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of the region is the feason why it has drawn many major international oceanographic
expeditions, such as those of the CHALLENGER (1872-75), the GAZELLE (1875), the
VALDIVIA (1899), the SIBOGA (I899-f900), the PLANET (1905-7), the SI.IELLIUS (I929-tO),
the ALBATROSS (1948), the SPENCER F. BAIRD (1947-50), and the CALATHEA (f951.). In
recent years, a few oceanographic cruises have been organized, locally or as part of some
cooperative regional studies, such as the lntecAovernmental Oceanographic Cornmission (lOC)
Cooperative Study of Kuroshio which also covers the South China Sea, and the International
Indian Ocean Expedition (llOE). We are thus fortunate to have a fairly good picture of the
general oceanographic characteristics of Lhese waters (Wyrtki, 196I; Soegiarto and Birowo,
L975\
Monsoonal influence on oceanoqraphic features
Located between the Asian and the Australian continents, the South-East Asian region
is strongly influenced by the monsoons. The South-East Asian waters are lhus ideal for
studying the effects of the monsoons on both waLer circulaLion and the seasonal distribution
of physical, chemical, and biological properties.
The equatorial pressure trough moves according to the position of lhe sun, crossing the
Equator twice each yean. In the norlhern summer, a low pressure area develops over the
Asian continent as an extension of the equatorial pressufe trough. In winter, a high pressure
area is formed over the continent, forming part of Lhe subtropical high pressure system. The
monsoons develop between the winler hemisphel'e rrhiqhrr and the "low" in the other
hemisphere. Because the pressure distribution is stationary, the winds are rather constanl,
especially over the sea. The wind forces ate, however, generally small. Storms and typhoons
are observed only over the northern parts of the South China Sea and the Philippines, over
lhe Andaman Sea, and north of Australia. During the intermonsoon period when the equatorial
lrough passes over the Equator, the winds over the region are generally extremely variable.
During the full monsoon the trough is deviated over land in the direction of the monsoon,
owing to thermal influences.
The north monsoon in South-East Asia lasts from December to February and the south
monsoon from June to Augusl. The rest of the yean fepresents the tnansition from the north
io the soulh monsoons (March-May) and from the south to the north rpnsoons
(September -November).
The variation of
corresponding variation of
the atmospheric circulation described above parallels the
the water circulalion. Because of the high constancy of the
monsoons and the regularity of appearances, the ocean currents show the same
characteristics. Just as the monsoons change direction twice a year and are praclically
neversed at the time of their strongest development, the oceanic circulation is also reversed
over large areas. This complete reversal is typical of the circulation in these waters. The
f ollowing is the description of the surf ace current systems in South-East Asian waters
(FAO/IPFC Secretariat, 1976).
When the south rnonsoon prevails, norLherly monsoon currents are dominant in the
middle portions of the South China Sea and Lhe Java Sea. The inflow of oceanic water is
strong through the Celebes Sea and the Flores Sea from the Pacific. The waler of the South
China Sea flows out through the SLrait of Taiwan (Formosa) and the Luzon Sbait. During the
north monsoon a southerly flow of water causes a eyclonic pattern of surface water
movement. In this season, the inflow of oceanic water is strong through the Taiwan Strait
and Luzon Strait. The outflow from the South China Sea is strong through the Flores Sea and
less strong, but with considerable volune, from the Celebes Sea to join the water mass from
the South China Sea flowing eastwards. In both monsoons, smaller amounts of water enter the
South China Sea through the Philippine Islands from Lhe Pacific and flow out to the Indian
Ocean through the Malacca Strait and the Sunda Strait.
As mentioned earlier, the water maeg of the South-East Asian region originates from
the Pacific Ocean. This is also clearly indicated by surface current patterns in.this region
(Figures 2 and J). The North Equatorial current flows westwards and, upon approaching the
Philippine Islandsr plits into two main branches: the nonthward branch becomes the Kuroshio,
and the southward branch the Mindanao current.