Oceanography

A. G. Ilahude

The Monsoons

Situated between two continents and two oceans, and straddling the equator, the Indonesian archipelago is an ideal place for the development of the monsoon. The low atmospheric pressure of the equatorial trough crosses the equator twice a year in accordance with the shifting position of the sun in respect to the equator. During the northern summer a low atmospheric pressure area is formed over Asia as an extension of the equatorial trough while over Australia a high pressure area is formed as an extension of the subtropical high. Between the low and the high, the monsoon winds develop, in which the air flows from Australia to Asia. During the northern winter the reverse is the case (Wyrtki 1961).

During the northern summer the direction of the winds is east and southeast south of the equator, south on the equator, and southwest north of the equator. The wind speed is generally 25 to 40 knots. This period is called the southeast season, and the monsoon is called the southeast monsoon in Indonesia. During the northern winter the wind reverses its direction to become north and northeast north of the equator, north on the equator, and northwest south of the equator. The speed is 30 to 45 knots. The period and the monsoon are called the northwest season and the northwest monsoon, respectively. Between the two seasons, transition periods develop in which the direction of the wind is variable and the speed is weak. In the Java Sea, to which the coastal region between Jakarta and Cirebon belongs, the northwest monsoon is usually accompanied by heavy rainfall, and is therefore called the rainy season, while the southeast monsoon is called the dry season. The terms "east monsoon" and "west monsoon" are sometimes used in place of "southeast monsoon" and "northwest monsoon."

The monsoons are capable of creating and maintaining surface sea currents in the Indonesian archipelago. This is because of the following factors: (1 ) the wind is steady, and (2) the axis of the wind more or less coincides with the axis of the seas formed by the South China Sea, Karimata Strait, Java Sea, Flores Sea, and Banda Sea.

Thus in the Java Sea the surface currents flow to the east during the northwest monsoon and to the west during the southeast monsoon. The velocity of the current is 25 - 38 cm/see during the northwest monsoon and 12 - 25 cm/see during the southeast monsoon (Wyrtki 1961). During the transition periods the currents are variable and relatively weak.

In the coastal region between Jakarta and Cirebon the velocity of the currents generally varies from 5 - 40 cm/sec. Several observations in Jakarta Bay have indicated that the direction of the currents is basically in accordance with the directions of the monsoon. Apart from the monsoon, the current direction is also affected by the land and sea breezes, and its speed is influenced by the tides. The current direction tends to deviate away from the coast during the land breeze and toward the coast during the sea breeze. The current speed is usually stronger during the rise and ebb of tides, and weaker during the time of high and low water (Anonymous 1975). The coastal currents may also be affected by the coastline and the bottom topography, but so far no study in this area has been carried out.

Tides and Waves

Observation of tides in Jakarta Bay indicates that in this region they are diurnal, with one high water and one low water daily. The F (Form zahl) value is generally higher than 3; that is, around 4.9. The tidal range varies between 27 and 97 cm. The rate of occurrence of a tidal range greater than 50 cm is 86 per cent while that for a tidal range less than 50 cm is 14 per cent. The amplitude and phase of the major component of tides, standardized against the 0 hour (West Indonesia Time) are as follows (Anonymous 1975):

The moon's influence on tides is observed mainly in tidal heights, the greatest range occurring during maximum lunar declination. Also in these regions the predominant diurnal tide may become mixed near minimum lunar declination (Anonymous 1972).

The tidal currents are generally weak, and merely strengthen or reduce the speed of non-tidal currents. Therefore they are only perceptible during the transition periods between monsoons, when the prevailing current systems are not fully developed (Anonymous 1972).

The velocity of tidal currents is believed to vary between 5 and 85 cm/see with the direction north and south in the Cirebon area, and west-northwest and east-southeast in the area between Tg. Krawang and Tg. Indramayu (Anonymous 1979).

Observation of waves in Jakarta Bay shows that wave characteristics (wave height H, wave period T, and wave wave-length L) in both northwest and southeast monsoons are determined by the wind velocity. These characteristics increase with increasing wind velocity. The values observed for the Jakarta Bay area are 1 - 10 dm for wave height, 1 - 8 sec for wave period, and 1 - 21 m for wave wave-length.

The direction of waves is dependent on the direction of the wind only for the offshore area. Close to the shore, the wave usually arrives perpendicular to the coastline due to refraction across the shallowing sea floor (Anonymous 1975).

