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This view was obtained by the NOAA-12 satellite on 1997 June 11 11:27 UT.
The sea surface temperature was computed from the thermal
infrared channels of the AVHRR image data, clouds and land are
from the near infrared channel. The sea surface temperature processing
includes cloud detection. Very sharp edges in the Gulf Stream are often
detected falsely as clouds, the threshhold was set to minimize that
here but some clouds have slipped through and appear as patches of
clustered cooler spots (the blue patches near the southeast corner and
the fine speckles near the bottom around -65 longitude are clouds. Similar
features elsewhere are also clouds).
A 3 day composite image of a wider area merges cloud free areas to give a good view of the northern Gulf Stream. The scale of these features is not always so obvious. A small image showing the gulf stream and eddies shift west by 12 degrees to overlay the eastern U.S. gives perhaps a new idea of the sizes. The image above shows a wealth of detail. Dr. Rick Chapman has provided a guest commentary on this image below. This unusually clear image of the Gulf Stream off the mid-Atlantic coast of the United States includes a wealth of interesting oceanographic phenomena. The North wall of the Gulf Stream, which is 5-10 degrees warmer than the shelf waters, is clearly evident where the green and orange colors meet. The explanation for the existence of the Gulf Stream (and other strong currents along the western boundaries of the world's oceans) was one of the great triumphs of oceanography during the first half of the 20th century. The overall rotation of the water in the North Atlantic basin is driven by zonal wind variations. This basin scale circulation intensifies near the western boundary due to the rotation of the Earth and a combination of other factors. The Gulf Stream is not a stable current, but meanders North and South on a variety of scales. Sometimes these meanders are quite small, taking the form of interfacial waves which appear to be breaking backwards relative to the true northeasterly flow (e.g. the small wave in the Gulf Stream wall at 74 deg E longitude, 36 deg N latitude). These waves are called semigeostrophic waves (Reference 1) because in one direction, namely across the stream, the balance of forces is primarily between the Coriolis force from the rotating Earth and pressure. When these forces are in balance the flow is said to be geostrophic. This balance occurs in only one direction near the Gulf Stream, so the waves are said to be semigeostrophic. In other locations, the meanders can become so large that the meander essentially pinches off a lens of warm water, which separates from the stream into the cooler shelf water. These are called warm core rings, two of which are visible as the large spiral circles above the stream at 65 deg E longitude and 69 deg E longitude. The warm core rings rotate clockwise in the Northern hemisphere with a typical period of several days. They are typically a lens-shaped body of water a few hundred meters deep and 50 to 200 km in diameter (Reference 2). They usually drift slowly west to southwest until they interact with the shelf or the Gulf Stream. (References 3 and 4) The effects of warm core rings were observed in tows taken across their diameter by early oceanographers, but it is fair to say that their true extent was not well understood until the advent of satellite imagery. Subsequent studies have explained much about their nature. For example, not only is the water warmer in the warm core ring than in the surrounding waters, but the biology is different. Biologists have observed that the springtime blooms within rings occur at a different time than for the surrounding waters, and that these blooms contain some species peculiar to the warmer waters of the Sargasso Sea that are not normally present in the cooler shelf waters (Reference 5). The interaction of a ring with the stream is quite interesting. When a ring gets pushed up against the stream, the stream reacts by ejecting a tendril of fluid which wraps clockwise around the rotating ring. Such an event is responsible for the spiral band of warm water encircling the warm core ring visible at 69 deg E longitude. The reasons for this interaction are quite complex, being intimately entwined with the behavior of fluids on a rotating Earth (Reference 6). Eventually most warm core rings are reabsorbed into the stream, usually after wandering around for 1 to 3 months. Just as stream meanders to the North can pinch off a warm core ring, stream meanders to the South can also pinch off, forming a cold core ring. These rings are often less visible because of the warmer water lying above them, but several remnant cold core rings are also evident in the image. 1. Kubokawa, A. and K. Hanawa, "A Theory of Semigeostrophic Gravity Waves and its Application to the Intrusion of a Density Current along a Coast: Part 1. Semigeostrophic Waves," Journal of the Oceanographic Society of Japan, Vol. 40, No. 4, pp.247-259, August 1984. 2. Flierl, G. R., "A Simple Model for the Structure of Warm and Cold Core Rings," Journal of Geophysical Research, Vol. 84, pp. 781-785, 1979. 3. Nof, D. "On the Beta-induced Movement of Isolated Baroclinic Eddies," Journal of Physical Oceanography, Vol. 11, pp. 1662-1672, 1981. 4. Killworth, P. D., "On the Motion of isolated Lenses on a Beta-Plane," Journal of Physical Oceanography, Vol. 13, pp. 368-376, 1983. 5. Davis, C. S., and P. H. Weibe, "Macrozooplankton Biomass in a Warm-Core Ring: Time Series Changes in Size Structure, Taxonomic Caomposition, and Vertical Distribution," Journal of Geophysical Research, Vol. 90, pp. 8871-8884, 1985. 6. Nof, D., "The Collision Between the Gulf Stream and Warm-Core Rings," Deep-Sea Research, Vol. 33, No. 3, pp. 359-378, 1986.
Ocean Remote Sensing Group tel: (301) 953-6000 x4209
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