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Analysing the South Atlantic After beating to windward between
two zones of high pressure, Club Med has found the South Atlantic trades. What
makes this region so special? We take a look. February 21, 2001 Many navigators refer to the zone of high pressure within the South Atlantic as the 'St Helena High', while its North Atlantic counter-part is typically named the 'Azores High', or the 'Bermuda High'. In summer, one North Atlantic High is centred over the Azores while another typically is centred over Bermuda. In that situation, a trough line separates the two, tracking north-east from the Gulf of Mexico to a centre of low pressure east of Newfoundland. Back to the South Atlantic Navigators are continuing to call the zone of high pressure within the South Atlantic Ocean the 'St Helena High', yet it is only centred over these islands in winter. Come summer, the zone of high pressure migrates south, and is often affected by a trough line tracking from southern Brazil south-east to waters south of Cape Town. In this scenario, there are two high pressure zones - one west of Cape Town (you could almost call it a Tristan da Cunha High) - and another east of Uruguay. Time to redefine the South Atlantic! When Club Med attempted to sail around the St Helena High, she really tried to sail around the Uruguay High. In the end she beat to windward in a northerly airstream between the two high pressure zones and directly underneath that trough line mentioned earlier. Some day soon, maxi-multihulls will, indeed, be able to slingshot around the Uruguay high and six days will fall from the Jules Verne 'round the world sailing record. But it will take some doing. Besides greater speed, such a boat would need luck on her side as she rounded Cape Horn. Ideally, the trough line separating the two South Atlantic high pressure zones would need to be connected to a low forming over South Georgia. That low would have to be strong enough for its frontal cloud to remain connected to the sub-tropical trough. At the same time it would have to track to the east without heading too far south. Further north, the Uruguay High would migrate across the South Atlantic, filling in behind the trough line. Our dream yacht, meanwhile, would have to be positioned east of the Falklands just as the connection between sub-tropical trough and frontal cloud band ignited. Racing downwind in south-west winds, our maxi-multihull would slingshot around the eastern flank of the Uruguay High, riding south, then south-east winds, before rounding the high to the north and catching favourable trades. Throughout this ordeal, our dream yacht would vary only a few miles from its rhum line with the North Atlantic. In fact, just thinking about it, Club Med could have done it this time around if the low to the south stalled on its eastward journey. Then, Club Med would have made the miles to sail successfully around the high's eastern flank. Someday it will happen. But in the meantime, the South Atlantic will continue to test routers and navigators battling to sail through fickle conditions. Back to The Race Club Med, meanwhile, is reaching in trade winds heading towards the equator and sticking to a course bringing it close to Recife, on the far eastern coast of Brazil. This strategy is all the more intriguing, given that Club Med's router Roger Badham alerted Weather-wise to the pitfalls of sailing too close to Brazil on the return leg up the Atlantic. "The east coast of Brazil is a tricky spot," he said. "In fact, anywhere north of 35 degrees south, along the eastern seaboard of South America is tricky. Heat lows come out and interact with fronts. Tropical systems can interact with frontal systems and trades along the top part of the Brazilian coast are tricky. They sometimes fire and sometimes don't." However, as of late the doldrums, or inter-tropical convergence zone, has stretched much further north and south than when the cats sailed through it in January. No doubt Badham plans to avoid the worst by sailing to the west where convergence between the two hemispheres is strongest. Innovation Explorer, meanwhile, is losing ground to Club Med while stuck in the windless zones of the South Atlantic High (at the moment there is only one, centred east of Tristan da Cunha). She is more than 1100 miles behind and looks like losing more ground before she collects the trades further north. Expect the distance to shorten as Club Med slows in the doldrums - and expect it to lengthen again as Club Med accelerates away in the North-east trades of the North Atlantic. Barring major catastrophe, Club Med has it in the bag - and deserves the win after a most professional voyage around the world. But do not write off the team aboard innovation Explorer for future regattas. Loick Peyron, Skip Novak and Roger Nilson make a superb team. And they know how to have a bloody good time doing it! (pass the claret, will you Skip?). In the meantime, expect Innovation Explorer to make some interesting decisions, weather-wise. After all, they have nothing to lose ...Warta Polpharma is 5600 miles astern, Team Adventure a further 300 miles back and Team Legato stuck in Wellington, a total of 7700 miles astern of Club Med. The race within the race Team Adventure took