Chinese military flights around Taiwan have increased in recent weeks.
Those flights are seen as Chinese efforts to test Taiwan and send a message to its partners, especially the US.
But the aircraft included in those operations hint at a larger Chinese effort to improve its military's capabilities.
China's military flights around Taiwan have intensified in recent weeks in what is seen as an effort to test Taiwan and to send a message to its partners, especially the US.
Amid those operations, China appears to be continuing an ongoing effort to improve its military's ability to fight below the waters off its coast.
In 2020, Chinese aircraft made a record number of flights into Taiwan's air-defense identification zone. China has stepped up those flights this year, adding combat aircraft and setting new records: 20 aircraft in one incursion on March 26, followed by 25 warplanes on April 12.
According to Taiwan's Defense Ministry, many of those flights take place off of southwest Taiwan and include variants of the Shaanxi Y-8 aircraft equipped for reconnaissance or anti-submarine warfare.
The latter capability is particularly important in those waters, where the shallow Taiwan Strait meets the deeper South China Sea. To the east, the Bashi Channel connects them to the Pacific through the Philippine Sea.
The South China Sea's deep waters are "favorable" for Chinese submarine activity, and the proximity to Taiwan is the reason for "the frequent presence" of anti-submarine aircraft, said Su Tzu-yun, director of the defense strategy and resource division at the Institute for National Defence and Security Studies, a Taiwanese state-backed think-tank.
The Bashi Channel "can be considered as an underwater corridor from which Chinese submarines can enter the Philippine Sea and launch strikes against the US West Coast," Su told Insider.
Operating and detecting submarines in that area depends on knowledge of water conditions there, which China is working to learn.
"If you want to fight a successful naval war, you had better get your hydrographic information right," said Lyle Goldstein, a research professor and expert on Chinese undersea warfare at the US Naval War College.
That information affects algorithms used in undersea warfare, and China has been "pulling out all the stops" to understand the currents, temperature, and salinity of those waters, Goldstein told Insider.
"The performance of all those systems is affected by those algorithms, and the Chinese know that," Goldstein said, "They are working absolutely overtime."
The geography around the first island chain - the islands immediately off East Asia, including Japan, Taiwan, and the Philippines - creates challenges and opportunities for submarine warfare.
"China has this kind of perennial strategic problem of geography," Goldstein said. "To egress their submarines is quite difficult because they have to maneuver through the island chains."
"You can bet they're working very hard on making it as hard as they can for the US to know when Chinese submarines are going out and how they do it," Goldstein added.
The shallowness of the Taiwan Strait inhibits submarine operations, but the currents, temperatures, and salinity create "a really tough acoustic environment," according to Bryan Clark, an expert on naval warfare at the Hudson Institute and a former submarine officer.
The latter conditions make it harder to detect submarines - an acute problem for US forces tasked with finding Chinese subs in a conflict. The Chinese, however, will likely "just wait for the US [submarines] to start doing something that they can detect," such as firing torpedoes or missiles, Clark said.
"China just in the last five years has started fielding units of serious anti-submarine aircraft," Goldstein said, citing the Gaoxin-6, an improved anti-submarine variant of the Y-8/9 aircraft.
"They had helicopters that could do anti-submarine warfare, but this is quite new, to have these large fixed-wing planes that drop on sonobuoys and look for submarines," Goldstein added.
China still has anti-submarine-warfare shortcomings. Its sonar and sonobuoys "aren't very sophisticated," Clark said. "They're easily two generations behind where the US and NATO are and ... at least one generation behind Japan."
The combination of assets and capabilities likely means China will focus on thwarting offensive operations by US subs rather than proactively hunting them, Clark said. Despite US submarines' familiarity with the area, the confined waters there would limit their ability to evade attacks, meaning they'd likely back off in the face of Chinese pressure.
"The idea would be if a potential submarine is detected, they would send these aircraft, like the Y-8, out there to drop weapons on them," Clark said. "They're going to make sure that their own submarines are not in the area and use that to suppress US submarine operations."
"The fact that the Chinese are developing this, essentially, [anti-submarine warfare] response capability means that US submarines are going to have a more limited impact on any confrontation with the Chinese over something like Taiwan," Clark added.
'China is extremely worried'
China's efforts to improve its maritime awareness aren't limited to waters around Taiwan.
Last year, Australian officials said a Chinese ship off that country's western coast was likely mapping routes Australian subs use to access the South China Sea. In December, Indonesian fisherman found a suspected Chinese underwater drone, which was seen as a sign of China trying gather information needed for submarine operations near Australia.
China is not alone in these efforts.
India, which recently clashed with China on their disputed land border, has stepped up efforts to monitor the Indian Ocean, especially around the Malacca Strait, which connects that ocean to the Pacific.
Last year, the US reportedly asked Indonesia to allow P-8 maritime patrol planes land and refuel there, giving them another operating location around the South China Sea; Indonesia denied the request. The recovery of underwater drones near China's coast also suggests the US is gathering data on the waters there.
"China is extremely worried about our submarine force ... because they know that it would be hard for them to hunt our submarines, and our submarines have quite a devastating punch," Goldstein said. "They're really trying to do everything they can to understand where our submarines are and how could Beijing possibly counter them."
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