Waves, tides, storm surges and currents along the coast of China.

6.4. 1. 1. 1 wave distribution

The nature of wind system and the geographical location along the coast of China determine the distribution characteristics of waves along the coast of China: northerly waves prevail in winter and southerly waves prevail in summer; The height of wind and waves in Bohai Sea and Beibu Gulf is small, and the wind and waves in Taiwan Province Strait are large. Waves from the Pacific Ocean can be introduced into the East China Sea and the Yellow Sea through Ryukyu Island Chain, and into the South China Sea through bashi channel. Surges can be generated in the Yellow Sea, the East China Sea and the South China Sea, and large waves are easily formed in the southeast waters of Jeju Island, the northeast waters of Taiwan Province Province Island and the southeast waters of Dongsha Island. When the gas cyclone is active, there will be big waves, which often appear in the East China Sea, the South China Sea and the Yellow Sea, as well as the Beibu Gulf and the Bohai Sea.

The month in which big waves of magnitude 5 (3m) or above occur with a frequency of not less than 20% is called the month in which big waves occur frequently, and the month in which the frequency is less than 20% is called the month in which big waves occur rarely. Now, the monthly wind waves in various sea areas in China are shown in Table 6.2. According to the relevant contents in Table 6.2, the monthly maximum wave directions in the adjacent sea areas of Chinese mainland are northerly and southerly, and the frequent and infrequent months of big waves in each sea area are summarized in Table 6.3. As can be seen from Table 6.3, the northerly waves and big waves along the eastern coast (central part) of China mainly occur in the winter half year.

Table 6.2 Direction and frequency of main wind waves in various sea areas of China. Distribution of wind waves above magnitude 5 in each month.

Source: Wang Ying et al. (1996), China Marine Geography, Science Press.

Table 6.3 Monthly distribution table of the month with the most waves from north to south, the month with many waves and the month with few waves.

6.4. 1. 1.2 Waves in Special Weather System

In winter, northerly winds prevail in China, and cold air and strong winds can control the vast sea area from Bohai Sea to the northern part of the South China Sea, affecting the central and even southern parts of the South China Sea. China is also located in the mid-latitude area, and temperate cyclones can be generated in the Bohai Sea, the Yellow Sea and the East China Sea from late autumn to early spring of the following year or moved into the above-mentioned sea areas after being generated on coastal land. In summer and autumn, tropical cyclones from the northwest Pacific often affect the South China Sea, the East China Sea, the Yellow Sea, and some even extend to the Bohai Sea. These are the main weather systems that constitute strong winds along the coast of China, and also the weather systems that produce major disastrous waves along the coast of China. These disastrous waves will not only break the seawall and flood the sea into the plain, but also lift a lot of sediment in some intertidal shoals and underwater bank slopes, and sometimes collapse on a large scale in some intertidal zones or underwater bank slopes with erosion scarps. The distribution characteristics of these disastrous waves are briefly described as follows.

A. waves formed by cold air and strong wind. In winter, northwest wind, north wind and northeast wind will appear in China sea area from north to south, forming a strong wind with 8-9 grades and stable wind direction, which lasts for several days or even exceeds 10 day. The waves in China Sea increase with the increase of wind zone from north to south, and the sea area where the cold front is located often forms a huge wave zone, with the wave height of 4 ~ 8m and the maximum wave height of 9 ~11m. From the Bohai Sea to the South China Sea, the waves formed by cold air and strong wind change in the northwest, north and northeast directions. These waves are the source of surges.

Note: Prepared according to the relevant information in Table 6.2.

B Waves formed by tropical cyclones (including tropical storms, strong tropical storms and typhoons). Tropical cyclone wind is a fast-moving counterclockwise wind system, with a large central wind speed, which can reach more than 12. The wind waves and waves in the tropical cyclone range are adjusted with the wind direction from time to time, which is beneficial to the wave generation on the right side of the cyclone moving direction, so the huge wave area mostly appears on the right side of the cyclone moving direction. There is a storm near the center of the cyclone, and the edge is expanding. A wild wave with cyclone wave height of17.8m was recorded in the East China Sea, and a wild wave with wave height of14m was also recorded in the South China Sea.

