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Southern sleigh ride Getting south proved more tricky than expected, but now that skippers are in the roaring 40s their downwind slide has begun in earnest December 6, 2000 Leading skipper Yves Parlier has broken the 24 hour solo Open 60 record upon reaching the roaring 40s, and with four of the frontrunners breathing down his neck, the pace of this race will not slow down any time soon. In fact, the Vendee Globe 2000 is shaping up to be a real delight for armchair viewers. No other solo global race has remained this tight so far into it. Even the last Whitbread - certainly the tightest ocean racing in history - was not this close at the end of the Cape Town leg. What we are seeing is some very talented skippers, top yacht design and proven equipment working together in harmony. Part of this successful equation relies on skippers responding quickly to each other's tactics. If Yves Parlier jibes and sails south, Mitch Desj follows suit, along with Roland Jourdain, Thomas Coville and Ellen MacArthur. There is less individual risk taking, fewer attempts to play the wild card when it comes to routing through weather systems. However, some still play it this way and Catherine Chabaud's odd decision to remain in the north while the rest of the fleet dives south has left many critics scratching their heads. She has done this before - heading east of the Cape Verde Islands on her approach to the ITCZ - and has won miles as a result. This time, based on the forecast model used by Virtual Spectator, Chabaud and her yacht Whirlpool look like they may end up smack dab in the middle of a Southern Ocean high. And there is nothing worse for racing yachts. Here, the wind dies but the swell doesn't. Rocking and rolling, even her friend the albatross will have to fold his wings and rest on the water's oily surface. The only movement will be the rest of the fleet sailing silently around her to the south. Ugh! Looking
back: Tricky trough slows While leading yachts are enjoying perfect downwind conditions now (winds 25 knots, seas relatively calm, swell long and low ... ), getting to this point proved tricky indeed. An upper level trough of low pressure has been a feature east of Argentina for many months, alternately strengthening and fading. Low pressure cells forming off southern Argentina swing north before heading east, and invariably become intertwined with this band of low pressure. The band gains strength from the low, and vice versa. When the trough is strong, it affects the surface pressure, compressing the St Helena High and creating winds just right for yachts between 25 and 35 degrees west, heading north west to south east towards the 40th parallel. But skippers beware: This trough also fades, and fails to drive surface winds. Instead, it resides in the upper atmosphere, merely dividing the South Atlantic's sub-tropical high pressure cell into two windless centres. This appears to have happened between November 30 and December 3. Skippers struggled to cope. Yves Parlier, not sure what would happen next, dove south in a brave bid to reach the steady winds below 40 degrees. While he made good his southing, he did not escape the light airs. He, and the rest of the fleet leaders, were back to the old routine: Many sail changes to take advantage of all opportunities. What did happen next, was that the trough strengthened, compressing isobars between it and the St Helena High, creating a gateway at last for skippers heading south. Winds from the north west freshened, and the leading yachts dove below the 40th parallel and picked up the westerly trades of the Southern Ocean. With winds rising steadily off a calm sea: Perfect conditions for breaking speed and distance records, which Parlier did in due course. Now, Mike Golding aboard Group 4 - handicapped by more than a week after he returned to Les Sables D'Olonne to repair his broken mast - has announced that he hopes to cut the corner of the St Helena High with the help of a developing low off Argentina. Sound familiar? We will wait and see, but if this upper level trough we have talked about continues to strut its stuff, Mike may end up dodging and weaving like his colleagues before him! © weather-wise.com 2000
Animated loop of South Atlantic
surface pressure Image courtesy of OceanWeather Yes, this is difficult to understand, but concentrate on the sequence, and you will see where it begins - The first frame holds for three seconds. In this first frame you can see the effects of the trough east of northern Argentina. This trough pushes into the South Atlantic High like a wedge, creating, in effect, two high pressure zones. Winds between the trough and high centred just west of Cape Town are perfect for the leading Vendee Globe yachts: They are located right between the trough and the high, in winds above 25 knots. Trouble is, the trough does not behave like a developing low pressure cell, and refuses to move east with the rest of the weather. It stalls, dumping yachts into a windless zone to its south and east. Animated loop of North and South Atlantic
showing cloud in infra-red
Image courtesy of Intellicast This one is a little harder to follow, because frames are taken every 48 hours, but you will get the picture. Note how the trough east of Argentina and Uruguay goes from strength, to weakness. You can see the dark blotches of the St Helena High, and also the North Atlantic High, pushed well south by a remarkable low to its north. As an aside, look how that north-east Atlantic low evolves into a multi-centred complex. The front crossing the Bay of Biscay must have packed a punch! Fleet positions from December 1-4 December 1: Leading yachts ride favourable north-west winds between the trough and the St Helena High. December 2: The trough refuses to move eastwards, dumping yachts in the windless centre of the high pressure cell. December 3: The trough, St Helena High and a developing low south of 50 degrees combine to create wind pressure where skippers need it! December 4: Leading skippers have worked their way below 40 degrees south to take advantage of steady westerly winds. What this frame doesn't show, is Catherine Chabaud's track - almost on a line with Cape Town, and well above the influence of westerly winds. Hmmm.. Picture courtesy of Virtual Spectator. Explore comment from the By Philippe Jeantot Yves Parlier (Aquitaine Innovations) has enjoyed
a solid lead since the Equator over the 20 strong international fleet still
racing in the Vendée Globe 2000. He has comfortably put 100 miles in between
himself and his nearest rival, Michel Desjoyeaux (PRB), who has been making his
life uneasy since the start. Parlier¹s boat has crossed the Greenwich Mean Time
first as well. December 2: The leading boats thought they would manage
to escape without slowing down after catching Westerly winds which would help
them reach the 40th degrees. But as far as weather forecasting nothing is never
sure. The system, a school case, hasn¹t developed like it should have had. |