The average depth the Andaman Sea is approximately 1,000 meters (3,200 ft), while the western and central areas are particularly deep at 900 to 3,000 meters (3,000–10,000 ft). The northern parts are much shallower due to the silt deposited by the Irrawaddy River, as are the coastal areas of Myanmar and Thailand due the continental shelf.
At an average salinity of 32 parts per thousand, the Andaman Sea is especially salty. Due to the fresh water entering the sea from the Irrawaddy River in the extreme north of the sea, slightly higher salinity occurs in southern areas near Thailand. The influx of cool, fresh water is a contributing factor to the development of low pressure systems and cyclones in the region (see cyclogenesis below).
Along with the Nicobar island chain, The Andaman Islands form a natural back-arc basin which defines the Andaman Sea. The western area of the sea is dynamic with seismic activity along a zig-zag north-south line where the seabed demarks the boundary between the Burma plate and the Sunda Plate.
As a result of the 2004 Indian Ocean earthquake, the sea floor was uplifted by several meters in some areas. This area is home to the only active volcano connected with the Indian subcontinent.
Deep sea currents
Deep sea currents in the Andaman change considerably with the monsoon seasons from south-easterly and easterly during winter months (December to March), and south-westerly and westerly during summer months (June to September). The changes in currents affect sea temperatures and salinity in various parts of the sea.
Peculiar to the Andaman Sea is the occurrence of ‘internal waves’, which are essentially underwater waves which can travel across the sea and sometimes surface to form the mysterious ripples recorded by early seafarers in the region. Caused by the mixing of different water temperatures and densities in relation with deep-sea currents, internal waves are comparable to oil and vinegar in a jar: when lightly shaken, a sub-surface wave forms where the different fluid densities meet.
Cyclogenesis is the birth of large spinning storms, a low-pressure weather phenomenon particularly dynamic to the Andaman Sea. (Kumar et al, 2008) Although cyclones are normally associated with a weather phenomena related to the equator, the Bay of Bengal and Andaman Sea are potentially energetic for the development of strong cyclonic storms and account for about seven percent of the total number of cyclones in the world annually (Mohanty et al., 1994).
In teaching Thai Geography for nearly a decade, I find that this topic, that is, giant storms often generated just northwest of Thailand's Andaman Coast, is rarely ever mentioned, save for my classroom discussions. With some of the largest and most destructive storms in the world are generated near Phuket, why aren't Phuketians aware?
In my experience, the reason is that these weather systems are not on most citizen's radar is that as they form, they actually draw the clouds and moisture away from Phuket, pulling them anti-clockwise into the center of the storm. Thus, as Andaman cyclones form, Phuket tends to have spectacularly clear weather. While this is certainly not always the case, according to historical records dating back to 1000 AD, not a single Andaman-born cyclone has moved towards Phuket. Rather these storms characteristically track in a west-northwesterly direction toward the Andaman Islands, making landfall in India, Bangladesh or Myanmar.
Exemptions to the rule include Pacific-born hurricanes that cross the Malaysian Peninsula and enter the Andaman Sea, such as Harriet in 1962, the deadliest tropical cyclone in Thai history, responsible for nearly 1,000 deaths.
Worthy of mention, cyclones born in the Andaman Sea have long lifespans and are among the most devastating in history (
. For example, Cyclone Nagris which hit Myanmar on May 2, 2008.
Thai surfers – watch for rare, yet epic, cyclone-generated north-westerly swells hitting southerly provinces either early or late in the surf season!