1. Physical oceanography of the shelf and upper slope
Ken Brink and Steve Lentz*
WHOI

2. Mean Flow
Lentz, 2008

3. mean Cross-shelf flow schematic
Lentz

4. As the water flows southwestward
It gets warmer
It gets saltier
Implies exchange at shelf edge

5. Shelf edge permanent front: Salinity
Linder and Gawarkiewicz

6. The front Also shows in temperature and density, but seasonal changes
Implies an along-front jet
Its existence implies limited exchange
Like a membrane that is
Easily distorted
Stretchable
Permeable
The front

7. Volume of shelf water expands and contracts
Mountain, 1991

8. How does the exchange happen? possibilities
Surface Ekman transport and compensation (wind-driven)
Bottom boundary layer
Warm Core Ring “suction”

9. Warm core rings

10. How does the exchange happen? possibilities
Surface Ekman transport and compensation (wind-driven)
Bottom boundary layer
Warm Core Ring “suction”
“S max” intrusions
How does the exchange happen? possibilities

11. S max intrusion
Burrage and Garvine

12. How does the exchange happen? possibilities
Surface Ekman transport and compensation (wind-driven)
Bottom boundary layer
Warm Core Ring “suction”
“S max” intrusions
Instabilities
How does the exchange happen? possibilities

13. Instabilities
Gawarkiewicz et al.

14. How does the exchange happen? possibilities
Surface Ekman transport and compensation (wind-driven)
Bottom boundary layer
Warm Core Ring “suction”
“S max” intrusions
Instabilities
Canyon processes
How does the exchange happen? possibilities

15. Measuring these exchanges is difficult!
The front is a very mobile feature: fixed assets are tricky
“Sloshing” is not exchange: vertical turbulent transports
Models and observations suggest a low correlation of (e.g.) v and T : O(0.1)
Low correlation means long/extensive observations to get a significant eddy flux
Nonstationary, inhomogeneous
Measuring these exchanges is difficult!

16. Related issue: vertical flow at front
Has been seen in a few snap-shots (dye)
Can be envisioned as an “internal Ekman layer”
Seems improbable to me: stratified, hence weak turbulence
Possibly: vorticity conservation and frontal meanders (as in Gulf Stream)
Also: wind-driven Ekman layer at a front
In any case: it may have biological significance (e.g., Ryan work)
Related issue: vertical flow at front

17. Shelf-edge productivity
Ryan et al., 1999
Shelf-edge productivity

18. Why is the front there? Some models exist, but there are problems
Are shelf-break fronts really common beyond northeastern North America?
Other fronts at the shelf edge are clearly associated with
a western boundary current (Georgia/Carolinas),
tidal mixing (Britain)
or buoyancy current (Norway or Alaska)
Why is the front there?

19. Looks so easy in some cases, such as with coastal upwelling (strongly alongshore winds).
Gives rapid flushing of the shelf, and eliminated strong contrasts between shelf and offshore waters.
Winds are not dominant here, and exchange is constrained
Yet, these exchanges are important
Cross-shelf exchange

20. Get the shelf-0cean exchange!
Biologically interesting, both locally and on shelf scale
Difficult observationally: bring new approaches
Take advantage of ever-improving modeling capabilities
Get the shelf-0cean exchange!

22. Temperature

23. Density