Warm water in the Gulf Stream meanders northward along the eastern U.S. coast, sometimes coiling away as rings. Last spring, these circular currents moved large amounts of warm water from the Gulf Stream into the Sargasso Sea, a large area of the North Atlantic Ocean with floating seaweed and associated marine life.
Amy Schubert, University of Rhode Island Graduate School of Oceanography, Narragansett, Rhode Island
We spotted an unusual phenomenon in the April and May, 1994, satellite images of sea surface temperatures in the Gulf Stream. South of North Carolina's outer banks (between 33°N and 34°N), a large quantity of warm water left the Gulf Stream - the current of warm ocean water that flows around Florida from the Gulf of Mexico and north along the eastern U.S. - and moved east toward the Sargasso Sea. Within two weeks, this water mass was observed as far east as 62°W. No similar event has been recorded in the 10-year archive of satellite images at the University of Rhode Island.
Fig. 1. Images of the Atlantic Ocean, east of North Carolina's barrier islands, taken in the spring of 1994 from a NOAA satellite carrying an advanced very high resolution radiometer. A: March 8, 8:00 p.m. GMT. Arrows indicate cold core rings. B: April 18, 9:00 a.m. GMT. Arrow indicates warm pool. C: April 26, 9:00 p.m. GMT. Right arrow indicates stream of warm water moving east. D: May 14, 8:00 p.m. GMT. Temperatures (Celsius) represented by color: orange, 26°-31°; yellow, 20°-25°; green, 12°-19°; blue, 7°-11°; red, 1°-6°. Black areas are cloud cover.
Like inland rivers, the Gulf Stream meanders (see the west edge of north-flowing Gulf Stream (light orange, Figure 1c). Occasionally, these meanders become sharp enough to pinch off and form a ring. When these rings form to the south of the Stream, they have cold water in their centers and are called "cold core rings." Cold core rings that have pinched off from the Gulf Stream usually propagate to the west, moving into and interacting with the Gulf Stream. Cold core rings interacting with the Gulf Stream sometimes pull water out of the Stream.
In the March 8 image (Figure 1a), a cold core ring at 31.5°N, 75°W (left arrow) can be seen pulling water out of the Gulf Stream. Normally, water pulled out of the Gulf Stream by a cold core ring moves around the ring. In this case, however, the water is pulled out of the ring centered at 31.5°N, 75°W by a second, more northern ring at 33.5°N, 73.5°W (right arrow) and transported to the northeast. Scientists reported a similar transfer of water between cold core rings in 1976.
The next image on April 18 (Figure 1b) shows the northern ring - rather than the southern ring - pulling water out of the Gulf Stream, forming a pool of warm water (orange, arrow) extending from 71.5°W to 73°W at meridian 33. By April 26 (Figure 1c) more warm water, pulled out of the stream by the northern ring, has moved east as far as 66°W (lower stream of orange, at right arrow). Three days later, the eastern tip of the water has moved southeast to 34°N, 62°W (visible at arrow, 34°N, 66°W, in cover photo. Judging by the displacement of the eastern tip, the water appears to be moving at about 1 m/s, as fast as the core of the Gulf Stream. Unfortunately, cloudy weather makes it difficult to study the decay of this feature; however, most of it seems to have decayed by May 14 (Figure 1d). Although some water is still being pulled around the ring, no warm water can be seen east of 71°W.
It is highly unusual to see so much Gulf Stream water in the Sargasso Sea. (The Sargasso Sea is a large North Atlantic area with floating seaweed and associated abundant marine life that lies between 20°-35°N and 30°-70°W, at the lower right corner of the images). A possible explanation is that a series of cold core rings acted as a conveyer belt, with each cold core ring moving the water farther to the east. The effect of this water on the biology and chemistry of the Sargasso Sea remains to be seen.
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