As the current flows generally southwards a number of temporary and fluctuating eddies form which give rise to erratic northerly flows - particularly closer to the coast. The largest of these eddies is about 450 Km in diameter and usually centred about off the coast in the Newcastle / Sydney region. Other smaller ones also form - either cyclonic or anti-cyclonic. The eddies are thought to be caused by undersea ridges and trenches and when flowing with the main current can have surface flows up to five knots. Such speeds were observed by the Endeavour in 1777.
The main body of water generally flows along the 200 metre line of the continental shelf which varies between 15 and 35 Km off shore. However the current has been found as close as 3-5 Km and then a day later 25 Km away and moving further out. Buoys tracked by satellite show the eddies to be anticlockwise and sometimes remain stationary for weeks on end. The huge volume of water in the current is considerably warmer than the average water temperature and therefore has a pronounced affect on our weather - especially when onshore winds blow during summer.
The general southerly set can be best avoided by keeping as close as possible to the coast where significant northerly currents on the western side of the eddies will often be found. Boats have been known to pick up 3 knot north easterly currents in Stockton Bight. Therefore for northerly passages it is generally best to hug the coast. Such a plan must take into account the relatively high density of fish-trap buoys - often joined by rope in groups of 4-6, close to the coast. The problem is worse at night and if the engine is used for propulsion a very good lookout must be kept for the menace of buoys tied together. When proceeding south the stage length will be an important factor in deciding whether it is worthwhile proceeding the necessary 15-25 Km or so out to sea to avoid a possible northerly current flow.
Fishermen can benefit from a knowledge of this current which brings the large fighting fish down from the tropics. The whereabouts of the current can be established by taking sea water temperatures
Finally a knowledge of the eddies and their movement is important for marine search and rescue operations. Being able to plot the future or past movement of a vessel in a current or eddy can be vital in a rescue.
The central axes of the main ocean basins are connected by the midocean ridge system, extensive mountain chains with inner troughs that are intersected by fracture zones. The midocean ridges are fundamental to understanding of the evolution of the ocean basins, are explained by Plate Tectonics. They are associated with earthquakes, volcanoes, and with hydrothermal vents that transfer hot chemically-rich fluid from the Earth's interior and are associated with unusual sulphide-dependent biological communities. From the midocean ridges, molten rock upwells and spreads internally, adding new material to the Earth's rigid crustal plates. The plates are moving apart, at a few centimetres per year. In areas where the plates overlap, such as the rim of the Pacific ocean, crust is subducted and returned to the mantle, forming trenches which reach depths exceeding 7 km (4.4mi). The deepest known depth, in the Mariana Trench east of the Philippines, is nearly 11 km (6.8 mi).
It is useful to distinguish between the shallow continental shelves and the deep ocean, but it must be kept in mind that even the deepest trenches are shallow relative to the diameter of the Earth: the depth-to-width ratio is about 1 to 1,000. The ocean, like the atmosphere, consists of a shallow layer of fluid held on the rotating Earth by the force of gravity.
The greatest depths of the ocean are to be found in the ocean trenches. The deepest known (and the deepest point on Earth) is 11,034 m (36,191 ft) deep, in the Challenger Deep of the Mariana Trench-far deeper than Mount Everest (8,848 m/29,028 ft) is high. The Challenger Deep was the site of a pioneering dive in a bathyscaphe (manned submersible) by Jacques Piccard in 1960. The water in the trenches is cold: its temperature being typically between 0° C and 2° C, but increasing slowly with depth because of the increasing pressure. Although sometimes poor in oxygen, the trenches constitute the deepest habitat for marine life (hadal fauna) including sea-cucumbers, anemones, polycheate worms, and some molluscs and crustacea. After the Mariana, the world's deepest ocean trenches are the Tonga (10,800 m/35,424 ft); the Philippine Trench (10,500 m/34,440 ft); and the Kurile-Kamchatka (10,500 m/34,440 ft), all in the Pacific. The longest are the Peru-Chile (5,900 km/3,658 mi); the Java (4,500 km/2,790 mi); and the Aleutian (3,700 km/2,294 mi).
