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What are fronts
- Narrow/sharp horizontal boundaries separating different water masses
– Density difference can drive geostrophic currents in along-front direction
- Occur at different spatial and temporal scales (from a few meters to hundreds of km long)
- Short-lived (days), although many fronts are quasi-stationary
- Cross-frontal differences up to 10-15°C and 2-3 in salinity
- High biological production → important for fisheries
Why fronts are important in the coastal ocean
Definition: abrupt change of a given property (T or S) over a relatively short distance
Fronts are common in the coastal ocean because of the heterogeneous nature of the coastal ocean water masses
Importance of fronts in the management of the coastal ocean:- Flow convergence at the surface: accumulation of floating materials (oil spill, vessels...)- Accumulation of nutrients: enhanced primary production → fisheries
In the open ocean: fronts can be 100 km wideIn an estuary: front can be a few meters wide
www.metoffice.gov.uk/http://eoimages.gsfc.nasa.gov
Types of fronts in the coastal ocean:
Upwelling front- The thermocline breaks the surface from upward water movement- Caused by winds: variability in wind will affect the strength of the front (and position)
Shelf break front- The result of differences in hydrographic properties between coastal ocean and open sea- The water of the coastal ocean: fresher, saltier, colder or warmer than the water of the adjacent deep ocean- Stationary; their mean position is entirely controlled by the location of the shelf break
Shallow sea front or tidal front- Result of two forces: heat from sun (stratification) and tides (mixing)- At the boundary: shallow sea front- Temperature in the homogeneous region: result of mixing the water column from the surface to the bottom
→ colder than the surface temperature in the stratified region→ warmer than the temperature in the layer below.
It therefore corresponds to the temperature found somewhere in the centre of the thermocline.
Estuarine fronts:
Plume front- Fresh water reaches the mouth region of an estuary and discharges into the ocean- The front around the plume is strongly convergent and turbulent- Accumulation of drifting material
Estuarine front- Not limited to the mouth of the estuary- Dynamically similar to shallow sea front (tide and salinity)- Tidal mixing and water depth determine front formation
Strong halocline
Strong current → vertical mixing
Focus on shallow water / tidal fronts
Position of the tidal fronts:
- Tidal fronts mark the transition point between regions that seasonally stratify and those that remain mixed
- The position of tidal fronts is easily seen in infrared satellite images because of their surface temperature signature
- Front position should follow contours of the critical value
h/u3
h: water depth u: strength of tidal currents
a) Ushant and English Channel frontb) Saint George's Channel frontc) Cardigan Bay frontd) Western Irish Sea front
http://www.liv.ac.uk
a
b
c
d
Weak tides
Strong tides
Tidal fronts change position due to:- Advection by tidal currents- Changes in tidal mixing (spring/neap cycle): fortnight stratification/mixing cycle
Mean flow:- Strong jet-like current (because of density gradient)- Meanders can become unstable: eddy detachment, water transfer- Weak flow perpendicular to the front (due to friction): convergence at surface, downwelling at the front
Typically the jet flows along the front to the left (N.H.)
Tidal fronts and influence on biology
Enhanced chlorophyll concentration in surface waters at fronts, linked to T gradient. Example of Ushant front (France)
Simpson and Sharpless, 2012
___ temperature
Also localised mixing in isolated island (e.g. Isles of Scilly below), due to enhanced stirring from tidal currents in shallow waters
Simpson and Sharpless, 2012
Possible mechanisms that lead to increased primary production at fronts
1 - Increased vertical mixing
2 - Periodic mixing due to tides
3 - Eddy transfer across front
4 - Weak mixing of nitrate driven by interfacial friction within the front
~ 80% of phytoplankton nitrate requirements
→ ~ 20% of phytoplankton nitrate requirements
Changes in light availability and nitrate supply across a tidal mixing front:
Identify these fronts
Shelf break front
Upwelling fronthttp://www.scielo.cl
a) Sea surface temperature (°C)b) salinityc) density (σt).
Top: Vertical density sections (σt) across the front, south Bottom: Vertical density sections (σt) across the front, north-east (bottom) of the Cascade.
Shelf break front
Plume front
Shelf break front
Shallow sea front
Upwelling front
