Volvo Ocean Race

The Gulf Stream Superhighway

The early stages of Leg 7 from Miami to Lisbon will see the teams looking for the best way to play the current of the Gulf Stream -- an intense, warm ocean current in the western North Atlantic Ocean. It moves north along the coast of Florida and then turns eastward off of North Carolina, flowing northeast across the Atlantic.

The Gulf Stream flows at a rate nearly 300 times faster than the typical flow of the Amazon River. The velocity of the current is fastest near the surface – the maximum speed is up to approximately 4.8kts; the average speed is around 3.5kts. The current slows to about 1kt as it widens to the north.

Transporting nearly four billion cubic feet of water per second, the Gulf Stream moves an amount of water that is greater than that carried by all of the world's rivers combined. It is powerful enough to be seen from space.

Beginning in the Caribbean and ending in the northern North Atlantic, the Gulf Stream is an extensive western boundary current (western boundary currents are found on the western side of all ocean basins). It plays an important role in the poleward transfer of heat and salt along with warming the European subcontinent. It is a result of the wind pattern acting on most of the North Atlantic Ocean – the combination of the easterly trade winds and the westerlies blowing at mid-latitudes cause the North Atlantic to rotate clockwise.

This basin-wide, clockwise flow is referred to as the subtropical gyre. Due to the Earth's rotation, the poleward flow in the western Atlantic is limited to a narrow current on the western boundary of the ocean basin.

The Gulf Stream begins its turn east slightly north of Cape Hatteras. This point changes throughout the year – in the autumn it shifts north, while in the winter and early spring it shifts south. Significant changes in its speed, meandering, and structure can be seen through various time scales as it travels northeast.

The meandering of the Gulf Stream intensifies east of Cape Hatteras reaching a maximum around 65 degrees west. Meanders often pinch-off from the current to form rings and eddies. It has been observed that, on average, the current sheds 22 warm-core rings and 35 cold-core rings per year.

For the Volvo Ocean Race boats this is critical – getting it wrong could mean facing short-term adverse current, while a competitor may have positive current, resulting in a compound velocity differential.

“The data we have form Tidetech is very essential here,” said Groupama sailing team navigator Jean-Luc Nélias. "For the first week of the race we are under the influence of the Gulf Stream, and we need information on these currents."

Once the Gulf Stream reaches the Grand Banks its structure changes from a single meandering front to several branching fronts. One branch bends north along the continental slope, eventually turning east between 50 degrees and 52 degrees north. The other branch flows southeast towards the mid-Atlantic ridge. These are called the North Atlantic Current and the Azores Current respectively. The region of the Gulf Stream's branch point is highly dynamic and subject to rapid change.

CURRENTS PROVING INFLUENTIAL

During the race so far, currents have had an influence in the straits of Gibraltar, off the southeast coast of Africa (the Agulhas Current), in the China Sea and most recently off the coast of Brazil (the Brazil Current).

Tom Addis, navigator with PUMA Ocean Racing powered by BERG, said it was crucial to have good data on currents.

“Tidetech is supplying all the current data we have access to on the boats and current is a big issue certainly for the first three or four days of this leg with the Gulf Stream so we'll be using that data constantly,” said Addis.

“The Gulf Stream can flow up to three or four knots and in light breezes that's a big percentage of our boat speed. It's going to be really important.”

Andrew Cape, navigator on race leaders Team Telefónica, agreed, saying:  "It's great to have the digital information, especially on this leg with the Gulf Stream. It's very important to get good data and we're certainly getting that."

For Telefónica skipper Iker Martínez, the information is useful not just to pick the fastest route but also to keep the boat safe.

“I think that the information is good,” he said. “Sometimes we use it to try to go quicker and at other times we use it to try to be safer. I think it’s information that you need to have and is good to. We use it, we like it and it’s another thing we can play with.”

Once the fleet begins to approach Portugal they may encounter south-going Iberian current but the influence here will be slight, if any.

This video shows the variation of movement in the Gulf Stream for 7-15 May 2012 – http://www.youtube.com/watch?v=Mw2XsDWUJHo&feature=plcp

Volvo Ocean Race meteorologist Gonzalo Infante said having the information available to navigators had been a benefit for the race.

“We are delighted to have Tidetech as an oceanographic data supplier," Infante said. It is the very first time that this data is included in a race weather package and we are sure that it has made the game more interesting for the sailors.”