Background Information: Oceans
Water vapor in the air eventually condenses and falls as rain, snow, sleet, or hail. Water that falls on land collects in rivers which carry it back to the ocean. The return of water to the ocean may be slowed when water becomes trapped in lakes, swamps, or openings in the rocks. However, most water returns to the ocean.
It is interesting to consider the events which led to the formation of the world's oceans although the present knowledge is not perfect. An individual ocean grows from an initial rift, reaching a maximum size, then shrinks and ultimately closes completely. The question of the origin of the ocean is really two problems: (a) from where did the water come and (b) how did it get its unique concentration of elements? Concerning the water on earth, there are three hypotheses to explain its origin:
- From the primordial atmosphere of the earth.
- From the decomposition of volcanic rock.
- From the incremental addition of water throughout geologic time.
The first hypothesis suggests that the primordial atmosphere condensed all at one time to form the ocean. If this really happened, one would expect the original components of this atmosphere to be present in the ocean in higher quantities than have been observed.
The second hypothesis suggests that when the earth consolidated, much of the original water was chemically bound into volcanic rock and subsequently has been removed by decomposition of these rocks to form the ocean. Experimental and field evidence indicates that volcanic rocks contain only 5 percent water, and even if all the water in the volcanic rocks of the earth's crust was removed, it would be less than 50 percent of the water in the ocean.
The third hypothesis for the origin of the water, that of incremental addition throughout geologic time, is generally the most accepted. This hypothesis proposes that ocean water was slowly, but not necessarily uniformly or continuously, added over geologic time. Probably, a large amount of the water was supplied relatively early in the earth's history due to chemical and physical processes associated with the early development of the earth.
It seemed that all the water and air which are now at the earth's surface were inside the earth at one time through a slow process of de-gassing from the earth's interior, about 2.5 billion years ago when the oceans and the atmosphere were formed. The volume of the earth is approximately 900 times the volume of water contained in the oceans. Hence, it might be possible that all the ocean water came from inside the earth. The origin of the ocean water is still questionable.
Distribution of Land and Water
Land and water are unevenly distributed on the earth. The continents which cover 29.2 percent of the earth's surface, break up the ocean into three basins: Atlantic, Pacific and Indian oceans.
The ocean basins cover 70.8 percent of the earth's surface. The Southern Hemisphere is dominated by ocean (80.9 percent) because of the connections of the ocean basins around Antarctica. Although the northern hemisphere contains most of the land, it is still dominated by ocean (60.7 percent).
Topography of the Ocean Floor
In the past, the ocean bottom was considered to be a smooth, plane surface where no erosion and little deposition occurred. Later oceanographers released how irregular the bottom of the ocean is. The first scientific attempt to map the ocean bottom was undertaken by the Challenger expedition which set out from England in 1872.
Volcanoes and volcanic islands are common features on the ocean floor, usually 1 km or more above the surrounding ocean floor. There are tens of thousands of volcanoes on the Pacific floor [for example]. Over millions of years, volcanoes subside and are eroded. Eventually they become submerged banks or the foundation for coral reefs.
The Pacific Ocean
The Pacific is the deepest and by far the largest ocean basin bordered by North and South America, Asia and Australia. The Pacific occupies more than one-third of the earth's surface. Most of the freshwater and sediment flowing into the Pacific comes primarily from Asia, although the diagram suggests that this addition is pitifully small.
Islands are abundant in the Pacific, especially in the southern and western regions. The smallest islands are low-lying sand islands, many of them associated with coral reefs. Most of the larger islands are of volcanic type. In the Western Pacific most of the larger islands are usually pieces of continents that were separated from Australia and Asia millions of years ago.
Early Use of the Sea
The sea played an important role in human affairs long before human history was recorded. Enumerable shells in refuse piles [called maddens] of ancient coastal villages shows that seafood was important in the villagers' diets. Some middens also contain bones of deep-sea animals, which suggests that boats may have been used in fishing well off-shore. Transportation by sea must also have played an important role in early human history, but little evidence remains of these early maritime activities. Wood, skins, reed-material, commonly used in primitive boats, are rarely preserved. The earliest models and ships known come from Egyptian tombs and Viking graves. All suggest that these peoples were highly proficient sailors.
