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Figure 6 : Surface temperature
Salinity
- 180
of South-East Asian
(in oC)
waters during June to SepLember
In contrast to the uniform Lemperature in South-Easl Asian waters' the salinity is
extremely variable. The high rainfall, the runoff of many large rivers, and the geographical
subdivisions of the seas are responsible for this characterisLic. The high rainf all lowers the
salinity of Lhe surface layer. The distribution of dischanges fnom land and the presence of
large bays and channels with lillle water exchange favour the formation of regions wilh very
low salinity, which conlribute to the general lowering of the salinity. The monsoons cause a
rainy and a dry season which then affecl the annual variation of salinity. But monsoons also
govern the seasonal water circulaLion. These interactions between different factors and
influences, the geographical siructures, runoff from rivers, evaporationr and circulation result
in a highly complex distnibution and marked variation of the salinily in Lhese waters.
Figure 7 shows the averaged surface salinity distribution in South-East Asian watens
during the norlh monsoon (December-May). ln general, low salinity prevails oven the Sunda
Shelf and the coaslal areas. This low salinity is mostly due to the heavy rainfall and ri ver
discharge. During the south monsoon (June-November), however, hiqh salinily prevai ls in
almost all South-East Asian waters (Fiqure 8). Ttu Java Sea is a good example of how the
monsoons affecl surface cinculation and salinity variation (Doty and Soegiarto, 1970)' During
the north monsoon, heavy rainfall occurs throughout the west-enn parts of the lndorresian
Archipelago. The heavy rainfall, combined vrith the large runoff of many ri vers from Sumatra.
Java, and Borneo, resu lts in a general lowering of the salinity level near the shore.
Sometimes even the )0 parls per thousand isohaline is pushed far towards the open sea. At
the same time, the surface current frorn the South Shina brings low salinity water into the
western Java Sea and pushes the high salinity wa[er easLwards. With Lhe onset of the south
monsoon, this low salinity water is pushed back westwards and replaced by waLer masses of
hiqher salinity from the Makassar Strait and the Flores Sea. By September, lhe water masses
of high salinity reach their maximal westward penetration.