Ombai Strait (, , ) is an international strait in Southeast Asia. It separates the Alor Archipelago (part of East Nusa Tenggara province) to the west from the islands of Wetar (part of Maluku Province) and Atauro (part of the nation of Timor Leste) to the east, and from Timor to the south, all these islands forming part of the Lesser Sunda Islands. The strait is also the western portion of a pair of international straits, the other one being Wetar Strait; the two straits combine to link the Pacific Ocean with the Indian Ocean.
Etymology Ombai is an alternative name for the island of Alor, in the Alor Archipelago on the other side of the strait's north, north western and western coastline.
In Tetum, the expression tasi feto () is often used to refer to the 'Ombai-Wetar Strait', which extends along most of Timor's northern shores. The counterpart of that body of water, the Timor Sea, which has larger waves, is more turbid, and washes the whole of Timor's southern coastline, is commonly referred to in Tetum as tasi mane ().
Geography
Ombai Strait is relatively narrow and deep (). It has complex and extreme bathymetry, ranging from narrow reef flats (often less than wide) partly covered with seagrass and coral, to depths within of the coast.
The strait separates the Alor Archipelago from the islands of Wetar, Atauro, and Timor in the Lesser Sunda Islands. It also straddles the boundary between the Banda Sea (in its north east) and the Savu Sea (in its south west).
The Alor Archipelago, and the western part of Timor except the coastal exclave of Oecusse, which is on the north coast of western Timor, are part of the province of East Nusa Tenggara, Indonesia. Wetar is part of the Maluku Province of Indonesia. Atauro, Oecusse, and the eastern part of Timor comprise the nation of Timor-Leste.
The southern limit of the Banda Sea runs along the southern edge of the strait from the eastern extremity of Timor along its north coast as far south west as longitude 125° East. From that point, the Banda Sea's western limit heads north to Alor Island, where it starts a further run, east, along Alor's south coast, around its east point, and beyond.
Meanwhile, Ombai Strait continues to run from longitude 125° East in a south westerly direction, now within the limits of the Savu Sea. It extends, as part of that sea, at least as far south west as Batek Island, at the north western tip of Oecusse.
From Ombai Strait's south western, Savu Sea, end, the strait heads in a generally north eastern direction, while gradually narrowing. At its narrowest point between the Alor Archipelago and Timor, it is wide, and its navigable waters are across.
Beyond the end of the channel at the strait's north eastern extremity, between the islands of Alor and Atauro, is the Flores Sea to the north west and north, and the Banda Sea to the north east. That channel is wide at its narrowest point, where the navigable waters are across. To the east of the strait, and linking with it south of Atauro, is Wetar Strait. To the east of the latter strait are the south eastern reaches of the Banda Sea, and the southernmost of the Maluku Islands.
As such, Ombai Strait is one of two deep water passages in the Indonesian archipelago that link the waters of the Pacific and Indian Oceans, the other one being Wetar Strait. The archipelago is the only interocean connection on earth at low latitudes, and the exchange of water between the two oceans is known as the Indonesian Throughflow (ITF).
The ITF flows in a generally north east to south west direction. Its main pathway runs from the Pacific Ocean into the Banda Sea via the Celebes Sea, Makassar Strait and the Java and Flores Seas. Some of the ITF then passes directly into Ombai Strait, and some of it flows there indirectly, around Wetar Island's eastern end, and then via Wetar Strait.
By this and other ITF pathways, nutrient-rich water from the Pacific Ocean runs into the Indian Ocean, at a rate 50 times faster than the discharge of the Amazon River. During El Niño, the water is cooler and inhibits rain. In Ombai Strait, there is also a counterflowing eastwards current, which affects the strait's overall mix of nutrients, temperatures and salinity levels.
The eastwards current is an extension of the South Java Current (SJC), and its deeper Undercurrent (SJUC), from the Savu Sea into the strait. During the dry season (March–November), the strong, prevailing westwards ITF currents in the strait are at an annual maximum at the surface in the southern part of the strait, and a front appears to trap the portion of the SJC inside the strait to within ~ of the strait's northern boundary. By contrast, the SJUC, when present, is observed across the entire strait. The ITF currents are at their weakest during the north-east monsoon rainy season (December–February), when there are also strong westerly (onshore) winds and waves.
Ecology Flora
Along the northern littoral of Timor-Leste, including the strait, the shallow coastal waters are dominated by a seagrass bed about in total area. Seagrass beds protect coral reefs from sedimentation and are feeding grounds for dugong (Dugong dugon), an endangered species. Seven genera of seagrass are known to exist off Timor-Leste's coastline as a whole.
Timor-Leste's coastal waters have also traditionally hosted mangrove forests. Together with seagrasses and coral reefs, they are the primary breeding grounds for many species of fish and shellfish. However, the country's mangrove cover has very substantially declined since 1940, and in Ombai Strait is now confined to the coastline and hinterland between and the Bay of Dili.
A survey of Tibar Bay carried out in 2016 found three mangrove species, namely Sonneratia alba, which was the dominant species, Avicennia marina and Rhizophora stylosa. The report of another survey, carried out in 2017, stated that the dominant species in the area was Sonneratia albia, and that there were some clusters of Rhizophora apiculata, Ceriops tagal, and Lumnitzera.
