Project Team Member

LT Anna C. Bryant, USN
 
 

Major Findings

The Gulf Stream has been a topic of study since it was first illustrated for the world by Benjamin Franklin and Timothy Folger. Since that time, the intensity of study has increased with the level of our technology and today focuses on Warm and Cold Core Rings in the mesoscale. There are more studies available that one person could reasonably read covering all aspects of the Rings, but this presentation focuses on those items of practical interest which weren’t fully covered in our physical coursework. A brief review of the basics is followed by some specific areas included are effects on Atlantic fishing, larvae dispersal, salt redistribution and nutrient pumping. Acoustic effects are covered in another presentation.

Cold Core rings separate from the Gulf Stream to the South, with each ring being made up, in the interior, of Slope water originating in the cooler waters of the Atlantic to the North of the Gulf Stream. Warm Core rings separate to the North and consist of warmer, Sargasso Sea water from the South. Rings have the following general characteristics:

Cold Core Rings (CCRs): Their life cycles can be a few months to a few years, but the overall average life of a Cold Core Ring (CCR) is on the order of 1.5 years. This cycle is largely determined by whether or not the CCR interacts with the Gulf Stream again. Repeat interactions can either end the ring or, on some interactions, merely eject the CCR again. The usual end of a CCR is through a terminal interaction with the Stream or through dissipation over time. If a ring dissipates it is through mixing with the Sargasso Sea water and losing its unique character combined with surface heating and mixing.

The essential character of CCRs is their initial lower salinity and temperature in relation to the surrounding waters. They also carry a greater load of nutrients and biota. This rapidly changes as the surface layers are mixed and warmed and with the constant drain on the nutrients. Both the species arriving with the ring and Sargasso species which opportunistically feed in the rings quickly reduce its nutrient load. New thoughts hold that some Sargasso species may not just benefit from the existence of rings, but require their occasional presence to maintain their numbers and size. In effect, that rings provide an essential nutrient boost to the area that Sargasso species have evolved to take advantage of. Other studies postulate that the travel and stress on species carried in the CCRs may be a link to evolution of Slope species.

Warm Core Rings (WCRs): WCRs tend to have a shorter life span due to their limited range once they are ejected to the North of the Gulf Stream. The continental shelf of North America lies in close proximity to the Northern edge of the Gulf Stream. Interaction with the Gulf Stream, often leading to dissipation, is also the frequent end to WCRs. However, interaction with the Shelf provides a unique set of circumstances which not only causes an increase in the rate of dissipation, but has enormous practical effects in terms of biology.

WCRs are characterized by their initially higher salinity and temperature in comparison to Slope water. They also contain a lesser nutrient load and substantially lower standing crop. What they do provide to the North Atlantic is salt. Salt redistribution, as well as heat, is only marginally understood when it comes to these rings. Opinions and results vary in terms of bulk amounts. Part of the reason for this variation is the natural variation in the number of rings around at any one time and the variation in the position of the Gulf Stream, as well as many other variables. Some results indicate that the salt redistribution between the Sargasso and the Atlantic north of the Gulf Stream is enough to be critical to the process of maintaining a steady state.

The most visible effect of the WCRs is their impact on fishing. When a WCR does interact with the shelf, a certain amount of turbulence and shearing is created at the level of the shelf. Since the WCR depth is much greater than the shelf depth, it can’t travel further and this turbulence and shearing is the result. Water from the ring is transported onto the shelf and shelf water is pulled off the shelf either into the ring or into deeper water. This small amount has potentially significant impact on recruitment and survival of many of the primary food fishes of the area.

Larval shock results from the sudden increase in temperature related to this transport. While it may not kill the larvae, it does effect their growth and decrease their viability. Recruitment is effected by transported the larvae or young fish off the shelf over deep water. When groundfish mature enough to make their trip to the bottom, their evolutionary imperative doesn’t include ensuring that they are not over sufficiently shallow water. In effect, they simply try to get to a bottom that simply isn’t there and remove themselves from future fishable populations. This effect can be related to the amount of time WCRs are in contact or proximity to the shelf and a distinct shortage of fishes. Catches decrease in both size and number the more WCRs come into contact with the shelf. This is probably the single most important biological effect directly impacting humans that I found in any of the papers.
 
 

The most important aspects of WCRs and CCRs include:

• CCRs provide a nutrient boost to the Sargasso Sea and bring in water of lesser salinity.
• WCRs provide salt to Slope water and may significantly impact fishing off the coast of northern North America.
• Evolution of species may be tied into their transport within rings.
• Salt balances between water north and south of the Gulf Stream is assisted by the transport provided by rings.

Figure 1. AVHRR image showing presence of newly formed CCR

Figure 2. ERS2 image showing the meanders and a CCR along the Gulf Stream

References

The Ring Group, 1981: "Gulf Stream Cold-Core Rings: Their Physics, Chemistry, and Biology." Science. Vol. 212, 1091-1100.

Olson, Donald B., 1991: "Rings in the Ocean." Annu. Rev. Earth Planet. Sci., Vol. 19, 183-311.

Olson, D. B., Schmitt, R. W., Kennelly, M., and Joyce, T. M., 1985: "A Two-Layer Diagnostic Model of the Long Term Physical Evolution of Warm Core Ring 82B." J. Geo. Research., Vol. 90, 8813-8822.

Web Sites

Classic CZCS Scenes

Fisheries Committee Meeting on Warm Core Gulf Rings

Roffs Commercial Fishing Satellite Info

NAVOCEANO Product Support

DEOS: Gulf Stream Velocities

Best of the Web (List of Sites)

AVHRR Home Page

Geosystems in Fairfax County Site

Live Weather Images
 

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