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Indian Seas & Ocean

Indian Seas & Ocean

Sehgal, K.*, Phadke, G. G.#,  Joshi, H. D.#, Ramteke, K. K.*, Sukhadhane, K. *, Naik, P., Nandanpawar, P. *, Katare, M.#, Bhendarker, M., Sonone, A., Vijay, A. R.#, Jagadeesh, T. D. #, Rather, M. A., Patel, D. M. #, Joshi, S. #

*Central Institute of Fisheries Education, Seven Bunglows, Versova, Andheri (W.), Mumbai, Maharashtra, India 400061.

#College of Fisheries, Mangalore, Karnataka Veterinary Animal and Fisheries Sciences University, Bidar, Karnataka, India 575 002.

Corresponding Author's e-mail: sehgal.cife@gmail.com

Introduction

An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 70.9% of the Earth's surface (~3.61 x 10 8 km 2) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.

More than half of this area is over 3,000 metres (9,800 ft) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt. Scientists estimate that 230,000 marine life forms of all types are currently known, but the total could be up to 10 times that number.

India is a peninsular country with water on 3 sides i.e.Arabian sea on the west, Bay Of Bengal on the east

Indian ocean in the south.

 

Arabian Sea

Northwestern branch of the Indian Ocean, covering 3,859,000 sq km/1,489,970 sq mi, with India to the east, Pakistan and Iran to the north, and the Arabian Peninsula and Somalia to the west. It is linked with the Red Sea via the Gulf of Aden, and with the Gulf via the Gulf of Oman. Its mean depth is 2,730 m/8,956 ft. The maximum width of the Arabian Sea is approximately 2,400 km (1,490 mi), and its maximum depth is 4,652 m (15,262 ft), in the Arabian Basin approximately at the same latitude as the southernmost tip of India.  The chief river flowing into the Arabian Sea is the Indus, which is linked with a large submarine canyon in the continental shelf. The sea is rich in fish.

The monsoons of the Arabian Sea are remarkably strong and steady in comparison to the storms that dominate midlatitude locations, making the Arabian Sea a nearly ideal laboratory for the study of steady wind-driven processes. An array of surface moorings was deployed from October 1994 through October 1995 yielding the first year-long time series of atmospheric and oceanic variables spanning both monsoons. The array, funded by the Office of Naval Research, included five moorings set in a 50-km square pattern centered at 15°30'N, 61°30'E. and was a collaboration among investigators from Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, University of Washington, Lamont-Doherty Earth Observatory, and University of California, Santa Barbara.

Two monsoons occur each year: the weaker NE monsoon in winter and the stronger SW monsoon in summer. As the SW monsoon strengthens in summer, the wind changes direction in a counterclockwise direction. At its strongest in July, the wind is concentrated in a feature often called the Findlater Jet. The location of the moored array was chosen to be near the climatological axis of the Findlater Jet in July to measure the strongest winds in the Arabian Sea. Typical wind speeds were 5 m/s during the NE monsoon, while speeds were more than 10 m/s throughout the SW monsoon. The air was generally cooler than the sea surface, except during the summer when the SW monsoon brought warm air from lower latitudes. The net heat flux is composed of latent, sensible, shortwave, and longwave fluxes, which were either measured directly or estimated using bulk parameterizations. The longest period of negative heat flux occurred during the SW monsoon, primarily due to latent heat losses.

From May to September there is strong upwelling in the East Arabian Current along Oman, accompanied by a 5° C or more lowering of coastal temperatures due to the cold upwelling water. This upwelling isn't as conducive to primary production as elsewhere due to the rapidly moving current removing much of the upwelled additional biomass before it can be utilized.

The processes determining the temperature and salinity of the upper ocean are advection, turbulent mixing, and air-sea heat flux. Each of these processes is apparent in the year-long time series of temperature. Two periods of mixed-layer deepening occurred, coincident with the beginning of the two monsoons in December and June. Min surface temperature about 24° to 25° is found during the month of Jan — Feb. and the max temp about 28° is found during the month of June to Nov. During rainy season salinity is found less that 35 ppt.

The deepest mixed layer of the year is about 100 m during the NE monsoon in January because of intense latent heat loss. In contrast, the much stronger SW monsoon produces a mixed layer of only 70 m. The spring inter-monsoon restratification isolates much of the winter's deep mixed layer from the atmosphere. This deep isothermal layer persists through summer after which it is apparently advected away.

