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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.

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LT Thomas B. Keefer

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  1. 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

  2. Shelf Break Fronts: What are they? LT Thomas B. Keefer OC4331/Summer 2005

  3. 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) LT Thomas B. Keefer OC4331/Summer 2005

  4. 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 LT Thomas B. Keefer OC4331/Summer 2005

  5. 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 Oceanic front Classification LT Thomas B. Keefer OC4331/Summer 2005

  6. Density vs. Density Compensating Density LT Thomas B. Keefer OC4331/Summer 2005 Dens. Comp.

  7. Can be prograde or retrograde Density fronts LT Thomas B. Keefer OC4331/Summer 2005 Shelf water is less dense than slope water Shelf water is more dense than slope water

  8. 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: LT Thomas B. Keefer OC4331/Summer 2005

  9. 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 Front Types: LT Thomas B. Keefer OC4331/Summer 2005

  10. 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 LT Thomas B. Keefer OC4331/Summer 2005

  11. 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 LT Thomas B. Keefer OC4331/Summer 2005

  12. 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 LT Thomas B. Keefer OC4331/Summer 2005

  13. 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 LT Thomas B. Keefer OC4331/Summer 2005

  14. Shelf Break Fronts: How do they form? • 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!! LT Thomas B. Keefer OC4331/Summer 2005

  15. Shelf Break Fronts: How do they form? • 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 LT Thomas B. Keefer OC4331/Summer 2005

  16. 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. LT Thomas B. Keefer OC4331/Summer 2005

  17. Shelf Break Fronts: Lets make a model! • 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. LT Thomas B. Keefer OC4331/Summer 2005

  18. Shelf Break Fronts: Lets make a model! • 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!! LT Thomas B. Keefer OC4331/Summer 2005

  19. Shelf Break Fronts: Lets make a model! LT Thomas B. Keefer OC4331/Summer 2005

  20. Shelf Break Fronts: Run the Model! Cross Shelf Velocity Profiles- Note Bottom LT Thomas B. Keefer OC4331/Summer 2005 X-50Km X-150Km

  21. Off shore flow On shore flow Shelf Break Fronts: Run the model! Shelf Break Area LT Thomas B. Keefer OC4331/Summer 2005 Convergence Zone X-150m

  22. Shelf Break Fronts: Run the model! LT Thomas B. Keefer OC4331/Summer 2005 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.

  23. Shelf Break Fronts: Run the model! LT Thomas B. Keefer OC4331/Summer 2005 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.

  24. 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 LT Thomas B. Keefer OC4331/Summer 2005

  25. 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. LT Thomas B. Keefer OC4331/Summer 2005

  26. Shelf Break Fronts: Does the Model Work? • Nantucket Shoals Experiment (1979-1980) LT Thomas B. Keefer OC4331/Summer 2005 Existing data from this experiment (conducted by Beardsley, Chapman, Brink, Ramp and Schlitz) was used for model verification

  27. Shelf Break Fronts: Does the Model Work? LT Thomas B. Keefer OC4331/Summer 2005 • 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)

  28. 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 LT Thomas B. Keefer OC4331/Summer 2005

  29. 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 LT Thomas B. Keefer OC4331/Summer 2005

  30. Shelf Break Fronts: QUESTIONS? LT Thomas B. Keefer OC4331/Summer 2005

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