Introduction to Lombok Strait

The Lombok Strait separates the Indonesian islands Bali and Lombok and is one of the most important straits through which water is exchanged between the Pacific Ocean and the Indian Ocean. This strait is one of the Indonesian throughflows that plays an important role in the global oceanic circulation. Transport through the Strait exhibits large seasonal variations due to variations in the atmospheric pressure gradient between the Pacific and the Indian Ocean which is a function of the monsoon. As a result, the seasonal currents through the strait are bidirectional. The main topography features inside the Lombok Strait are an island (Nusapenida) and a sill between this island and the smaller Lombok islands in the southern mouth of the strait. The current pattern consists of the superposition of the main flow and the tidal flow. In the upper 100 meters the current velocity reaches 1.5 m/s at the center of the strait and 3.0 m/s in the sill region. The ERS SAR images of this region reveal that the packets of internal waves propagate both to the north into the Java Sea where they reach the shallow waters surrounding Pulau-Pulau Kangean and to the south into the Indian Ocean. In the deep waters (deeper than 500 m) south and north of the Lombok Strait the crests of the internal waves have a regular circular shape. The propagation velocity of the internal waves in the Java Sea is about 2 m/s. In the Indian Ocean the internal wave pattern is quite irregular due to the variable currents in this sea area. Sometimes 2-3 systems of internal waves which propagate in different directions are observed in this region . Available CTD profiles (Murray and Arief, 1990) show a well-mixed surface layer and a salinity maximum at the thermocline depth. South of the sill, in situ hydrological observations detected a well-developed thermal plume with steep thermal gradients near its boundary. The plume intrudes over 30 km into the Indian Ocean. Possibly the curved bright bands which are visible south of the strait in image 4 and 5 are sea surface manifestations of this plume.