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Free at last! Desjoyeaux's 640 mile lead is whittled down to one-tenth of that after a frustrating week in the South Atlantic High January 24, 2001 The weather models predicted it: As series of big depressions forming east of the Falklands pushed strongly into the South Atlantic High, otherwise known as the St Helena High, forcing it to the north at about the same pace as leader Michael Desjoyeaux. As a result, Desjoyeaux's comfortable 640 mile lead has melted 'like snow in the sun' as Ellen MacArthur, Marc Thiercelin, Roland Jourdain, Dominic Wavre and Thomas Coville all sneak up from behind. In fact, these six skippers were within 630 miles of each other at midday yesterday with Catherine Chabaud in seventh nearly 1400 miles behind the leader. Frustrating! To make things more interesting, frontal systems extending north from the deep lows in the south have linked with monsoonal troughs over the Amazon basin, effectively splitting the South Atlantic High in two. Sailors immediately north east of these fronts bash into headwinds while those to their south west surf downwind. Those directly under the frontal trough, wallow in light airs. And, of course, anyone caught under the centre of the two high pressure centres will watch all the action sail around them. Trying to plot a course through this minefield requires more luck than skill! However, Desjoyeaux is not the only one grumbling in his coffee. "The weather is really complicated, complained Marc Thiercelin after battling 60 knot headwinds blowing out of a determined squall cloud, on January 19. "I was upwind when the others were downwind, he noted. "So obviously, they are catching up with me. I am getting closer to the head of the fleet but I cant watch my back. "It's all very vague, there are things
happening I can't even see on my wind files." Terry Hart, superintendent meteorologist with the Australian National Meteorological Operations Centre, oversees the seven day forecasts for the bureau and says modeling - which incorporates observations from all over the world - is very good at predicting big events, but no so crash hot at forecasting smaller events. "Very localised events are too chaotic for the machines to predict," he says. Into the trades Now that the South Atlantic High is finally letting go of its prey, Desjoyeaux and MacArthur, and those behind, can look forward to trade winds for at least 1000 miles before they hit the doldrums. MacArthur spent an hour yesterday morning moving moveable items to starboard, trimming her boat for the next leg. As she points out, it is unlikely she will tack for several days and it is essential to keep weight where it is needed: On the windward side and away from the ends, to reduce pitching. Negotiating the trades won't be plain sailing, however. Skippers are in a position to sail closer to the Brazilian coast - tempting in light of the dominant South Atlantic High. However, respected Australian weather router Roger Badham has this to say: "The east coast of Brazil is a tricky spot. 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." Navigating the doldrums We watch with baited breath how the inter-tropical convergence zone will behave for the Vendee fleet on their return north. Here's what a couple expert routers, currently working with The Race, 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 Fleet positions Here leader Michael Desjoyeaux (dark red line) is about to cross his old track coming down the Atlantic in late November. You can see the isobars to his south-west indicate a frontal cloud band stretching north west from a low centred on latitude 50. That front has split the South Atlantic High in two, bringing mixed conditions to the fleet. Fleet positions Desjoyeaux and MacArthur (blue line) exiting the high and reaching the first of the south-east trades - at almost 20 degrees south. As the vigorous low in the Southern Ocean travels east, and a less vigorous westerly flow takes over, the South Atlantic High starts to assume its summer position, centred about 35 degrees south. The indentation in the isobars is a monsoonal trough extending out from the Brazilian coast. Models predict it to develop into a weak low and head south-east with little affect on the Vendee Globe fleet. Note Thomas Coville (bright yellow line) way off to the west. He told race headquarters he didn't want to be there - there is too much upwind bashing, with 35 knots reported. But watch to see where he goes from here. Too far west, is not best according to leading weather routers. Pictures courtesy of Virtual Spectator.
South Atlantic infra red You can see a frontal cloud band stretching north west from a low in the South Atlantic and connecting with a monsoonal trough over southern Brazil. This front has split the South Atlantic High, while its parent storm to the south has helped to push the high northwards. East of Uruguay this front is bordered by white squall clouds, indicating that this frontal trough is indeed affecting surface winds! Marc Thiercelin reported 60 knots gusts from the north in one squall the day before. Areas of light winds are west of South Africa - where no infra red visible cloud exists at all, and east of Uruguay. Trade wind squall clouds are faintly visible north of the centre of the South Atlantic High. Thunderstorms in the doldrums show up as red dots, indicating very high, and thus very cold, cloud tops. Image courtesy of Intellicast.
Five day wind forecast for the
Atlantic
Image courtesy of NOAA. For fleet leaders, trade winds are forecast to fill in right across the Atlantic - no surprise there - but the South Atlantic High looks like it will remain a tricky thing for those coming up through it - Catherine Chabaud, Josh Hall and Mike Golding.
More great reading: Catch up on all the past action in our race archive. Follow the action between the world's fastest multihulls in The Race Vendee Globe Race headquarters Michel Desjoyeaux's home page: Ellen MacArthur (Kingfisher) : Re-live the Sydney to Hobart yacht race! Break ice in the Ross Sea! |