Some trenches, like those off western South America, are found seaward of a continental platform. Others, like the Mariana, which is 402 km (250 mi) south-west of Guam in the north-western Pacific, are associated with an island arc. Although trenches and island arcs cover only about 1 per cent of the Earth's surface, they, like ocean ridges, are fundamental to the understanding of plate tectonics. As ocean ridges are formed when two crustal plates separate, so ocean trenches form when two crustal plates come together, one being forced below the other. When an oceanic plate meets a continental plate, the oceanic plate (being more dense) subsides or subducts beneath the continent-forming a trench at sea and creating a range of volcanic mountains on land.
The Andes are thought to have been created about 80 million years ago by subduction along the coast of Chile; the accompanying Peru-Chile Trench has a maximum depth of 7,635 m (25,042 ft). Other examples are found off Indonesia, Kamchatka, and Japan. When two oceanic plates meet, the subducted plate melts back into the mantle and this results in a trench and a roughly parallel arc of volcanic islands. The Mariana Trench is thought to have been formed in this way and is associated with an arc of volcanic islands, including Guam. Other examples of island arcs include the Aleutian, Izu Bonin, Tonga-Kermadec, the Lesser Antilles, and South Sandwich. The mechanism invoked in the formation of trenches and their associated island arcs is not fully understood but is important in our understanding of the structure of the Earth. Volcanism in the island arc system produces rocks like those that make up the continents-therefore continental evolution is related to that of trenches
Typically, the period of a long swell wave is 20 seconds, its length 624 m (2,040 ft), and its speed, 31.2 m/sec (102.3 ft/sec); a swell wave has a period of 10 seconds, a length of 156 m (510 ft), and a speed of 15.6 m/sec (51 ft/sec); and a wind sea wave has a period of 7 seconds, a length of 76 m (246 ft), and a speed of 10.9 m/sec (35.7 ft/sec). Waves in bays have rather shorter periods of about 3 seconds, and are 14 m (45 ft) long, with a speed of 4.7 m/sec (15.4 ft/sec); and ripples on ponds have periods of 0.5 seconds, and are 0.4 m (1.3 ft) long, with a speed of 0.8 m/sec (2.6 ft/sec).
The wave height of the dominant waves (the vertical distance between a trough and the next crest) is more difficult to understand. It is controlled mainly by the strength of the wind, the time for which it has been blowing, and the fetch, which is the length of water over which the wind has blown. In the open sea the fetch is often long, and the strong winds of a depression blowing for a long time can generate long, high wind waves. These travel in various directions (mainly in the same direction as the wind) until they leave the area of strong winds (the generating area) and thereafter travel as swell, which is the term used for a wave that is no longer under the influence of the wind.
As the swell waves travel away from the generating area, the energy of the waves with the largest wavelength and longest period travels fastest (at half the wave speed), and therefore arrives first at a distant coastal wave-measuring station. By monitoring the time at which the slower swell waves arrive, the distance to the generating area can be estimated. It is found that swell waves can be detected at large distances from where they were generated. Some have been found to travel almost half-way round the world.
Conditions in the generating area are more difficult, because of the effects of the wind and of waves breaking to form whitecaps as a result of air bubbles in the water. Useful results can be obtained by treating the ever-changing surface as the sum of a large number of non-interacting, simple waves. Methods are available for sorting out a complicated pattern into its component parts, by assessing the energy (and sometimes the direction of travel) associated with simple waves of a given period. A diagram of wave energy against the period or, more commonly, against the reciprocal of the period (1/period), the frequency, is known as a wave spectrum.
The development of the wave spectrum in relation to wind speed and fetch is now reasonably well understood; given good meteorological wind forecasts, computer models can produce useful forecasts of wave conditions. Their output can be checked against visual observations by mariners, against instruments on some research ships and on buoys, and against wave heights obtained using a radar altimeter in an orbiting satellite.