so long to get to Wellington, thanks to zones of emerging low pressure, followed by windless high pressure, that Warta Polpharma and Team Legato caught up enough to make a race out of the trailing trio. But that has changed somewhat. Team Adventure laid over in Wellington to re-repair their delaminated front beam while Warta Polpharma headed beyond for the Southern Ocean. Team Legato, meanwhile, has also laid up in Wellington to repair injured crew and remains there as of today. Currently, Warta Polpharma is 1000 miles east of Wellington with Team Adventure 300 miles astern. Navigating the doldrums We watch with baited breath how the inter-tropical convergence zone will behave for The Race fleet on their return north. Here's what a couple expert routers have to say on the subject: Ken Campbell, Commanders Weather (Team Adventure): "In equatorial regions, I do not know of any way of forecasting cloud development. Basically, we look for a spot with a narrow or narrowest band of concentrated clouds and go for it. To a certain extent it is hope and pray, but that is weather forecasting anyway!" Do you take into account upper level winds carrying moisture from the "I use all the high resolution satellite pictures (infra red, water vapor and visible) for the mean inter-tropical convergence zone position, real time satellite derived winds and model-predicted winds. "We try to skirt all the major cumulonimbus (there are light winds directly underneath these thunderstorms) and use the edges (where downdrafts flow outwards), especially at night. Upper level winds and jet-stream cloud to the north are not used at all." So there you are: All that science and training, and still the doldrums are something of a mystery! Just a note on thunderstorms in the tropics. Weak winds at upper levels generally prevents these boiling blobs of moisture from spinning, or rotating, so downdrafts and updrafts have no way of remaining separate. Warm air updrafts feed these storms and without them they die. As a result, cumulonimbus in the tropics typically explodes high into the sky during the heat of the day only to collapse down on itself. © Jamie Brown/weather-wise.com 2001 Synoptic situation and fleet
positions Picture courtesy of Virtual Spectator. Club Med streaks to an 1100 mile lead as enjoys gentle trade winds off the east coast of Brazil. Here she is sailing at 18.8 knots in just 11.6 knots of breeze. Innovation Explorer, meanwhile, gets a taste of light northerlies as she navigates through high pressure to the south. Four day wind and synoptic
forecast Image courtesy of NOAA. Take a look at this model, and you can see why Roger Badham has encouraged Club Med to head west. It seems the inter-tropical convergence zone really goes quite in the east, spreading almost all the way to Brazil! Innovation Explorer might do well to follow in Club Med's footsteps for the moment. Note how the trough extending south-east from southern Brazil ignites as a low passes to its south. Case history: Synoptic situation and fleet
positions Club Med was beating against northerly winds for several days before this position report, but here has managed to climb far enough to the north to avoid any ridge of high pressure to her east. She has slackened sheets and is close reaching doing 12.3 knots in 17.5 knots of wind. The day before she was close hauled doing 6.1 knots in the same winds. Picture courtesy of Virtual Spectator. South Atlantic in infra-red Images courtesy of Intellicast Check out the band of cloud, stretching from southern Brazil, into the South Atlantic. This is a trough line - warm, moist air from the Amazon Basin embedded within a pool of cooler air at upper altitudes. At the surface is weak low pressure. Because there is no obvious low pressure system to the south, this trough seems to just sit there, not going anywhere, and failing to generate a low at surface level. If a deep low helped to fuel this trough line by combining energy from its frontal cloud, things would happen: The trough would move with the low to the east, and surface pressure would be more affected. This trough always seems to hang around this corner of the world - a known cyclo-genesis region, and heavily influences the shape of high pressure zones in the South Atlantic. On this day, Club Med was beating to windward underneath the trough, between two zones of high pressure. The Uruguay High can be seen as a dark blue patch (clear air) east of Uruguay, while the 'Tristan da Cunha' high shows as mottled cloud - a mix of stratocumulous and clear air west of Cape Town. To the north of these two highs are south-east trades, showing as mottled squall clouds, cumulous, within a sea of deep blue. South Atlantic surface pressure The two zones of high pressure are clearly seen here in this surface pressure map. A zone of low pressure exists directly under the trough, but fails to materialise into a storm and absolutely refuses to move to the east, as one would expect. More great reading: Catch up on all the past action in our race archive. Here it from the horse's mouth at race headquarters. re-live the action between the world's toughest solo skippers in the Vendee Globe. Re-live the Sydney to Hobart yacht race! Break ice in the Ross Sea! Re-live Bernard Stamm's record-breaking Atlantic crossing |