The Era When the Silence in the Sea Disappears

In early February, the submarine acoustic signature captured by the Japan Maritime Self-Defense Force's P-1 patrol aircraft over the Tsushima Strait matched the characteristics of the Chinese Navy's Type 039A submarines recorded during the author's active service. However, the signal strength was below expectations. This data suggests an improvement in stealth capabilities.

The anti-submarine warfare (ASW) capabilities of the Japan Maritime Self-Defense Force have long been regarded as the "silent pride" of the Japan-U.S. alliance. During the Cold War, the technology for tracking Soviet Pacific Fleet nuclear submarines was at the highest level in the world, and the SOSUS network deployed in the Tsugaru Strait and Soya Strait was essential for the defense of the first island chain. However, by the mid-2020s, this superiority is facing structural challenges.

The first challenge is the qualitative change in the targets. The Chinese Navy has not only doubled its submarine force over the past decade but has also made significant improvements in stealth. Analysis suggests that the noise level of the Type 093B attack submarine has decreased by more than 30 decibels compared to earlier first-generation nuclear submarines. This means a substantial reduction in detection range. While the JMSDF's P-1 patrol aircraft is equipped with excellent magnetic anomaly detection devices, it would require more than double the flight time to capture a stealthy submarine in the vast East China Sea.

The second challenge is the resource constraints within the JMSDF itself. Currently, the JMSDF operates about 70 P-1 and P-3C patrol aircraft, but their operational rate has been declining in recent years. In addition to aging airframes, a more serious issue is the shortage of experienced pilots and acoustic analysts. At least five years of experience is required to interpret submarine acoustic signatures, but the JMSDF's personnel recruitment plan has only reached 85% of its target. The introduction of unmanned patrol aircraft is being considered to compensate for this, but it will take at least five years for them to be operationally deployed.

The third issue is the aging underwater monitoring infrastructure. The fixed sonar network in the Tsugaru and Tsushima Straits was laid in the 1980s and has not been updated. The U.S. military's SOSUS network faces similar challenges, and while "joint enhancement of underwater monitoring capabilities" was on the agenda at the 2024 Japan-U.S. defense ministerial talks, no specific budget measures have been indicated. There are no fixed monitoring systems in the waters around the Nansei Islands, creating a favorable "corridor" for Chinese submarines.

Interestingly, there is a disparity in the perception of this situation within the JMSDF. The staff office maintains the position that "we can respond through technological updates and Japan-U.S. cooperation," but there is a strong sense of crisis at the operational level. An unofficial statement from a destroyer squadron commander noted, "We are currently equipped to track Soviet submarines from the 1980s while facing Chinese submarines from the 2020s."

The Ministry of Defense's budget proposal for fiscal year 2026 includes 12 billion yen for the procurement of new sonobuoys, which is only 60% of the required amount. Additionally, the development of an acoustic analysis system utilizing AI is underway, but the target year for practical application is 2028. Until then, the JMSDF will continue to rely on the "human ear."

In the Japan-U.S. joint exercise "Keen Sword," ASW training is conducted annually, but the assumed scenarios still primarily focus on "tracking a single submarine." In actual emergencies, multiple stealth submarines are likely to operate simultaneously, and unmanned underwater vehicles may be deployed as decoys. Training for this "saturation attack" is insufficient.

The South Korean Navy faces similar challenges and has decided to introduce the latest AN/SQQ-89 sonar system from the United States by the end of 2025. Meanwhile, Japan's insistence on domestic equipment has delayed standardization with the U.S. This has also become a barrier to information sharing among Japan, the U.S., and South Korea.

The real issue is not the lack of technology or equipment, but the delay in political decision-making that has treated the decline of ASW capabilities as an "invisible crisis." Submarines do not reveal themselves during peacetime, and their threat is difficult to quantify. This is why, in budget allocations, they have been overshadowed by more glamorous destroyers and missile systems. However, if the first island chain is breached, Japan's sea lanes will be instantly threatened. In an era where underwater silence is disappearing, we must seriously reconsider our ability to hear the sounds that go unheard.