The Bay of Bengal is a triangular shaped bay in the northeast part of the Indian Ocean. The bay is 2,172,000km2 and is feed by a number of large rivers including the Padma (a distributor from the Ganges). Sea temperature and salinity change dramatically across this area. The reason for this change will be discus in this report. Comparisons will be made between the Bay of Bengal and the Arabian Sea. The Arabian Sea is a region of the Indian Ocean; it too is fed by a number of rivers and is attached to the Red Sea, a seawater inlet which undergoes a vast amount of evaporation, it is therefore the most saline body of water in the world. The factors which cause difference in temperature and salinity between the Bay of Bengal and the Arabian Sea will also be discussed.
Variations in O values for the Bay of Bengal
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Data for the Bay of Bengal
Figure 2 shows a change in O values from the northern end of the Bay of Bengal to the open Indian Ocean. The values increase dramatically from, -3.4ppm to -2.6ppm in the open ocean.
Figure 3 shows an increase in O values form -3.6ppm in the Northern part of the bay, to -1.7 in the open Indian Ocean close the equator. The latitude values seen in Figure 3 can be compared to the aerial photo in Figure 1 showing longitude and latitude.
Interpretation for the bay of Bengal
O is a measure of the ration of stable isotopes. The data from foraminifera and ice cores is used as a proxy for temperature. Foraminifera have calcium rich shells, (CaCO3) the ratio of O to O shows the temperature of sea water at the time of solidification. These values can vary slightly due to salinity.
Rainwater is enriched in O; it is a lighter molecule and is preferential evaporated from seawater. Atmospheric vapour is therefore enriched in O, where as ocean surface becomes depleted especially in costal regions.
The Bay of Bengal has lower O values than in the open ocean. This cannot be due to poor circulation, as in this case the water would become enriched in this molecule. The rivers which flow into the bay are causing a depletion of O close the river mouth. Further away, towards the open ocean, the river is causing less of an effect and therefore O is increased.
Data for the Arabian Sea
Figure 4: A Google Earth map showing longitude and latitude lines for the Arabian Sea.
Source: Google Earth
Figure 5: A graph to show O values vs latitude for the Arabian Sea.
Figure 5 shows that the concentration of O decreases towards the equator from the Arabian Sea into the Indian Ocean.
Interpretation for the Arabian Sea
The Arabian Sea has a higher concentration of O near to the coast compared to the Bay of Bengal. This can be seen in Figure 5. The reason for this is the greater circulation of water in the open ocean. The Red Sea also flows into the Arabian Sea. The Red sea is a water inlet which undergoes large amounts of evaporation. It is the most saline body of water in the world. This could also be causing the coast to be more enriched in O. Like the Bay of Bengal several rivers flow into the Arabian Sea bringing a fresh water input rich in O.
The effect of salinity and temperature on O values
Figure 6: A graph to show salinity vs latitude for the Bay of Bengal
Figure 7: A graph to show temperature vs latitude for the Bay of Bengal
Figure 6 shows that in the Bay of Bengal salinity increases from the Coast into the open Indian Ocean. Figure 7 shows temperature compared to latitude in the Bay of Bengal. The graph shows that temperature increases from the equator into the bay. This pattern however is not seen at Longitude 95.5 where the temperature decreases.
Figure 8: A graph to show salinity vs latitude for the Arabian Sea.
Figure 9: A graph to show temperature vs latitude for the Arabian Sea
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Figure 8 shows that in the Arabian Sea salinity increases with latitude. The exception being at longitude 75.5, here the salinity decreased closer to the coast. Figure 9 shows temperature compared to latitude in the Arabian Sea. With increasing latitude northwards away from the equator temperature decreased.
Figure 10: Left is a diagram which represents salinity in the Bay of Bengal and the Arabian Sea. Right is a diagram which represents surface temperature of the two areas.