Waves get their energy from the wind: some of it is dissipated in whitecaps and turbulence, while some travels to the coast to be dissipated in breakers and in friction on the shore. To convert some of this wave energy to electricity is an attractive possibility and several types of mechanical devices have been designed to make use of wave power. However, as the engineering problems are considerable, none is yet in routine operation.
When the swell from wind waves has a wavelength less than the water depth, it is known as a short wave. There are many other classes of wave on and in (and at the boundaries) of the oceans, notably the long waves, which are those whose wavelength is greater than the water depth. The speed of travel of long waves does not depend on their wavelength but only on the water depth. Long waves are found on many scales from the oscillations of sea-level scales in harbours and bays to the extremes of the oceanwide tides. An important example is a tsunami.
SOME WAVES FEARED !
SOME WAVES REVERED !
In the deep ocean, the wave is virtually imperceptible, usually less than 1 metre (3 feet) high. Upon entering shallow coastal waters, however, a tsunami is forced to slow down and suddenly grows in height. By the time it reaches the shore, it may be a towering wall of water 15 metres (50 feet) high or more, capable of destroying entire coastal settlements.
On 17 February 1996 an earthquake measuring 7.0 on the Richter scale occurred off the coast of Indonesia's Biak Island, north of New Guinea, causing 6-metre (21-foot) waves to wash over the coast. The death toll reached 102, and 50 people were reported missing. More than 3,000 homes on Biak and the surrounding islands were washed away by the tsunami. Scientists can predict when and where tsunamis will occur by determining the focus and strength of the precipitating earthquake. Residents in low-lying coastal areas may have time to move to higher ground after warning of a distant underwater earthquake, but they may be caught off guard by a nearby offshore quake.
On the evening of June 15, 1896, the north-east coast of Hondo, the main island of Japan, was struck by tsunami, born from an earthquake, which was more destructive of life and property than any earthquake convulsion of this century in that empire. The whole coastline of the San-Riku, the three provinces of Rikuren, Rikuchu, and Rikuoka, from the island of Kinkwazan, 38° 20' north, northward for 175 miles, was laid waste by a great wave moving from the east and south, that varied in recorded height from 10 to 50 feet. A few survivors, who saw it advancing in the darkness, report its height as 80 to 100 feet. With a difference of but thirty minutes in time between the southern and northern points, it struck the San-Riku coast and obliterated towns and villages, killed 26,975 people out of the original population, and grievously wounded the 5,390 survivors. It washed away and wrecked 9,313 houses, stranded some 300 larger craft-steamers, schooners, and junks-and crushed or carried away 10,000 fishing boats, destroying property to the value of six million yen. Thousands of acres of arable land were turned to waste, projecting rocks offshore were broken, overturned, or moved hundreds of yards, shallows and bars were formed, and in some localities the entire shoreline was changed.
They were all seafaring communities along this coast strip and fishing was the chief industry. The shipment of sea products to the great ports was the main connection with the outer world. A high mountain range bars communication with the trunk railway line of the island, and this picturesque, fiord-cut coast is so remote and so isolated that only two foreigners had been seen in the region in ten years, with the exception of a French mission priest. With telegraph offices, instruments, and operators carried away, word came slowly to Tokyo, and with 50 to 100 miles of mountain roads between the nearest railway station and the seacoast aid was long in reaching the wretched survivors. When adequate idea of the calamity reached the capital and the cities, men-of-war, soldiers, sappers, surgeons, and nurses were quickly dispatched, and public sympathy found expression in contributions through the different newspapers, amounting to more than 250,000 yen, for the relief of the injured. The Japanese journalists and photographers were quickly on their way, and the vernacular press soon fed the public full of horrors.