      Measurements of water velocity and wind allow the examination of the momentum transfer to the ocean by the monsoons. If the ocean were driven only by local wind then the current would be nearly steady during the monsoons. However, the strongest variations in current are apparently unrelated to local wind, and are likely geostrophic. The shear, as measured by self-contained Acoustic Doppler Current Profilers, reveal some interesting features:

(1) The mixed layer is relatively unsheared during periods of deepening, as assumed in the simplest bulk mixed-layer models;

(2) Thin regions of high shear are located just beneath the mixed-layer base;

(3) High shear well beneath the mixed layer coinciding with a change in current direction in early August is an indication of a geostrophic current.

Bay of Bengal

Bay of Bengala northern extended arm of the Indian Ocean, is located between latitudes 5°N and 22°N and longitudes 80°E and 100°E. It is bounded in the west by the east coasts of Sri Lanka and India, on the north by the deltaic region of the Ganges-Brahmaputra-Meghna river system, and on the east by the Myanmar peninsula extended up to the Andaman-Nicobar ridges. The southern boundary of the Bay is approximately along the line drawn from Dondra Head in the south of Sri Lanka to the north tip of Sumatra. The Bay occupies an area of about 2.2 million sq km and the average depth is 2,600m with a maximum depth of 5,258m. Bangladesh is situated at the head of the Bay of Bengal.

Bottom topography:characterised by a broad U-shaped basin with its south opening to the Indian Ocean. A thick uniform abyssal plain occupies almost the entire Bay of Bengal gently sloping southward at an angle of 8°-10°. In many places underwater valleys dissect this plain mass.

Continental Shelf: The width of the continental shelf off the coast of Bangladesh varies considerably. It is less than 100 km off the south coast between Hiron Point and the swatch of no ground and more than 250 km off the coast of cox's bazar. Sediments are fine seaward and westward with the thickest accumulation of mud near the submarine canyon, the Swatch of no Ground. The shallow part (less than 20m) of the continental shelf off the coast of chittagong and teknaf is covered by sand and the intertidal areas show well-developed sandy beaches. The shallower part of southern continental shelf off the coast of the sundarbans, patuakhali and noakhali is covered by silt and clay; and extensive muddy tidal flats are developed along the shoreline.

Swatch of no Ground: also known as Ganga Trough. Swatch of no Ground has a comparatively flat floor 5 to 7 km wide and walls of about 12° inclination. At the edge of the shelf, depths in the trough are about 1,200m. The Swatch of no Ground has a seaward continuation for almost 2,000 km down the Bay of Bengal in the form of fan valleys with levees. The sandbars and ridges near the mouth of the ganges-brahmaputra delta pointing toward the Swatch of no Ground showing sediments are tunnelled through this trough into the deeper part of the Bay of Bengal.

Sunda Trench:also known as Java Trench. Running parallel along the west side of the arc of the Nicobar and Andaman islands it is extended northward up to 10°N into the Bay and joins the eastern limit of the Himalayan range. It originated tectonically at the junction of the Indian and Myanmar plates.

Ninety East Ridge: major feature of the Indian Ocean which runs in a north-south direction approximately along the longitude 90°E. It lies at the immediate outboard of the Sunda Trench between the Bengal Fan and the Nicobar Fan. The Ninety East Ridge has existed since early in the formation of the Bay of Bengal.

Eighty-five Ridge:a ridge along 85°E longitude. More than 5 km thick sediments have been deposited on either sides of the ridge. The main turbidity current channel of the subaerialdrainage pattern lies immediately east of the buried ridge.

Bengal Deep Sea Fan:the world's largest submarine fan, also known as Bengal Fan. Together with its eastern lobe, the Nicobar fan, it covers an area of 3106 sq km. It is 2,800 to 3,000 km long, 830 to 1,430 km wide and more than 16 km thick beneath the northern Bay of Bengal. Sediments are tunnelled to the fan via a delta-front trough, the Swatch of no Ground. It can be divided into three parts: upper fan, middle fan and lower fan.

Geographical characteristics:Hydrological conditions surface hydrology of the Bay of Bengal is basically determined by the monsoon winds and to some extent by the hydrological characteristics of the open part of the Indian Ocean. Fresh water from the rivers largely influences the coastal northern part of the Bay. The rivers of Bangladesh discharge the vast amount of 1,222 million cubic metres of fresh water (excluding evaporation, deep percolation losses and evapotranspiration) into the Bay. The temperature, salinity and density of the water of the southern part of the Bay of Bengal is, almost the same as in the open part of the ocean. In the coastal region of the Bay and in the northeastern part of the Andaman Sea where a significant influence of river water is present, the temperature and salinity are seen to be different from the open part of the Bay. The waves and ripples entering from the southern part of the Bay provide the energy for mixing the water and consequently bring uniformity in its chemical and physical properties. Tidal action is also very great in the shallow coastal zones.

