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Physical oceanography of the shelf and upper slope

Physical oceanography of the shelf and upper slope. Ken Brink and Steve Lentz* WHOI. Lentz, 2008. Mean Flow. Lentz. mean Cross-shelf flow schematic. It gets warmer It gets saltier Implies exchange at shelf edge. As the water flows southwestward. Linder and Gawarkiewicz.

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Physical oceanography of the shelf and upper slope

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

  2. Lentz, 2008 Mean Flow

  3. Lentz meanCross-shelf flow schematic

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

  5. Linder and Gawarkiewicz Shelf edge permanent front: Salinity

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

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

  9. Warm core rings

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

  11. Burrage and Garvine S max intrusion

  12. 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. Gawarkiewicz et al. Instabilities

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

  18. 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. 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!

  21. Temperature

  22. Density

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