Introductory Physical Oceanography (MAR 555) - Fall 2009

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Presentation transcript:

Introductory Physical Oceanography (MAR 555) - Fall 2009 Miles A. Sundermeyer Unit 13: Fronts Assigned Reading: Matthias Tomczak notes, Ch9: http://www.es.flinders.edu.au/~mattom/ShelfCoast/chapter09.html

Key Concepts: Definition of a Front Shelf-break Fronts Tidal Mixing Fronts Upwelling Fronts River Plume Fronts

Definition of a Front A front is a region where properties change markedly over a relatively short distance. The scale depends on the scale of the process responsible for the front's existence. In deep ocean up to 100 km wide In estuaries can be just a few meters wide Can be pro-grade or retro-grade Key feature is change of some hydrographic property – T, S or both - across front is much larger than over the same distance on either side of the front Alternatively, the horizontal gradient of a property goes through a marked maximum.

Fronts - General Example: Middle Atlantic Bight Mavor and Bisagni, 2001

Fronts – Density vs. Density Compensated Density front (top) and a density-compensated front (bottom). The stable stratification in the density-compensated front of this example is temperature; it is compensated by salinity. Another possibility would be to have fresh, cold water above saline, warm water.

Shelf-Break Fronts Coastal boundary O(h) ~ O(h) Lx << Ly Motion is constrained to shelf; Significant time- and spatial variations. z Ly Continental Shelf Shelf Break Lx x h 200 m 4000 m 100-200 km 10 km

Shelf-Break Fronts (cont’d) Maintained by freshwater input from point sources along the coast Establishes a geostrophically balanced flow along the shelf break Can be density compensated, or not If not, cross front scale set by: From Pickart (2000)

Tidal Mixing Fronts – Simpson-Hunter Parameter H/U3 H: Water depth (Simpson and Hunter , 1974) H/U3 H: Water depth U: Mean tidal current velocity (turbulent dissipation) The tidal mixing front is typically found at approximately:

Tidal Mixing Fronts Example: Irish Sea Satellite images of sea surface temperature of the European shelf seas, June 1990. White is cold, black is warm; white streaks on the left of images are clouds. Fronts are seen as regions of strong temperature change and therefore displayed as boundaries between dark and white. Letters refer to named fronts: A: western Irish Sea front, B: Celtic Sea front, C: Islay Front, D: Scilly Isles front, E: Ushant (Ouessant) front.

Tidal Mixing Fronts (cont’d) Example: Irish Sea http://www.es.flinders.edu.au/~mattom/ShelfCoast/chapter09.html Compare the location of the 2.5 contour to the frontal locations on the previous satellite image

Tidal Mixing Fronts (cont’d) Example: Georges Bank

Tidal Mixing Fronts (cont’d) Example: Georges Bank

Tidal Mixing Fronts (cont’d) Example: Georges Bank Tidal mixing front Shelfbreak front

Upwelling Fronts Driven by along-shore winds, which drive surface m.l. off-shore. Upwelling carries deep, heavier water to surface near shore Resulting horizontal density gradient advances offshore.

Upwelling Fronts – Example: California Coast http://airsea-www.jpl.nasa.gov/cos/theory/upwelling1.gif

Upwelling Fronts (cont’d) Local wind Ekman transport f-plane Near-shore zone y Upwelling favorable wind drives off-shore flow at surface Set down of sea surface at coast Onshore flow in bbl Downwind along-shore geostrophic flow in interior

River Plume Fronts Boundary between fresh and salt water y Typically turning region in near-field Geostrophically balanced along-shore current in far field Turn according to Coriolis unless in mean along-shore current y Shelf x River u u v

River Plume Fronts – Near Surface vs. Surface to Bottom Decoupled from bottom boundary layer Coupled to bottom boundary layer a: Near-surface front Frontal zone b: Surface-bottom front Frontal zone x x Low-salinity water low-salinity water high salinity water high salinity water Mixing caused by shear instability at the interface between low and high salinity waters Mixing controlled by dynamics of the bottom boundary layer.

River Plume Fronts – Example: Columbia River Plume Front http://www.nwfsc.noaa.gov/research/divisions/fed/images/ocean_river.jpg http://makani.coas.oregonstate.edu/rise/images/SAR_Aug09_2002_transects_dudley_small.jpg

Key Concepts: Definition of a Front Shelf-break Fronts Tidal Mixing Fronts Upwelling Fronts River Plume Fronts