Figure 10 shows a high salinity of over 35.5pps in the Arabian Sea compared to the Indian Ocean. The Bay of Bengal has a low salinity of around 33.5pps which is lower than the Indian Ocean. The annual temperature diagram to the right shows an overall stable temperature of 28.5oC in the Bay of Bengal, Arabian Sea and the Indian Ocean.
Salinity depends upon how much water there is in the oceans, rather than salt (Wolfgang, Session 13). The salinity of the oceans remains at a fairly constant level of around 35%. This can vary slightly due to locally large inputs of fresh waters via rivers and by large amount of evaporation. O and salinity increase proportionally. This is the result of evaporation of O in shallow seas where circulation is less effective. This can be seen for the Arabian Sea in Figure 5 and 8.
Figure 3 and 6 for the salinity and O of the Bay of Bengal shows an opposite trend to that of the Arabian Sea. This may be due to a large amount of fresh water entering the bay from rivers. Effective circulation of water may also cause such a trend. O and salinity strongly correlate as they are both effected by evaporation, fresh water input and circulation.
Sea surface temperature remains fairly constant over an area as a result of the slow rate of change. Figure 10 shows a stable annual temperature of 28.5oC for the Bay of Bengal, Arabian Sea and Indian Ocean. Figure 7 and 9 are graphs to show temperatures in these areas. The temperature appears to increase inland in the Bay of Bengal with the opposite effect in the Arabian Sea. This variation may be due to proximity to land and warm tidal currents.
O values as result of temperature across the Bay of Bengal
T(°C) = 17.04 - 4.34 x (Otest - Oseawater) + 0. x (Otest - Oseawater)2
An average O value for the northern and southern part of the Bay of Bengal has been obtained from the ratio of O and O is the calcite shells of foraminifera.
OSMOW is equal to 0%
North: T(°C) = 17.04 - 4.34 x (-3 - 0) + 0. x (-3 - 0)2 = 31.5°C
South: T(°C) = 17.04 - 4.34 x (-2.619 - 0) + 0. x (-2.619 - 0)2 = 29.5°C
Sea surface temperature for the Bay of Bengal varies by 2oC from north to south. This result is supported by the increase O gradient to the south seen in figure 2. This means small changes in temperature can affect the gradient.
Difference in O values between the present day and Last Glacial Maximum
Figure 11: A graph to show the difference in O between the present and at LGM.
Figure 12: Left is a diagram of O values for the Bay of Bengal at present. Right is a diagram of O at the last glacial Maximum.
From Figure 11 the difference in O from present day to LGM can be seen. In the present day O values decrease closer to the coast of the Bay of Bengal. This pattern is repeated at the LGM however here the overall levels of O are higher and less varied. This can also be seen in Figure 12.
The last glacial maximum was 20,000 years ago. At this period O values where higher than at present day. This is the result of evaporation of O causing the concentration of O to increase. The water that is evaporated into the atmosphere falls at higher latitudes away from the tropics. This results in levels of O to become further concentrated. Rivers would be frozen over at this time and therefore O would no longer be flowing into the bay, bringing with it O. All these factors result in the higher O values during LGM compared to at the present day. The geology of the area appears to have stayed the same aver the last 20,000 years and therefore this cannot have caused the rise. Global currents would be affected by ice sheets so circulation of oceans would have been less effective at this time. This may be the reason for the low amount of variation between levels of O at the equator and the coast during LGM.
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The Bay of Bengal has low levels of O. This level increase southwards towards the open Indian Ocean. The reason for the depletion in O is the input of water as a result of rivers like the Ganges. This fresh water has travelled across land picking up O which flows into the bay. Close the river mouth O values would be at their lowest. In contrast the Arabian Sea has high levels of O which decrease southwards towards the equator. The reason for this high concentration is evaporation causing the loss of O, input from the Red Sea which is rich is salt. The Indian Ocean has a lower concentration of O due to currents circulating molecules throughout the world.