There were old traditions of such earthquake waves on this coast, one of two centuries ago doing some damage, and a tsunami of forty years ago and a lesser one of 1892 flooding the streets of Kamaishi and driving people to upper floors and the roofs of their houses. The barometer gave no warning, no indication of any unusual conditions on June 15, and the occurrence of thirteen light earthquake shocks during the day excited no comment. Rain had fallen in the morning and afternoon, and with a temperature of 80° to 90° the damp atmosphere was very oppressive. The villagers on that remote coast adhered to the old calendar in observing their local fetes and holidays, and on that fifth day of the fifth moon had been celebrating the Girls' Festival. Rain had driven them indoors with the darkness, and nearly all were in their houses at eight o'clock, when, with a rumbling as of heavy cannonading out at sea, a roar, and the crash and crackling of timbers, they were suddenly engulfed in the swirling waters. Only a few survivors on all that length of coast saw the advancing wave, one of them telling that the water first receded some 600 yards from ghastly white sands and then the Wave stood like a black wall 80 feet in height, with phosphorescent lights gleaming along its crest. Others, hearing a distant roar, saw a dark shadow seaward and ran to high ground, crying "Tsunami Tsunami!" Some who ran to the upper stories of their houses for safety were drowned, crushed, or imprisoned there, only a few breaking through the roofs or escaping after the water subsided.
Shallow water and outlying islands broke the force of the wave in some places, and in long, narrow inlets or fiords the giant roller was broken into two, three, and even six waves, that crashed upon the shore in succession. Ships and junks were carried one and two miles inland, left on hilltops, treetops, and in the midst of fields uninjured or mixed up with the ruins of houses, the rest engulfed or swept seaward. Where the wave entered a fiord or bay it bore everything along to the head of the ravine or valley and left the mass of debris in a heap at the end. Where the coast was low and faced the open ocean the wave washed in and, retreating, carried everything back with it. Many survivors, swept away by the waters, were cast ashore on outlying islands, or seized bits of wreckage and kept afloat. On the open coast the wave came and withdrew within five minutes, while in long inlets the waters boiled and surged for nearly a half hour before subsiding. The best swimmers were helpless in the first swirl of water, and nearly all the bodies recovered were frightfully battered and mutilated, rolled over and driven against rocks, struck by and crushed between timbers. The force of the wave cut down groves of large pine trees to short stumps, snapped thick granite posts of temple gates and carried the stone cross-beams 300 yards away. Many people were lost through running back to save others or to save their valuables. In a moment the waters disappeared, leaving a black, empty level where the populous village had bean a few minutes before. One hundred and fifty people were found cast away on one island offshore. From two large villages on one bay only thirty young men survived, hardy, muscular young fishermen and powerful swimmers, yet in other places the strongest perished, and the aged and infirm, cripples, and tiny children were miraculously preserved. The wave flooded the cells of Okachi prison and the jailers broke the bolts and let the 195 convicts free. Only two convicts attempted to escape, the others waiting in good order until marched to the high ground by their keepers. Japanese men-of-war cruised for a week off Kamaishi, recovering bodies daily. The Japanese system of census enumeration is so complete and minute that the name of every person who lost his life was soon known, and the Official Gazette was able to state that out of a population of 6,529 at Kamaishi 4,985 were lost and 500 injured, while 953 dwellings and 867 warehouses and other structures were destroyed or carried away, and 176 ships carried inland or swept out and lost.
The survivors were so stunned with the appalling disaster that few could do anything for themselves or others. With houses, nets, and fishing-boats carried away and the fish retreating to further and deeper waters, starvation faced them, and, the great heat continuing while so many bodies were strewn along shore and imprisoned in ruins, the atmosphere fast became poisonous. The north-coast people are opposed to cremation and insisted on earth burial, which delayed the disposal of the dead and augmented the danger of pestilence. Disinfectants were sent in quantity, and the work of recovery and burial was so pressing that soldiers were put to it after all available coolies had been impressed. The Red Cross Society, with its hospitals and nurses, had difficulty in caring for all the wounded, the greater number of whom, besides requiring surgical aid, were suffering from pneumonia and internal inflammations consequent upon their long exposure in wet clothing without shelter and from the brine, fish oil, and sand breathed in and swallowed while in the first tumult of waters. Besides the generous relief fund subscribed by the people, the government made large assignments from its available funds and sent stores of provisions, clothing, tools, etc., to the 60,000 homeless, ruined, bereaved, and starving people of the San-Riku coast.
