What is the ebb and flow of the tide?

People who live by the sea or often fish by the sea must be very used to the regular fluctuation of seawater level twice a day. This is the trend. The tidal range along the Pacific coast of Japan is 1.5m, and that along the Sea of Japan is about 40cm. This is because the Sea of Japan is a closed sea area, so the tidal range is smaller than that of the Pacific Ocean.

tide

The largest tide in the world is in the Bay of Fundy, Canada. When the tide ebbs, a wet plain is formed, and the ship seems to hit the rocks.

The highest tide in East Asia is located in Incheon, South Korea, about 10 meter. Incheon has a sluice, so ships can go in and out even at low tide.

Tides are caused by the gravity of the moon, the sun and other celestial bodies changing with their positions relative to the earth. For example, the moon is in the south, and the gravity increases, while the other side of the earth is relatively reduced, and the seawater is attracted to the side with high gravity, forming a climax.

Fundy Bay, Canada

The mass of the sun is 25 million times that of the moon, but its distance from the earth is 400 times that of the moon, so the gravity that causes tides is only half that of the moon, and this gravity is only about1/90,000 of the gravity of the earth.

climax

The period of the moon's revolution is 24 hours and 50 minutes, so the interval between two tides a day is 12 hours and 25 minutes. When the moon and the sun are in the same direction, the tide becomes larger, which is called "spring tide". Compared with the earth, the change period of the direction of the moon and the sun is about 29.53 days, so there is a big tide at the full moon and the new moon, and a small tide a week later.

If the earth's surface is completely covered with seawater, the moon will cause a tide of 55 cm, and the sun will cause a tide of 24 cm, with a spring tide of 79 cm. The rise of the water surface produces water waves that spread in all directions, with the same propagation speed as the tsunami √gh, and the speed is 7 12 km/h at a depth of 400 meters.

The magnitude of tides in different sea areas is mainly due to the phenomenon of "* * *". In a rectangular ocean with a certain depth, when the length of the rectangle is just the tidal wavelength of 1/2, it will cause * * * sound. The wavelength of the semidiurnal tide at a depth of 400 meters is 8900 kilometers. If there is a sea with a length of 4450 kilometers, it will produce a * * * sound with very large amplitude. But there is no such ocean.

However, when the water depth becomes shallow, the wave velocity will also decrease, and the vibration with the same tidal cycle may occur in a short distance. The location of the world's largest tide mentioned above, Fendi Bay, is a vibration system formed with Boston Bay at the other end, which will form a huge tidal range with the tide.

When the sea surface changes, the sea water also flows, especially because of the long wavelength of tidal waves, which leads to the movement of the sea water at the bottom of the sea. This is the trend. In the coast or strait, the speed may be as high as several meters per second, but it is very small in the deep sea. The speed near the bottom of the Japanese trench is only about 2 cm/s. ..

Japan trench

In addition, although some ocean currents will not cause the sea surface to move up and down, they will change with the tidal cycle. When the tidal current passes through the mountains under the sea, it will produce internal tidal waves, which sometimes exceed the tidal current in the open sea in scale and speed.

Now that we know the cause of the tide, we can make a tidal forecast. It can be calculated according to the seabed topography or the actual observed values in various places. The height of the sea surface also changes with the change of ocean current or air pressure. Every time the air pressure drops 100 Pa, the sea surface rises 1 cm. The change of sea level varies with the accuracy of ocean forecast or weather forecast.