Dive Atlas Of The World: An Illustrated Reference To The Best Sites - Jack Jackson (2016)
THE INDIAN OCEAN
Introduction by Jack Jackson
THE INDIAN OCEAN IS THE THIRD-LARGEST BODY OF WATER in the world after the Pacific and Atlantic Oceans. It covers roughly 20 per cent of the earth’s water surface and is the youngest and physically most complex of the three major oceans. No natural boundary separates the Indian Ocean from the Atlantic, Antarctic and Pacific Oceans. The 20° meridian east of Greenwich, through Cape Agulhas at the southern end of Africa, is used to denote separation from the Atlantic Ocean south of Africa and the 147° meridian east of Greenwich, separates it from the Pacific Ocean south of Australia. The northeastern border is difficult to define. The one most commonly accepted runs northwest across the Timor Sea from Australia’s Cape Londonderry, along the southern shores of the Lesser Sunda Islands and the island of Java, and then across the Sunda Strait to the shores of Sumatra. Between Sumatra and the Malay Peninsula the boundary is usually drawn across the Singapore Strait. The Antarctic Ocean, often called the Southern Ocean, includes all oceanic areas surrounding Antarctica, south of latitude 55° south.
Including the Gulf of Aqaba, the Red Sea, the Arabian (Persian) Gulf, the Gulf of Oman, the Arabian Sea, the Bay of Bengal, the Strait of Malacca, the Great Australian Bight and the Bass Straight, the Indian Ocean covers an area of 73,556,000 sq km (28,400,000 sq miles).
Bordered in the north by southern Asia, in the west by the Arabian Peninsula and Africa, in the east by the Malay Peninsula, Indonesia and Australia and in the south by Antarctica, the Indian Ocean is extremely varied in its coastal habitat, reef development and species diversity.
In the north, the Gulf of Aqaba and the Red Sea are more saline and warmer than one would expect from their geographical position. Volcanic activity at the seafloor increases the water temperature and the desert coastline has no large river systems flowing out. This has led to healthy reef development and good species diversity.
During ice ages, lower sea levels isolated the Arabian (Persian) Gulf, Gulf of Aden and the Red Sea from the Indian Ocean, precluding the interchange of species and allowing some 15 per cent of the species here to evolve endemically. Even when the ice melted and sea levels rose again, the sandy bottom and upwelling along the coast of Somalia limited the development of reefs and reef species there. This lower species diversity reinforces the isolation of reef species to the north. In the Arabian Gulf, the sandy bottom, upwelling and extremes of winter and summer temperatures inhibit reef growth. The result is that species diversity is lower than in the Red Sea and the Gulf of Aqaba, but there is still a high degree of endemism.
Indian Ocean Oriental Sweetlips (Plectorhinchus vittatus) can be found singly, but they often also occur in large shoals.
The coast of Pakistan has low winter temperatures. Most of the coast from India to Myanmar (Burma) has massive seasonal outflows of sediment-laden freshwater from large river systems. Reef development along these coasts is limited. The Andaman Sea, southern Myanmar and western Thailand have huge granite boulders and small reefs. Northwest Malaysia has a few reefs and good species diversity, but suffers from agricultural run-off. The Strait of Malacca has agricultural run-off and heavy shipping movements. A restricted water mass, the run-off also lowers its salinity, but the few small reefs found in the Strait of Malacca have good species diversity. Indonesia has some of the best coral and species diversity in the world. The west and south coasts of Australia are not as good as the Great Barrier Reef in the Pacific, but there is fine diving all the way round to the Bass Strait and Tasmania where the water is cold enough for kelp.
Some of the islands off southwest India, Sri Lanka, the Maldives, East Africa, Comoros and Madagascar have suffered recent coral bleaching, but there is still good species diversity. The islands on the Mascarene Ridge, Mauritius, Réunion, Rodrigues and the Seychelles, have less species diversity, but a degree of endemism. The east African coast has coral reefs as far south as Durban. Juvenile tropical inshore fish may be swept seasonally by the current as far south as the Cape of Good Hope. Mozambique and South Africa’s coastline ranges from coral reefs to waters as cold and rough as that of Europe.
The Indian Ocean has the fewest seas. To the northwest are the Red Sea, Arabian Gulf, gulfs of Aden and Oman and the Arabian Sea. To the northeast are the Andaman Sea and the Bay of Bengal. The Great Australian Bight is off the southern coast of Australia. The Indian Ocean only goes as far north as the Red Sea - it cannot reach a temperate or cold region. It has fewer islands. Madagascar is the world’s fourth-largest island and Sumatra is the fifth. The Seychelles, Socotra, and Sri Lanka are continental fragments, while all other islands are volcanic. It is the only ocean whose surface circulation lacks symmetry and, in the north, reverses semiannually.
The semiannual reversals of wind direction, known as monsoons from the Arabic word mausim for season, vary in their date of onset and intensity. Monsoon dynamics are modulated by the El Niño/Southern Oscillation Phenomenon of the South Pacific and cyclones that form over the open ocean.
