Physical oceanography of the shelf and upper slope
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PHYSICAL OCEANOGRAPHY OF THE SHELF AND UPPER SLOPE
Ken Brink and Steve Lentz*WHOI
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Lentz, 2008
MEAN FLOW
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Lentz
MEANCROSS-SHELF FLOW SCHEMATIC
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• It gets warmer• It gets saltier• Implies exchange at shelf edge
AS THE WATER FLOWS SOUTHWESTWARD
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Linder and Gawarkiewicz
SHELF EDGE PERMANENT FRONT: SALINITY
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• 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
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Mountain, 1991
VOLUME OF SHELF WATER EXPANDS AND CONTRACTS
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• Surface Ekman transport and compensation (wind-driven)
• Bottom boundary layer
• Warm Core Ring “suction”
HOW DOES THE EXCHANGE HAPPEN?
POSSIBILITIES
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WARM CORE RINGS
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• Surface Ekman transport and compensation (wind-driven)
• Bottom boundary layer
• Warm Core Ring “suction”
• “S max” intrusions
HOW DOES THE EXCHANGE HAPPEN?
POSSIBILITIES
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Burrage and Garvine
S MAX INTRUSION
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• Surface Ekman transport and compensation (wind-driven)
• Bottom boundary layer
• Warm Core Ring “suction”
• “S max” intrusions• Instabilities
HOW DOES THE EXCHANGE HAPPEN?
POSSIBILITIES
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Gawarkiewicz et al.
INSTABILITIES
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• 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
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• 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!
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• 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
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Ryan et al., 1999
SHELF-EDGE PRODUCTIVITY
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• 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?
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• 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
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• 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!
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TEMPERATURE
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DENSITY