Visitors to the Outer Hebrides Islands off the west coast of Scotland might be surprised to find palmetto trees, transplanted from warmer climes, surviving at 58 ° latitude. Across the Atlantic, along the coast of Labrador, icebergs drift southward to waters south of 50°. This environmental disparity illustrates that latitude and altitude are not the only determining factors of local climate. The ocean and its currents play an important role in the weather and climate of coastal and inland regions.
In 1770 Benjamin Franklin drew a map illustrating the course of the Gulf Stream, the dominant surface current of the Atlantic Ocean. This strong flow moves north along the east coast of North America from the West Indies, past Newfoundland, where it melts icebergs delivered from Labrador, and crosses the Atlantic to Scotland and Scandinavia, where its warm waters allow palmettos to survive and provide for mild temperatures along the Arctic Coast of Norway. The major currents in the Gulf Stream system are the Florida Current, the Canaries Current, the North Atlantic Drift, the North Equatorial Current, and the Norwegian Current in the Northern Hemisphere, with the Brazil Current and the Benguela Current in the Southern Hemisphere. In the Pacific Ocean, the major currents are the Kuroshio Current, the California Current, and the North Pacific Current in the Northern Hemisphere and the Humboldt Current in the Southern Hemisphere. This last current, also called the Peru Current, is named for Alexander von Humboldt, who observed South America's Pacific coast and collected data while on a five-year expedition (1799-1804).
Not all ocean currents are as stable and permanent as the major Atlantic and Pacific currents. The El Niño phenomenon, for instance, occasionally develops off the coast of South America. It is signaled by an increase in the sea temperature near the equator off the coast of South America. Low air pressure becomes entrenched in the eastern Pacific, resulting in storms that can cause widespread flooding in South America. Meanwhile, areas in the western Pacific are dominated by high pressure, which prevents the annual monsoons from reaching southern Asia and can result in severe droughts in this area. Disruptive climatic changes in other parts of the world are also associated with El Niño. This phenomenon is still little understood by scientists.
The hydrosphere is similar to its counterpart, the atmosphere, in that the same forces influence circulation within it. Winds at the ocean surface are responsible for moving the upper ocean layers. The Earth's rotation, resulting in the Coriolis force, causes a circular deflection of currents near the ocean surface; ocean currents generally move in a clockwise direction north of the equator and counterclockwise south of it. Density differences cause the vertical movement of water. Frictional forces cause drag where the flow interacts with the atmosphere, shorelines and ocean bottom.
The result of all these interactions is a large-scale circulation cell that is something like a conveyor belt that flows between the different oceans of the world. The water in surface currents is warmed in equatorial regions and then moves laterally poleward. Once at the poles, water descends as it is cooled, forming deep ocean currents of very cold, dense water. These currents creep slowly along the seafloor--the trip can take up to 1,500 years--but eventually rise again in more temperate areas of the globe.
The development and improvement of the marine chronometer and sextant in the 1700s and 1800s made it possible to accurately locate and map the ocean currents. However, at the time, the reason for their particular courses was unknown. During the 1920s, Swedish oceanographer Vagn Walfird Ekman discovered that the Coriolis force was deflecting the surface currents to the right of the actual wind direction. The upper ocean layer is referred to as the Ekman layer. Subsurface exploration, first made possible with the invention of the bathysphere in the 1930s, then later with the bathyscaphe in the 1950s, gave scientists a new understanding of ocean water transport. Today, deep-sea photography has resulted in new understanding about deep ocean currents, the mere existence of which was not widely accepted by scientists until the 1960s.
The Sun and Moon also interact with the ocean's fluid environment. Due to the Moon's proximity, the lunar tide is about 2X the solar tide. The gravitational pull from both cause complex tidal variations. The Moon orbits the Earth every 24 hours and 50 minutes, while the earth completes one rotation about every 24 hours. This means that the timing of the lunar and solar tides are usually out of sync with each other. The difference in the tidal height between high and low tide is called the tidal range. The greatest tidal range, the spring tides, occurs when the Moon and Sun are aligned on the same or opposing sides of the Earth, that is, during new and full moons. The lowest tidal range, the neap tides, occurs when the Moon and Sun are at right angles to the Earth--during the first quarter and third quarter moons. Springs and neaps occur at roughly seven-day intervals.
In most areas of the world, there are two high tides interspersed with two low tides each day. If on a daily basis, both high tides (or low tides) are of approximately the same size, this is called a semi-diurnal tide system. If on the other hand, one high tide is significantly higher than the other, this is called a mixed tide system. In some areas there is only one high tide and one low tide daily. This is known as a diurnal tide system. The shape of the coastline and the latitude determine the tidal system. The shape of the coastline also helps determine the tidal range. Tide range in the Bay of Fundy, Canada, exceeds 50 feet (15 m) and causes the St. John River in New Brunswick to reverse its flow. Likewise, vigorous currents called tidal bores occur in the passages leading into Puget Sound and the Strait of Georgia in the Pacific Northwest. European countries, experiencing a shortage of natural resources, have begun harnessing the tides of the North Sea for power generation.
One of the first persons to theorize about tides was Aristotle, who in 350 B.C. noted the rising and falling of the sea and associated them with the Moon's position. A significant contribution to tidal theory was made in 1687 by Isaac Newton, who in his Principia mathematica presented his notion of universal gravity and explained the role of the Sun and the Moon in producing ocean tides. Daniel Bernoulli increased scientific understanding with his 1738 book on hydrodynamics, as did Pierre Laplace, who introduced hydrodynamics into tidal theory in 1774. The English philosopher and scientist, William Whewell, developed tidal charts during the mid-1800s that could be used to anticipate the level and timing of tides.
Tidal waves are not associated with the tides. Their greatest similarity to tides is that their most profound effect is in narrow harbors and passages. Called tsunamis or seismic sea waves by scientists, these waves are the result of shock waves broadcast outward from earthquake epicenters and active volcanoes. They most commonly occur around the geologically active Pacific Rim.
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