What caused an unexpected, record-breaking tidal event off the east coast of China?

Late Monday night, along the coast of the Bohai Sea in northeast China, the water began to rise as it normally does when the tide rises.

But the water continued to rise.

Under the cover of darkness, the tide swept across several coastal regions in Liaoning and Hebei provinces, as well as the city of Tianjin, which is situated just over 100 km (62 miles) southeast of the capital Beijing. The waters quickly flooded inland areas, triggering emergency responses and breaking official records.

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“In the absence of obvious winds and waves, a sudden rise in water levels over such a large area has never been recorded, either nationally or internationally,” said Fu Cifu, head of the Storm Forecast Office at the National Center for Marine Environmental Prediction. , in an interview with state news agency Xinhua on Tuesday.

The event was strong and sustained, with water levels remaining about 1 meter (39 inches) above normal for more than 20 hours, with several tide gauge stations in Liaoning reporting record levels.

But the impact was also felt in many other parts of the country. Reverse seawater flows have emerged along the coast of the Yellow Sea in Jiangsu province as well as the coast of the East China Sea in southeastern Fujian province. In the South China Sea, including the Hong Kong and Macau regions, tides were 30 cm (12 inches) higher than normal.

Residents up and down the coast shared videos online showing submerged roads, with some joking that their villages suddenly looked like Venice. In the cities of Dalian, Yingkou, Panjin, Jinzhou and Huludao, buildings were flooded, forcing residents to move to higher areas.

“This serves as a reminder to us. In the context of global climate anomalies, extreme weather events can occur silently and without any warning. We need more research on new types of natural disasters,” Fu said.

As the waters rose, the Ministry of Natural Resources initiated a level 4 emergency response for marine disasters. Five teams of experts were sent to Liaoning, Tianjin, Hebei and Shandong to inspect and assess high water levels in local areas and assess damage.

An amateur group of meteorology enthusiasts known as the “China Weather Fans” published a video suggesting that the event was primarily the result of a rare confluence of astronomical, meteorological, oceanographic and topographic factors.

“Firstly, October 20th marked the strongest astronomical tide of the year. Secondly, the recent supermoon has just ended, raising tide levels by another 20cm. Thirdly, the recent weakening of a cold air mass in the north has caused a drop in atmospheric pressure at sea level, naturally leading to a rise in sea height. Finally, a tidal wave generated in the Pacific Ocean coincided with these changes by reaching Bohai Bay,” the group said in the video.

According to an unnamed expert from the Ministry of Natural Resources, the unusual rise in water levels was mainly due to a combination of storm surges and astronomical tides that began a few days earlier.

A flow of warm, moist air moved north along the Yellow River, colliding with the cold front moving south in the Bohai region, forming a cyclone – similar to a typhoon – that caused an early storm.

“The oscillation started in Bohai Bay on October 19, propagating south to Shandong and then returning to Liaoning, where it met high tide, leading to a seawater reflux phenomenon,” said the expert.

By Wednesday, floodwaters in the worst-affected areas such as the city of Panjin had largely receded, traffic had returned to normal and no casualties had been reported.

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