C. waves formed by extratropical cyclones. Although the activity range of temperate cyclones is not as good as that of tropical cyclones, the damage to the waves formed by them can not be ignored because of its strong suddenness and long season. Its maximum wind speed can be greater than 10, and it can produce 7m wild waves in Bohai Sea and 8m wild waves in Yellow Sea and East China Sea.

6.4. 1.2 Tides and storm surges off the coast of China.

6.4. 1.2. 1 China offshore tide.

According to the research of Shen Yujiang (1980) and Ding (1985), the tidal vibration in China offshore is mainly caused by tidal waves from the Pacific Ocean, and the independent tidal force from the sun and the moon is very small. As can be seen from Figure 6. 1, the Pacific tidal waves enter the East China Sea and the South China Sea from Ryukyu Island Arc and bashi channel respectively, and form tidal vibration and tidal wave movement in the East China Sea, the Yellow Sea and the Bohai Sea.

Fig. 6. 1 Schematic diagram of tidal waves introduced into China sea area.

Ocean tidal waves are composed of many tidal waves with different frequencies and amplitudes. Among the tidal components, the sum of the amplitudes of main lunar semidiurnal component (M2), main lunar semidiurnal component (S2), lunar synthetic diurnal component (K 1) and main lunar day component (O 1) generally accounts for about 70% of the actual tidal wave amplitude. Among them, M2 component tide and K 1 component tide have the largest amplitude and proportion among semidiurnal tide and diurnal tide, respectively, so these two components can be used as representatives to understand the activities of semidiurnal tide and diurnal tide in China sea area.

Figure 6.2 reflects the basic characteristics of M2 tidal wave movement in Bohai Sea, Yellow Sea and East China Sea. After the tidal wave shown in Figure 6.2 is introduced from the Ryukyu Island Arc, most of it propagates to the Yellow Sea through the East China Sea and then enters the Bohai Sea. A few of them formed a so-called "degenerate rotating tidal wave system" along the northern coast of Taiwan Province Province, and entered the Taiwan Province Strait in a left-handed direction.

Fig. 6.2 M2 tidal map of Bohai Sea, Yellow Sea and East China Sea (according to Wang Ying et al., 1996) The solid line is the tidal time line (8 o'clock in the east), and the dotted line is the constant amplitude line (cm).

There are two counter-clockwise tidal wave systems with M2 component in Bohai Sea and Yellow Sea respectively. The first two non-tidal points are located about 13km outside Liaodong Bay and the Yellow River Estuary, while the last two non-tidal points are located near Chengshantou of Shandong Peninsula and off Lianyungang.

The semidiurnal tide in the Taiwan Province Strait, one flows southward from the East China Sea, the other flows into the South China Sea from the Pacific Ocean via bashi channel, and then flows northward into the Taiwan Province Strait. The two branches are at the Jinmen-Ma bus junction in Taiwan Province, showing the nature of standing wave (D,1983; Zheng Wenzhen et al., 1982).

Compared with the semidiurnal tides, the diurnal tides in the Bohai Sea, the Yellow Sea and the East China Sea have weaker vibration (see Figures 6.3 and 6.4). Due to the long period, diurnal tidal components (including K 1 and O 1) can only form a counterclockwise tidal wave system in the Bohai Sea and the Yellow Sea. In addition, the diurnal tidal wave movement in the whole Bohai Sea, the Yellow Sea, the East China Sea and the north-central part has the nature of standing wave (D, 1984).

The energy of tidal movement in the South China Sea mainly comes from tidal waves coming from the Pacific Ocean through the bashi channel, followed by independent tides generated by the tidal force in the whole sea area, and the superposition of the two constitutes the tidal system in the South China Sea. A notable feature of tidal vibration in the South China Sea is that the diurnal tidal wave is larger than the semidiurnal tidal wave (see Figures 6.5 and 6.6). The combined tidal range of two main diurnal tides, K 1 and O 1 ~1.5m, and the combined tidal range of two main semidiurnal tides, M2 and S2, is 0.5 ~ 1m in most sea areas of the South China Sea.

