LT Thomas B. Keefer

Shelf Break Fronts: How much do we really know?

A brief overview of the physical properties, formation and modeling of Shelf Break Fronts

LT Thomas B. Keefer

Shelf Break Fronts: What are they?

LT Thomas B. Keefer OC4331/Summer 2005

Front (in General): Region where the rate of change of a certain physical property is an order of magnitude greater than the surrounding areas in the medium (the ocean in this case).

(For this discussion, we will look primarily at

temperature, salinity and density)


Front Properties:

•Quasi Permanent•Resist destruction by turbulence

•Need a frontogenesis mechanism

•Strong gradient•Temperature, density and Salinity

•Convergent Flow•Cause intense advection of 'different' water into a region

•Flow can be across isohalines or isotherms


Oceanic front Classification


•Density Front•Strong density gradient in frontal zone

•Front causes water movement-flow follows isopycnals

•Oceanic density fronts are analogous to atmospheric fronts

•Shelf break fronts are of this type

•Density-Compensating Front•Small or non-existent density gradient at the front

•Interleaving and Intrusion present

•Two properties of the front compensate in terms of density across the front

•Shelf break fronts are of this type As Well

Density vs. Density Compensating


Densi

tyD

ens.

Com

p.

Density fronts


•Can be prograde or retrograde

Shelf water is more dense than slope water

Shelf water is less dense than slope water

Front Types:


•Planetary•Only open ocean front

•Separation of two water masses of different temperatures

•Dramatic influence on air/sea mechanisms (heat budget)

•Upwelling•Regions of long steady winds

•Strong upwelling present

•Shallow Sea•Separation of well mixed and stratified waters

•Caused by competing forces of solar radiation (stratifies) and tidal outflow/inflow (mixing)

Front Types:


•Plume•Estuarine- fan of fresh water into the salty ocean

•Separation of two water masses of different Salinities

•Strong turbulence in area of fresh/salty mixing

•Estuarine•Similar to plume- separation of fresh and salty water

•Vertical stratification caused by buoyancy (not outflow)

•Shelf Break•Focus of this brief

•Not very well understood (not very common)

•Caused by the flow characteristics at the shelf break

Shelf Break Fronts:


•Separation of warm, saline slope water from cool, fresh shelf water

•Can be density or Density Compensating

•Restricted to the very edge of the shelf and are stationary in the cross-shelf

•Present most of the time and will re-form quickly after short disturbance

•Drastically affect transfer of heat, salt and momentum from the shelf to slope region

Shelf Break Fronts: Is it really that hard?


•Not present Everywhere•Mid Atlantic Bight, E. Bering Sea, Celtic Sea

•Data collection can be difficult (shelf break)

•All coastal (shallow) front mechanics are difficult

• non-linear nature of the near shore

Shelf Break Fronts: Is it really that important?


•Critical for understanding the budget between shelf and slope

•Heat

•Salt

•Momentum

•Eco-Systems • Fronts stop cross shelf transport of particulate matter

•Extremely rich biologically

•Commercial interest to fisheries

Shelf Break Fronts: How do they form?


•Initial Conditions

•Linear stratification at shelf break

•No horizontal density gradients

•Along shelf flow into frontogenesis region

•This is the 'forcing'

•Seaward (cross shelf) flow of bottom water

•Caused by to ekman Flux

•Transports light water under heavy

•Overturning causes well mixed density field in the shelf break area



•In a phrase: 'Positive feedback Mechanism'

•Current Conditions

•Cross shelf flow still exists

•Well mixed frontal zone at the break

•Still stratified on both sides of the front development area

•Now there is a large horizontal density gradient seaward of the shelf (well mixed shelf and stratified slop water mixing at the break)

•Off-shore flow along the bottom still exists!!



•Off shore transport across break goes on

•Confined to bottom layer

•Particles ascend isopycnals as they move offshore

•Need a replacement mechanism for this water that is moving up

•Water is downwelled over the shelf to replace the upwelled water over the slope

•This is a self perpetuating situation that remains in place and continues (pos. Feedback)

•Now we have a shelf break front

Shelf Break Fronts: Lets make a model!