During the northern winter, from November to April, high atmospheric pressure develops over India due to the cold, falling air. This combines with the low-pressure system north of Australia to cause the northeast monsoon and the northeast-to-southwest winds and currents that bring a wet season to southern Indonesia and northern Australia. These winds generate the North Equatorial Current, which carries water towards the coast of Africa.
In the northern summer, from May to October, low atmospheric pressure develops over Asia from hot, rising air. This results in the southwest monsoon and southwest-to-northeast winds. The southwest monsoon brings heavy rain to the Indian subcontinent and a wet season in south Asia.
In the southern hemisphere, the winds are generally milder, but summer storms near Mauritius can be severe. Trade winds drive a broad, circular system of currents (a gyre) of warm water anticlockwise. The South Equatorial current carries water towards Africa, where the portion not connected with the monsoon wind system curls south below Madagascar and reinforces the Agulhas Current. In winter this combined southward flowing current can reach speeds of 180km (112 miles) per day along the edge of South Africa’s continental shelf. At this speed it can more than double the height of waves travelling north from storms in the Antarctic Ocean and divers have to put up with a high-voltage, camera-damaging experience as their boats are launched through the surf.
The old Greek freighter Paraportiani sank in a storm at the southern end of Pemba in the late 1960s. Lying in shallow water she is well colonized with marine growth.
Tropical cyclones occur during May/June and October/November in the northern Indian Ocean and January/February in the south. When the monsoon winds change, cyclones sometimes strike the shores of the Arabian Sea and the Bay of Bengal.
Sailing ships plying the important India and southeast Asia trade route made full use of the monsoon trade winds. They made the eastward journey with the winds of the southwest monsoon and used the winds of the northeast monsoon for their return between November and March. Nowadays, ships have a shorter passage through the Suez Canal.
All three types of tides are found in the Indian Ocean, but semidiurnal tides are the most common, prevailing in east Africa as far north as the equator and in the Bay of Bengal. Tide types are mixed in the Arabian Sea and the inner part of the Arabian Gulf. Southwest Australia, the coast of Thailand in the Andaman Sea and the south shore of the central Arabian Gulf have small areas of diurnal tides. Tidal ranges vary considerably - Mauritius has a tidal range of only 50cm (20in) on springs, while Australia’s Port Hedland has 600cm (19ft), and Rangoon, 520cm (17ft).
The average depth of the Indian Ocean is 3890m (12,760ft). Its deepest point, in the Sunda Deep off the southern coast of Java, Indonesia, in the Java Trench, is 7450m (24,442 feet). This is thought to mark the line of subduction where the Australian Plate goes below the Eurasian Plate. The related volcanic activity is high. The eruption of Krakatoa in 1883 was heard as far away as Australia.
The African, Indian, and Antarctic tectonic plates diverge in the Indian Ocean as the mid-oceanic ridge. Shaped like an inverted Y, the stem runs north and then west as the Carlsberg Ridge to join the rift system of the Red Sea. One arm extends around southern Africa to connect with the Mid-Atlantic Ridge, while the other extends south around Australia to connect with the East Pacific Rise. This underwater chain of mountains represents strong volcanic activity. At the Red Sea the northern part of Africa is moving apart from Arabia. In geological time it will probably become wide enough to be an ocean.
Anthias, mostly Pseudanthias species, are known as Fairy Basslets or Goldies in some countries. Usually small and brightly coloured, they feed on zooplankton above reefs.
All bottom water originates from outside the Indian Ocean’s boundaries. Below the surface currents, deep water movement is sluggish. The highly saline water from the Arabian Gulf and the Red Sea sinks below the fresher surface water to form the North Indian High Salinity Intermediate Water between 610m (2000ft) and 1000m (3300ft). This layer spreads east into the Bay of Bengal and as far south as Madagascar and Sumatra. Below this, the Antarctic Intermediate Water continues to 1500m (5000ft). A current from the north Atlantic, the North Atlantic Deep Water, flows between 1500 (5000ft) and 3050m (10,000ft) and below 3050m (10,000ft) the Antarctic Bottom Water comes from the Weddell Sea. These cold layers flow slowly northward from the Antarctic Circumpolar Region, becoming extremely low in oxygen in the north.
Some Turbinaria species of stony corals have highly convoluted growth to collect the maximum amount of sunlight.
The Indian Ocean is nearly as good as the Pacific Ocean for species diversity, but overfishing is a problem. Destructive fishing methods have ruined coral reefs in places. Indonesia, whose reefs straddle the Indian and Pacific Oceans, has the greatest share of coral reefs in the world (18 per cent) of which 82 per cent are under threat. Diver numbers and indiscriminate anchoring have damaged some areas, but are now being addressed.
Cultivation, hunting and guano mining on land used to be the main threats to the environment. Nowadays agricultural, domestic and industrial waste flowing into nearshore waters, oil spills from normal tanker operations and occasional large-scale tanker catastrophes are the major problems.
The sight of a traditional dhow sailing across the deep waters of the Pemba Channel. A living reminder of Pemba’s past.
An idyllic tropical setting for Pemba’s first marine reserve, Mesali Island is situated on the west coast, surrounded by colourful coral reefs that sustain a diverse ecosystem.