Andaman Sea

The Andaman Sea is a marginal sea of the northeastern Indian Ocean bounded by the coastlines of Myanmar and Thailand along the Gulf of Martaban and the west side of the Malay Peninsula, and separated from the Bay of Bengal to its west by the Andaman Islands and the Nicobar Islands. Its southern end is at Breueh Island just north of Sumatra, with the Strait of Malacca further southeast.
Traditionally, the sea has been used for fishery and transportation of goods between the coastal countries and its coral reefs and islands, which are popular tourist destinations. The fishery and tourist infrastructure was severely damaged by the December 2004 Indian Ocean earthquake and tsunami.

Geography

Location

The Andaman Sea, which extends over 92°E to 100°E and 4°N to 20°N, occupies a very significant position in the Indian Ocean, yet remained unexplored for a long period. To the south of Myanmar, west of Thailand, and north of Indonesia, this sea is separated from the Bay of Bengal by the Andaman and Nicobar Islands and an associated chain of sea mounts along the Indo-Burmese plate boundary. The Strait of Malacca forms the southern exitway of the basin, which is wide and deep.

Extent

The International Hydrographic Organization defines the limits of the "Andaman or Burma Sea" as follows:

On the Southwest. A line running from "Oedjong Raja" in Sumatra to Poeloe Bras and on through the Western Islands of the Nicobar Group to Sandy Point in Little Andaman Island, in such a way that all the narrow waters appertain to the Burma Sea.
On the Northwest. The Eastern limit of the Bay of Bengal through the larger islands of the Andaman group, in such a way that all the narrow waters between the islands lie to the Eastward of the line and are excluded from the Bay of Bengal, as far as a point in Little Andaman Island in latitude 10°48'N, longitude 92°24'E].
On the Southeast. A line joining Lem Voalan in Siam , and Pedropunt in Sumatra.

Oedjong means "cape" and Lem means "point in the Dutch language on maps of the Netherlands East Indies. Lem Voalan is the southern extremity of Goh Puket.

Exclusive economic zone

in Andaman Sea:

Number	Country	Area
1	' - Andaman and Nicobar Islands	659,590
2	' - Mainland	511,389
3	' - Andaman Sea	118,714
4	' - Northeast Aceh	76,500
Total	Andaman Sea	-

Channels

See Channels in Bay of Bengal, Laccadive Sea, and Arabian Sea.

Geology

The northern and eastern side of the basin is shallow, as the continental shelf off the coast of Myanmar and Thailand extends over . About 45 percent of the basin area is shallower, which is the direct consequence of the presence of the wider shelf. The continental slope which follows the eastern shelf is quite steep between 9°N and 14°N. Here, the perspective view of the submarine topography sectioned along 95°E exposes the abrupt rise in depth of sea by about within a short horizontal distance of a degree. Isobaths corresponding to are also shown in the figure to emphasize the steepness of the slope. Further, it may be noted that the deep ocean is also not free from sea mounts; hence only around 15 percent of the total area is deeper than.
The northern and eastern parts are shallower than due to the silt deposited by the Irrawaddy River. This major river flows into the sea from the north through Myanmar. The western and central areas are deep. Less than 5% of the sea is deeper than, and in a system of submarine valleys east of the Andaman-Nicobar Ridge, the depth exceeds. The sea floor is covered with pebbles, gravel, and sand.
The western boundary of the Andaman Sea is marked by islands and sea mounts, with straits or passages of variable depths that control the entry and exit of water to the Bay of Bengal. There is a drastic change in water depth over a short distance of, as one moves from the Bay of Bengal to the vicinity of islands and further into the Andaman Sea. Water is exchanged between the Andaman Sea and the Bay of Bengal through the straits between the Andaman and Nicobar Islands. Out of these, the most important straits are Preparis Channel, Ten Degree Channel, and Great Channel. PC is the widest but shallowest of the three and separates south Myanmar from north Andaman. TDC is deep and lies between Little Andaman and Car Nicobar. GC is deep and separates Great Nicobar from Banda Aceh.

Ocean floor tectonics

Running in a rough north–south line on the seabed of the Andaman Sea is the boundary between two tectonic plates, the Burma Plate and the Sunda Plate. These plates are believed to have formerly been part of the larger Eurasian Plate, but were formed when transform fault activity intensified as the Indian Plate began its substantive collision with the Eurasian continent. As a result, a back-arc basin center was created, which began to form the marginal basin which would become the Andaman Sea, the current stages of which commenced approximately 3–4 million years ago.
The boundary between two major tectonic plates results in high seismic activity in the region. Numerous earthquakes have been recorded, and at least six, in 1797, 1833, 1861, 2004, 2005, and 2007, had a magnitude of 8.4 or higher. On 26 December 2004, a large portion of the boundary between the Burma plate and the Indo-Australian plate slipped, causing the 2004 Indian Ocean earthquake. This megathrust earthquake had a magnitude of 9.3. Between of the boundary underwent thrust faulting and shifted by about, with the sea floor being uplifted several meters. This rise in the sea floor generated a massive tsunami with an estimated height of that killed approximately 280,000 people along the coast of the Indian Ocean. The initial quake was followed by a series of aftershocks along the arc of the Andaman and Nicobar Islands. The entire event severely damaged the fishing infrastructure.

