The transfer of upper ocean waters from the Pacific Ocean to the Indian Ocean and through the Indonesian Seas is call the Indonesian Throughflow.
For any person who wants to learn to scuba dive a basic understanding of the Indonesian Throughflow is necessary. Scuba diving can be great in some areas of the Indonesian Throughflow than others, so it is essential to pay attention to this factor. The sea level on the side of the Pacific Ocean is 150 millimetres above average and to the south, the Indian Ocean, the sea level is 150 millimetres below average. Both are worthy of being noted.
The trade winds and ocean currents that run in opposite directions cause the 30 centimetre difference. Imagine the largest movement of water on the planet which flows through the Indonesian archipelago from the Pacific Ocean to the north-east to the Indian Ocean in the south-west and that describes this difference.
Cubic meters and gallons are not justified enough to describe the Throughflow. Because of the inability to express the volumes of water the Norwegian scientist, Harald Sverdrup, came up with the “Sverdrup” which describes: one million cubic meters of water per second. In other words, think of a river that is 100 metres wide, 10 metres deep and flows at speeds of 4 knots, then imagine 500 of those rivers all combined together and you will have the description one Sverdrup!
One Sverdrup is a lot of water. Imagine 20 to 22 Sverdrups, or 10,000 rivers and that is the estimation of the amount of water flowing through the Indonesian Throughflow is between 20 to 22 Sverdrups, or 10,000 of those rivers. This massive flow of water makes its way around the chain of islands along the bottom part of the Indonesian archipelago, or hte Lesser Sundas.
The Lombok Strait offers the most direct path to the Indian Ocean. Throughflow passes through that conduit and equals about 1500 rivers.
This vital water carries the eggs and larvae life of the marine life of the Indo-Pacific. This is over 4000 identified species. There is an abundance of biodiversity and explains Bali’s beautiful reefs and dive sites. The Throughflow touches around Raja Ampat in Irian Jaya, Halmahera and North Sulawesi and brings the greater part of eggs & larvae.
Just as on Earth, in the ocean everything that dies contributes to its natural environment. It will decompose and settle to the bottom to aid in becoming a nutrient of the ocean. It’s decomposition creates a rich layer of nutrients and making for a rich and diverse environment that thrives. The same event happens to the south, within the deep basins of the Banda Sea and the Lesser Sundar Islands.
The Indonesian archipelagos underwater topography includes trenches, troughs and basins – it is very complex and deep and not as simple or shallow as one would think. Around the Lesser Sundas it is particularly complex. The Indonesian Throughflow ebbs and flows to create upwellings that carry streams of nutrients to the reefs of eastern Bali.
Of course, just like seasons, the Indonesian Throughflow varies seasonally in response to the monsoons. Variations in the throughflow affect the currents.
The Indian ocean and the Pacific Ocean, two large pools of warm water, transfer warm, low salinity water from the Western Pacific to the Indian Ocean. This is despite its origin in the Pacific, where the temperatures are as warm as 29 degrees Celsius and the temperature in the Thru-Flow is a ‘cold’ 12 – 16 degrees.
Diving at Nusa Penida is considered to be cold water diving – as many divers may know. The reason for this is possibly because of the colder Pacific water exchanged via the throughflow.
The monsoon season is June, July, and August and during that time the Indonesian Throughflow is strongest.
The Makassar Strait and Lifamola Passage play a role with the Indonesian waters. The Indonesian waters are connected to the Indian Ocean via the Lombok Strait, the Sape Strait (that runs between Eastern Sumbawa and the island of Komodo), the Ombai Strait (which separates the islands of Kalabati and Timor at Alor), and the Timor Strait (that divides the island of Timor and North Australia). The North Pacific water that flows through the Makassar Strait. The waters below are mostly of hte South Pacific origin. The Pacific Ocean waters are mixed before being sent to the Indian Ocean.
Where the Indonesian Throughflow enters the strait, it is not just “crushed” by the channel made by Bali’s eastern coast and Lombok’s South-Western isthmus, however in the meantime, the base climbs suddenly. While the profundity midpoints 1500 meters as it enters the strait, it climbs to around 400 meters at its inside, simply where the “crush” in the middle of Bali and Lombok is the tightest and the 3 islands of Penida, Ceningan and Lembongan remained in its route, to instantly drop to 1500 meters again and into the Lombok Basin (3000 meters) simply a couple of kilometers further south, and on to the Java Trench where the profundity duplicates afresh (6000 meters) into the Indian Ocean.
It is hardly surprising the currents around the Penida dive sites are ripping.This area’s dive sites are considered by many as the most exciting dive sites in the world.
At this junction the flow enters the Lombok Strait and the great Whale Sharks determine their migrating pattern. It is fascinating that the White Sharks determine their way toward a new home based on this flow. The White Sharks have a great route: joining the South Equatorial Current, then arriving in the Maldives, Madagascar, the East African Coast and the Cape of Good Hope.
Not to say that all Whale Sharks head in the same direction. Some Whale Sharks head east or west to enter the Indian Ocean. Many Whale Sharks head further north and end up in the South China Sea and the Philippines. There is also thought and speculation that they end up in the North Pacific. Those whale sharks heading east might enter the Indian Ocean via the Timor passage.
The Indonesian Throughflow provides some great dive sites. As a result of the Throughflow the biodiversity is vast and abundant. The conditions prevail and are favorable for both life in the sea and for divers who love it.