This hotter air rises up at the equator and as colder air moves in to take its place, winds begin to blow and push the ocean into waves and currents. Wind is not the only factor that affects ocean currents. The Earth is a sphere that spins on its axis in a counterclockwise direction when seen from the North Pole. The further towards one of the poles you move from the equator, the shorter the distance around the Earth. This means that objects on the equator move faster than objects further from the equator.
While wind or an ocean current moves, the Earth is spinning underneath it. As a result, an object moving north or south along the Earth will appear to move in a curve, instead of in a straight line. Wind or water that travels toward the poles from the equator is deflected to the east, while wind or water that travels toward the equator from the poles gets bent to the west.
The Coriolis Effect bends the direction of surface currents. The third major factor that determines the direction of surface currents is the shape of ocean basins Figure When a surface current collides with land, it changes the direction of the currents.
Imagine pushing the water in a bathtub towards the end of the tub. When the water reaches the edge, it has to change direction. Figure Currents are created by wind, and their directions are determined by the Coriolis effect and the shape of ocean basins. Surface currents play a large role in determining climate. These currents bring warm water from the equator to cooler parts of the ocean; they transfer heat energy. The Gulf Stream is an ocean current that transports warm water from the equator past the east coast of North America and across the Atlantic to Europe.
It is about kilometers wide and about a kilometer deep. If the Gulf Stream were severely disrupted, temperatures would plunge in Europe. Surface currents occur close to the surface of the ocean and mostly affect the photic zone.
Deep within the ocean, equally important currents exist that are called deep currents. These currents are not created by wind, but instead by differences in density of masses of water.
The onset and offset of the oscillation are still not fully understood. Deep ocean circulation is primarily driven by density differences.
It is called thermohaline circulation , because density differences are due to temperature and salinity. However, the water masses moving around by thermohaline circulation are huge. Density gradients alone are not sufficient for sustaining the deep ocean circulation. Upwelling and mixing processes, to bring deep ocean water back to the surface, are required too [8].
The density of surface water increases when frigid air blows during winter across the ocean at high latitudes. The water density increases further by evaporation and by salt expulsion when sea ice is formed. From these regions, a cold deep water layer spreads over the entire ocean basins. The thermohaline circulation moves water masses around between the different ocean basins [9] [10].
The conveyor belt is fed in the northern North Atlantic with high-salinity water due to evaporation supplied by the Gulf Stream , which sinks to great depth after cooling down in the Arctic region, forming the North Atlantic Deep Water NADW. The replacement of this dense sinking water generates a continuous surface flow feeding the conveyor belt.
This current compensates for the net northward surface flow in the Atlantic Ocean. The cold dense water from the Antarctic zone fills the deep water layer in these oceans and then gradually rises and mixes with the surface waters of the Indian and Pacific oceans. The mixing of deep ocean water is promoted by strong surface winds, by tides, by upwelling and by abyssal circulation [12] [8]. The circulation is finally completed by a warm surface return current to the Atlantic Ocean that passes south of Africa and America, see figure 5.
The whole trip takes more than 1, years to complete. To maintain the large-scale thermohaline circulation of the ocean, it has been estimated that about 2. It has long been recognized that winds and tides are two important sources of mechanical energy to drive the ocean interior mixing.
Although most of the tidal energy from Moon and Sun on the global ocean is dissipated in the shallow seas, perhaps 1. The breaking of internal waves is believed to be a principal contributor to pelagic turbulence.
They are thought to play an important role in diapycnal mixing to sustain the global system of thermohaline circulation. However, recent calculations on the basis of observations suggest that the wind power is only 0. In recent decades, biogenic mixing is thought to be another significant contributor to ocean mixing Katija and Dabiri, [17].
From small zooplankton to large mammals, swimming animals are capable of carrying bottom water with them as they migrate upward, and that movement indeed creates an inversion that results in ocean mixing.
The global power input from this process is estimated in the order of a TW of energy, comparable with levels caused by winds and tides. After all, each day, billions of tiny krill and some jellyfish migrate hundreds of meters from the deep ocean toward the surface where they feed.
The deep ocean is a huge storehouse of heat, carbon, oxygen and nutrients. Deep ocean circulation regulates uptake, distribution and release of these elements. The low overturning rate stabilizes our global climate.
By carrying oxygen into the deeper layers it supports the largest habitat on earth. Present theories for explaining the deep ocean circulation predict that global warming will have a negative impact on the deep ocean circulation.
Most studies have focused on the northern Atlantic [18]. The formation of dense sinking surface water in the Arctic region will be counteracted by a higher atmospheric temperature and by release of fresh water by ice melting.
The feeding of the Atlantic Meridional Overturning Circulation , which drives warm Gulf Stream waters to the north, will thus be reduced. Thus a density of 1. T-S diagrams can be used to identify water masses. Since each major water mass has its own characteristic range of temperatures and salinities, a deep water sample that falls into that range can presumably have come from that water mass. Figure 9. To investigate water masses, oceanographers can take a series of temperature and salinity measurements over a range of depths at a particular location.
If the water column was highly stratified and there was no mixing between or within the layers, as the probe was lowered your would get a series of constant temperature and salinity readings as you moved through the first water mass, followed by a sudden jump to another set of different but constant readings as you moved through the next water mass.
Plotting temperature vs. However, in reality, the water masses will show some mixing within and between layers. So as the probes are lowered, they will encounter water that shows traits intermediate between the two points. Therefore, with increasing depth, the points on the T-S diagram will gradually move from one point to the other, creating a line connecting the two points, illustrating mixing between those two water masses. AABW is the deepest water mass, at depths of about m.
Notice that as the recordings get deeper in Figure 9. This is because the densest water should be located at the bottom, with the other layers stratified according to their density, otherwise the water column would be unstable. The bottom water from the Weddell Sea and Greenland Sea does not just circulate through the Atlantic. Together these water masses move eastwards into the Indian and Pacific Oceans. The deep Common Water moves northwards into the Pacific and Indian Oceans and gradually mixes with the warmer water, causing it to eventually rise to the surface.
As surface water, it makes its way back to the North Atlantic through the surface currents of the Pacific and Indian Oceans. For one, it is vital to the transport of heat around the globe, bringing warm water towards the poles, and cold water to the tropics, stabilizing temperature in both environments. The conveyor belt also helps deliver oxygen to deep water habitats. The deep water began as cold surface water that was saturated with oxygen, and when it sank it brought that oxygen to depth.
Thermohaline circulation carries this oxygen-rich deep water throughout the oceans, where the oxygen will be used by deep water organisms. Bottom water in the Atlantic is relatively high in oxygen, as it still retains much of its original oxygen content, but as it travels over the seafloor the oxygen is used up, so that deep water in the Pacific Ocean has much less oxygen than deep Atlantic water, with Indian Ocean water somewhere in between.
At the same time, deep water will accumulate nutrients as organic matter sinks and decomposes.
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