Volcanic activity

Within the sea, to the east of the main Great Andaman island group, lies Barren Island, the only currently active volcano associated with the Indian subcontinent. This island volcano is in diameter and rises above sea level. Its recent activity resumed in 1991 after a quiet period of almost 200 years. It is caused by the ongoing subduction of the India plate beneath the Andaman island arc, which forces magma to rise in this location of the Burma plate. The last eruption started on 13 May 2008 and continues. The volcanic island of Narcondam, which lies further north, was also formed by this process. No records exist of its activity.

Sediments to the Sea

Collectively, the modern Ayeyarwady and Thanlwin There is little modern sediment accumulating on the shelf immediately off the Ayeyarwady River mouths. In contrast, a major mud wedge with a distal depocenter, up to in thickness, has been deposited seaward in the Gulf of Martaban, extending to approximately water depth into the Martaban Depression. Further, 2) There is no evidence showing that modern sediment has accumulated or is transported into the Martaban Canyon; 3) a mud drape/blanket is wrapping around the narrow western Myanmar Shelf in the eastern Bay of Bengal. The thickness of the mud deposit is up to nearshore and gradually thins to the slope at water depth, and likely escapes into the deep Andaman Trench; 4) The estimated total amount of Holocene sediments deposited offshore is approximately. If we assume this has mainly accumulated since the middle Holocene highstand Unlike other large river systems in Asia, such as the Yangtze and Mekong, this study indicates a bi-directional transport and depositional pattern controlled by the local currents that are influenced by tides, and seasonally varying monsoons winds and waves.

Climate

The climate of the Andaman Sea is determined by the monsoons of southeast Asia, as the prevailing winds reverse with the start of either season. The region experiences north-easterlies with an average windspeed of from November to February. During these months, the western part of the domain experiences maximum wind intensity. It weakens by March–April and reverses to strong south-westerlies from May to September, with mean wind speeds touching in June, July, and August, distributed near-uniformly over the entire basin. Wind speeds plummet by October and switch back to north-easterlies from November.
Air temperature is stable over the year at in February and in August. Precipitation is as high as /year and mostly occurs in summer. Sea currents are south-easterly and easterly in winter and south-westerly and westerly in summer. The average surface water temperature is in February and in May. The water temperature is constant at at the depths of and below. Salinity is 31.5–32.5‰ in summer and 30.0–33.0‰ in winter in the southern part. In the northern part, it decreases to 20–25‰ due to the inflow of freshwater from the Irrawaddy River. Tides are semidiurnal with an amplitude of up to.
The effect of wind stress on the ocean surface can be explained by wind stress curl. The net divergence of water in the ocean mixed layer results in Ekman pumping. The contrast between the two seasons elicits a very strong negative pumping velocity of more than per day along the north coast of Indonesia from May to September. This may signify coastal downwelling in the summer. It is also observed that the region develops a weak but positive pumping velocity of less than per day at the mouth of GC in winter.

Current and wave fluid dynamics

Generally, currents are found to be stronger in the south than any other part of the basin. An intense surface outflux through GC, of the order of, occurs during summers and winters. While this flow is directed westwards in winter, it is southwards along the west coast of Indonesia in summer. On the other hand, the TDC has strong surface influx in summer, which weakens by October. This is followed by a sturdy outflux in winter, which wanes by the month of April. Although the surface flow through PC is generally inward during summer monsoon, the preceding and succeeding months experience outflow. During April and October, when the effects of local winds are minimal, Andaman Sea experiences the intensification of meridional surface currents in the poleward direction along the continental slope on the eastern side of the basin. This is characteristic of the propagation of Kelvin Waves.
It is observed that the water level rises in the basin between April and November with the maximum rate of piling up of water during April and October. The rise in sea surface height is attributed to rainfall, fresh water influx from rivers, and inflow of water through the three major straits. The first two of these are quantifiable and are hence expressed in volumes of water for comparison. From this, the expected influx through the straits could be deduced. A possible fourth factor, evaporative losses, is negligible in comparison. It is found that the SSH of the basin is primarily determined by the transport of water through the straits. The contributions from rainfall and rivers become substantial only during summer. Hence, a net inward flow occurs through the straits between April and November, followed by a net outward transport until March.
The basin has a very high rate of transport of water through the straits in April and October. This is a period of equatorial Wyrtki jets, which hit the coast of Sumatra and reflect back as Rossby waves and coastal Kelvin waves. These Kelvin waves are guided along the eastern boundary of Indian Ocean, and a part of this signal propagates into the Andaman Sea. The northern coast of Sumatra is the first to be affected. The isotherm which deepens during the same period is suggestive of the downwelling nature of Kelvin waves. The waves further propagate along the eastern boundary of the Andaman Sea, which is confirmed by the differential deepening of the 20-degree isotherm along longitudes 94°E and 97°E. These longitudes are chosen so that one represents the western part of the basin and the other along the steep continental slope on the eastern side of the basin. It is observed that both these longitudes experience deepening of the isotherms in April and October, but the effect is more pronounced at 97°E. This is a concrete signature of downwelling in the basin and is definitely not forced locally as the winds are weaker during this period. This confirms unequivocally that the sudden burst of water into the basin through the straits, the intensification of eastern boundary currents and the coincidental deepening of isotherms in April and October are the direct consequence of the propagation of downwelling Kelvin waves in the Andaman Sea, remotely forced by equatorial Wyrtki jets. The evolution of vorticity in the basin is suggestive of strong shear in the flow during different times of the year, and further indicates the presence of low frequency geophysical waves and other transient eddies.