•Early models don’t work well

•Problems developing required along shelf velocity sheer (forcing) using only geostrophy (Ou 1983; Hseuh and Cushman- Roisin 1983)

•Friction layer can be hard to model accurately but this is key

•Frontogenesis mechanism understanding weak to non-existent/ initial thoughts probably misguided.



•New Model Goal: ".. To understand the three-dimensional circulation that results after allowing a highly idealized inflow to adjust dynamically, under the influence of bottom friction, vertical mixing and stratification"

•Note: this model was not designed to actually be a shelf break front model solely but rather models the shelf break area. when the dynamics are modeled and run, the front development naturally takes place.



•Start with Primitive Equations for linear momentum

•INSERT EQNS HERE

•Define Variables

•Ignore momentum advection based on Rd

•Describe continuity equation in 3-d

•Rigid lid surface condition

•Hydrostatic pressure field

•Free slip condition at coastal boundary

•Set up physical domain of the modeled area

•START THE PARTY!!



Shelf Break Fronts: Run the Model!


X-50Km X-150Km

Cross Shelf Velocity Profiles- Note Bottom

Off shore flow On shore flow

Shelf Break Fronts: Run the model!


X-150m

Shelf Break Area

Convergence Zone



The cross section of the shelf/slope interface shows the development of the horizontal density stratification. From left to right the cross shelf stations are at 50km, 100km and 150km. This is the primary driving force in building and maintaining these fronts.



the horizontal density stratification is evident here. This density gradient is the reason that the particles ride upward on the seaward side of the shelf. The compensation for this on the shelf side is the primary development mechanism.

Shelf Break Fronts: Vary the input Parameters


•model run profiled was for a 'standard' case but other cases were developed by Gawarkiewicz and Chapman during their modeling phase.

•Details are beyond the scope of this brief

•Some conclusions are important

•Existence of the initial cross shore flow is needed; magnitude less important

•Changes in initial stratification change width of front but not drastically

•Front structure most dependant on bottom topography

Shelf Break Fronts: Does the Model Work?


•The model is based on a few simplifications that may or may not be valid.

•Neglect wind and eddy forcing

•Neglect buoyancy flux induced by local fresh water run off areas

•Constant eddy flux and diffusivity

•Steady, idealized upstream boundary conditions

•These simplifications, due to the refusal to handle a seasonal pycnocline, most accurately represent a winter regime.



•Nantucket Shoals Experiment (1979-1980)

Existing data from this experiment (conducted by Beardsley, Chapman, Brink, Ramp and Schlitz) was used for model verification



•Cross sections from Nantucket Shoals experiment.

•Left summer; right winter

•Summer has more prevalent front but front still exists in winter as a horizontal density gradient

•Relative strength reasoned to be a function of initial degree of vertical stratification (seasonal Pycnocline caused by surface heating of shallower shelf water)

Shelf Break Fronts: Conclusions


•Shallow water dynamics are difficult to model but very important to understand from ecological, scientific and commercial standpoints

•Shelf break fronts play a key role in the composition of the near shelf environment in several parts of the world

•Provided there is some along slope flow and favorable topography, shelf break fronts will form through a positive feedback mechanism between the slope and shelf waters

•Shelf break fronts are present in both winter and summer seasons but grow stronger in summer due to the greater stratification of the shelf water

•The model designed by Gawarkiewicz and chapman, 1992 handles a highly idealized (near winter) case very well

Shelf Break Fronts: References:


•Journal of Physical Oceanography volume 15 pp 713-748 "The role of Stratification in the Formation and Maintenance of Shelf Break Fronts"

Glen Gawarkiewicz and David C. Chapman

Woods Hole Oceanographic Institution

•Journal of Physical Oceanography Volume 22 pp 753-772 "The Nantucket Shoals Flux Experiment (NSFE79)"

Robert C. Beardsley, David C. Chapman and Kenneth H. Brink

Woods Hole Oceanographic Institution

•Course Text: http://oceanografia.cicese.mx/cursos/sco/chapter09.htm

Shelf Break Fronts: QUESTIONS?


LT Thomas B. Keefer

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