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13 February 2005, 09:42 pm
Southern Hemisphere Weather
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Oryx Quest

The yachts racing the Oryx Quest 2005 have now entered the Southern Hemisphere where a new set of weather obstacles awaits them. This is the first time that a major ocean race has sailed the length of the Indian Ocean and it's all new to the skippers, their navigators, and on-shore weather routers.
Unlike traditional races where there is a lot of empirical data, the Indian Ocean is almost a vast unknown, especially for racing sailors. On all the boats there is a lot of number crunching, debating, and finger-in-the-air guessing as they race south heading for the cold waters of the Southern Ocean.

Unlike the northern part of the Indian Ocean where the weather is interrupted by numerous land masses, the southern section offers a much more understandable pattern. Like the South Atlantic, it is usually dominated by a large region of high pressure that has winds circulating in an anti-clockwise direction around its center. Unlike the South Atlantic High, this one is easily kicked around and strong low pressure systems that drift up from the Southern Ocean, or ones that spin off the African coast can easily reduce the size of the high, split it in half, or shove it south. In other words it's a moving target.

High pressure means good weather. The sun shines brightly and the sea is usually flat calm under a cloudless sky. In other words, it's good beach weather but not great for sailing, at least not near the center of the high. Away from the center it's another story as the wind rotation around the system provides the much touted trade winds. It's where some great sailing can be found, and on a maxi-multihull, it's where the big day's runs happen. The key is to watch the on board barometer; if it starts to rise it's time to alter course to move away from the center of the high. On board Doha 2006 Will OXLEY, the navigator, spends much of his time analyzing weather data and merges it with the boats performance data to advise skipper Brian THOMPSON on the best route to sail. It's an inexact science because the weather is constantly changing, meaning that even the best laid plans need to be a work in progress. Oxlee is aided by satellite imagery that captures images of the area above the racecourse. From the vantage of outer space he can see cloud patterns, study sea temperature, wave heights, and get a general feel for the large scale weather patterns. Oxlee discussed the various means of analyzing weather in his weekend log.

'We use a variety of meteorological models to predict the areas of calm and wind,' he wrote, 'but none do a particularly good job of handling the complex weather situation in this part of the world, so our focus is on visible and infrared satellite imagery as well as quikscat, a satellite that is able to determine wind speed and direction by analysing the sea surface texture. This is a sun synchronous satellite meaning that we only get two passes a day; one in the morning and one just before dark. There is a delay as the data is posted and made available on the web, and our weather router onshore and I always wait anxiously for the next image as this will play a major role in the next 12 hours routing. The onshore router sends a text message to our Iridim phone when the image is available, and then I go online to download it. The satellite has a blind spot in each pass and two days ago this blind spot was right over us which is Murphy's Law in action. So far this strategy has been relatively kind to us and for now we are still making good progress south of the equator.'

Meteo France, the weather experts for the Oryx Quest 2005, have pinpointed the center of a stationary low pressure system located just south of the fleet. The wind rotates in a clockwise direction around low pressure in the Southern Hemisphere meaning that the yachts north of the low will be enjoying winds from the north. It's clear from looking at the different routes the boats are sailing that they are gybing downwind. At 10:00 GMT Doha 2006 had altered course to starboard and was heading due south, while Geronimo, 80 miles to the north, was still sailing a southeasterly course on port tack.

The area of high pressure is quite well formed to the south of the fleet and moderate southeasterly winds of around 20 knots will greet the sailors once they break free of the effects of the small low. Below the high intense low pressure systems swirl. Some may skew the high with others may simply break it apart. How these trouble-making lows effect the high is what makes this game that much more interesting for the navigators. The next week will fun to watch as the boats negotiate the vagaries of high pressure and tag on to the edges of lows to catapult them south.
Brian Hancock
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