Expert weather guru, Chris Tibbs takes a look at Southern Ocean storms and considers how sailors can best take advantage - or avoid- them
Any colour you want as long as it’s grey! That’s my overriding memory of life in the Southern Ocean – we tend to forget the sun reflecting off icebergs and the aurora of the Southern Lights on the horizon at night, because it is the relentless succession of low-pressure systems rattling around the world that dominate the weather.
This gives periods of low cloud and rain to be followed by an active cold front and squally conditions. A temporary lull may occur as a ridge of high pressure builds… .before the next low is upon us and the pattern repeats until we escape from the area.
For many sailors the Southern Ocean is the holy grail of racing, with its reputation for fearsome storms and monster waves. But what makes it different from other storms and oceans? The remoteness, for one: knowing that the only help you’re likely to get is from other competitors and at times you’re closer to the International Space Station than any other person (beyond fellow competitors). The Oceanic Pole of Inaccessibility is at 48° 52.6’S and 123° 23.6’W and is the point on the globe furthest from any land (over 1,400 miles) – and it feels it!
I have been fortunate to race around the world three times; twice in the Whitbread Race and also as skipper in the BT Global Challenge (racing the wrong way around the world) and the depressions were bigger and fiercer than others I have experienced in other parts of the world.
Racing strategy indicates heading south to reduce the distance along the great circle route, and also to take advantage of the circumpolar current that continually circles the globe, without getting south of the low pressure systems and into head winds. In the days of the heavy IOR yachts of the early Whitbread races, the more wind you had the faster you went, with the cold and ice being the limiting factors.
Modern around the world races now have ice limits to prevent yachts getting too far south and into the areas where icebergs are known to be (though the Jules Verne record attempt does not); in addition modern racing yachts will go faster in lighter wind strengths and sea states.
Lows develop from different air masses coming together but not mixing, giving the different sectors of a depression quite different characteristics. Warm tropical maritime air will give low cloud, rain and drizzle with poor visibility, then as you are overtaken by the cold front this gives way to squalls with hailstones likely and gusty conditions.
Post-front showers and squalls will come through like a freight train with significant increases in wind. There is also a noticeable drop in temperature as the wind backs more to the south, although there may well be sunny spells.
With no land to stop the progression of the lows in the south they tend to develop and travel across large tracts of ocean before they eventually mature and decline. With a large area of cold water and, more importantly, cold air above to the south and warm, moist air to the north there is plenty of energy and temperature contrast to set things going. Once developed they are driven by the jet stream and continue their track for thousands of miles – compared to the depressions of the north Atlantic, for example, where after a couple of thousand miles the depressions reach Europe and decline.
The southern winter
Study has shown that the southern polar jet stream moves progressively further south during the southern hemisphere winter to around 60°S and becomes clearly separated from the sub-tropical jet stream near 30°S. However, during the summer months it moves north and becomes less well split from the sub-tropical jet stream; often a single jet stream can be found near 40°S.
This indicates that the later in the season racing yachts circumnavigate, the further north the tracks of depressions are likely to be and in general the less aggressive they are, so a more favourable passage through the Southern Ocean is likely to be in March than December.
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Cold, active fronts
Not only are lows likely to be bigger in the Southern Ocean and cover large areas, they will generally be shown with just a cold front; we will still get conditions usually associated with a warm sector, it is just that there is not a well-defined warm front. Fronts define the boundaries between different air masses and while no two lows are ever identical, so it’s also true that no two cold fronts are the same.
In the Atlantic it’s not always the case that the cold front is the more active front. However, in the Southern Ocean it is nearly always the case that the cold front will be the more active, with strong winds, squalls and hail. I once experienced a 100-knot squall in the Southern Ocean and on a number of occasions fronts passed through with 80-knot squalls. These active fronts are known as ana cold fronts, named due to the warmer air ascending up the frontal boundary, generating large cumulonimbus clouds and intense squalls.
Ahead of the front we can expect a period of steady wind with drizzle and poor visibility. This is the time yachts can make good progress – the faster the boat, the longer they can keep ahead of the cold front. As the front gets closer the wind will become gustier with bands of heavy rain. This is where life becomes more interesting with gusty wind making for more testing sailing.
It is this relatively quiet period that sailors want to really maximise, where big mileages can be made in comparatively flat seas. However, it’s never straightforward, as it’s a balancing act between positioning yourself for the approaching front, not getting too far south to the strongest wind and the restrictions of ice gates, and not too far north into lighter winds and a longer route. Ana cold fronts are vicious but not all the same: prefrontal troughs can give a wild ride ahead of the front but if the front is in decline you may get spat out the back with lighter wind after the squalls.
If the lows were consistent and regular it would be possible to plot a passage through, but they are not. It’s important to keep an eye out for secondary lows. These are small lows that develop on the cold front and can deepen quickly. If one develops to the north the best result is a period of calmer conditions as it passes through, the worst case scenario being a period of violent headwinds if caught on the wrong side of the new low.
Secondary lows are best spotted on satellite imagery by a change in cloud patterns and should be in forecasts, but are not always picked up in weather models until too late.
On one race in the Indian Ocean a secondary low developed close to the north of us. Conditions went from a westerly gale to nothing as the low passed close overhead with a horrendous sea state, then almost instantaneous headwinds of 50 knots had us scrambling for storm sails, all in one four-hour watch!
The waves are unfettered by an almost infinite fetch and after a few days of near gale force westerly or north-westerly winds, followed by a backing of the wind and gales from the south-west, the result is a sea state like a washing machine, only more dangerous.
Even when we think the end is in sight and Cape Horn beckons, at 56°S it may be the furthest south we get on the race. It is a bottleneck of weather and waves. Quite often a large, slow moving low develops west of the Antarctic Peninsula. The landmass of South America funnels the wind, predominantly north-westerly here, and the seas are the highest of anywhere.
The large slow moving low spins off secondary lows with great regularity. These secondary lows can be active and further north than previous lows. Throw into the mix that to the south-west of Cape Horn the ocean floor rises from 4,000m to 100m within a few kilometres, and the resulting large waves and swell, with a rapidly shallowing water, can make the exit from the Southern Ocean particularly exciting. It was here on my second Whitbread that the last storm of the Southern Ocean overtook us.
Classic pre-front conditions had given a fast run towards the Horn though we’d been pushed further south than we really wanted to be. However, the backing wind after the front would take care of this. Like any cold front we were ready with reduced sail for the expected squalls – what we were not ready for was close to 100 knots. The front arrived and with it the mainsail disintegrated as we tried to put in the third reef, with just a blast reacher (jib top) the log was showing 20 knots and the anemometer was up against the stops.
As the sun came out all that could be seen was white water and, close by, a fishing boat! What they must have thought of crews sailing by Cape Horn for pleasure will never be known. After a couple of hours of squalls we were back to normal as we passed through the Le Mare Straits – we got away with it by the skin of our teeth because an uncontrolled broach in those conditions could have been disastrous.
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