There is so much racing going on right now it is crazy! Our friends at Tidetech sent this exclusive analysis of the tidal challenges facing the VOR fleet when they take off for leg 7…
The Gulf Stream will have a potentially profound tactical influence in leg seven of the Volvo Ocean Race – multiple tactical analyses of the leg from Miami to Lisbon – using different GRIB weather models and many wind and current speed/direction variables –show that the Gulf Stream could dominate tactics for the first half of the leg.
Tidetech managing director Penny Haire explained Tidetech’s analysis of the tactical options for Leg 7 relating to current.
“The Volvo boats are incredibly sensitive to wind speed and are normally hunting pressure, but the dominant factor for the first 40-50 per cent of this leg will be current rather than pressure.
“The Gulf Stream flow ranges in speed from 2kts to 5kts from Miami up to at least the Grand Banks, Newfoundland. To stay in the current means a big detour north away from the direct route to Lisbon… just about every routing scenario we have tried shows that this must be done until at least 400nm northeast of Bermuda.
“Complicating the decisions is the extreme volatility of the Gulf Stream as it leaves Cape Hatteras. Large eddies form unpredictably – much like you’d see in a river – breaking off each edge. Getting it wrong could mean facing short-term adverse current, while a competitor may have positive current, resulting in a compound velocity differential.
“The tactics for this leg may not be about whether to be in the Gulf Stream or not, but when to leave it – that may be the point where the leg is decided.”
ABOUT THE GULF STREAM
The Gulf Stream is 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.
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 PROVE INFLUENTIAL
During the race so far, currents have have had an influence in the straits of Gibraltar, off the south-east coast of Africa (the Aghulas Current), in the China Sea and most recently off the coast of Brazil (the Brazil Current).
This video shows the variation of movement in the Gulf Stream for 7-15 May 2012 – .