Презентация на тему Fronts in shallow seas

regions where properties change markedly over a relatively short

distance.

Another way of expressing this is that fronts are regions where the horizontal gradient of a property goes through a maximum.

Six types of fronts are usually distinguished in oceanography.

Three of these are found in the coastal ocean.

Two of these are found in estuaries.

The sixth type of front is restricted to the deep ocean.

horizontal variations, bounded by
narrow regions where horizontal gradients are

large >>
Fronts are the narrow high-gradient regions.

i.e.Front or Frontal zone: sloping interface between two
water bodies with different water properties (density, temperature, salinity)

• Examples:
Western boundary currents (e.g. Gulf Stream) 1000’s of kms; T difference of 10° C over 50 km;
Tidal flow transient fronts typically a few kms, T difference only 1-2°C

contours indicate surfaces of constant property values; to take

an example, in a temperature front the contours would be isotherms, cold would be purple, warm would be gold.

Water movement is towards the front at the surface, producing a convergence into the front. It can be seen that water moves across the contours (eg isotherms), so strong mixing, indicated by yellow arrows, must occur in the front.

The frontal zone is the region of rapid property (e.g. temperature) change at the surface. The frontal axis is the location of the largest horizontal property (e.g. temperature) gradient.

Dynamics of ocean fronts

Where there is convergence there will be a front, and where there is a front there has to be flow convergence.

in the Alboran Sea.
T and S are not good

indicators for vertical water movement because they determine the stability of the water column . Downward movement in the frontal zone is clearly seen in optical properties. Here fluorescence in a frontal region of the western Mediterranean Sea.

and a density-compensated front (bottom).

The stable stratification component

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.

deep ocean
• They span the width of entire ocean

basins

• Associated with features of planetary wind system

• Fronts are found in the Circumpolar Current and between
the subtropical and subpolar oceanic gyres of the northern
Hemisphere.

• Have great impact on air/sea interaction processes

• Create atmospheric conditions due to T differences across
a planetary front.

(Pacific coastal area)

Planetary fronts
examples

and direction of the wind.
Most important upwelling regions are

located in the Trade Wind region where winds are relatively uniform in strength and direction

region off the Mauritanian coast showing the upwelling front

and the drift of a buoy deployed to mark freshly upwelled water.

This figure shows the drift of the buoy. The buoy had a subsurface drogue attached and therefore served as an indicator of water movement for the upper mixed layer.

Black contours and colours indicate water depth, contouring interval is 250 m.

Thick lines indicate buoy drifts. The first drift (cyan) lasted 6 days, until the buoy had reached the upwelling front (at the black dot). It was then recovered and redeployed for a second drift (magenta), which lasted 3 days. By that time the buoy had again reached the front.

Note that the buoy crosses the continental slope at an offshore angle, in agreement
with upwelling dynamics.

Upwelling front

the prograde front the bottom slope and the slope

of the front are of the same sign, in the retrograde front they have the opposite sign.

the coastal ocean and the open sea.

• In contrast

to upwelling fronts and shallow sea fronts, shelf break fronts are more or less stationary.

• Position controlled by the location of the shelf break, departures from this position only through eddy formation.

Shelf break fronts

USA. Winter situation
south of Rhode Island. Stratification on the

shelf minimal during
winter, T > 4° inshore to 8° near the front. Salinity from 32.5 to 33.5.

by competition between
vertical distribution of solar energy (stratifies the

water column) and the vertical turbulent mixing generated by tidal stresses at the seabed (acting against the thermal stratification)

Shallow and/or tidally energetic regions of temperate shelf seas remain vertically mixed throughout the year, as the strong tidal stirring is always able to overcome the tendency to stratification caused by surface heating.

In deeper and/or less tidally energetic regions the tidal stirring is not able to counter the effects of surface heating in summer, ant the water column will stratify.

Tidal mixing fronts mark the boundaries between these different regions.

along-front mean flow, convergence and downwelling at the front,

upwelling the wel-mixed side, and frontal eddies, some of which close on themselves to form isolated patches.

H/u3. , where H is the water column depth and u the amplitude of tidal currents

sea front, showing the temperature field, the associated current

along the front and the surface convergence. The deeper water is to the right.

Centre: temperature in the deeper water as a function of depth, indicating the colour code for the surface mixed layer, thermocline and lower layer.

Right: Observed temperature field in a shallow sea front. The deeper water is on the left. The data are from the front in the Irish Sea.

temperature, chlorophyll-a, silicate and nitrate.

of the stratification observed during summer; the thermocline splits

on both sides of Bass Strait as a result of tidal mixing in shallow water.

Centre: log10(h/u3) calculated from a numerical tidal model.

Right: log10(Ep) calculated from the observed density field. The darkest regions are unstratified.

Ouessant (Ushant) front along the French coast during neap

tide (left) and during spring tide (right).

represents the weak nutrient flux into the base of

the thermocline on the stratified side of front. This flux supports production within the subsurface maximum, but production in the surface layer is nutrient limited.

Arrow B represents increased nutrient flux through the weaker stratification at the front potentially supporting the near-surface productivity at the front.

Arrow C represents the strong vertical nutrient exchange within the mixed water column. While this will maintain high nutrient concentrations through the depth, the same mixing continuously removes algae from the photic zone and so production will be light limited.

h/u3. , where h is the water column depth and u the amplitude of tidal currents

lunar month (28 days) between
spring and neap tides

• As

current speed increases the energy available for mixing increases,
which causes the level of turbulence to increase.

• The depth of water where the turbulence is energetic enough to break
down the stratification will increase >> boundary between the mixed
and stratified water will move towards deeper water, decreasing the
area of stratified water.

• When the tidal currents decrease, the turbulence declines and the front
moves towards shallower water, allowing the area of stratified water to
increase again.

• The newly stratified water behind the advancing front will contain
nutrient levels characteristic of the previously mixed water.

cold and warm water together in a circular fashion.
Created

by instabilities of the front, which allow small departures fron the geostrophic balance to grow at the expense of the potential and/or kinetic energy in the front.

Frontal eddies are of interest as it is often suggested that they cause water to be exchanged across the front and thereby contributing a significant flux of nutrients

through a tidally mixed front with vertical arrows indicating

the
increase in the rate of vertical eddy diffusion (low in the stratified side because vertical
diffusion through the pycnocline is low and high in the well mixed side caused by tidal
stirring) from the stratified side to the mixed cold, high-nutrient layer into the fullymixed
region.

form where relatively fresh water reaches the mouth region

of an estuary and discharges into the oceanic environment.

the estuary mouth. The front around the plume Is

strongly convergent and turbulent. Here fresh water is absorbed into the oceanic environment. Plume fronts are often indicated by an accumulation of drifting material such as leaves or foam.

to the mouth of an estuary. They may run

parallel to the banks of the estuary at some distance.

Dynamically they are a miniature version of the shallow sea front in the sense that tidal mixing competes against buoyancy generated stability of the water column, the difference being that in an estuary the stability is maintained by salinity rather than temperature.