6.4. 1.2.2 storm surge along the coast of China

"Storm surge" refers to the phenomenon that the sea surface deviates from the normal astronomical tide under the action of storm force field. Tropical cyclone, strong cold and high pressure system and temperate cyclone are the main storms that lead to storm surge in China's coastal areas.

Every year, there are about 20 tropical cyclones affecting China's sea areas, coastal areas and even inland areas, about 5 ~ 6 strong cold air affecting northern China and nearly 50 temperate cyclones affecting China's sea areas. On average, there are 14 storm surges 1m in China's coastal areas every year, and two extra-large storm surges increase water by more than 2m, which constitutes a major disaster every two years on average. The coastal area of China is a frequent area of various storm surges in the world. The distribution of storm surge along the coast of China is as follows.

Fig. 6.3 K 1 tidal map of Bohai Sea, Yellow Sea and East China Sea (according to Wang Ying et al., 1996), with the solid line as the tidal time line (8 o'clock in the east) and the dotted line as the constant amplitude line (cm).

Fig. 6.4 O 1 tidal map of Bohai Sea, Yellow Sea and East China Sea (according to Wang Ying et al., 1996) The solid line is the tidal time line (8 o'clock in the east) and the dotted line is the constant amplitude line (cm).

Fig. 6.5 The tidal map of M2 component in the South China Sea (according to Wang Ying et al., 1996) shows the tidal time line (8 o'clock in the east) with a solid line and the isobar line with a dashed line.

Figure 6.6 Tidal chart of South China Sea K 1 (according to Wang Ying et al., 1996) The solid line is the tidal time line (8 o'clock in the east), and the dotted line is the constant amplitude line.

A. Yellow Sea and Bohai Sea. In Jiangsu coastal areas, storm surges are mainly caused by tropical cyclones or temperate cyclones, among which tropical cyclones are the main ones. Such storm surges occur more than once a year. During typhoon 8 1 14, the maximum water increase of Lusi (Ⅳ), Sheyang Estuary and Xiaoyangkou reached 2.38m, 2.95m and 3.8 1m respectively.

In Bohai Sea, cold wave and strong wind are the main causes of storm surge, and the main weather situation is "cold high combined with cyclone" and "transverse cold high". The former usually occurs in spring and autumn, while the latter usually occurs in late autumn and early winter. On the south bank of Bohai Sea, this storm not only increased water frequently, but also increased water significantly. During the period of 1958 ~ 1974, the water increase was 6 1 time, with an average of 3.4 times a year, 23 times more than 2m and 2 times more than 3m.

B. East China Sea. The storm surge in this area is mainly tropical storm tide. Among them, 6.8 tropical cyclones attack and affect Fujian, 3.7 tropical cyclones attack and affect the coastal areas of Zhejiang, and the tropical cyclones around the Yangtze River estuary average about 1 time per year.

Since 1949 in Zhejiang, the typhoon water increase has exceeded 1m for more than 20 times. Among them, when the strong typhoon hit1August 0, 956, the maximum water increase in Zhenhai was 2.33m, and that in Ganpu was more than 4 m.

C. South China Sea area. The average number of tropical cyclones affecting the South China Sea and its coastal areas is as high as 13 per year, accounting for about three-fifths of the total number of tropical cyclones affecting China. Among them, the coastal areas of Guangdong land 5 times a year on average, and the coastal areas of Guangxi land 2.5 times a year on average. The annual impact lasts for 6 months (from mid-May to mid-October165438+/kloc-0).

During the 23 years from 1955 to 1977, there were as many as 63 typhoons with water increase of more than 0.5m, and there were 1 1 typhoon with water increase of 3.02m: from1957. There are 18 typhoons exceeding lm, and there are1/kloc-0 typhoons exceeding/kloc-0.5m, with a maximum of 2.45m Zhanjiang has an extraordinary flood peak of 5.94m (the third place in the world record at present).