Temperature

The water temperature in the coastal area between Jakarta and Cirebon is high, as it is in ail tropical regions. It ranges between 27.1° and 29.7°C, with two maxima and two minima annually. The primary maximum with a temperature of 29.1 29.7°C is generally found in April or May, the months of the first transition monsoon. The secondary maximum with a temperature of 28.6 - 29.2 C occurs in October or November, the period of the second transition monsoon.

The primary minimum temperature of 27.4 - 28.2 C is usually found in December or January, during the northwest monsoon, while the secondary minimum of 27.5 28.3 C occurs in August, during the southeast monsoon.

In exceptional cases the temperature may reach a highest value of 30.5°C or a lowest value of 26.5°C.

The water temperature is about equal to the air temperature at sea level, which also has two maxima and two minima annually. The winds that flow during the monsoons influence the water temperature greatly. Similarly, rainfall during the northwest monsoon and evaporation during the southeast monsoon also reduce the temperature of the water. This is true not only of the coastal region between Jakarta and Cirebon, but also of the whole Java Sea (Whyrtki 1957).

In the Jakarta Bay area, Arief (unpublished) found a similar annual course of surface water and air temperatures. In addition he also found the temperature in Jakarta Bay, which can be considered to be representative of the whole coastal zone between Jakarta and Cirebon, to be 0.8 C higher than in the open Java Sea. Thi's is believed to be due to the landmass effect of Java.

Muluk et al. (1976) reported that the water temperature in the main river of Citarum is 26.0 - 28.0°C during the rainy season of the northwest monsoon and 27.0 - 30.0°C during the dry season of the southeast monsoon. In the tributary rivers of the Citarum, the temperature is 26.0- 27.0 C and 29.0 - 30.0°C, respectively.

Salinity

The salinity in the coastal area between Jakarta and Cirebon generally varies between 31.0‰ and 33.0‰.

Due to the influence of the high salinity of the Pacific waters that come to the region through the South China Sea in the northeast monsoon and through the Flores Sea in the southeast monsoon, two maxima and two minima of salinity are usually observed in this region annually. This information is inferred from the papers of Soeriaatmadja (1956) and llabude et al. (1975). The primary maximum, with salinity value of 32.5 - 33.0 %o, is usually found in November, due to the mixing of the coastal water with Flores Sea water of around 34.2%o, brought to the region by the southeast monsoon current. This maximum is also strengthened by the dryness of the monsoon in this period. The secondary maximum usually occurs in May, with salinity 31.7 - 32.0 %o and is due to the mixing of the coastal water with the South China Sea water of around 32.5‰ salinity.

The primary minimum, usually found in February with salinity value of 30.6 - 31.0%o, is caused by the heavy rains that occur during the northeast monsoon. The secondary minimum with value of 31.0 - 31.4%o is usually found in July and is due to the influence of the low-salinity water mass of 31.0 - 31.8%o formed by the mixing of sea water with the river water from South Kalimantan. The formation of this low salinity is discussed by Sjarif (1959).

Apart from the seasonal factor, the salinity in the coastal region between Jakarta and Cirebon is also influenced by the river drainage, especially in Jakarta Bay. In front of the river mouths within this bay, the surface salinity can be as low as 15 %o ( llahude and Soepangat 1977).

In the various depositional environments in the Cimanuk Delta, Hehanussa etal. found that salinity varies between 22.1 and 38.9 ‰. Most of the values lie between 30.0 and 33.7 ‰.

In the rivers themselves the salinity is usually low. Muluk et al. (1976) found the salinity in the main flow of the Citarum River to be 0 to 0.5‰ during the rainy season and 0 to 31.0 ‰ during the dry season. A. J. van Bennekom and R. de Vries "Personal communication) found salinity values of 3.4 %o for the Ciujung River and 1.8 %o for the Cilontar River.

Seston

Measurements on seston (suspended particulate matter) have also been carried out in the coastal area and in the rivers of West Java. Nontji and Soepangat (1977) found the value of seston in Jakarta Bay to be 13.1 - 186.49 mg/l near the river mouths and 5.0 - 19.4 mg/l in the offshore areas. They found that the value of seston has a high negative correlation with the salinity of the water. The seasonal variation of rainfall on land may also cause variation of the seston concentration in Jakarta Bay.

