08 March, 2009

The Indonesian Through-Flow

The following post is being kindly contributed by Patrick Schwarz.

Patrick Schwarz is a Dive Instructor and owner and founder of the Scuba Seraya Resort at Tulamben on Bali’s north-east coast. He has lived in Tulamben and dived and learned to love the underwater realm around Bali Since 1997
patrick@scubaseraya.com http://www.scubaseraya.com/

The Indonesian Through-Flow:

Ocean currents that rush thru the Indonesian Archipelago from the tropical Western Pacific into the South Indian Ocean To understand why the waters around Bali are so rich, one only has to look at the geography of the area.
We have all heard of the Wallace line – a bio-geographical border separating the Australian and Asian biospheres – that runs between the islands of Bali and Lombok thru a body of water known as the Lombok Strait. Tropical temperatures, run-offs of nutrient rich volcanic fresh water streams, steep shores dropping to great depths and endless sunshine filled days, enhancing photo-synthesis, are all contributing factors. But the single most important reason why the underwater realm of this area is so unique and biologically diverse is a phenomenon known as The Indonesian Through-Flow.
Have a look at the satellite picture below: To the north of the Lombok Strait, we find ourselves in the Pacific Ocean. To the south is the Indian Ocean. These are the two largest pools of warm water in the global oceans: the one of the Western Pacific, and the other of the Eastern Indian Ocean. The Thru-Flow is transferring warm, low salinity water from the Western Pacific to the Indian Ocean. Scientists believe that 100% of the volume of water exchanged between the two oceans is carried by the Indonesian Through-Flow.
Despite its origin in the Pacific, where surface temperatures are as warm as 29 degrees Celsius, the temperature in the Thru-Flow is a ‘cold’ 12 – 16 degrees.
This also suggests that a lot of deeper, colder Pacific water is being exchanged via this Thru-Flow. It also explains why diving at Nusa Penida, which is located at the centre of a funnel between Bali and Lombok through which the Thru-Flow travels at great speeds, is considered ‘cold water diving’ for the spoiled lot among us that is used to a balmy average of 29 degrees temperatures that prevail at most other dive sites. On a recent dive at Crystal Bay on the south side of the island of Penida (facing yet another small strait the breadth of a mere river between Penida and Ceningan islands) a quick check with my dive computer (whilst finding myself face to face with a 2 ½m Mola-Mola Oceanic Sunfish at 40m depth and in an increasing current) revealed an ambient water temperature of just 17 degrees Celsius. Similar drastic temperature drops are also noticed when diving at Gili Selang, Bali’s easternmost extremity, and the trenches off western Lombok’s Gili Trawangan Island.
In recent years, scientists at the CSIRO Marine and Atmospheric Research (http://www.csiro.au/) organization of Australia that primarily studies Ocean eddies that influence Australia’s weather patterns and marine ecologies, have been extensively engaged in the observation of the Indonesian Through-Flow, using the latest technology available in oceanographic monitoring equipment that has been deployed across the straits and passages of the Indonesian archipelago. Water entering the internal Indonesian seas via the Makassar Strait and Lifamola Passage originates in both, the North-Pacific and the South Pacific Oceans. It exits into the Indian Ocean primarily via the Lombok Strait, the Sape Strait that runs between Eastern Sumbawa and the island of Komodo, the Ombai Strait that separates the islands of Kalabati and Timor at Alor, and the Timor Strait separating the island of Timor and North Australia. The Indonesian Through-Flow consists primarily of North Pacific water flowing thru Makassar Strait in the upper 500m, while water below is mostly of South Pacific origin. Whilst rushing thru the Indonesian Archipelago, the Pacific Ocean waters are being mixed due to upwelling and interaction with surface waters before being ‘exported’ into the Indian Ocean. Moorings that were deployed in the Makassar Strait by CSIRO from 1996 to 1998 showed a southward flow of a mass of water equaling that of 90 Amazon Rivers! The outflow measured going thru the various passages varied from a low of less than 45 Amazon River ‘equals’ (when down welling Indian Ocean winds reversed the flow) to more than 250 Amazon River ‘equals’ during the month of August of the La Nina year of 1998. Most of the ‘exit’ thru the passages occurs in a surface layer from just below the surface to 300m depth.
