Meteorologist Chris Tibbs explains everything you should know about the Agulhas Current before you sail the Indian Ocean
On any round-the-world cruise by the sunny route, there is the dilemma of how to cross the Indian Ocean. For me the Red Sea Route has no appeal whatsoever and although the reports of piracy have reduced it is not a risk many want to take.
Weather-wise there is a need to get out of the central part of the Indian Ocean before the tropical cyclone (hurricane) season gets going in November, although around Mauritius and Reunion you’re unlikely to find anything nasty until December. The majority of yachts therefore leave Reunion Island for South Africa in November.
You need to consider the approaching cyclone season moving into summer months, versus the Southern Ocean depressions throwing up active cold fronts that will reach north to Mozambique and Madagascar. The later you leave the passage to South Africa, the less active these fronts should be, but the higher the risk of tropical cyclones.
There are few yacht-friendly ports in Madagascar and Foreign Office travel advice does not make comfortable reading, so most yachts make the passage in one hop. With around 1,400 miles to Durban, or slightly less to Richards Bay, you need a weather window of about ten days.
These tend to be the favourite arrival ports, being further north than other South African ports, warmer and away from the Southern Ocean – although the fronts can still be very aggressive and secondary lows may form on them.
Article continues below…
What is the jet stream? The jet stream is a ‘river’ of strong wind high in the atmosphere which is…
There are a number of yacht races around the world in which strategic decisions on whether to go into or…
Here is the rub: a 10-day forecast is at best an indication and in reality the weather may be quite different. As you head south-west into more turbulent weather you will be planning your arrival in South Africa, and the areas of typically worst weather, on a long-range forecast.
This is of course nothing new for anyone making a long passage, but what makes this rather more interesting, from a meteorological point of view, and concerning for sailors is the Agulhas Current.
Like the Gulf Stream in the North Atlantic the Agulhas Current is a western boundary current, but being in the southern hemisphere it brings warm water south. This river of warm water meets the cold conditions of the Southern Ocean and adds energy to developing storms.
Southern Ocean cold fronts move quickly along the coast from the south, and fuelled by warm water they will often interact with low pressure over the land intensifying the front and spawning secondary lows. Behind the front are gale force south-south-westerly winds, which in itself is bad enough, but add a south-bound current and the wind over current conditions can quickly become dangerous.
Just as UK sailors would never dream of going through the Portland Race in a storm, so you also need to avoid the Agulhas Current in wind over current situations that can produce boat-breaking waves of freak proportions.
But approaching South Africa you have to go through it: the challenge is therefore to arrive at a time when the ‘normal’ north-easterly winds are blowing with the current, and avoid a front – which is easier said than done. Observations are sparse so sailors have to rely on model predictions and, although South Africa produces an analysis chart for the area, there are no forecast ones.
This means generally relying on GRIB files, which when offshore require big downloads via a satellite phone or SSB to get a large enough picture for an extended period. There’s then the dilemma in what to do if a cold front is on its way.
The Agulhas Current runs close to the shore along the continental shelf and can build up to speeds of 5 knots. It is generally not very wide, usually less than 50 miles, but can be double this. Its position can be downloaded as model currents in the form of GRIB files and there are satellite-derived positions for it as well. I usually use the RTOFS model as it is readily available through saildocs.
There are hard decisions to make if approaching the coast at the time of a cold front. Having to heave-to and stop, 100 miles or so from safety, is frustrating after a long passage. However, trying to cross the stream is a whole lot harder and could mean breakages and damage.
Last year’s World ARC saw exactly this scenario. After suffering some gear failure and fuel problems one yacht missed the weather window. With options restricted to pressing on into a wind against current situation, heaving-to, or finding a safe port to the north it was a hard choice for the skipper. Fortunately the yacht arrived safely, but only a couple of hours ahead of gale force south-westerlies, which were predicted to reach 50 knots later in the day.
About the author
Chris Tibbs is a meteorologist and weather router, as well as a professional sailor and navigator, forecasting for Olympic teams and the ARC rally.