Temperature: The mean annual temperature of the surface water is about 28°C. The maximum temperature is observed in May (30°C) and the minimum (25°C) in January-February. But the annual variation in temperature is not great, about 2°C in the south and 5°C in the north.

Salinity: The surface salinity in the open part of the Bay oscillates from 32% to 34.5% (parts per thousand, ie grams per kilogram of sea water) and in the coastal region varies from 10% to 25%. But at the river mouths, the surface salinity decreases to 5% or even less. The coastal water is significantly diluted throughout the year, although the river water is greatly reduced during winter. Along the coast of the Ganges-Brahmaputra Delta, salinity decreases to 1% during summer and increases up to 15% to 20% in winter. Salinity gradually increases from the coast towards the open part of the Bay. The surface salinity at the mouths of some large rivers like the ganges, brahmaputra, Irrawaddy and some Indian rivers like the Krishna, Godavari, Cauvery and Mahanadi varies widely from one day to another, especially in summer. Salinity of water also changes vertically. The influence of the fresh water is experienced up to depths of 200-300m. From the surface, the salinity gradually increases downward and at about 200-300m it reaches 35% and at about 500m the salinity is more than 35.10%, but at 1,000m it decreases slightly and attains 34.95%. With further increase of depth salinity decreases and at 4,500m it is close to 34.7%.

Tides: The semi-diurnal type of tides, ie two high and two low tides during the period of 24 hours and 52 minutes. The highest tide is seen where the influence of bottom relief and the configuration of the coast are prominent, ie in shallow water and in the Bay and estuary. The average height of tidal waves at the coast of Sri Lanka is 0.7m and in the deltaic coast of the Ganges it is 4.71m. In the Bay of Bengal tidal currents specially develop in the mouths of the rivers, like the Hooghly and the meghna.

Colour and water transparency: The colour of the water in the open part of the Bay is dark blue which gradually changes to light blue to greenish towards the coast. Transparency is great, 40-50m in some places. In the central part of the Bay of Bengal, the anticyclone circulation is generated and in the centre of this lies the zone of convergence. This region is characterised as a rule by high transparency of water. Regions of low transparency and turbid water are available in the limited area of the pre-deltaic part of the rivers Ganges and Brahmaputra.

Sea Level: Due to the influence of water density and wind the seasonal changes of the sea level in the Bay are remarkable and one of the highest in the world. The range at Khidirpur is 166 cm, at Kolkata 130 cm and at Chittagong 118 cm. But towards the southwestern coast at Madras and Vishakhapatnam [Vishakhapatnam] the range is small compared to the northern and northeastern coasts of the Bay. The lowest variation of sea level at the southeastern coast of India is due to its geographical location at the edge of a comparatively deep sea.

Ocean Current: Surface circulation is found to be generally clockwise from January to July and counter-clockwise from August to December, in accordance with the reversible monsoon wind systems. The flow is not constant and depends on the strength and duration of the winds. The effects of a strong wind blowing for a few consecutive days are reflected in the rate of flow. Currents to the northeast generally persist longer and flow at greater speed because of the stronger southwest monsoons. An important vertical circulation in the Bay of Bengal is up-welling. In this process, sub-surface water is brought toward the surface, and conversely a downward displacement is called down-welling or sinking.

Up-welling and down-welling are seasonal, being created by monsoon winds that blow from the southwest during the summer, then reverse direction and come from the northeast during the winter. The persistence of the monsoon, especially from the southwest and the orientation of the coasts cause up-welling to occur along most of the east coast of India. That is why in the east coast of India the up-welling takes place in summer and down welling in winter, and in the eastern part of the Bay of Bengal and in the Myanmar coast, up-welling occurs in winter and the down-welling in summer. However, the duration and intensity of vertical movement of water on both sides of the Bay of Bengal is not as great as on the Somali or North and South American coasts. But it does have a profound effect on the food economy of the sea through its influence on chemical properties and biological populations. [Subash Chandra Das]

Biological characteristics:The occurrence of marine species - both plants and animals - has largely been controlled by the physico-chemical properties of ocean water. Water discharges from the surrounding river catchments carry huge influx of sediments full of nutrients to the Bay, particularly along the near shore region. This has turned the Bay into a fertile marine fishing ground of the region. The near-shore up-welling zone not only has a high yield of nutrients, but also is a high primary production area for the phytoplankton and related zooplankton zones.