When we look at the depth charts of the Lombok Strait, the enormous energies of this Indonesian Through-Flow phenomenon are reaffirmed. Where the Thru-Flow enters the strait, it is not only ‘squeezed’ by the funnel created by Bali’s eastern coast (with Gili Selang at its extremity) and Lombok’s south-western isthmus, but at the same time, the bottom rises abruptly. While the depth averages 1500m as it enters the strait, it ‘shallows’ to around 400m at its centre, just where the ‘squeeze’ between Bali and Lombok is the tightest and the 3 islands of Penida, Ceningan and Lembongan stand in its way, to immediately drop to 1500m again and into the Lombok Basin (3000m) just a few kilometers further south, and on to the Java Trench where the depth doubles once more (6000m) into the Indian Ocean. No wonder the currents around the Penida dive sites are often described as ‘ripping’, and that’s putting it mildly. I do not wish to shout the idea into the open; I love the natural beauty of the Penida area too much for that, as some of its dive sites there are considered by many – together with Komodo and Raja Empat – as probably the most exciting anywhere in the world. Yet the tidal energies that prevail in the area probably would be sufficient to supply most of Indonesia, if not a large part of Asia, with electrical power. It is at this junction – where the flow enters the Lombok Strait – that the long haul migrating pattern of Whale Sharks is determined. The majority of them journey through the Indonesian archipelago riding the Thru-Flow to join the South Equatorial Current and eventually show up in the Maldives, Madagascar, and the African East Coast and round the Cape of Good Hope. Yet some of them ‘don’t make it’ in that direction and instead head east or west to perhaps enter the Indian Ocean via the Sunda Strait which separates Java and Sumatra, past the Island of Krakatoa. Some might even continue heading further north and end up in the South China Sea and the Philippines and perhaps even the North Pacific again. Those heading east might enter the Indian Ocean via the Timor passage. Whether whale sharks decide to continue into the Indian Ocean or head West (those are the few we occasionally encounter in the Tulamben area) out of their own free will or whether they are being ushered say, by an eddy created by prevailing weather patterns such as when a down welling in the Indian Ocean reduces or even reverses the Indonesian Through-Flow, we just don’t know. We still have got so much to learn!
Even less is known about the migrating patterns of the Mola-Mola (Oceanic Sunfish). They are not pure filter feeders like the Whale Shark; besides zooplankton, they also eat fish, mollusks, jellyfish and crustaceans. Although one would assume that they follow similar patterns, there is a difference. Whale Sharks are mostly found in shallower, warmer surface waters. Mola- Mola, on the other hand, are found in or near cold currents only, with the deepest recorded sighting being that of a Mola-Mola feeding at a depth of 480m. The ‘season’ when we can see them around Penida Island is during the southeast Monsoon (June-November) when the currents run cold. My wild guess is that the Mola-Mola are being ‘catapulted’ by the Indonesian Through-Flow thru the Lombok Strait, and when the bottom rises around the islands and they find shelter from the current in an eddy that is cold enough for comfort, they stop so that banner fish (a reef fish variety) can clean them by picking parasites off their encrusted skins. No one knows where they move from here – whether they follow the Thru Flow into the Indian Ocean and onto long haul transoceanic voyages – or linger regionally. Intensive research into Mola-Mola has only just begun. The satellite signal emitting float from one specimen that had been tagged in October 2006 near Nusa Penida has re-surfaced 800 km to the east near Sapu island, off the main island of Sumbawa in January 2009. If this one particular fellow becomes part of a general migrating pattern that might evolve as more of these tags eventually re-surface from the deep, one would assume that they are regional rather than inter-oceanic travelers. Again, we still have got so much to learn!
As it enters the Indian Ocean, the Indonesian Thru-Flow feeds the South Equatorial Current, the dominant westward flow across the South Indian Ocean. A shallow component of the South Equatorial Current flows back eastwards towards Australia. There it feeds the south flowing Leeuwin Current, whose fresh supply of warm waters profoundly impacts Western Australia’s coastal climate. It appears that, thanks to the phenomenon called The Indonesian Through-Flow, not only do we have phenomenal dive sites in ‘this neck of the woods’, but bio-geographically unaware landlubbers across the Indonesian Archipelago and Western Australia have a lot more in common than most would think.

Photo credits

Trough Flow map : CSIRO Marine and Atmospheric Research (www.csiro.au)


tara gebleg said...

Hi, I'm a student majoring in Oceanography, i want to ask you, where did you found the reference for your post? I'm interested on Indonesian Through-flow.

Frog Man said...

Hi Tara, I have already been in touch with the author to flag up your question. Best regards. Frog Man

Anonymous said...

Hello Tara, that's a wonderful subject you are majoring in! Wish I was younger...
The information I got on the Indonesian Thru Flow is primarily thru personal observations while diving the area for many years, talking to other (mostly dive) professionals and articles in nature and science and dive magazines. From a scientific stand point, the most information may be obtained from the CSIRO Marine and Atmospheric Research Institution in Australia. Go to their website www.csiro.com. Good luck with your studies, and let me know when you come up with something exciting, as I intend to learn more on the subject myself. Patrick