bottom water sinks and spreads toward the equator, acting as a conveyor belt. The return flow occurs due to a slow pycnocline over the entire ocean and through upwelling areas. The thermohaline circulation system is responsible for the vertical movement of oceanwater and for the global movement of all oceanwater.
However, this passage of water has a 1000 to 2000-year recycling time, which is subject to change from ice cap melting. Sea levels are slowly rising due to climate change, causing flooding, freshwater reduction, stronger tropical storms, and marine biodiversity loss. Moreover, the oceans are full of plastic and waste pollution, forming giant floating patches of garbage at the center of gyres. As I said previously, oceans are vast and formidable entities, but humanity has been treating them like enemies.
Kump, L. R., Kasting, J. F., and R. G. Crane. 2010. The Earth System. Prentice Hall. 3rd Edition.