Atlantic Ocean | pdl-inc.info
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This is called the hydrostatic equation, which is a good approximation for the equation of motion for forces acting along the vertical.
Horizontal differences in density due to variations of temperature and salinity measured along a specific depth cause the hydrostatic pressure to vary along a horizontal plane or geopotential surface, a surface perpendicular to the direction of the gravity acceleration. Horizontal gradients of pressure, though much smaller than vertical changes in pressure, give rise to ocean currents.
In a homogeneous ocean, which would have a constant potential density, horizontal pressure differences are possible only if the sea surface is tilted. In this case, surfaces of equal pressure, called isobaric surfaces, are tilted in the deeper layers by the same amount as the sea surface. This is referred to as the barotropic field of mass.
The unchanged pressure gradient gives rise to a current speed independent of depth. The oceans of the world, however, are not homogeneous. Horizontal variations in temperature and salinity cause the horizontal pressure gradient to vary with depth.
This is the baroclinic field of mass, which leads to currents that vary with depth. The horizontal pressure gradient in the ocean is a combination of these two mass fields. Since the absolute value of pressure is not measured at all depths in the ocean, the sea surface slope is presented relative to that of a deep isobaric surface; it is assumed that the deep isobaric surface is level.
Since the wind-driven circulation attenuates with increasing depth, an associated decrease of isobaric tilt with increasing depth is expected. Representation of the sea surface relief relative to a deep reference surface is a good representation of the absolute shape of the sea surface.
The total relief of the sea surface amounts to about 2 metres about 6. This pressure head drives the surface circulation.
To an observer in space, a moving body would continue to move in a straight line unless the motion were acted upon by some other force. To an Earth-bound observer, however, this motion cannot be along a straight line because the reference frame is the rotating Earth. An apparent deflection of the path of the moving object would be seen. If the turntable rotated counterclockwise, the apparent deflection would be to the right of the direction of the moving object, relative to the observer fixed on the turntable.
This remarkable effect is evident in the behaviour of ocean currents. It is called the Coriolis force, named after Gustave-Gaspard Coriolisa 19th-century French engineer and mathematician. For Earth, horizontal deflections due to the rotational induced Coriolis force act on particles moving in any horizontal direction. There also are apparent vertical forces, but these are of minor importance to ocean currents.
Because Earth rotates from west to east about its axis, an observer in the Northern Hemisphere would notice a deflection of a moving body toward the right. In the Southern Hemisphere, this deflection would be toward the left. It can be shown that the Coriolis force always acts perpendicular to motion.
Frictional forces Movement of water through the oceans is slowed by friction, with surrounding fluid moving at a different velocity. A faster-moving fluid layer tends to drag along a slower-moving layer, and a slower-moving layer will tend to reduce the speed of a faster-moving layer.
This momentum transfer between the layers is referred to as frictional forces. The wind blowing over the sea surface transfers momentum to the water.
This frictional force at the sea surface i. Currents moving along the ocean floor and the sides of the ocean also are subject to the influence of boundary-layer friction.
The motionless ocean floor removes momentum from the circulation of the ocean waters. Geostrophic currents For most of the ocean volume away from the boundary layers, which have a characteristic thickness of metres about feetfrictional forces are of minor importance, and the equation of motion for horizontal forces can be expressed as a simple balance of horizontal pressure gradient and Coriolis force. This is called geostrophic balance. On a nonrotating Earth, water would be accelerated by a horizontal pressure gradient and would flow from high to low pressure.
Cape Point is Where Two Oceans Meet: Cape Town South Africa
On the rotating Earth, however, the Coriolis force deflects the motion, and the acceleration ceases only when the speed, U, of the current is just fast enough to produce a Coriolis force that can exactly balance the horizontal pressure-gradient force. From this balance it follows that the current direction must be perpendicular to the pressure gradient because the Coriolis force always acts perpendicular to the motion.
In the Northern Hemisphere this direction is such that the high pressure is to the right when looking in current direction, while in the Southern Hemisphere it is to the left. This type of current is called a geostrophic current. The simple equation given above provides the basis for an indirect method of computing ocean currents. The relief of the sea surface also defines the streamlines paths of the geostrophic current at the surface relative to the deep reference level.
The hills represent high pressure, and the valleys stand for low pressure. Clockwise rotation in the Northern Hemisphere with higher pressure in the centre of rotation is called anticyclonic motion. Counterclockwise rotation with lower pressure in its centre is cyclonic motion. In the Southern Hemisphere the sense of rotation is the opposite, because the effect of the Coriolis force has changed its sign of deflection.
Ekman layer The wind exerts stress on the ocean surface proportional to the square of the wind speed and in the direction of the wind, setting the surface water in motion. This motion extends to a depth of about metres in what is called the Ekman layer, after the Swedish oceanographer V. Walfrid Ekmanwho in deduced these results in a theoretical model constructed to help explain observations of wind drift in the Arctic.
Within the oceanic Ekman layer the wind stress is balanced by the Coriolis force and frictional forces. This phenomenon is called Ekman transportand its effects are widely observed in the oceans.
Since the wind varies from place to place, so does the Ekman transport, forming convergence and divergence zones of surface water. A region of convergence forces surface water downward in a process called downwellingwhile a region of divergence draws water from below into the surface Ekman layer in a process known as upwelling.
