PHYSICAL OCEANOGRAPHY OF THE SHELF AND UPPER SLOPE

Ken Brink and Steve Lentz*WHOI

Lentz, 2008

MEAN FLOW

Lentz

MEANCROSS-SHELF FLOW SCHEMATIC

• It gets warmer• It gets saltier• Implies exchange at shelf edge

AS THE WATER FLOWS SOUTHWESTWARD

Linder and Gawarkiewicz

SHELF EDGE PERMANENT FRONT: SALINITY

• 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

Mountain, 1991

VOLUME OF SHELF WATER EXPANDS AND CONTRACTS

• Surface Ekman transport and compensation (wind-driven)

• Bottom boundary layer

• Warm Core Ring “suction”

HOW DOES THE EXCHANGE HAPPEN?

POSSIBILITIES

WARM CORE RINGS

• Surface Ekman transport and compensation (wind-driven)

• Bottom boundary layer

• Warm Core Ring “suction”

• “S max” intrusions

HOW DOES THE EXCHANGE HAPPEN?

POSSIBILITIES

Burrage and Garvine

S MAX INTRUSION

• Surface Ekman transport and compensation (wind-driven)

• Bottom boundary layer

• Warm Core Ring “suction”

• “S max” intrusions• Instabilities

HOW DOES THE EXCHANGE HAPPEN?

POSSIBILITIES

Gawarkiewicz et al.

INSTABILITIES

• 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

• 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!

• 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

Ryan et al., 1999

SHELF-EDGE PRODUCTIVITY

• 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

• 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