In Hong Kong, US aircraft carriers are like migratory birds - they come and go each winter.
This year, however, the giant flat-topped ships steaming through the mists of the East Lamma channel have been conspicuous by their absence.
Unusually, the United States did not seek Beijing's permission for even a single visit, even though two carrier strike groups have been operating in the western Pacific in recent months.
As reported in the South China Morning Post, a scheduled visit last October was scrapped and no request submitted. Officially, the US Navy said the reason was 'operational' rather than anything else - the same reason offered for their absence in more recent months.
Yet diplomatic observers and military analysts across the region wonder whether the reason has had a political element, too - some note that it was hard to believe Beijing would approve a visit, given the just-thawed frost that had fallen on the Sino-US military relationship in the past year, even if visits of lesser ships were permitted.
For decades carrier strike groups have been the ultimate symbol of US military primacy in East Asia - and regular visitors to Hong Kong since the 1960s. But now they also represent a strategic presence that must operate in an ever-more complex environment presented by China's own military rise.
Beijing, after all, has repeatedly expressed concern about the continued presence of US aircraft carriers off its coasts over the past year as Washington prepared to send the USS George Washington into the Yellow Sea following North Korean attacks on South Korea.
Even former Hong Kong chief executive Tung Chee-hwa echoed Beijing's concerns, telling US Pacific commander Admiral Robert Willard, during a discreet dinner in Hong Kong in August, that US carriers in the Yellow Sea were 'too close for comfort'.
A fortnight ago, the commander of the US Seventh Fleet spoke of the strategic and symbolic importance of his Japanese-based carrier, the USS George Washington, in a speech to the Asia Society in Hong Kong.
Stressing the 'enduring presence' of the US military commitment to the region, Vice Admiral Scott Van Buskirk listed the carrier as part of enhanced deployments and expanded military diplomacy - all part of his stated attempt to overturn perceptions that the US is a declining power.
Afterwards, Van Buskirk also said he was 'confident and comfortable' that an aircraft carrier would be welcome in Hong Kong soon. 'I look forward to that occurring here in the near future,' he said.
While US official reports and many regional academic surveys generally highlight the operational impact China's military advancement will have on the US and its allies decades ahead, the carriers are also a symbol of more immediate pressures.
In a fresh study for Australia's private Kokoda Foundation, well-connected military scholar Dr Ross Babbage warns that 'the assumption that US and allied naval surface vessels can operate with high security in all parts of the western Pacific is no longer valid'.
Babbage points to not just the growth of China's fleet of surface ships and submarines, but its expanding cruise and ballistic missile programmes and improved satellites and radars that allow over-the-horizon targeting.
Ships and aircraft 'operating in much of this theatre will in many circumstances be detected, tracked and potentially also targeted', warns the report, titled 'Australia's Strategic Edge in 2030'.
China would be capable of destroying US bases on Guam and in Japan 'within a few hours', it adds.
While his report deals with a 20-year time frame, seeking to foster a debate over how Australia should best respond as one of the closest US allies in the region, Babbage said in an interview that the risks and costs of operating within 1,500 nautical miles of the Chinese coast were already being felt.
And carriers highlighted that.
'Right now I don't think there is much that the US feels it can't do in terms of its traditional operations - except if things were tense [militarily], they would be very careful about putting a carrier within 1,500 miles of the Chinese coast,' he said. 'That in itself is quite significant.'
His comments were echoed by Dr Sam Bateman, a scholar based at Singapore's Nanyang Technological University, who said it had been apparent for more than a decade that US commanders had been 'circumspect' about putting carriers too close to Taiwan or the mainland coast.
The two carrier strike groups deployed by the administration of former US president Bill Clinton at the height of the 1996 Taiwan Strait crisis did not get as close to the island as widely believed, Bateman said. They certainly did not travel through the strait, as some reports suggested.
'In a crisis situation, what they would be most worried about would be a Chinese submarine,' Bateman, a senior fellow at the university's S.Rajaratnam School of International Studies, said.