The above describes the distribution of waves, tides and storm surges off the coast of China. It is worth noting that there are two tidal current systems in the sea area east of Sheyang-Lvsi in northern Jiangsu. One is the forward wave of the Pacific Ocean, which reaches the Yellow Sea through the offshore continental shelf of China; On the other hand, the forward wave of the Pacific Ocean passes through the continental shelf along the coast of China and meets the Shandong Peninsula, forming a reflected wave. Forward wave and reflected wave converge to form radial tidal current field. According to the data of Wang Ying (2002), the average tidal range in this area is 4.5m, and the maximum tidal range is 6.5m, but the measured tidal range is 9.28m, which indicates that the tidal current in this area is very strong. On the other hand, the coastal beaches of Sheyang-Lvsi in northern Jiangsu are mainly silty sand. Under the action of waves, swells, tides, storm surges and tidal bore, a large amount of sediment is easily lifted into suspended load. At the same time, there are many sandbars along Sheyang-Lvsi coast in northern Jiangsu (see section 6.4.3), where waves tend to converge and energy is concentrated; Especially in winter, northerly waves and big waves prevail, the wind field is stable, and the strong waves make the previously accumulated sediment overturned and suspended again. As will be pointed out below (see section 6.4.2), the remote sensing image shows that the maximum width and sediment concentration of turbidity current in China offshore appear in the sea area east of Sheyang-Lvsi in northern Jiangsu, which is the main reason.

6.4. 1.3 China coastal current

As can be seen from Figure 6.7, Kuroshio, Yellow Sea Warm Current, Taiwan Province Province Warm Current, Coastal Current and Monsoon Drifting have great influence on China's offshore waters. They are briefly described as follows.

(1) Kuroshio. The northern equatorial current originating from the Pacific Ocean. The northern equatorial airflow flows westward to the east coast of the Philippines and is divided into two streams: one is southward; The other branch (main body) flows northward (Kuroshio), enters the East China Sea from the northeast of Taiwan Province Province, then turns to NE, leaves the East China Sea through the Tulaga Strait and re-enters the Pacific Ocean.

The amplitude of the Kuroshio current is relatively narrow, about 150km, and the width of the Kuroshio mainstream greater than 0.8 knots varies between 70 ~ 1 100 km, and its thickness in the East China Sea is 800 ~ 1000 m ... If the starting surface is 700m, the average flow of the Kuroshio in the East China Sea is about.

(2) Yellow Sea Warm Current. In the area south of latitude 30 and east of longitude125, the Kuroshio main branch diverges into a warm current flowing to the Sea of Japan, and the warm current flowing to Duima branches off into a tributary flowing from the East China Sea to the Yellow Sea in the south of Jeju Island. The warm current runs through the whole Yellow Sea from south to north and finally enters the Bohai Sea. The Yellow Sea warm current can directly drive Bohai Bay after leaving Kuroshio, which is closely related to the whole process of tidal wave.

This warm current enters the southern part of the Yellow Sea at a speed of about 0.2 knots. On the way northward, influenced by the seabed topography, it continuously branches to the coast, and there is a branch to the west at about 34 north latitude, which causes the turbidity current along the coast east of Rudong in northern Jiangsu to be obviously abnormal (see Figure 6.9 below). About 37 north latitude, there is a larger tributary on the east and west sides, which joins the coastal current of the west coast of Korea and the coastal current of the Yellow Sea in northern Jiangsu respectively. The main part of the Yellow Sea Warm Current enters the Bohai Sea through the Laotieshan Waterway and divides into two tributaries when it reaches the northwest of the Bohai Sea. The northern branch flows northward into Liaodong Bay along the northwest coast of Liaodong Bay, and the southern branch flows southward into the sea with the runoff from the Yellow River and other places, forming a Bohai coastal current flowing along the northern part of Shandong Peninsula.

