89weather briefing
CHRIS TIBBS ON THE AGULHAS CURRENT
PRACTICAL
CHRIS TIBBS is a meteorologist
and weather router, as well as a
professional sailor and navigator,
forecasting for Olympic teams
{ and the ARC rally
}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.Tough choices
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.n 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.
Long-range forecasts
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
The Agulhas Current brings warm water south
to meet the cold Southern Ocean, fuelling
storms. Left: GRIB file highlights a cold front
off South AfricaO