In the river water, Muluk et al. (1976) reported the values of 95 342 mg/l during the rainy season and 27 - 30 mg/l during the dry season in the Citarum River. A. J. van Bennekom and R. de Vries (personal communication) reported the value of 270 mg/l and 350 mg/l, respectively, for the Ciujung and Cilontar rivers, West Java, during the rainy season (December 1978).

Dissolved Oxygen and pH

Edi Legowo (unpublished) has reported the result of LONLlPl's observation on dissolved oxygen in Jakarta Bay. He found that the oxygen concentration during the northwest monsoon varied between 4.08 and 4.83 ml/l and in the southeast monsoon varied between 3.14 and 5.35 ml/l, the average values being 4.46 and 4.45 ml/l, respectively.

The dissolved oxygen varied between 4.46 and 5.21 ml/l in the northwest monsoon and 4.42 and 4.58 ml/l in the southeast monsoon, with the average values of 4.62 and 4.51 ml/l, respectively. The percentage of oxygen saturation was 96.3 per cent for the northwest monsoon and 98.7 per cent for the southeast monsoon. Supersaturation values from 110 - 120 per cent were usually found during the southeast monsoon.

Muluk et al. (1976) found dissolved oxygen in the Citarum River to vary between 1.4 and 4.8 ml/l with percentage saturation of 19.1 to 61.9 per cent.

The pH in Jakarta Bay was found by Santoso et al. (1977) to be 7.5 - 8.2 near river mouths, and 7.9 - 8.2 in the offshore area, which is considered normal. In the Citarum River the pH value ranges between 6.2 and 7.7 (Muluk et al. 1976).

Trace Elements

Studies of water quality have also been carried out in the coastal region of West Java (Table 1). The results indicate that for certain elements, such as lead, copper, and mercury, the sea water has a larger concentration than the river water.

Santoso et al.., using the criteria of the U.S. Federal Water Pollution Control Administration of 1968, decided that this concentration is larger than the standard set by the Administration. They believe that the high concentration of trace elements in Jakarta Bay is due to the outflow of industrial wastes and that such high concentration can have a deleterious effect on the marine organisms in the region.

Table 1. Values of Trace Elements in the Coastal Region between Jakarta and Cirebon

* LON-LIPI = National Institute of Oceanology, LIPI (unpublished data)
**tr= trace

Table 2. Values of Nutrients in the Coastal Region of West Java

The Nutrients

The concentration of nutrients of the sea water, consisting of phosphate-phosphorus, nitrate-nitrogen, and silicate-silicon, has been studied by LON-LIPI in various Indonesian waters, including the Jakarta Bay area. The results of this study show that the nutrient values generally vary from 0-0.02 mg/l for phosphate, 0 - 0.03 mg/l for nitrate, and 0 - 0.42 mg/l for silicate (Table 2). The concentration of nutrients is very much affected by the fresh-water drainage from the land, and therefore it varies seasonally, the high concentration usually being found during the rainy season of the northwest monsoon.

Measurements of nutrients in the river water indicate that the rivers have higher concentrations of nutrients than the sea. For example Santoso et al. (1977) reported values of 0 - 0.42 mg/l and 0 - 0.22 mg/l for phosphate and nitrate, respectively, for the river mouths in Jakarta Bay. Muluk et al. (1976) found the concentration of 0.02 - 0.21 mg/l, trace-0.08 mg/l, and 0.40 - 46.25 mg/l for phosphate, nitrate, and silicate, respectively in the Citarum River, while van Bennekom and de Vries (personal communication) found the values to be 0.01 0.12 mg/l, 0.10 - 0.23 mg/l, and 15.8 - 21.00 mg/l for phosphate, nitrate, and silicate, respectively, in the Ciujung and Cisadane rivers (Table 2).

References

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Ilahude, A. G., and 1. Soepangat 1977. Pengamatan hidrologi di Teluk Jakarta, Januari 1977. Monitoring Teluk Jakarta, Laporan Pelayaran No. 6 Lembaga Oseanologi Nasional-LIPI, Jakarta 9 - 41.

Ilahude, A.G.; I Soepangat; den P. Sianipar 1978. Pengamatan hidrologi Teluk Jakarta, Januari 1978. Monitoring Teluk Jakarta, Laporan Pelayaran No. 10. Lembaga Oseanologi Nasional-LIPI Jakarta: 6 - 43.

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_________1961. Physical oceanography of the Southeast Asian Waters. Scripps Institution of Oceanography, Naga Report 2:1 - 95.