(3) Warm current in Taiwan Province Province. This is a tributary of the northward branch of the Kuroshio trunk in the northeast sea area of Taiwan Province Province, with high temperature and high salt all the year round. After it separated from the Kuroshio trunk, it moved northward along the bottom slope of the continental shelf in the East China Sea, and its speed gradually weakened due to the influence of seabed topography along the way. The surface of ocean current is easily affected by monsoon, and the middle and lower layers are relatively stable, heading north all the year round. Its front is mixed with the southward coastal current outside the Yangtze River estuary, and then it turns to the northeast. Part of the seawater flows into the Ma Dui warm current, and the other part flows into the Yellow Sea warm current. When the northeast wind prevails in winter, seawater is forced to be transported to the shore, and the direction of warm current is opposite to that of coastal current. At this time, the strength of the warm current in Taiwan Province Province weakened, and the current amplitude narrowed. A westward inclined front is formed between the warm current and the coastal current in Zhejiang offshore, with the coastal current flowing south to west and the warm current flowing north to east. When the southwest monsoon prevails in summer, seawater is forced to be transported offshore. At this time, the warm current and coastal current in Taiwan Province Province are in the same direction, merging into one. The current is wide and powerful, covering almost the shallow water area in the west of the East China Sea.

Fig. 6.7 Schematic diagram of offshore current distribution in China [according to Figure 6. 18(2006 54 38+0) of Modern Physical Geography edited by Wang Jian and topographic data cited from http://www.ngdc.noaa.gov].

(a) Distribution of offshore currents in winter half year; (2) Current distribution in summer half year

(4) South China Sea monsoon drift. According to Sun Xiangping (1996), the surface current direction of the South China Sea changes with the change of its monsoon, so it is called South China Sea monsoon drift (see Figure 6.8). From mid-June of 5438+00 to April of the following year, the northeast monsoon drifted, mainly near South China, Vietnam and Malay Peninsula, and flowed into the Java Sea through Karimata and Kaspar Strait. Part of the Kuroshio also entered the South China Sea through buses and Balintang Strait, and turned southwest. Generally, the surface velocity in the north (South China Sea) is 1 ~ 1.5 knots, and that in the south is 1 knot. The mainstream velocity is generally 2 knots, with a maximum of 3 knots. When the southwest monsoon prevails from June to August, the current turns to the northeast, and the surface current velocity in most of the southern sea areas is relatively low, generally 0.5 knots, except for the coastal waters and straits of Vietnam. The strong current area appears in the Malay Peninsula and the southern coast of Vietnam, with a velocity of 1 knot, with a maximum of 2 knots. To the northern part of the South China Sea, most of the seawater flows out of the South China Sea through the bashi channel and Balintang Strait, and flows into the mainstream of Kuroshio, while a small part continues to flow northward into the Taiwan Province Strait.

Fig. 6.8 South China Sea monsoon drift map in winter and summer (according to Wang Ying et al., 1996).

According to the data of the Comprehensive Investigation Office of the Oceanographic Group of the State Science and Technology Commission (1964), in the Beibu Gulf, due to the influence of the easterly wind and the northeast monsoon, the seawater moves counterclockwise in winter, with a velocity of 0.2 ~ 0.4 knots. There is still a large counterclockwise circulation in Quanwan. In summer, under the influence of southerly winds, the seawater moves clockwise, forming a circulation with a velocity of about 1 knot, which is opposite to the direction of winter current. In autumn, the sea water on the west side of the bay has turned counterclockwise, and only Leizhou Peninsula is still clockwise.

(5) Coastal current. The coastal currents along the coast of Chinese mainland can be divided into north coastal currents and south coastal currents, which are roughly bounded by the standing waves distributed in the Taiwan Province Strait.

