The Andaman Sea
The Andaman Sea is a salad bowl of high-salinity water topped with waves of mixed types and sources, outlined by an unpredictable volcanic ridge, and characterized by mysteriously deep ocean upwellings and currents, internal waves, and a stealthy world-leader in cyclogenesis.
In this Learning Adventure, I investigate the Andaman Sea and surfing in Thailand, including bathymetry, tides, wave types and directions, and swell windows.
In 2006, after seeing the waves in Phuket while on holiday, I wondered if I might be able to live and work on the island.
Prior to visiting Phuket, I had been a post-graduate student in Taiwan, and it had been great; but how about studying in Phuket? After learning about the International MBA program in Hospitality and Tourism Management being offered at the Prince of Songkla University, Phuket Campus, I decided to apply. The courses started in April 2007, just in time for the surf season on the Andaman Coast.
I packed my bags – and surfboards.
I soon realized that surf tourism was a new and growing industry in Phuket, and my thesis adviser agreed it was an excellent research topic. Besides, what surfer wouldn't feel great strapping their surfboards on the car and heading to the beach to conduct field research?
Surf travel was in my blood.
Environmental concerns and the protection of surf sites had always been important to me, and my master's thesis progressed into a coastal resource assessment of Phuket surf beaches. I looked at all sorts of issues, including water pollution, marine debris, the tin mining industry, water safety, and much more.
Eventually, my work on the Andaman Coast led to a doctoral scholarship, and in the years following, I developed the Surf Resource Sustainability Index (SRSI), earning me a Ph.D. in Environmental Management.
If you feel motivated to learn more about these topics, please visit my Surf Tourism Research page.
The Southwest Monsoon
The Southwest Monsoon is the driving force behind the surf on Thailand's Andaman Coast. The month of May signals the onset of steady westerly/ southwesterly winds, occasionally gusty and accompanied by fast-moving squalls and heavy rain. However, gloomy skies and heavy downpours tend to pass quickly, replaced by pillowy convection-born cumulonimbus clouds, shimmering sunbeams, and consistent head-high surf.
The Southwest Monsoon tapers off during October, giving way to cooler northeasterly winds during November. During this period, lucky locals may experience a few days with off-shore wind pushing up the faces of clean, Indian Ocean groundswells (see sections below on wave types and swell windows).
The Andaman weather wheel, shown here, illustrates from the inner cycle outward: monsoon season, approximate average rainfall in millimeters, wind types and directions, and expected storm activity and skies.
Andaman Surf Meteorology
Surf on the Andaman coast comes from a wide-range of sources and directions, and various wave types are generated by particular sets of weather phenomena. This is to say that depending on how, when, and where waves are generated, those arriving at Thailand’s Andaman coast beaches differ significantly.
Windsea, windswell and groundswell
In this article, three types of waves are discussed in terms of swell period, referring mainly to wave interval, that is, the amount of time it takes for two consecutive wave crests to pass through a determined point. The definitions offered here are slightly adjusted to better understand what we actually see on Thailand's Andaman Coast. Exact definitions can be found across various surf forecasting websites.
- Windsea refers to waves breaking very close together, perhaps just 6 to 8 seconds apart.
- Windswell refers to mainly to short period swells averaging around 9-12 seconds apart.
- Groundswell mainly refers to longer period swells, averaging 14 seconds or more.
In the widest sense, waves are generated at different distances from the coast. Waves resulting from weather patterns occurring near the Andaman Coast generally create a windsea condition. Windsea refers to waves accompanied by the wind which generated them and may look like waves breaking one right after another, resulting in mixed wave heights, a common sight at Phuket Beaches on stormy days.
Once the windsea condition passes, and the wind dies down, a rideable windswell may remain for several hours or several days.
In contrast, groundswells generated by weather systems in the Indian Ocean may travel great distances, pass through the The Great Channel, between Banda Ache and Great Nicobar Island, and provide clean, long-period surfing waves.
If comparing the consistent, almost daily windswell arriving at Thai beaches during the Southwest monsoon, groundswells are characterized as more powerful, offering longer rides, and having more time between waves. While different wave types may be common knowledge among surfers, types of windswells and groundswells vary considerably in Thailand based on a number of factors.
For example, swells with a 15-second or more wave period, generated as far away as Madagascar in the Southern Indian Ocean, as compared to swells with a 10-second wave period generated south of Sri Lanka in the southern Bay of Bengal. Low pressure systems in the Bay of Bengal have potential to push large waves through The Great Channel or Ten Degree Channel (see maps below) with considerable force, creating big surf in Phuket, similar to what one might expect in Indonesia or Hawaii.
Worthy of note, The Great Channel is much wider, deeper, and open to Indian ocean wave activity compared to The Ten Degree Channel.
The three most obvious sources of ocean swell activity and associated swell directions relative to the Andaman coast of Thailand are as follows:
- Monsoonal wind flow which propagates southwesterly to westerly windsea and windswell.
- Groundswell generated in the southern or central Indian Ocean which produces southwesterly swells.
- Regional cyclonic activity, including tropical depressions, storms, and cyclones, which may propagate a variety of swell types and directions.
Each type of weather phenomena and its associated swell type and direction create various surfing conditions on the Andaman Coast which may range in size and ‘surfability’ from one coastal area to another. Swell direction is highly significant given that the swell window for each province varies considerably.
