page_title: Andaman Sea
section_title: Current and wave fluid dynamics
hierarchical_section_title: Andaman Sea / Current and wave fluid dynamics
caption_attribution_description: English: Temporal variation of depth of 20 degree isotherm (95 E to 96 E averaged) in metres.
context_page_description: The Andaman Sea is a marginal sea of northeastern Indian Ocean bounded by the coastlines of Myanmar and Thailand along the Gulf of Martaban and 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 southernmost end is defined by Breueh Island, an island just north of Sumatra, and communicates with the Malacca Strait.
  Traditionally, the sea has been used for fishery and transportation of goods between the coastal countries and its coral reefs and islands are popular tourist destinations. The fishery and tourist infrastructure was severely damaged by the 2004 Indian Ocean earthquake and tsunami.
context_section_description: 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 40 cm/s, 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 (strong outflow in October, but weak outflow in April). 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 (marked by the steep slope of the curve). The rise in sea surface height (SSH) 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 (= SSH anomaly – Rainfall – River Influx) could be deduced. A possible fourth factor, evaporative losses, is negligible in comparison. (Previous studies show that the annual mean freshwater gain (precipitation minus evaporation) of the Andaman Sea is 120 cm per year.) 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 20 °C 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 (averaged over latitudes 8°N and 13°N). These longitudes are chosen so that one represents the western part of the basin (94°E) and the other along the steep continental slope on the eastern side of the basin (97°E). 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 (isotherms deepen by 30m in April and 10m in October). 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 (such as westward propagating Rossby waves) and other transient eddies.
caption: Temporal variation of depth of 20-degree isotherm (95°E to 96°E averaged) in metres
exif: {}