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(f) Senri-diurnal type: Dailyr two high and two low waters of almost equal heightr e.9.r
Bagan Siapi-api (Riau Archipelago) and the Malacca Strait.
(2) Mixed tide, prevailing semi-diurnel: Daily, two high and two low waters, but different
both in height and the time of high water; e.9., Sandakan and Sulu Sea.
(l) Mixed tide, prevailing diurnel: Only one high and me low water daily, but there are
also regimes with two high and two low waters which differ appreciably in height and
in the time of occurrence of the high water; e.9., Hm Nie Nieu, Vietnam.
(4) Diurnal Tide: Only one high and one low water daily; e.g., Gutf of Tmkin.
Since in the Indian Ocean the semi-diurnal type of tide is pnedominant, the Andaman
Sea' the Malacca Strait, and Lhe shelf off northwest Australie have semi-diurnal tides. Other
regions, such as the south coast of Sumatra, Java, and the Nusa Tenggara Islands, have mixed
tides with prevailing semi-diurnal. In the western Pacific Ocean, the diurnal tide is dominant.
Hor,rrever, this changes as soon as the tides enter South-Eest Asian waters. Alrnost the entire
South China Sea, for example, experience a mixed, prevailing diurnal tide. In other waters,
the diurnal tide is strengthened. Thus, in the Gulf of Thailand, the Gulf of Tonkin, the
waters between Sumatra and Borneo, and the Java Sea, an almost purely diurnal tide is
observed. In eontrast, over the eastern parts of the Indonesian Archipelago the mixed,
prevailing semi-diurnal tide is dorninant.
General properties of the water
Diacontinuity layer
Since the South-East Asian region is located near the Equator, the zurfaee water is
characterized by high temperat.ures. This property combined with the influence of low salinity
reduces the density of the surface water rather markedly. The large excess of rainfall over
evaporation causes an average salinity of less than J4 parts per Lhousand within a region
enclosed by a line running from Sri Lanka, off the islands of Sumatra, Java, Celebes, and
Philippines to Taiwan (Wyrtki, f961). The density of this water always nemains below Sigma t
22.0 (or a specific gravity of about 1.022). This light tropical surface water contrasts
strongly with Lhe cold water rnasses in deeper layers with Sigma t of 27.7 to 27.8. The
transition between these two water masses, usuatly called the discontinuity layer, takes place
between 100 and f00 melres. This stable discontinuity layer pnactically prevents any vertical
exchange of water. It is only in a few places and in certain seasons lhat this stable
stratification is disturbed, eithen by extremely strong wind or by upwelling proeesses.
The properties of the water at the surface normally extend downwards to a certain
depth before the transition to colder water takes place. This upper homogeneous layer of
water is mixed by the action of winds and, in some cases, by currents and tides. Below this
layer' a change of waten properties, especially an increase in the density, begins, at first
gradually' later rather rapidly until a maximal density gradient is reached at the centre of
the discontinuity layer. Below this, the density of the water continues to increase, but rather
slowly, until it reaches the deep cold water.
The dept.h of the homogenous layer ls of interest in dynamic oceanography and is of
relevance in marine productivity. Over the Sunda and Sahul shelves the homogeneous layer
reaches the botlom of the shallow parts. In the deeper parts, high density water is found
below the homogeneous layer at a depth of about 40 metres. In the Celebes, Sulu, and Flores
seas, the homogeneous layer is at about. the same depth throughout lhe year. However, in the
South China Sea and in the Eastenn Archipblago, the seasonal variation is more pronounced.
In the Banda Sea and the Arafuna Sea, the depth of the homogeneous layer is about 20 to 50
melres in October and down to about ]00 metres in March. A similar range is reconded for
the South China Sea where the homogeneous layer is only about l0 to 40 rnetres deep in the
south monsoon, but increases to 70-90 metres during the strong north monsoon.
The thickness of the discontinuity layer is affected primanily by dynamic processes. In
a large eddyr the warm water masses are pressed downwards, causing e thick discontinuity
layer. In the eastern archipelago, the discontinuity oven the year is of a uniform thickness of
about 100 metres. Tfre sharpest discont,inuity, at 20 metres depth, is found off the Sumatra
coast in October, when the Counten-Current turns along a broad fronL to the south.