The islands of Bali and Lombok may only be separated by a distance of about 20 kilometers through the Lombok Strait, but an insurmountable depth has kept the Indonesian archipelago ecologically disjointed for millions of years.

The Lombok Strait is an important part of the larger Indonesian Throughflow, connecting the Pacific and Indian Oceans through enormous amounts of water transfers between each sea. North of the strait, in the Java Sea, there are depths of about 1,000 meters, while to the south, in the Indian Ocean, the depth drops to 3 to 4 kilometers. Deep and wide seas narrow and shallow with a minimum depth of 250 meters across the strait and a width of about 20 kilometers, generating a huge constriction of the water flow – leading to intense and dangerous currents.

Figure 1: Lombok Strait bathymetry contours (depth shown in meters). Source: Indonesian Journal of Geography

Local inhabitants of these waters have known for thousands of years of the enormous and dangerous currents generated by this flow, with extreme caution required to cross the strait. The local fauna also reflects this inaccessible stretch of water. East of the strait (so Lombok and on to Australia) colorful and exotic birds such as cockatoos and parrots and marsupials can be found, more characteristic of Australia’s fauna. On the other side (that is, Bali and beyond, towards Asia) you can find far fewer colorful birds, replaced by pheasants and chickens that like to live in the jungle that spans the Indonesian archipelago. Large terrestrial mammals native to Asia such as the famous and almost extinct tigers can (or used to) be found on this side of the strait.

However, the flora have been able to overcome the treacherous strait, with plant seeds floating across without concern for travel time – a feeling similar to those who have experienced the slow and bumpy ferry between Bali and Lombok.

This distinct contrast was first noticed by the British naturalist Alfred Russel Wallace in 1859 – and later named after him: the Wallace line. The border runs across the Lombok Strait, through the Makassar Strait between Borneo and Sulawesi and east of the Philippine Islands. The split marks the separation between the Sunda Shelf (connecting Bali, Borneo, Sumatra and Java) and the Sahul Shelf (connecting Australia to islands to the north). During the Pleistocene, shallow seas across the region were easily traversed with sea levels up to 120 meters lower than present day, allowing free-flowing faunal migration within each region. The depth of Lombok Strait remained prohibitive as it is today, locking species on both sides of the Wallace Line.

Image: The probable landmasses of Sunda and Sahul when sea level was up to 120 meters lower than today, with the Wallace Line (along with the Weber and Lydekker Lines – other faunal boundaries). Source: Wikipedia Commons

Between the wet season (during the austral summer) which sees extensive cloud cover and convection, and the dry season (austral winter) which sees persistent trade winds, the seas around Indonesia become increasingly smooth. This allows a short period between seasons for space weather satellites to see amazing oceanic processes take hold. Around midday in early November, when the sun is directly overhead, internal ocean waves generated through the Lombok Strait can be seen from space.

Video: Satellite imagery video showing two sets of internal waves propagating north of the Lombok Strait on November 4.

As the tide enters the strait, the tightening bottleneck between Lombok and Nusa Penida, together with the powerful current from the Indonesia Throughflow, generate these internal oceanic waves. Zooming out reveals a number of these wave sets generated by each high tide, moving at a speed of about 100 kilometers a day.

Video: Satellite image video showing multiple sets of internal waves propagating north of Lombok Strait, along with weaker waves at the top of the image propagating northwest from the Sape Strait between east and west Nusa Tenggara in the east, on November 5.

Although these waves generally pass unnoticed under crossing ships, the effects of the tide pushing in and out of these narrow water passages create treacherous conditions. If you zoom back into the images, turbulent waters can be seen after the tide around Bali, Nusa Penida and Lombok. Avid scuba and snorkelers who have dived in the waters around these parts to see the various reefs and many manta rays have experienced the dramatic tidal currents that swirl around these islands.

Video: Satellite image video showing a set of internal waves propagating north of the Lombok Strait on November 3 with turbulent water following the current from the tide.

With the huge tourism load on the main island of Bali, tourists are heading across the Lombok Strait more often, increasing demand from operators and crowding the powerful waters between the islands. The deep waters also make the Lombok Strait an attractive alternative to the busy Straits of Malacca (between Sumatra and Malaysia) and Sunda (between Sumatra and Java), making sailors more tempted to battle the powerful currents – increasing maritime traffic across these turbulent waters.