The greatest ocean voyagers of all times were believed to be Polynesians. With primitive ships and without compasses, the Polynesians in the centuries before 1000 AD located the tiny islands spread over 14 million square kilometers of the Pacific ocean, from New Zealand to Hawaii and Easter Island, and colonized nearly all of them. The exact starting point of these migrations is not known, and is still a matter of controversy. In fact, Polynesians later constructed elaborate doublehulled vessels. The largest of these had living quarters for people and animals and were used in transoceanic voyages to colonize the islands of the Pacific. The Micronesians also colonized many of the larger islands of the western Pacific.
We know very little about the seafaring traditions of these peoples, as there are no written records. In many cases our information comes from the accounts of the first European explorers to contact them. There is ample evidence, however, that they were skillful sailors and navigators. One example of their navigational skills is the technique of stick charts used by Micronesians. Shells mark the locations of islands, and the bamboo strips show wave patterns. Stars, cloud pattern, and winds were also used by these skilled navigators to find their way between islands.
Action of Wind on Surface Water
It is easy to show that the winds and currents are intimately related. But exactly how winds drive ocean currents is not so obvious. When wind blows over the ocean, it drags on the water surface and energy is transferred from wind to the surface layers.
The greater the speed of the wind, the greater the frictional force acting on the sea surface, and the stronger the surface current. Generally speaking the current speed is typically about 2-3 percent of the wind speed. The Norwegian scientist and explorer, Fridtjof Nansen observed in the 1890s that the surface water is transported 20-40' to the right of the original direction of the wind direction (in the Northern Hemisphere) as a result of the rotation of the earth [Coriolis effect]. The opposite effect will occur in the Southern Hemisphere.
Later work by Ekman showed that the Nansen's observation applied only to the surface skin of the ocean and that the progressive deflection caused by the Coriolis effect in an ocean driven from above results in the top 100 meters or so of the water column moving at right angles to the surface wind: to the left in the Southern Hemisphere and to the right in the Northern Hemisphere.. The surface layer responds to the winds by moving much like a slab. The process has the name, Ekman Transport.
The wind system exerts a stress on the ocean's surface and produces the wind-driven circulation of the ocean. The easterly trade winds from the equatorial currents are common to all oceans. In the Atlantic and Pacific oceans these currents are intersected by land and they are deflected to the North and South. These deflected currents travel along the Western parts of the oceans and have the name, western boundary currents, which are among the largest and strongest currents in the ocean.
The western boundary currents are due in large measure to the variation of Coriolis force with latitude. These currents are driven across the ocean by the westerly wind and form currents that flow back into the equatorial region, thus completing the large gyre. Gyres of this type occur in the subtropic regions of the north and south Pacific, the north and south of the Atlantic and the South Indian Oceans. The northern and southern gyres of the oceans are separated by eastward flowing counter currents. Parts of some of these large currents may break off as loops or meanders, called Rings. These rings have almost a life of their own and may exist for several years.
Winds and Pressure Gradient
The wind-driven circulation of the ocean results from differences in water pressure. These differences result mainly from changes in the slope of the sea surface due to winds. Winds blowing over the water cause the water to move and build up in the direction that the wind is blowing. This creates a pressure difference between the high and low areas [Pressure is higher where the water is piled up]. The pressure difference generates a force that tends to push the surface back towards the region of lower pressure. In other words, the water wants to go downhill or down the slope. Because of the Coriolis force [effect of the rotating earth], however, the moving water or current is deflected to the left in the Southern Hemisphere [to the right in the Northern Hemisphere]. If the pressure difference is balanced by the Coriolis force, the current is called a Geostrophic Current. Most of the surface currents in the ocean are geostrophic but some near surface currents may be in part nongeostrophic. These are calle
A sea surface slope up towards the east results in horizontal pressure gradient force towards the west. Initially, this causes motion down the pressure gradient, but because the Coriolis force acts at right angles to the direction of motion, the equilibrium position is one in which the direction of flow is at right angles to the pressure gradient (for Southern Hemisphere).
In the ocean, due to Ekman's theory, westerlies (winds) transport water to the south in the Northern hemisphere [and to the north in the Southern Hemisphere]. At the same time, the trades (winds) transport water to the north in the Northern Hemisphere [and to the south in the Southern Hemisphere]. This results in raising of sea level in mid latitudes and geostrophic currents arise on the slope.
Wind Stress and Vertical Water Movement
Wind stress at the sea surface not only causes horizontal movement of water, it is also leads to vertical motion. To understand such vertical movements, we have to refer to the Ekman transport again. The net movement of the surface layer in the Northern Hemisphere as tested by Ekman, is directed 90' to the right of the wind. The reverse occurs for the Southern Hemisphere where many Pacific island countries exist. In order to replace the moving surface water of a coastal area, deep water will rise and the vertical water movement takes place.
