1. Introductory Physical Oceanography (MAR 555) - Fall 2009
Miles A. Sundermeyer
Unit 13: Fronts
Assigned Reading: Matthias Tomczak notes, Ch9:

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

3. 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.

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

5. 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.

6. 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 km
10 km

7. 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)

8. 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:

9. 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.

10. Tidal Mixing Fronts (cont’d)
Example: Irish Sea
Compare the location of the 2.5 contour to the frontal locations on the previous satellite image

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

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

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

14. 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.

15. Upwelling Fronts – Example: California Coast

16. 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

17. 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

18. 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.

19. River Plume Fronts – Example: Columbia River Plume Front

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