The wave was plainly felt two hours later on the shores of the island of Yesso, 200 miles north of the center of disturbance on the San-Riku coast, the water advancing 80 feet beyond high-tide mark on the beach at Hakodate. Eight hours later there was a great disturbance of the waters on the shores of the Bonin islands, more than 700 miles southward, the water rising three or four feet and retreating violently. Six hours later, on the shores of Kaui, the most northern of the Hawaiian islands, distant 3,390 miles, the waters receded violently and washed on shore in a wave some inches above the normal height.
The plainest inference has been that the great wave was the result of an eruption, explosion, or other disturbance in the bed of the sea, 500 or 600 miles off the San-Riku coast. The most popular theory is that it resulted from the caving-in of some part of the wall or bed of the great "Tuscarora Deep," one of the greatest depressions of the ocean bed in the world, discovered in 1874 by the present Rear-Admiral Belknap. U. S. N., while in command of the U. S. S. Tuscarora, engaged in deep-sea surveys.The "Tuscarora Deep" is nearly five and one-third stature miles in depth, being exceeded, so far as known, only by the still more profound depths discovered last year in the South Pacific by Commander A. F. Balfour, of the British Navy.That disturbances were taking place in this tremendous abyss was again suggested at six o'clock on the morning of July 4, when the Canadian Pacific Railway Company's mail steamer Empress of Japan, sailing directly over it in a smooth sea, was shaken as if a propeller blade had been lost or the ship had struck an obstruction. Every one was roused by the peculiar shock, but no visible explanation was furnished.
BOXING DAY 2004 TSUNAMI CAUSED BY THE EARTHQUAKE WITH THE EPICENTRE NEAR NE JAVA
The two scenes here show the large tsunami wave arriving with an interested spectator group and shortly after, as the enormity of the wave becomes apparent, how panic sets in.
# The area covered by sea is about 361,740,000 sq km (139670 sq miles)
# The Pacific, the largest ocean, covers about 46% of the world's ocean - an estimated 165,250,000 sq km (63,800,000 sq miles)
# There is no such thing as "the seven seas". All oceans are linked into one global sea.
# About 1% of all food comes from the sea (40,000,000 tonnes)
# There are an estimated 25,000 species of fish in the global sea.
# The deepest known part in the oceans is the Mariannas Trench in the Pacific which is 11,033m deep. (36,198ft). The Tonga Kermadec Trench to the north of New Zealand is about 10,500m deep (35,000ft).
# Petroleum, salt, bromide, sulphur and magnesium are among the many minerals recovered from the oceans. # Diamonds are known to exist on the sea bed as well as manganese nodules, copper and cobalt. 5 litres of sea water contains 113g of salt (1/4 lb).
# Wave movement is felt to a depth of about 34m (112ft) in big seas.
# The average wave height on the open seas is around 4m.
# The highest wave recorded was 34m (112ft) in the Pacific Ocean during a hurricane in 1933.
# The highest tsunami recorded was 530m high (1740ft) caused by a landslide in Alaska in 1958.
# In 1883 a tsunami created by the eruption of the Krakatoa volcano in Java caused 36,000 deaths and was recorded as far away as Britain.
# In 1946 a wave estimated to be 30.5m (100ft) built up after an earthquake in the Aleutian Trench and raced across the Pacific at an estimated speed of 800kph (500mph) to devastate Hawaii five hours later, lifting the sea level some 16m (50ft).
# The greatest tidal range in the world is experienced in the Bay of Fundy, Canada, where the average tides rise 16.3m (53.5ft). In Australia the highest tides are experienced along the north-west coast where a rise of 13m (42ft) is common.
This
picture shows a huge procession of 50ft waves produced by wind gusting up
to 80Kts. It was taken in the Pacific Ocean off southern Chile
