Bay of Bengal 02
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 conditions 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.
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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 conditions 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.
http://banglapedia.net
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