Physical oceanography of the shelf and upper slope

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Presentation transcript:

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

Mean Flow Lentz, 2008

mean Cross-shelf flow schematic Lentz

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

Shelf edge permanent front: Salinity Linder and Gawarkiewicz

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

Volume of shelf water expands and contracts Mountain, 1991

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

Warm core rings

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

S max intrusion Burrage and Garvine

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

Instabilities Gawarkiewicz et al.

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

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!

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

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

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?

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

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!

Temperature

Density