CCAR - Colorado Center for Astrodynamics Research

Satellite Oceanography : Applications :

Navigating the Bermuda Races

Suzanna Barth and Bob Leben

Figure 1. Oceanographic map showing the sea surface height anomaly (SSHA).

Sailboat races to Bermuda
take place every June and are some of the world's premiere sailing events. Hundreds of sailors gather to race the across the Gulfstream to Bermuda, starting off from either Marion, Massachusetts or Newport, Rhode Island. This demanding route across one of the ocean's most powerful currents has posed navigational and sailing challenges to sport sailors since 1906. Even today, sailors aboard older boats such as the wooden-hulled 1930 schooner, Mistress (see above left), as well as state-of-the-art racing sloops test their sailing skills against each other ... and the ocean currents and weather.

Navigational charts prepared
using data collected by ocean and weather satellite systems are an important tool used by the tactical teams aboard each sailboat. One such charting service is provided by Jenifer Clark, a veteran with 30 years of experience who is known as the "Gulfstream Lady". She uses satellite measurements of sea surface temperatures and ocean height to create charts of the ever changing eddies, meanders, and currents associated with the Gulf Stream. Currents rush across the sailing route at speeds over 4 knots. On race day these charts can be the difference between winning and losing, since a sailboat that makes 5 knots in still water can almost double its speed by taking advantage of favorable currents and avoiding unfavorable ones!

The tactical strategies
employed by sport sailors rely heavily on knowledge of the location of the Gulfstream current and its associated eddies. Large warm eddies are found north of the Stream and rotate clockwise with swirl speeds of up to 1 or 2 knots. While south of the Stream, counterclockwise rotating cold eddies are found with currents averaging up to 2 to 3 knots. The diameters of these eddies average between 50 - 60 nautical miles, although some eddies may be as large as 150 nm, they are unstable and last only a few days. A sail boat must be positioned on the correct side of an eddy to take advantage of the direction of circulation of the swirling eddy flow. The winds and waves near the Gulfstream also factor into tactical planning. It is important for ships to avoid situations in which the Stream's current opposes the direction of the wind direction, a condition in which dangerously high waves can result along the Gulf Stream north wall.

Figure 2. Oceanographic map showing ocean frontal information (J. Clark). Double click on the image to view a four-day animation of the frontal movement.

Sailors use Jenifer Clark's product (Figure 2) to locate the warm and cold eddies, as well as the Gulf Stream. Arrows indicate the direction of flow; Sea surface temperatures are in degrees F and show warm-core and cold-core eddies that correspond to topographic highs and lows in the altimetry maps. The altimeter data are invaluable for depicting the locations of cold-core eddies, which are heavier than the surrounding water and have a tendency to sink below the surface. The thermal infrared imagery only sees the surface or skin temperature of the ocean. Because the cold-core eddies often are right below the surface, they are not detectable by their sea surface temperature signature. Also, cloud cover blocks the satellite from reading the sea surface temperature, so that the eddies are no longer visible. However, the microwave radar altimeter measurements can "see" through the clouds. Jenifer Clark uses the topography maps to identify the eddies, and their direction of rotation.

In the 1986 Newport to Bermuda race, cold eddies were recorded rotating at speeds as high as 7 knots! During this race, one competitor was on the eastern side of the cold eddy. He was headed south but the intense current was pushing him backwards!

Figure 3. Oceanographic map showing ocean frontal information and the race's rhumb line.

The straight line from Marion to Bermuda is the rhumb line for the race in 1999 (Figure 3). Note how well the warm and cold eddies correlate between the thermal infrared image (Figure 4) and the TOPEX topographic map (Figure 5). A warm eddy rotating clockwise near 39N 69W in the thermal infrared image shows up in the TOPEX map as a surface being 20 cm higher that the surrounding water (concentric red circles). South of the Stream in the thermal infrared image is a very large cold eddy (rotating counterclockwise) centered near 35N 67W, directly on the rhumb line. The TOPEX image depicts the cold eddy location with sea heights 30 cm below the surrounding water (concentric blue circles).

Figure 4. Gulf Stream Sea Surface Temperatures (SSTs) derived from infrared data from NOAA/AVHRR satellites. Figure 5. Gulf Stream Sea Surface Height Anomalies (SSHAs) derived from radar data from the TOPEX/Poseidon satellite.

Figure 4 shows Jenifer Clark's Gulfstream analysis, derived in part from the false colored thermal infrared imagery from NOAA polar orbiting satellites, and Figure 5 shows the TOPEX analysis of sea surface height anomalies . The warm eddy signatures can be seen as concentric red circles, in both images, indicating sea heights above the mean level of 10-15 cm near 39N 70W. The cold eddy located at 32N 74W shows sea heights are 20 cm below the surrounding water while the cold eddy near 35N 70W has sea heights depressed 10 cm below the surrounding water. The cold eddy near 36N 66W shows that the sea heights are 30 cm below the surrounding water. Since these sea heights are so distinct, the implication is that the current speed associated with this eddy is more intense than the others noted.

Suzanna Barth is a Graduate Research Assistant at CCAR. Bob Leben is a Research Associate Professor in the Department of Aerospace Engineering Sciences. Both are actively involved in satellite altimetry research at CCAR.

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