Ocean waters and currents

Ocean waters and currents

Ocean waters and currents depends upon lots of things as below.

Composition and Temperature 

If you've ever had a chance to swim in the ocean, you may have noticed that you float much more easily in ocean water than you do in freshwater. That’s because ocean water contains an average of 3.5% dissolved salt; in contrast, typical freshwater contains less than 0.02% salt. The dissolved ions fit between water molecules without changing the volume of the water, so adding salt to water increases the water’s density, and you float higher in a denser liquid. 

There’s so much salt in the ocean that if all the water suddenly evaporated, a 60-m-thick layer of salt would coat the ocean floor. This layer would consist of about 75% halite (NaCl) with lesser amounts of gypsum (CaSO4s(2O), anhydrite (CaSO4), and other salts. Oceanographers refer to the concentration of salt in water as salinity. Although ocean salinity averages 3.5%, measurements from around the world demonstrate that salinity varies with location, ranging from about 1.0% to about 4.1%. Salinity reflects the balance between the addition of freshwater by rivers or rain and the removal of freshwater by evaporation, for when seawater evaporates, salt stays behind; salinity also depends on water temperature, for warmer water can hold more salt in solution than can cold water. 

When the Titanic sank after striking an iceberg in the North Atlantic, most of the unlucky passengers and crew who jumped or fell into the sea died within minutes because the seawater temperature at the site of the tragedy approached freezing, and cold water removes heat from a body very rapidly. Yet swimmers can play for hours in the Caribbean, where sea-surface temperatures reach 28C (83F). Though the average global sea-surface temperature hovers around 17C, it ranges between freezing near the poles to almost 35C in restricted tropical seas. The correlation of average temperature with latitude exists because the intensity of solar radiation varies with latitude. 

Water temperature in the ocean varies markedly with depth. Waters warmed by the Sun are less dense and tend to remain at the surface. An abrupt thermocline below which water temperatures decrease sharply, reaching near freezing at the sea floor appears at a depth of about 300 m in the tropics. There is no pronounced thermocline in polar seas, since surface waters there are already so cold.

The Coriolis Effect 

Imagine that you have a huge cannon aim it due south and fire a projectile from the North Pole to a target on the equator (figure below a). If the Earth were standing still, the shot would follow a line of longitude. But the Earth isn't standing still. It rotates counter-clockwise around its “axis” (an imaginary line that passes through the planet’s centre and its geographic poles). To an observer in space, an object at the pole doesn't move at all as the Earth spins because it is sitting on the axis, but an object  at the equator moves at about 1,665 km/h (1,035 mph). Because of this difference, the target on the equator will have moved by the time the projectile reaches it. In fact, to an observer standing on the Earth and moving with it, the projectile follows a curving trajectory. The same phenomenon happens if you place the cannon on the equator and fire the projectile due north (figure below b)the projectile’s path curves because the projectile moves eastwards progressively faster than the land beneath while moving north. (The same phenomenon, of course, happens in the southern hemisphere, but in reverse.) This behaviour is called the Coriolis effect, after the French engineer who, in 1835, described its consequences. Because of the Coriolis effect, north-flowing currents in the northern hemisphere deflect to the east, and south flowing currents deflect to the west.

The Coriolis effect because the velocity of a point at the equator, in the direction of the Earth's spin, is greater than that of a point near the poles.

Currents: Rivers in the Sea 

Since first setting sail on the open ocean, people have known that the water of the ocean does not stand still, but rather flows or circulates at velocities of up to several kilometers per hour in fairly well-defined streams called currents. Oceanographic studies demonstrate that circulation in the sea occurs at two levels: surface currents affect the upper hundred meters of water, and deep currents keep the remainder of the water column in motion. 

 The major surface currents of the world’s oceans.

Surface currents occur in all the world’s oceans (figure above). They result from interaction between the sea surface and the wind as moving air molecules shear across the surface of the water, the friction between air and water drags the water along with it. The Earth’s rotation, however, generates the Coriolis effect, a phenomenon that causes surface currents in the northern hemisphere to veer toward the right and surface currents in the southern hemisphere to veer toward the left of the average wind direction.  Across the width of an ocean, the Coriolis effect causes surface currents to make a complete loop, known as a gyre. Surface water may become trapped for a long time in the centre of the gyre, where currents hardly exist, so these regions tend to accumulate floating plastics, sludge, and seaweed. The “Sargasso Sea,” named for a kind of floating seaweed, lies at the centre of the North Atlantic gyre, and the “Great Pacific Garbage Patch,” an accumulation of floating plastic and trash, lies at the centre of the North Pacific gyre. Figure above is a simplification of currents interactions of currents with coastlines create chains of eddies, in which water circulates in small loops (figure below a–c).

 The complexity of the ocean’s currents. An animation by NASA, based on data collected over a two-year period, shows the details of eddies and swirls in the ocean, and emphasizes that currents interact with the coasts.

Surface water and deeper water in the ocean exchange at a number of locations. Specifically, in downwelling zones, surface water sinks, and in upwelling zones, deeper water rises. Downwelling and upwelling occur for a number of reasons. For example, in places where winds blow surface water shoreward, an oversupply of water develops along the coast, so surface water must sink to make room. And where winds blow surface water away from the shore, a deficit of water develops along the coast, so deeper water must rise to fill the gap. Upwelling of deeper water also occurs near the equator, where winds blow steadily from east to west, because the Coriolis effect deflects surface currents to the right in the northern hemisphere and to the left in the southern hemisphere, thereby leading to the development of a water deficit along the equator. The resulting rise of cool, nutrient-rich water fosters an abundance of life in equatorial water. 

Global-scale upwelling and downwelling of ocean currents.

Contrasts in water density, caused by differences in temperature and salinity, can also drive upwelling and downwelling. We refer to the rising and sinking of water driven by such density contrasts as thermohaline circulation. During thermohaline circulation, denser (cold and/or saltier) water sinks, whereas water that is less dense (warm and/or less salty) rises. As a result, the cold water in polar regions sinks and flows back along the bottom of the ocean toward the equator. This process divides the ocean vertically into a number of distinct water masses, which mix only very slowly with one another. In the Atlantic Ocean, for example, the Antarctic Bottom Water sinks along the coast of Antarctica, and the North Atlantic Deep Water sinks in the north polar region (figure above a). The combination of surface currents and thermohaline circulation, like a conveyor belt, moves water and heat among the various ocean basins and moderates global climate (figure above b).

Figures credited to Stephen Marshak.

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