For example, provinces north of Phuket are open to southerly and southwesterly swell directions, compared to provinces south of Phuket, which are mainly exposed to westerly swell directions or rarely occurring northerly swells resulting from regional cyclonic activity (see Andaman Coast Swell Windows, below).
Classification of Storms in Thai Waters
The classification of weather and large storms varies from country to country around the world. For example, a tropical storm in one country may be considered a tropical depression in another country. Consequently, communication and clarification regarding the exchange of data among various national weather bureaus is of the utmost importance, especially when issuing storm warnings and in terms of public safety awareness.
In Thai waters, the following criteria apply:
- Tropical depression is categorized as a weather system which produces winds up to 59 km/hr
- Tropical storm produces winds of 60-119 km/hr
- Cyclone produces winds of over 119 km/hr
Andaman Coast Swell Windows
Primarily a factor of geography, Sumatra and the Nicobar Islands block or shadow the vast majority Indian Ocean swells from reaching Thailand’s Andaman Coast. Consequently, the surf along Thailand’s Andaman Coast is generally much smaller on a given day compared to the South-Western coasts of Sumatra, the Mentawai Islands, Java, Bali, etc., which are highly exposed to the Indian Ocean.
The news isn't all bad, a gap between Banda Aceh and Great Nicobar Island offer a ‘swell window’, an opening through which waves can pass through, such as between islands or around points of land.
In order for Indian Ocean swells to reach Phuket, they must pass through The Great Channel, a swell window limited from roughly 230 degrees west-southwest through 245 degrees west-southwest depending on a given swell direction. Although a narrow window, the Banda Aceh/Nicobar gap allows enough Indian Ocean surf though to provide year-round surprises for local surfers.
As in navigation, wind and wave directions for meteorology and swell directions follow the numbers of the compass (a 360° circle) where 0/360° is North, 90° is East, 180° is South, and 270° is West. Waves traveling from a particular source or direction are labeled as coming from that direction in terms of the compass relative to the point of arrival. This is to say that if Phuket is the arrival point, we can set the center of the compass over Phuket and measure the direction of the incoming swell (see map below).
Thailand’s Andaman Coast occasionally receives big surf generated in the northern Indian Ocean and southern Bay of Bengal. Lucky days for surfers are those when the swell direction finds an open window enters the Andaman Sea at full-face value. Such was the case for the July 2008 Kalim Surfing Contest, which saw clean overhead waves on the day of the finals. The unusually big surf came from low pressure system not far from Sri Lanka.
Ocean swells passing through The Great Channel or The Ten Degree Channel 'refract' or bend, thus changing direction upon entering the Andaman Sea and may reach coastal areas north and south of Phuket (see groundswell windows and wave refraction map).
The science of wave refraction also helps to explain why the 2004 Indian Ocean Tsunami, generated on the opposite side of Sumatra from Phuket, was able to bend around Banda Aceh, the western tip of Sumatra, and strike the Malaysian Peninsula. As tsunamis travel at great ocean depths, speeds, and volumes, the rate of refraction is additively great.
Andaman Coastal Bathymetry
Bathymetry, or seafloor topography, varies at different latitudes along Thailand’s Andaman Coast and this greatly affects wave speeds and heights. Waves approaching a particular coast from deep water travel faster and carry more energy and power than waves approaching over shallow water, such as when they pass over a continental shelf before reaching the shore.
Notably, the deepest water on Thailand’s Andaman Coast is found near Phuket; hence, Phuket generally has the most powerful waves regardless of the fact that provinces to the north may have a better swell window to the southern Indian Ocean.
All six of Thailand's Andaman provinces have a continental shelf. The shelf averages approximately 100 km wide in the north (Ranong Province), narrowing to 25 km in the middle (Phuket Province) and widening to about 130 km in the south (Satun Province).
Tides along Thailand’s Andaman Coast are semi-diurnal, meaning there are two high tides and two low tides daily, with spring heights of up to approximately 3.6 meters and neap tides down to approximately .6 meter.
Generally, the maximum tidal amplitude, or the magnitude of change in an oscillating tidal variable, in Phuket is approximately 3 meters; however in some areas of the Andaman Sea, amplitudes can reach as much as 7 meters.
The few reef breaks along the Andaman Coast are highly ‘tide dependent’ in terms of surfing. For example, these areas may become exposed reefs on low tide and have rideable waves on medium to high tides.
Conversely, waves at many beach breaks become too thick and slow on high tides, and are often better surfed on incoming or medium tides.
Exceptions to the rules occur when the waves are big – when indeed, anything goes!
Mysterious Andaman Sea
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!
Thank you for visiting my Surfing Thailand page.
I hope you enjoy my photos and the information in the links provided. If you feel motivated to travel to the Thailand, please let me know – I’d love to hear from you.
Online resources by author
- Climate Change and the Environment in Thailand...
- Coastal Currents in Phuket...
- Coastal Resource and Surfing in Thailand...
- Rare Earth vs. Rare Surf: Malaysian ‘Rare Earth’ Refinery...
- Surfer-lifesavers of Phuket...
- Surfing Rayong: Wave magnet of the Eastern Gulf...
- Surfing the Southern Gulf...
- The Phuket Ocean Safety Guide...
- Trash talking: Exploring Marine Debris on the Andaman Coast...
- Water Safety Awareness for the Hospitality & Tourism Industry...