sundermeyer mar 555 fall, 2009 1 unit 13: fronts introductory physical oceanography (mar 555) - fall...

SundermeyerMAR 555 Fall, 2009

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Unit 13: Fronts

Introductory Physical Oceanography (MAR 555) - Fall 2009

Miles A. Sundermeyer

Assigned Reading: Matthias Tomczak notes, Ch9:http://www.es.flinders.edu.au/~mattom/ShelfCoast/chapter09.html


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Key Concepts:1. Definition of a Front

2. Shelf-break Fronts

3. Tidal Mixing Fronts

4. Upwelling Fronts

5. River Plume Fronts


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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.

Definition of a Front

In deep ocean up to 100 km wide

In estuaries can be just a few meters wide

Can be pro-grade or retro-gradeKey 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.


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Mavor and Bisagni, 2001

Fronts - GeneralExample: Middle Atlantic Bight


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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.

Fronts – Density vs. Density Compensated


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100-200 km

10 km

h

Ly

x

y

z

4000 m

200 m

Lx

0

Coastal boundary

O(h) ~ O(h)

Lx << Ly

Motion is constrained to shelf;

Significant time- and spatial variations.

Shelf-Break Fronts


7From Pickart (2000)

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:

f

hgRd

'


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H/U3

H: Water depth

U: Mean tidal current velocity

(Simpson and Hunter , 1974)

log

3

10

UCD

D

H

Dt

t

The tidal mixing front is typically found at approximately:

2 log10 tD

H

Tidal Mixing Fronts – Simpson-Hunter Parameter

(turbulent dissipation)


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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 FrontsExample: Irish Sea


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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

http://www.es.flinders.edu.au/~mattom/ShelfCoast/chapter09.html


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


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1.2 ,9.1 log10 tD

H



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Tidal mixing front Shelfbreak front



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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.


15http://airsea-www.jpl.nasa.gov/cos/theory/upwelling1.gif

Upwelling Fronts – Example: California Coast


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Local wind

bS ~

Ekman transport

f-plane

Near-shorezone

y

Upwelling Fronts (cont’d)

• 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


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u u

v

River

Shelf

yg /

fu fu

xg /fv

River Plume Fronts

• Boundary between fresh and salt water

• Typically turning region in near-field

• Geostrophically balanced along-shore current in far field

• Turn according to Coriolis unless in mean along-shore current

xy


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xLow-salinity water

high salinity water

a: Near-surface front Frontal zone

x

low-salinity waterhigh salinity water

b: Surface-bottom front Frontal zone

Mixing caused by shear instability at the interface between low and high salinity waters

Decoupled from bottom boundary layer Coupled to bottom boundary layer

Mixing controlled by dynamics of the bottom boundary layer.

River Plume Fronts – Near Surface vs. Surface to Bottom


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http://makani.coas.oregonstate.edu/rise/images/SAR_Aug09_2002_transects_dudley_small.jpg

http://www.nwfsc.noaa.gov/research/divisions/fed/images/ocean_river.jpg

River Plume Fronts – Example: Columbia River Plume Front


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Key Concepts:1. Definition of a Front

2. Shelf-break Fronts

3. Tidal Mixing Fronts

4. Upwelling Fronts

5. River Plume Fronts

sundermeyer mar 555 fall, 2009 1 unit 13: fronts introductory physical oceanography (mar 555) - fall...

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