Oskar Kihlborg/Volvo Ocean Race
Saturday 18 October 2008
As the boats head south toward the equator, they are rapidly limiting their choices for the transit through the Doldrums.
Over the last 48 hours we have seen the fleet cruising south, with fresh NE'ly winds and good boat speeds. To their west, Tropical Storm Omar has been strengthening; however, this storm is too far north and west to have any impact on the fleet and the navigators continue to focus on the North Atlantic high.
With the given position of the high, any teams that broke for the west saw lighter winds and a slightly worse sailing angle. Thus the pack has stayed together heading south into the Cape Verde Islands. Unfortunately, the fleet is quickly sailing out of the trades and approaching the Doldrums. The next 24 hours are sure to bring interesting changes in the conditions.
Before we talk about the tactics for the Doldrums, it might be useful to have a better understanding of why the winds are so light in this region.
More generally called the Inter Tropic Convergence Zone (ITCZ) or Equatorial Trough, the Doldrums are not a permanent feature. In simple terms, they can be defined as the area that gets the most direct heating from the sun.
As we have mentioned in previous reports it is a stable feature in the Atlantic, although it moves north to south with the seasons. As a result of the direct sunlight, the surface temperatures are very warm and most of the air is moving vertically rather than horizontally.
This makes for light surface winds and a localized area of low pressure or a surface trough. In addition the direct sunlight and warm temperatures cause massive evaporation from the sea surface. The combination of homogenous air and high humidity means that any change in stability can cause massive thunderstorms.
As of midday Friday, there was reasonable model agreement for the approach to the Doldrums, with a tropical wave of low pressure moving east, frequent squalls, and only a few days of light winds expected. There has been good agreement between the ECMWF and UKMO, and both models support a route west of the great circle course.
The teams that choose to believe and follow these models will likely approach the Doldrums from the north, sailing to 13N and 30W before heading due south for the next 500+ nautical miles.
These may see a period of light winds (as low as 5 knots), but for much of the next 48 hours the winds will be in the low teens (10-14 knots).
On the other hand the GFS favors a course closer to the rhumb line (shortest course between two points).
This route would means the boats sail less total distance (about 100 miles less), but they also risk being in lighter winds or more squalls. This model favors the boats crossing the Doldrums closer to 28W.
While the model guidance is very important, the light winds and frequent convection (thunderstorms) found in the tropics make satellite imagery central to the navigator's decisions.
With the E'ly wave moving through the boats' sailing area, the teams can expect frequent squalls and some heavy rain over the next few days.
Ideally the teams will try to avoid sailing through any squalls; however, if used right, the outflow from a squall could give any team in the right place a large advantage.
In the next weather update I will talk a little more about how the teams can try to use any local squalls to their advantage.
With the equatorial crossing expect for early next week it will not be long before the teams round the first scoring gate.
Jennifer Lilly