The 2016 survey report estimated the total area of mangrove in the bay to be ; the 2017 survey report's estimate was . According to the latter report, tree density was relatively low, at around 100–400 trees per hectare, and the mangrove cluster was heavily degraded, due to direct and indirect human activities. The former activities included fish pond and salt pan development, mangrove cutting, and cattle grazing. The latter activities were sea level rise, and siltation due to erosion in the hills on the landward side of the mangroves.
In parts of Ombai Strait, and especially close to the coast of Alor Island, elevated phytoplankton blooms or concentrations (and therefore availability of food for other species) have been observed, particularly during the dry south-east monsoon. The physical structure of the strait makes it likely that such concentrations are driven by fronts and vertical flows caused by some combination of tidal currents or topographic effects.
A study published in 2002 noted persistent flow-induced upwelling and frontal features in the strait, together with subjective evidence of extremely strong tides. The study concluded that the upwelling and frontal features coincided with large phytoplankton blooms and significantly affected the concentration and distribution of chlorophyll in the Savu Sea. According to the study report, it is likely that the mean flow associated with the ITF also plays a role in causing the blooms.
Fauna The Ombai-Wetar corridor is heavily used by whales, dolphins, whale sharks (Rhincodon typus) and manta rays. The two straits combine to form a major migratory route for these species between the Pacific and Indian Oceans. Timor-Leste, on the south side of the two straits, is a recognized global hotspot for whales and dolphins.
Ombai Strait in particular has been identified as a critical habitat for sperm whales (Physeter macrocephalus), blue whales (Balaenoptera musculus), giant oceanic manta rays (Manta birostris) and sea turtles (Cheloniidae), amongst other endangered, threatened and protected species. Other species that have been observed consistently in the strait include false killer whales (Pseudorca crassidens), and spinner dolphins (Stenella longirostris), both of them in huge pods made up of large numbers of individuals.
The report of a 2013 survey of Tibar Bay, on Ombai Strait's southern shore not far west of Dili, Timor-Leste, noted that a local dive tourism operator had made frequent sightings of dugong along the coast immediately east of the bay, and that there were also anecdotal reports of saltwater crocodiles in the bay.
A short distance further east, at Tasitolu, dugongs are often observed, although detailed information regarding their habitat location, size, and characteristics in the area was not available. The other sea life at Tasitolu includes cephalopods, crustaceans, frogfish, harlequin ghost pipefish, small octopuses, rhinopias, sea horses, and soft coral crabs.
Ombai Strait is also part of the Coral Triangle, which is recognised as a global centre of marine biodiversity.Allen, G. R. 2007 Conservation hotspots of biodiversity and endemism for Indo-Pacific coral reef fishes. Aquatic Conserv: Mar. Freshw. Ecosyst. The coastal zones of Timor-Leste have important areas of coral reef ecosystems, such as the ones at Tasitolu and in the Bay of Dili. At Tasitolu, the reefs west of Dili Rock are live, healthy and diverse, but those to the east of the rock have lower coverage and more reef rubble. At the Port of Dili inside the Bay of Dili, there are at least four different species of coral.
The Dili area, on the southern edge of the strait, is a recognised nesting ground for sea turtles, of three different species: hawksbill sea turtle (Eretmochelys imbricata; Critically Endangered (CR), according to the International Union for Conservation of Nature (IUCN) Red List), olive ridley sea turtle (Lepidochelys olivacea; Vulnerable (VU)), and green sea turtle (Chelonia mydas; Endangered (EN)). On both the Tasi Tolu Beach and the Beto Tasi Beach near Dili's international airport, there is regular hatching of sea turtles. However, in most parts of Timor-Leste sea turtles are overharvested for their eggs, skin, meat and carapace for handicraft making.
The northern edge of the strait abuts two Important Bird Areas (IBAs), Gunung Muna (on Alor, Indonesia), and Atauro (Timor-Leste). The southern edge of the strait is also the northern shore of four East Timorese IBAs. From west to east, they are Be Malae (just to the west of the Loes River), Maubara (a short distance east of the village of Maubara), Tasitolu (just to the west of Dili), and Areia Branca no Dolok Oan (on the eastern side of the Bay of Dili).
Humans The village of Lamalera (pop. 2,500) on the south coast of Lembata, facing the south western entrance to the strait, is known for its hunting of sperm whale and other deep-sea species. The hunting has been taking place for at least six centuries, and is allowed under International Whaling Commission regulations concerning aboriginal whaling. However, conservationists worry that commercial whaling also takes place, and that hunters use their engine-powered boats all year round to catch other protected species such as manta rays, orcas (Orcinus orca), dolphins and oceanic sharks.
Ombai-Wetar Strait Hope Spot On World Oceans Day 2020, the Ombai-Wetar Strait was designated as a Mission Blue Hope Spot. The designation recognises the importance of conserving the open waters on the north side of Timor-Leste, with their globally-significant coral reefs and marine biodiversity. It also emphasises the potential of the two straits for sustainable marine ecotourism development.
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