Elaine Bunting takes a look at Southern Ocean storms and how round the world yacht try to harness the brutal forces of low pressure systems
Solo sailors call it ‘the South’, as if to emphasise its alien difference. The Southern Ocean is a place most of us have never been to and never wish to visit, a realm of cold grey skies and raging winds that eternally circulate round the bottom of the world.
It is an extreme environment on every level. Immense Southern Ocean storms, often 2,000 miles from north to south, can affect four million square miles of ocean at a time. They are fast-moving and frequent – on average there is one gale per week south of 50°S, with the accompanying violent polar fronts sweeping though the Roaring Forties and Furious Fifties every few days.
At the sea surface, wave heights of 4-5m occur 50 per cent of the time below 50°S, but these mean heights take no account of conflicting wave trains or the heaped, breaking crests of a conjoined swell. Below the surface, a vast never-ending conveyor belt of cold water travels eastwards. At the latitude of the Forties and Fifties it is mainly wind-driven current, but further south the powerful Antarctic Circumpolar Current transports around 140 million cubic metres of water a second, three times more than all the rivers of the world combined.
Observations from space are giving a better picture of a remote region and demystifying the Southern Ocean: infrared images, real-time wind observations from microwave-equipped satellites and the latest synthetic aperture radar able to detect ice, even through cloud or in a rough sea. On the water, there is a swelling fund of first-hand experience among professional sailors.
Above: ocean surface winds, as detected by US NOAA advanced scatterometer in geostationary orbit
However, the reality is unchanged and the battle that sailors face in this hostile territory is extreme. Given the unpredictable dynamic energy of the Southern Ocean, it is preposterous bordering on arrogant to think that any yacht design could ever be completely safe.
But as our understanding increases and yachts get faster, sailors have a new possibility, of harnessing energy on the periphery of weather systems. If judged right, skirting the storms has a threefold virtue: less wind, flatter seas and faster speeds to minimise the number of weather systems encountered and the time spent at risk.
That’s the theory, but as we’ll see, riding a Southern Ocean low pressure system is a tightrope strategy.
Violent lows and little rascals
A Southern Ocean low is fundamentally a mirror image of one we might experience in the North Atlantic, but with some important differences. In the North Atlantic a depression has a limited lifespan because it can only travel 3,000 miles. Down here, storms are unhindered by land other than the intrusion of the tip of South America and that gives them time to build up a huge fetch. In the most intense north-westerly quadrant there can easily be winds of 70-80 knots and 15-20m seas.
The other significant difference is that, as these systems mature, the cold front overtakes the warm front. Although technically an occluded front, it is nearly always drawn as a single, cold front.
“These ‘ana’ fronts are very active, very intense, with bigger windshifts,” says Mike Broughton, a well-known racing sailor and weather forecaster. “In 20 minutes you can go from north-west winds at 50 knots to south-west at 20, and the wave trains from the north-north-west and south-west become very difficult to negotiate.”
In the absence of a warm sector, many more secondary lows develop along the front – “little rascals” Broughton calls them. “They can spin up quickly and develop like a slingshot, so it can be difficult to pinpoint where they’ll track. They can’t be underestimated and seem to form much more frequently on these active fronts than we see in the Northern Hemisphere. They make life so much more difficult when negotiating lows.”
The downhill slalom
Back in the ‘old’ days of the Whitbread maxis, the route through the Southern Ocean was radically different from that taken by today’s Volvo Ocean Race. There were no waypoints and yachts headed deep south, as far as crews dared, to shorten the distance. In 1978, John Ridgway took Debenhams down to 66°S and was confronted by pack ice. He called all hands and the shaken crew had to sail back they way they’d come.
It made sense for the maxis to head deep: they were after big winds and the speeds achievable from surfing the big Southern Ocean rollers. Even after waypoints were introduced, crews pushed south. Navigator Roger Nilson remembers one of the early Volvo Ocean Races:
“We counted 110 icebergs in three days,” he remembers. “It was Russian Roulette. I would never do that again.”
That has all changed – the tactics are different because the yachts are so much quicker and every round the world race today has its route hauled north by ice gates. Apart from rounding Cape Horn, at 56°S, there is no need for these yachts to go deep south and every reason to avoid it.
What had basically happened is the degrees of freedom on the course have been restricted, but this can poses knock-on difficulties of another nature. These gates can become a navigation problem for sailors, forcing their route rather than allowing them to take best advantage of the Southern Ocean low pressure systems.
By and large, what crews are trying to do in the South is set themselves up to the optimum true wind angle, from around 115-135°. The best places to get those breezes are in the pre-frontal zone in the north-west, or in the south-west behind the system.
The key is to avoid really strong winds and the sea state that goes with them – better to be in smoother seas where they can plane without the surface action of waves.
Sea state is a more difficult one. Routeing software does a good job in predicting wind strength and angle but it doesn’t do so well in factoring in the role of sea state. The days of seeking out surfing conditions are well and truly over, though, and it is very rare for modern racing yachts to go faster with waves than without. Wave direction also has a huge effect, and yachts only have to experience a 10-20° change in swell angle relative to the wind direction to be much less uncomfortable and slower.
In the ideal pre-frontal zone, in flatter seas and in the right pressure band, the idea is to hitch onto the system and ride ahead of it for as long as possible. In this respect, monohulls have become closer to multihulls, whose ability to travel as quickly or even faster than a weather system can give them the freedom to slide in and out of pressure. But typically, navigators are trading off where they would like to be against how fast they can VMC (velocity made good against the course to the mark) down the track.
The Cape Horn trap
No matter how you dial into the weather systems, and no matter how far north organisers place waypoints or gates, everyone has to come down to 56°S eventually. Cape Horn extends down into the firing line of lows and this is where the options for positioning close down and crews can rapidly run out of runway. Timing is everything.
Timing is critical on this leg and if navigators allow themselves to get chased too far south they can get trapped. Sometimes these yachts have a limited range of angles and, with the spinnaker up, can come up by only 3-4° or down by the same, or less.
Cape Horn can also present rough seas, as the long fetch and big swells of the Southern Ocean heap up on shallower water. There are also some navigation challenges as there are offlying islands.
And once the corner is turned there can be high winds in the South Atlantic. But the real stress is over. No longer are crews remote from land. Every mariner, of every age, has always passed Cape Horn and left it behind with a great sigh of relief.
That is why you sometimes see photos of sailors toasting the ocean with champagne at the Horn. It is a traditional gesture of gratitude to the sea for granting them a safe passage.