Tidal Currents
Tidal currents are horizontal water movements associated with the total rise and fall of the sea surface. The relative strength of the tidal currents is generally proportional to the tidal range of the day. For instance, a spring tide is usually accompanied by stronger tidal currents than a neap tide.
In general, tidal currents are the strongest currents in coastal regions. In the open ocean, tidal currents exhibit a rotary pattern of movement. Readers are requested to refer to the more advanced text for the details of tidal currents.
Why Measure Sea Level?
There are many reasons for measuring sea level. These vary from the immediate operational requirement of ship navigation to long term predictions of global sea level change due to climate variations. Technicians, engineers and scientists may have different requirements for accuracy and availability of the measurements, but they are all concerned with the same parameter, the average level of the sea surface, after eliminating short-period waves, relative to some fixed datum.
Ocean Influence on Weather
Apart from the general influence, large masses of unusually warm or cold surface waters in a particular part of ocean influence the weather on land. Sometimes these water masses are thousands of kilometers across and 2-3 hundred meters thick. They persist for years as they are moved across the ocean by currents. The vast size of these water bodies and the heat energy they contain apparently steer major weather systems and storms as the latter move across the ocean and onto land.
It is important to remember that the temperature and salinity of surface waters in the ocean are determined by the amount of evaporation and precipitation. So as a result, sea surface temperature will vary from place to place and from time to time. The same is true of local salinity although change here takes place on a slower time scale.
For example, persistent cold surface water masses in the central Pacific cause shifts in the prevailing westerly winds blowing across the United States. Some scientists believe that the long drought in Africa may be related to the unusually warm surface water in the North Atlantic. If the causes of these relationships could be determined, predictions of certain climate changes could be made, using measurements of sea surface temperature.
Bands of equal sea surface temperature tend to be oriented east-west and symmetrical about the equator. Temperatures are highest along the equator because of the warming of the Earth in the tropics. They gradually become cooler towards the poles. In equatorial regions, water and air temperatures change little seasonally. The small changes in polar ocean water temperatures are due to the year round presence of ice.
Climate Patterns in the Oceans
The open oceans can be divided into climate regions with relatively stable boundaries which run generally east-west. Temperature and salinity of surface waters are functions of the amount of solar energy received and the amounts of evaporation and precipitation. In equatorial regions the major air movement is vertical as air rises, so that winds are weak. Surface waters are warm, but heavy precipitation keeps salinity relatively low. Sailors once referred to this region as the doldrums because their sailing ships were becalmed by the lack of winds. Meteorologists name the region, the Intertropical Convergence Zone (ITCZ) because it is the zone between the tropics where the trade winds converge.
Tropical regions are characterized by northeasterly trade winds in the northern hemisphere and Southeasterly trade winds in the southern hemisphere. These winds push the equatorial currents and create moderately rough seas. Relatively little precipitation falls at higher latitudes within tropical regions, but precipitation increases towards the equator. Hurricanes and typhoons which carry large quantities of heat into higher latitudes are initiated here as tropical storms.
The belts of high pressure are centered in the Subtropical Regions. The dry air descending on the subtropics results in little precipitation and a high rate of evaporation, producing the highest salinity's in the open ocean. Winds are weak in the open ocean as are currents. However, strong boundary currents flow north and south, particularly along the western margins of the subtropics.
The temperate regions are characterized by seasonal temperature changes and strong westerly winds blowing from the southwest in the northern hemisphere and from the northwest in the southern hemisphere. Severe storms are common, especially during winter, and precipitation is heavy.
The subpolar ocean is covered in winter by sea ice that melts away, for the most part, in summer. Icebergs, which are blocks of frozen fresh water detached from the polar ice caps, are common, and the surface temperature seldom exceeds 5 degrees Celsius in the summer months.
Surface temperatures remain at or near freezing point in polar areas, which are covered with ice throughout most of the year. In these areas, which include the Arctic ocean and the ocean adjacent to Antarctica, there is no sunlight during the winter and no night during the summer.
Brief Description on El Niño: The Christ Child
So far we have discussed how the atmosphere and ocean change annually, seasonally, and locally in general. There is yet another type of astmosphere-ocean interaction and variability that has a profound impact on the equatorial ocean and on the weather throughout much of the Southern Hemisphere.
Normally, the wind-driven Humboldt Current flowing northwards parallel to the coast of South America to its right [in the Southern Hemisphere] presents a favorable case for coastal upwelling. The cold water drawn from the ocean depths normally dominates the cold climate and also supports nutrient rich fishing grounds.
