The JS is a shallow shelf sea, located between Kalimantan Strait in the north, Java in the south, Sumatra in the west, and the Flores Sea in the east. It is entirely located on the Sunda Shelf and the sea bottom is almost flat with an average depth of only 46 m. It is connected to the Indian Ocean by the Sunda Strait in the west and Bali Strait in the east, both straits being shallow and relatively narrow with limited water exchange.
Being shallow, its temperature and salinity is strongly affected by the monsoon (Kida & Richards, 2009;Napitu, Gordon, & Pujiana, 2015). In winter, the sea surface temperature (SST) is around 29.0°C and the sea surface salinity (SSS) is between 31.3 and 32.0, whereas in summer, the SST only reaches 28.5°C (Qu, Du, Strachan, Meyers, & Slingo, 2005) and SSS shows a strong west-east increase from 32.0 to 34.0 (Wyrtki, 1961), indicating the intrusion of more oceanic water during the SE monsoon.
The sea surface height (SSH) in the JS is directly related to monsoon activity. In winter, SSH in the southern South China Sea and in adjacent western JS increases due to the blocking effect, caused by the south-eastward flow across the very shallow Sunda Shelf. In summer, the SE monsoon pushes the water to the northern South China Sea and the resulting outflow through the Kalimantan Straits leads to a falling SSH. The amplitude of this seasonal signal is around 20 cm (Tkalich, Vethamony, Luu, & Babu, 2013). Superimposed to this is a geostrophic adjustment, leading to a northward SSH gradient in winter and a southward one in summer.
The JS is influenced by semidiurnal tides from the Indian Ocean and a diurnal tide from the Pacific, which intrudes through the Makassar Strait and Molucca Sea (Wyrtki, 1961). Due to the shallowness of the JS, both tidal waves are relatively slow, needing >1 day to traverse it. The M 2 tidal amplitude is not >30 cm while the amplitude of the K 1 tide is significantly larger at over 60 cm close to the western and eastern boundaries, and <10 cm in the central JS (Ding, Bao, Yu, & Kuang, 2012). Tidal currents are approximately 0.5 m s -1 (Ray, Egbert, & Erofeeva, 2005), causing a significant anticlockwise rotation in the western Java Sea (Ding et al., 2012).
Currents in the JS show a pronounced seasonality (Gordon, 2005). In winter, water enters from the South China Sea through the Kalimantan Strait, moving eastward to the Flores Sea. In summer, currents are reversed exhibiting a weaker, westward flow. These general flow patterns do not show a strong spatial variability, though in bays close to the coast (like Jakarta Bay), minor countercurrents occur.
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