Upwelling and downwelling also occur where the wind blows parallel to a coastline. The principal upwelling regions of the world are along the eastern boundary of the subtropical ocean waters, as, for example, the coastal region of Peru and northwestern Africa. Upwelling in these regions cools the surface water and brings nutrient-rich subsurface water into the sunlit layer of the ocean, resulting in a biologically productive region. Upwelling and high productivity also are found along divergence zones at the Equator and around Antarctica.
The primary downwelling regions are in the subtropical ocean waters—e. Such areas are devoid of nutrients and are poor in marine life.
The vertical movements of ocean waters into or out of the base of the Ekman layer amount to less than 1 metre about 3. Within an upwelling region, the water column below the Ekman layer is drawn upward. This process, with conservation of angular momentum on the rotating Earth, induces the water column to drift toward the poles. Conversely, downwelling forces water into the water column below the Ekman layer, inducing drift toward the Equator. An additional consequence of upwelling and downwelling for stratified waters is to create a baroclinic field of mass.
Surface water is less dense than deeper water. Ekman convergences have the effect of accumulating less dense surface water. This water floats above the surrounding water, forming a hill in sea level and driving an anticyclonic geostrophic current that extends well below the Ekman layer.
Divergences do the opposite: This induces a depression in sea level with a cyclonic geostrophic current. The ocean current pattern produced by the wind-induced Ekman transport is called the Sverdrup transportafter the Norwegian oceanographer H. Some of these canyons extend along the continental rises and farther into the abyssal plains as deep-sea channels.
This involved little guesswork because the idea of sonar is straight forward with pulses being sent from the vessel, which bounce off the ocean floor, then return to the vessel. The Laurentian Abyss is found off the eastern coast of Canada.
Path of the thermohaline circulation. Purple paths represent deep-water currents, while blue paths represent surface currents. Maximum temperatures occur north of the equator, and minimum values are found in the polar regions.
The south tides in the Atlantic Ocean are semi-diurnal; that is, two high tides occur during each 24 lunar hours.
Vasco da Gama - Wikipedia
Epidemic diseases dramatically reshaped American societies. They facilitated European conquest, encouraged Americans to convert to Christianity, shattered connections to local traditions and histories, and caused the demise of some tribes and ethnicities altogether and the reformulation of others.
But pathogens were not the only travelers on European ships across the Atlantic. Plants and animals wreaked their own havoc. Pigs, cows, sheep, goats, and horses all damaged native crops that had not previously required protection from large domestic animals. America, in return, offered new food crops to the rest of the world. Maize, tomatoes, peppers, gourds, peanuts, and beans were American crops that transformed diets worldwide.
Although American populations plummeted in the wake of contact, the diffusion of American food crops ultimately led to an increase in the world's population. And, finally, insects traveled across the Atlantic, none more destructive than diseasebearers such as the Aedes aegypti or the Anopheles mosquitoes, both of which flourished in the transformed arable lands of the tropics and among populations of newly arrived Europeans. The most precious commodities were minerals: The Spanish fleet system, which saw all the riches of America travel to Spain in a convoy of ships, flaunted this wealth for all to see.
The discovery of gold in Brazil at Minas Gerais in the s similarly tantalized people with the promise of quick riches. Other commodities, especially food crops such as sugar, rice, and grains; luxury consumables such as tobacco and chocolate; dye goods such as indigo, madder, and cochineal; naval stores ; and pelts, while less immediately lucrative, were in the long run of considerable economic and cultural value.
These commodities transformed European tastes, diets, and economies; reoriented indigenous economies; depleted environmental and human resources; and generated enormous labor demands. The vital trades that emerged contributed to new cities in America: In Europe cities grew as a direct result of the wealth and activity of Atlantic trade, as was true for Seville, Glasgow an important tobacco trading centerBristol, Liverpool, and Nantes.
Some commodities, such as sugar, created new worlds of their own. Sugar did not require the Atlantic Ocean for familiarity among Europeans, who encountered it as a luxury commodity used as a spice from their first forays to the eastern Mediterranean. But sugar's migration out of the Mediterranean and into areas of the south Atlantic well suited for its cultivation and modified to enhance the environment for production—particularly Brazil and the Caribbean—meant that the crop moved from a luxury to a staple.
Sugar, moreover, demanded laborers who could be forced to work around the clock to satisfy sugar's cycle: For other commodities, such as pelts or dyewood, Europeans initially tried to trade with indigenous people.
It is easy to overestimate the power of European traders and the appeal of their commodities. While much that Europeans offered was useful, in semisedentary societies there was a natural limit to the number of goods people wanted to transport with them from one home to another.
Moreover, recipients of trade goods altered their function: Indigenous people did not trade unthinkingly. European weaponry, for example, had limited utility in some conditions of indigenous conflict. A musket would not fire in the rain; at night, a musket flash would reveal the location of a hidden attacker. And weapons required constant maintenance. Thus indigenous people adapted European commodities for their own use.
When the barter economy no longer enabled Europeans to extract the commodities and, later, the plantation labor they required, they resorted to slavery, as was the case in Brazil. The range of commodities identified in the Americas was great, and the extraction of some commodities prompted profound environmental and social transformations.
In Peru, Indians were compelled to toil in the silver mines, a debilitating and deadly labor. In North America, the French quest for pelts altered indigenous cultures and economies. Among the Montagnais of North America, for example, women produced 65 percent of daily calories through their farming activities, and held a significant position in society because of the value of the food they produced.
The Montagnais, moreover, were matrilocal. But as hunters, men controlled access to furs, and thus controlled trade with Europeans.