A. north coast current. The flow direction is from north to south all year round. As for the cause of the North Sea shore current, we think it is not the traditional view that it is mainly formed by river runoff into the sea (Sun Dawen,1992; Sun Xiangping, 1996, etc. We think that the reverse diversion of the Yellow Sea warm current on the way northward and the backflow when it reaches the northwest of Bohai Sea are the main parts of the northern coastal current, and the river runoff into the sea mainly dilutes the salinity of the coastal current locally. On the way to the north, the Yellow Sea warm current is affected by the seabed topography and constantly diverges to both sides in the opposite direction. When it reaches the northwest of the Bohai Sea, it is blocked by land, and according to the requirements of mass conservation, it is bound to form a backflow to its two sides (see Figure 6.7(a)): one branch runs northeast along Liaodong Peninsula-west coast of Korean Peninsula-south end of Korean Peninsula; The other branch goes south, and the counterclockwise tidal wave system formed by the tidal components of the Bohai Sea and the Yellow Sea M2, K 1 and 0 1 is constantly strengthened. It goes south along the Shandong Peninsula, passes through the coastal areas of Jiangsu, Zhejiang and Shanghai, and finally joins the southern coastal current at the southern end of the Taiwan Province Strait. Along the way, it is mixed with diluted water from Haihe River, Yellow River, Guanhe River, Sheyang River, Yangtze River and other rivers entering the sea, forming the northern coastal current along the coast (middle) of the eastern mainland of China. The distribution and intensity of the north coastal current change with the change of winter and summer: in winter half a year (roughly from September to April of the following year, ***8 months), affected by strong northeast wind, the scale of the north coastal current increases and the velocity accelerates; In the summer half year (May to August, about 4 months), the scale of the northern coastal current is reduced, the velocity is slowed down, and it completely disappears in the northern Fujian sea area (see Figure 6.7(b)).

B. South Coast Current. It mainly comes from monsoon drift, so the flow direction changes with the change of monsoon drift. In the winter half year (10 to April of the following year, * * * more than 7 months), the flow direction is from north to south. It starts from the southern tip of the Taiwan Province Strait, along the coast of Guangdong and Guangxi, and then goes south along the eastern coast of Indo-China Peninsula. In the summer half year (April-August and September, more than 5 months), the flow direction is from south to north. From the waters near Cam Ranh Bay in the south to the waters near Xiamen in Fujian in the north, it finally disappears (consistent with the standing wave area of the Taiwan Province Strait).

The biggest difference between the south coastal current and the north coastal current is as follows: ① The south coastal current is mainly formed by wind and waves, while the north coastal current is mainly related to the diversion of the Yellow Sea warm current on its way northward and the backflow when it reaches the northwest of the Bohai Sea, and is constantly strengthened by the counterclockwise tidal wave system formed by M2, K 1 and 0 1 tidal components in the Bohai Sea and the Yellow Sea. ② The direction of the southern coastal current changes with the change of wind and waves, while the direction of the northern coastal current remains unchanged from north to south. ③ The current potential on the south bank is weak and small, while the current potential on the north bank is strong and large.

To sum up, the Yellow Sea warm current and monsoon drift have played an important role in the formation and change of coastal currents in China. However, tidal waves also have a great influence on China's coastal currents. The most significant influences are as follows: ① During the summer half year, the coastal currents in the northern Fujian Sea disappeared, which is related to the tidal waves coming directly from the vertical coast of the East China Sea. Because the northern coast is popular in the waters near this coastal section at this time, the driving force for progress is not very strong. Naturally, this tidal wave coming straight to the coast has become an insurmountable "hurdle". (2) In the Taiwan Province Strait, tidal waves from south to north meet in the southern part of the Strait, resulting in the formation of a standing wave across the Strait in the border area, which has become an insurmountable obstacle for the South China Sea coast to flow southward and northward during the summer half year. (3) The sea area east of Jiangsu Changsi is the intersection of advancing tidal waves and rotating tidal waves, which leads to the formation of a complex tidal system in this sea area and is also strongly disturbed by the natural north coast. ④ In the coastal area where the water depth is small and r is equal to λ/25 wavelength, the tidal current becomes a reciprocating flow, which has a great influence on the movement of coastal current. In addition, when encountering storm surges and large-scale swells, the normal coastal current pattern will also be completely disrupted.