On a gray, rainy, cold fall day some years ago I set sail from Norfolk, VA, headed to Bermuda. I expected cloudy and rainy weather for a few more days, until I had at least crossed the Gulf Stream and come under the influence of the Bermuda-Azores High. But less than a day out of the Chesapeake Bay I found myself under clear skies speckled with puffy cumulus clouds, buffeted by a warm southerly wind. This dramatic change in sailing conditions came on so quickly that I was troubled at first. What could bring on such a rapid weather change, I wondered?
Then I looked at the water. I no longer saw the brown-gray color of shelf and slope water; I was looking instead at azure-blue and turquoise-green tropical water. I took the water temperature and found it to be nearly 70 degrees Fahrenheit—far too high for coastal waters in the fall. I began to wonder, where was I?
Then it came to me. I was sitting in the middle of a Gulf Stream eddy, a warm eddy most likely, that had wandered far to the west of the Gulf Stream's main flow. Actually, I was not sure if it was an eddy, a meander, or an extrusion. This pool of warm water was certainly not the main Gulf Stream since I was still over the continental shelf, and there was little set and drift to my boat's track. Not having access to Gulf Stream charts at the time I could not confirm my analysis, so I just accepted this good weather and kept on sailing toward Bermuda.
This delightful eddy-induced weather stayed with me until I was safely across the actual Gulf Stream. In thinking back on this trip I realize now the great influence that Gulf Stream dynamics (and other major ocean currents as well) have on maritime weather. The location, the current flow, and various other aspects of the Gulf Stream now play an increasingly important role in route planning to and from all ports along the East Coast of the US. Features such as eddies, meanders, and extrusions can each effect winds, waves, visibility, as well as set and drift.
Timely and accurate Gulf Stream information has not always been easily available, however, in the past few years, with the expansion of the Internet as well as an increase in the numbers and performance of NOAA's environmental satellites, there is now an abundance of accurate, real-time information available to mariners. By the far the most accurate and most current information can be found in the high-resolution satellite images available on NOAA's satellite-active archive website. These images, referred to as HRPT (High Resolution Picture Transmission) have a pixel resolution of 1 km (.5 n.m.) and are available on the Internet (www.saa.noaa.gov) within an hour of a NOAA satellite pass. NOAA has three operational POES satellites continuously circling the earth in polar orbits, and these collectively provides new images approximately every 90 minutes.
Because the image files that you can download from the NOAA website contain actual captured data, they allow overlays of color, temperature analysis, and the input of GPS position data. To demonstrate the detail and analysis potential of these images, I have selected several images that I recently downloaded.
On June 11, this year, a distinct warm eddy was located near 41.5N and 58.5W. This eddy shows a symmetrical shape and significant temperature gradients, with its center at 62 degrees F and its outer ring at 55 degrees F. Note along the eddy's northern edge a temperature change from the mid-40-degree range to 60 degrees F over a distance of just a few miles. This area showed a strong easterly current inside the warm eddy and a strong westerly current outside the eddy due to this noticeable temperature change over a relatively short distance.
To illustrate another dramatic change, I downloaded a temperature profile on May 20 that indicated the Gulf Stream's north wall. Outside the stream the water temperature registered 68 degrees, while inside the stream the temperature was 73 degrees F. This change occurred between 32.665 and 32.671 latitude, or 0.3 n.m! There was certain to be current induced activity in this region.
|"The strongest currents are found along the eddy's edges where there's the greatest temperature change over the shortest distance."|
A full temperature profile cross section is shown below. Outside this eddy on the north side temperatures are near 57.7 degrees F and to the south 56 degrees F. At the eddy's core temperatures are 60 degrees F plus. From north to south this eddy measures 62 n.m. in diameter (42.4 N to 40.997 N). Strongest currents are found along the eddy's edges where the temperature profile shows the greatest temperature change over the shortest distance.
These sample infrared satellite images bring out details in the Gulf Stream's features and dynamics that we have rarely been able to see up to now. Since these features produce strong currents and influence local weather and waves it is important to obtain images such as these NOAA HRPT infrared photos while making routing and departure plans.
Not long ago I showed several of these images to a sailor who crossed the stream during May, but who did not have any Gulf Stream information. When we overlaid his vessel's track on the images and discussed the weather he experienced, it immediately became apparent he had crossed over a large warm eddy that had contributed to fog and choppy localized seas. If he had used this data before entering the area, he might have been able to make a course alteration in order to avoid the warm eddy's counter current and the fog it induced.
Thanks to improvements in both communications technology and NOAA's improved environmental satellites sailors now have a wealth of usable and easily accessible weather and oceanographic information available to them. Make use of this data for faster and safer passages!