Fishing:The hydrological condition of the Bay of Bengal is favourable for a variety of shrimps and fishes. Although fishes remain scattered in the Bay in some places they get concentrated and constitute important fishing grounds.

Four fishing grounds have been identified so far. They are south patches, south of south patches, middle ground and Swatch of no Ground.

South patches: Located at 91.30°E to 92.10°E and 20.55°S to 21.52°S, having a total area of 3,662 sq km. Depth ranging from 10m to 100m, but 90% of the total area is less than 40m deep. Bottom sediment is sandy or slightly muddy sand. Nearest distance of the ground from Chittagong and Cox's Bazar is 40 km and 10 km respectively. Salinity in surface water ranges from 26% to 32% and 30% to 35% in bottom water. Water temperature varies between 20 and 28°C.

South of south patches: Located at 91.30°E to 92.20°E and 20.15°S 20.50°S, having an area of 2,538 sq km. The nearest boundary of this area is 5 km from Teknaf. Depth ranges from 10m to 100m. Within this ground 75% of the area is more than 40m deep. Bottom is sandy or muddy sand. Surface salinity ranges from 18% to 34% and bottom water salinity from 28% to 38%. Water temperature ranges between 22°C and 30°C.

Middle ground : Located at 90.20°E to 91.30°E and 20.25°S to 21.20°S, having a total area of about 4,600 sq km. The nearest distance from Cox's Bazar is about 65 km. The depth of 70% of the total area is more than 40m. Bottom sediment is soft mud or muddy sand. Surface salinity ranges from 22% to 34% and bottom salinity 28% to 35%. Water temperature is between 26°C and 28°C.

Swatch of no Ground: Located at 89.35°E to 90.10°E and 20.55°S to 21.55°S, about 30 km away from Dublarchar and 40 km from Sunarchar. Total area is about 3,800 sq km, of which 70% is more than 40m deep. Overall depth of the area ranges from 10m to 100m. Bottom sediment consists of muddy sand. Surface salinity is 28% to 34%, while the bottom salinity is 30% to 35%. Water temperature falls within 24°C to 30°C.

All these fishing grounds are potential reserves for fish and shrimp. Most of the known commercial species of shrimps and fishes are harvested from these areas by trawlers or mechanized fishing boats. Commercially important shrimp and fish species include tiger shrimp, karuma shrimp, cat fish, Bombay duck, snapper, flounder, Indian salmon, crocker, seabream, jawfish, mullet, pomfret, ribbon fish, anchovy, hilsa, oil sardine.

 

Indian Ocean

The Indian Ocean is the third largest of the world's oceanic divisions, covering about 20% of the water on the Earth's surface. It is bounded on the north by Asia (including the Indian subcontinent, after which it is named); on the west by Africa; on the east by Indochina, the Sunda Islands, and Australia; and on the south by the Southern Ocean (or, traditionally, by Antarctica). One component of the all-encompassing World Ocean, the Indian Ocean is delineated from the Atlantic Ocean by the 20° east meridian running south from Cape Agulhas, and from the Pacific by the 147° east meridian. The northernmost extent of the Indian Ocean is approximately 30° north latitude in the Persian Gulf and, thus, has asymmetric ocean circulation.

The ocean's volume is estimated to be 292,131,000 cubic kilometers (70,086,000 mi³). Small islands dot the continental rims. Island nations within the ocean are Madagascar (formerly Malagasy Republic), the world's fourth largest island; Comoros; Seychelles; Maldives; Mauritius; and Sri Lanka. Indonesia borders it on the east. The ocean's importance as a transit route between Asia and Africa has made it a scene of conflict. Because of its size, however, no nation had successfully dominated most of it until the early 1800s when the United Kingdomcontrolled much of the surrounding land. After the decline of the British Empire, the ocean has been dominated by India and Australia.

Ocean between Africa and Australia, with India to the north, and the southern boundary being an arbitrary line from Cape Agulhas to south Tasmania; area 73,500,000 sq km/28,370,000 sq mi; average depth 3,872 m/12,708 ft. The greatest depth is the Java Trench 7,725 m/25,353 ft. It includes two great bays on either side of the Indian peninsula, the Bay of Bengal to the east, and the Arabian Sea with the gulfs of Aden and Oman to the west.

