East Sea file: Physical features

VietNamNet Bridge – There is growing concern about increased tension in the East Sea after China illegally placed its drilling rig Haiyang Shiyou-981 in Vietnam’s exclusive economic zone in the sea.

East Sea, drilling rig Haiyang Shiyou-981, Gulf of Tonkin

VOV online introduces a series of stories featuring the East Sea’s development potential, Vietnam’s establishment of its sovereignty over the Hoang Sa (Paracel) and Truong Sa (Spratly) archipelagos, and current territorial disputes in the area.


The East Sea is a semi-enclosed sea covering an area of about 3.5 million km2, extending from latitude 3° North to latitude 26° North and from longitude 100° East to 121° East.

Besides Vietnam, the East Sea is surrounded by nine other countries and territories, including China, the Philippines, Indonesia, Brunei, Malaysia, Singapore, Thailand, Cambodia, and Taiwan.

According to preliminary estimates, the East Sea exerts a direct impact on the lives of some 300 million people in these countries. It is not only an important strategy area for the countries in the region but also the Asia-Pacific and the Americas.

Vietnam touches the East Sea on three sides: East, South, and Southwest. Its sea areas and the continental shelf are part of the East Sea spreading along the 3,260 km-long coast, from Quang Ninh to Kien Giang with many beautiful beaches such as Tra Co, Do Son, Sam Son, Cua Lo, Cam Ranh, and Vung Tau.

Thus, for every 100 km2 there is 1 km of coastline, this ratio is 6 times higher than the rate of the coastal countries of the world (the world average is 1 km of coast for every 600 km2). No place in Vietnam’s inland areas is more than 500 km away from the sea.

Vietnam has internal waters, territorial sea, contiguous zones, exclusive economic zones, and continental shelf. In the middle of the East Sea there are Hoang Sa (Paracel) and Truong Sa (Spratly) archipelagos and 2,577 small and large islands at various distances from the shore which form a defensive line to protect and control the country’s waters and continental shelves.

Seabed topography

In most of the world’s oceans and seas, eight major types of seabed topography are found: continental shelves, continental slopes, continental feet, ocean basins, island arcs, deep grooves, underground hills and underground mountain ranges, all of which find their similar appearance in the East Sea’s seabed.

The East Sea’s continental shelf accounts for over 50% of the area, distributing at a depth of less than 200m.

Continental slopes account for about 25%, the remaining area is distributed at a greater depth of 2,000m over the entral basin, the deep grooves, shoals, island arcs and underground mountain ranges.

Between the north and the south deep sunken part, the East Sea is deeply connected by a central trough with underground mountain ranges at its edge.

Salinity and temperature regime

Temperature and salt content (salinity) are the two most basic physical characteristics of sea water, dominating every marine hydro-thermodynamic process, while ensuring the existence and development of aquatic life in the sea.

Unlike inland water, sea water is characterised by its salinity. The average salinity of the world ocean water is 35‰ (i.e.35g of solid minerals dissolved in 01 kg of sea water). Salinity variations depend on water temperatures, marine meteorology, geographical locations and marine dynamic processes.

The salinity of the East Sea’s surface water layer ranges from 32 to 34.5ppt (excluding estuaries). The salinity area of high values is in the Northeastern part of the East Sea, where there is an exchange with the Pacific water bloc through Basi and Taiwan Straits, which is subject to less influence of continental water and strong sea surface evaporation.

January to March is the time with the highest salinity. The low salinity areas are coastal areas due to the strong impact of the continental waters. The lowest salinity occurs in summer, June-August, due to heavy rains on the sea and large quantities of continental water discharge.

The thermo-saline distribution of the sea water reflects the hydromass structure and the ocean dynamic regimes. The water surface layer of the East Sea contains different blocks: cold and light inshore water blocks, northeast offshore water block, south offshore block, and the summer upwelling block.

Between the blocks are hydrographic fronts with very high thermo-saline horizontal gradient characteristics. Fish schools often concentrate near the hydrographic fronts. The volatility of the fronts leads to migration of extractable fish schools and other types of sea creatures.

The East Sea flow

The East Sea surface water flow is the result of the sea-atmosphere interaction process. The flow observed on the surface is the mix of three components: wind flow, the local switch flow and tidal flow. The first two components are very difficult to determine, however, it is possible to use actual measurement data and mathematic models to quantify them.

Combining both methods, the East Sea flow regime map has been developed characterised for two seasons (winter and summer), reflecting the fundamental law of the under surface layer circulation under the action of the monsoon regime.

In the Gulf of Tonkin, a cyclonic circulation has always existed as well as a strong current towards the south along the coast (in the winter, this flow penetrates deep into the waters of Binh Thuan and beyond).

In the summer, under the influence of the Southeast monsoon, the strong southwest flow was formed along the Southern part of the East Sea and South Central to the North and sees the flow from the North at about 16° South, they deflect to the East to the central sea area, creating large-scale vortices in the North and South of the East Sea.

The East Sea tide

The East Sea tidal regime results from tidal waves from the Pacific and Indian Oceans passing through large straits dominated by the complicated topography of the sea. The Pacific tides are semi-diurnal in the nature. Entering the East Sea, they become predominantly diurnal and their amplitude increases significantly.

On the map of the East Sea’s tidal nature drawn by Nguyen Ngoc Thuy, the daily cycle part takes up most of the space of the East Sea. The typical regular tide is observed in Hon Dau (Haiphong), and Hon Gai (Quang Ninh), which is the region that has the largest tidal amplitude of the East Sea (4m).

The tidal regime along the coast of Vietnam is very diverse and fluctuating. Here, one can observe all four types of tides: regular diurnal, irregular diurnal, semi-diurnal and irregular diurnal. The tidal characteristics along the coast of Vietnam are as follows:

+ The sea from Quang Ninh to Thanh Hoa has a predominantly diurnal and irregular diurnal regime, the tide height varies within 3-4m.

+ The sea from Ha Tinh to Quang Binh has a largely irregular diurnal regime, it is observed that the water comes in and recedes once a day for only about 15 days a month, and the height of the tidal water level varies within 1.2-2.5m.

+ The seas of Cua Tung-Thuan An-Quang Nam-Danang has a tidal regime which is considered to be the most complex and prone to irregular semi-diurnal tides, including Thuan An where there is a regular semi-diurnal regime, i.e. during most of the month the tides come and leave twice a day but the tide height only reaches about 0.5m. Meanwhile, the height of high tides of Thuan An fluctuates from 0.5-1.2m.

+ The seas from Quy Nhon to Nha Trang, the tide repeats irregular diurnal regime, it is observed that the water comes in and recedes once a day about 18-22 days a month, and the tide level increases by 1.2-2.0m.

+ The seas from the region of Ham Tan-Vung Tau-Ca Mau, the tide is of irregular semi-diurnal nature, it is observed that the water comes in and recedes twice a day, but they are irregular in amplitude and time. The height of the tidal water level here increases to 2.0-3.5m (close to the value of the Gulf of Tonkin).

+ The sea of Southwestern coastal region (Gulf of Thailand) has irregular daily cycles with a small tidal height (<1.0m).

(To be continued…)


East Sea, drilling rig Haiyang Shiyou-981, Gulf of Tonkin