There is a current from the equatorial zone heading to the southeastern Pacific ocean bringing warm water towards the coast of South America in December or January of a typical year, especially to the normally cold coast of Ecuador and Peru. The local fishermen named this current, El Niño, because of its proximity to Christmas and the tropical fauna and exotic flora it carries along.
The special meaning of El Niño during that period is the Christ Child although the direct meaning is the baby boy. The term El Niño was initially given to the annual Christmas warm current flowing off the Peru coast. At regular intervals of as little as two or as much as seven to ten years, this otherwise benevolent current becomes very warm, and brings catastrophe since it limits or terminates the upwelling mechanism. Scientists now reserve the term El Niño for these dramatic events.
A simple definition for El Niño is that it is an unusual warming in the sea surface temperature off Peru in South America. It has been noticed that major warmings of a much larger scale tend to occur every two to seven years.
Although historical usage prompts a definition of El Niño in terms of conditions off the South American coast, these changes are connected directly to changes across the entire tropical Pacific and indirectly to changes throughout the world's atmosphere and ocean. Unusual weather conditions in various parts of the world are believed to be associated with El Niño Whether El Niño is the actual cause of all of them is still unknown. However, the El Niño events are perturbations of the ocean-atmosphere system. It is not known whether the perturbations originate in the atmosphere or the ocean, since one set leads to the other and vice versa.
One El Niño evolves very differently from another. However, they usually last less than 2 years and have a relatively consistent pattern of sea surface temperature. Normally, the waters in the western Pacific are quite warm and those in the eastern Pacific are very cold. However, during the initial stages of an El Niño, the sea surface begins to warm in the eastern Pacific off the west coast of South America. This generally happens during March to May of the first year of the El Niño.
The central Pacific begins to warm up significantly as the warm water moves westward along the equator. The mature stage of El Niño occurs from December through February of the second year of the El Niño This stage is characterized by a substantial warming in the central Pacific and a slight cooling off South America.
Generally, during an El Niño period, the central Pacific experiences unusually high rainfall. Correspondingly, the western Pacific undergoes drought conditions. The devastating tropical cyclones that hit Pacific islands in the last decade occurred most in the El Niño years.
As the El Niño conditions subside, a significant cooling of the sea surface occurs in the eastern Pacific and lasts about one year. This cooling is referred to as a La Niña (baby girl) and its effects on the atmosphere is opposite to that of El Niño.
Conclusions
Due to the remarkable properties of ocean water, the climate everywhere on earth is extremely mild compared to that of our neighboring planets. Both the ocean and atmosphere are several thousands times wider than their thickness. It makes their dimensions like those of a thin sheet of paper. Like two neighboring pages in a closed book, these two fluid layers are intimately connected and their behaviors are closely related.
One consequence of the interaction of ocean and atmosphere is that each drives the other. Wind and weather are driven by the heat that the atmosphere receives from the earth (mostly the ocean) beneath it. Some of this energy is returned to the ocean in the form of wind-driven currents. The wind and weather also produce the denser water that sinks and flows through the dark deep regions of the ocean. Because the ocean is so massive, however, its motions are much steadier and more predictable than those of the atmosphere.
Some Useful Tidal Terminology
Sea Level: A measurable quantity, the results of all influences that affect the height of the sea surface (moon, sun, atmospheric pressure, winds, thermal effect, vertical land movement, seismic activity, oceanographic effect such as El Niño, etc.), above a defined datum (reference level).
Residual: Equal to (observed sea level - tide). The residual is that part of the observed change in the sea level which is not due to tides. It is due to meteorological or seismic activity, generated locally or perhaps many thousands of kilometers away (e.g., tsunamis).
Tidal Range: The difference in elevation between the highest and lowest tide levels.
Spring Tides: Maximum tidal range which is larger than the average tidal range, usually occurs near times of full and new moons, especially where tides are predominantly semi-diurnal in character.
Neap Tides: Minimal tidal range which takes place during the first and three quarters of the moons, in the case of semi-diumal tides.
Diurnal Tides: Daily tides, one high and one low waters in a day.
Semi-Diurnal Tides: Half-daily tides, two high and two low waters in a day.
Tidal Period: 24 hour 50 minute or 12 hour 25 minute for the Lunar tide.
Flood Tide: Usually refers to currents on the rising tides.
Ebb Tide: Usually refers to currents on the falling tides.
