1. Gulf Stream Warm Core RingsBy
Ayan Chaudhuri

2. Topics Overview Climatology Physical mechanisms Biological Impacts
Conclusion

3. Overview
Form in the Slope Water region between the continental shelf of northwest Atlantic and the north wall of the Gulf Stream (GS)
km width, m depth
Contain parcels of water that spin off from a shoreward tending GS meander and entrain warm salty Sargasso Sea water in their core, encircled by even warmer GS water around its fringes.
Flow velocities are strongest at the surface progressively decreasing with depth
Source: John Hopkins Remote Sensing Lab

4. Overview
Excite perturbations and influence exchanges across the shelf slope boundary
Entrainment could occur either directly through rapid mixing or through formation of streamers/jets around the ring
Interact with ambient waters through mixing and stirring. Possible mechanisms
- Double diffusion
- Internal wave breaking
- Frontal instabilities
Significantly impact biological and chemical distributions
Source: John Hopkins Remote Sensing Lab

5. Climatology Ring formation concentrated between 63-66 W and 39-40 N
Vicinity of the New England seamounts
Ring destruction
- usually by re-absorption
- occasionally by dissipation
Two regions, 71-75W and 60-69W (re-absorption)
Both regions coincide with highest meandering frequency of the GS

6. Climatology
Rings usually translate to the west and then southwest through the slopewater at speeds of 4-6 km/day, reducing in size through mixing with ambient waters.
Most of the rings move erratically in some point of their traversal
Secondary ring formations occur when a growing meander overruns a neighboring ring eliminating its surface expression, however spawns it back due to meandering instabilities.

7. Physical Mechanisms
The isothermal and isohaline surfaces slope down into the ring from the periphery toward the center, depicting the isolation of warm Sargasso water near the surface and center of the ring.
Resultant pressure gradient is balanced by Coriolis Force.
An anti-cyclonic geostrophic flow is established.

8. Physical Mechanisms
Governing Equation
fv =1/r * dp/dx
fu =-1/r * dp/dy
where,
f = Coriolis Force
u,v = horizontal velocity components
dp/dx, dp/dy = horizontal pressure gradients
The initial pulse of ring water must be large enough (< 20km) such that the outward pressure gradient can be balanced by the Coriolis force before the water parcel gets dispersed

9. Physical Mechanisms
Energy in two forms: (1) Available Potential Energy (APE) and (2) Kinetic Energy (KE)
APE: thermocline depression that is characteristic of hydrographic profiles observed in WCRS.
- Accounts for 95% of available energy
- Concentrated at the ring center
KE: form of circulation in the ring
- Peaks at the surface
- Peaks midway between center and edge
As ring traverses, APE gets converted to KE leading to total energy loss.

10. Physical Mechanisms Energy loss induces shoaling of the thermocline.
As the ring decays and slows down, the Coriolis force associated with the dispersing water re-establishes the tangential motion.
Balance of Coriolis force and outward pressure gradient gets continously adjusted to maintain geostrophic flow
Other factors affecting losses in APE include:
-entrainment of colder shelf/slope waters into the core of the ring
-shearing away of deep rings water through interaction with bottom topography
-episodic wind events
-atmospheric cooling
-wave mixing

11. Physical Mechanisms
Fierl, 1977, used the principle of conversation of potential vorticity to model ring movement
Governing Equation:
f + F/ Dh = 0
where,
f = planetary vorticity
F = relative vorticity
h = thickness of the thermostad
Northward flow along the western edge of the ring led to an increase in planetary vorticity (f), causing the water column to stretch and result in deepening the thermocline. The resultant depression lowers the pressure in the region and hence induces a geostrophic westward flow.

12. Biological Impacts
Physical dynamics associated with WCRS are reflected in the biological distribution of the ring.
Gulf Stream and Sargasso Sea waters are depleted of nutrients as compared to continental slope and shelf waters.
Are warm core rings also nutrient limited ?

13. Biological Impacts The ring is nutrient limited during its genesis
Ring undergoes nutrient enrichment through entrainment (shown by Schiltz et. al.,1989, on ring 81-E ) and diffusion with ambient waters.
The rotary motion of rings due to resultant geostrophic forces are believed to be the basis for nutrient enrichment in the high velocity region.
The edge of the ring forms a frontal band which is considered very productive. This was shown by Peele et.al,1985, on ring 81-E.

14. Biological Impacts

15. Biological Impacts

16. Biological Impacts

17. Biological Impacts
Bishop et. al,1990, investigated particulate matter and chlorophyll in ring 82B and surrounding waters in spring.
Suggested biogenic particles present in the core of the ring were strongly influenced by day-to-day variation in air-sea heat flux.
Suggested that transient stratification through vernal heating and contrasting mixing through wind events and deep convective mixing were important for ring productivity.
Frontal zones at the ring periphery had maximum particle concentrations.

18. Biological Impacts

19. Conclusion
Warm Core Rings are a regular feature in the Slope Sea region.
They significantly impact ambient waters through mixing and dispersion processes.
They consistently entrain shelf waters and influence the nutrient budgets for the shelf and slope regions.
Their biological impact need to be better understood.