 

Background: The Indian Ocean is the third largest of the world's five oceans (after the Pacific Ocean and Atlantic Ocean, but larger than the Southern Ocean and Arctic Ocean). Four critically important access waterways are the Suez Canal (Egypt), Bab el Mandeb (Djibouti-Yemen), Strait of Hormuz (Iran-Oman), and Strait of Malacca (Indonesia-Malaysia). The decision by the International Hydrographic Organization in the spring of 2000 to delimit a fifth ocean, the Southern Ocean, removed the portion of the Indian Ocean south of 60 degrees south latitude.

 

Location: Body of water between Africa, the Southern Ocean, Asia, and Australia

Geographic coordinates: 20° 00 S, 80° 00 E

Area:Total 68.556 million sq km

Note: Includes Andaman Sea, Arabian Sea, Bay of Bengal, Flores Sea, Great Australian Bight, Gulf of Aden, Gulf of Oman, Java Sea, Mozambique Channel, Persian Gulf, Red Sea, Savu Sea, Strait of Malacca, Timor Sea, and other tributary water bodies.

Area - comparative: About 5.5 times the size of the US

Coastline: 66,526 km

Climate: Northeast monsoon (December to April), southwest monsoon (June to October); tropical cyclones occur during May/June and October/November in the northern Indian Ocean and January/February in the southern Indian Ocean.

Terrain: Surface dominated by counterclockwise gyre (broad, circular system of currents) in the southern Indian Ocean; unique reversal of surface currents in the northern Indian Ocean; low atmospheric pressure over southwest Asia from hot, rising, summer air results in the southwest monsoon and southwest-to-northeast winds and currents, while high pressure over northern Asia from cold, falling, winter air results in the northeast monsoon and northeast-to-southwest winds and currents; ocean floor is dominated by the Mid-Indian Ocean Ridge and subdivided by the Southeast Indian Ocean Ridge, Southwest Indian Ocean Ridge.

Elevation extremes:

Lowest point: Java Trench -7,258 m

Highest point: sea level 0 m

 

References:

Banse, K. 1968. Hydrography of the Arabian Sea shelf of India and Pakistan and effects on demersal fishes. Deep-Sea Res., 15, 45-79.

Barkley, R. A. (1972) Johnston Atoll's wake. Journal of Marine Research30, 201-216.

Behrman, D. 1981. Assault on the Largest Unknown: The International Indian Ocean Expedition 1959-65. UNESCO Press, 96 pp.

Brown, O. B. and R. H. Evans. 1981. Interannual variability of the Arabian Sea surface temperature. pp. 135-143

Luther, M. E., and J. J. O'Brien. 1985. A model of the seasonal circulation in the Arabian Sea forced by observed winds. Prog. Oceanogr., 14, 353-385.

Rudnick, D. L., R. A. Weller, C. C. Eriksen, T. D. Dickey, J. Marra, and C. Langdon, 1997: Moored instruments weather Arabian Sea monsoons, yield data. Eos, Trans. Amer. Geophys. Union,78, 117, 120-121.

Tomczak, M. and J. S. Godfrey (1994): Regional Oceanography: an IntroductionPergamon, Oxford, 442 pp

Pugh, D. T. (1987) Tides, surges and mean sea-level. John Wiley & Sons, Chichester. 472 pp.

McCreary, J. P., and P. J. Kundu. 1989. A numerical investigation of sea surface temperature in the Arabian Sea. J. Geophys. Res., 94, 16097-16114.

Weller, R. A., D. L. Rudnick, C. C. Eriksen, K. L. Polzin, N. S. Oakey, J. W. Toole, R. W. Schmitt, and R. T. Pollard, 1991: Forced ocean response during the Frontal Air-Sea Interaction Experiment. J. Geophys. Res.,96, 8611-8638.

Rudnick, D. L., and R. A. Weller, 1993: Observations of superinertial and near-inertial wind-driven flow. J. Phys. Oceanogr.,23, 2351-2359.

Lee, C. M., and D. L. Rudnick, 1996: The upper ocean response to surface heating. J. Phys. Oceanogr.,26, 466-480.

Limits of Oceans and Seas. International Hydrographic Organization Special Publication No. 23, 1953.

 

Molinari, R. L., D. Olson, and G. Reverdin. 1990. Surface current distributions in the tropical Indian Ocean from compilations of surface buoy trajectories. J. Geophys. Res., 95, 7217-7238.

 

Vannucci, M. and D. Navas. 1973. On the ecology of Indian Ocean Hydromedusae. Indian Ocean Biological Center Handbook 5: 1-54.

 

 


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