'China's cruise missiles and the development of a ballistic missile that can target a ship may get a lot of attention, but in many ways China's submarines are the greatest threat - it would only take one to get through and fire a torpedo. There is an old adage: the easiest way to sink a ship is to put a hole in its bottom, and that remains true to this day.'
US carriers always sail flanked by a strike group, a vast protective screen of aircraft, surface ships and, often, fast-attack submarines to create a defensive shield of surveillance and weaponry, including missile defences. It is a concentration of military power that no other nation can yet muster.
They form part of an approach that can be seen as strategic reassurance of old US allies and newer friends, just as the public comments of Van Buskirk and other senior US military brass are geared to ensuring fears of US decline are eased while the China threat is not publicly overplayed.
In Hong Kong, for example, Van Buskirk repeatedly stressed that the US did not see China as a 'direct threat'. 'The US has a broad, deep and complex relationship with China, and much of that relationship is very positive. Indeed, to look at China through the lens of an adversary would be counter-productive,' he said.
But talk privately with both US and regional naval officials and a different picture emerges. Conversations are routinely dominated by China, from intensifying US deployments, to increased co-operation among friendly navies to the need to improve battle plans and strategies and better understand Chinese intentions and potential weaknesses.
'Make no mistake,' said one senior US naval official, 'it is all about China now.' US officials travelling the region have been quietly stressing to regional counterparts that there is nothing they cannot do operationally despite China's build-up. They also stress continuing US strengths in technological advance, reach and experience, as well as various potential measures to counter specific Chinese threats. They also repeatedly stress that Chinese objections to US activities in international waters near its coasts will be matched by increased deployments.
In his report, analyst Babbage notes that regional security planners are 'reluctantly asking whether the United States will rise to the challenge'.
'Generally in closed-door discussions, the credibility of the United States is being questioned and Washington's political capital is starting to erode,' he warns.
US re-engagement was also backed by the need not just to expand its footprint, but better disperse a variety of military assets and command and control facilities from heavily concentrated areas such as Japan and Guam. Australia would be one option, he notes.
And if carriers will increasingly struggle to move close to the Chinese coast during times of crisis, the ageing US fleet of nuclear-powered attack and missile submarines will ensure the US holds an advantage over China off its own coast for years to come, according to some analysts.
William Murray, a former US submariner and an associate research professor at the US Naval War College, said US submarine crews still had the advantage of stealth and experience.
'For many years to come, I think US submarines will still be able to conduct missions that an adversary could only ignore at their peril,' Murray said, adding that his views did not necessarily reflect those of the US Navy or government.
'Finding a quiet submarine operated by an experienced crew remains something that is still exceptionally difficult - it can be done, but it is a complex, time-consuming and costly business. And then there is the problem of trying to learn how to do it when you don't have completely quiet submarines yourself to exercise with.'
While he was unable to say how close US submarines might routinely operate to the Chinese coast, he said he did not believe any recent Chinese naval developments - whether its growth of advanced surface ships, its own submarines or military satellites - would affect US capabilities. 'For the US, submarines are still something that offer a high degree of flexibility, almost an immunity.'
Murray said that for all China's naval modernisation and two decades of double-digit growth in defence spending, there were still significant gaps in its programmes and ability. While the People's Liberation Army was expanding fleets of fast attack submarines - the so-called hunter-killers - and larger, ballistic missile submarines, it appeared to lack a truly quiet submarine. It also had yet to develop extensive anti-submarine warfare systems.
'China is just not there yet... It is a reminder that the broader effort is one that requires decades of sustained dedication,' Murray said. 'It cannot be done overnight, no matter what the expenditure.
'At the risk of sounding a little bombastic, it is also a fact that one US fast-attack submarine could, with its torpedoes alone, take out the modern Chinese fleet of surface ships ... that's 20 to 25 vessels. It might take time, but they could do it.
'To my mind, that is a conventional deterrent. This is a war that must never be fought, so that is why such a deterrent is needed.'
When asked if submarines formed the key, from Washington's perspective, of unlocking an increasingly complex Chinese access-denial strategy in East Asia, he said: 'They might be. They do seem to offer this potential. The downside, of course, is that effective submarine programmes are tremendously expensive.'
Greg Torode is the South China Morning Post's chief Asia correspondent