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.
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 91.30¼E to 92.10¼E and 20.55¼S to 21.52¼S, 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 28¼C.
South of south patches:Located at 91.30¼E to 92.20¼E and 20.15¼S 20.50¼S,
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 22¼C and 30¼C.
Middle ground: Located at 90.20¼E to 91.30¼E and 20.25¼S to 21.20¼S, 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 26¼C and 28¼C.
Swatch of no Ground:Located at 89.35¼E to 90.10¼E and 20.55¼S to 21.55¼S,
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 24¼C to 30¼C.
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
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