Japan has increased anti-submarine patrols in international waters, just outside Japanese territorial waters. Chinese submarines are apparently exercising there more frequently, looking for Japanese, South Korean and American warships to play tag with. The U.S. has also redirected more of its space based naval search capabilities to assist the Japanese.
Chinese Song class diesel electric and Han class nuclear powered boats were detected and tracked recently. One of each of these was spotted stalking the American carrier USS George Washington, as it headed to South Korea for a visit.
Remember, China isn't near to having the ability to defeat our Navy and control the western Pacific. Heck, the Chinese aren't near the ability to beat the Japanese navy alone.
But China doesn't need to beat us. China needs to beat the clock by keeping us at arm's length from Taiwan long enough for China to defeat the Taiwanese. Do that and the Chinese defeat Taiwan.
For this purpose, the Chinese don't even need to keep their submarines quiet. Indeed, the Chinese would want to be discovered. If the Chinese want to make us too cautious to race in and defend Taiwan, the Chinese would want us to be aware that their submarines will line the route from Japan to Taiwan.
A logical deduction from this desire to buy time is that China will not attempt any military action designed solely to intimidate Taiwan like a missile barrage or blockade. Either of those attacks would not require a rapid American response--only a certain American response--to bolster Taiwanese morale to endure for the short run until help arrives.
Only a full-scale invasion makes any sense to me as a Chinese military option to absorb Taiwan. And China's submarines are a speed bump on the road to Taiwan, clearly marked for our Navy to see.
