2. Fronts in the coastal seas
2.1. General characteristics of fronts
Fronts are regions with enhanced gradients of hydrographic or
biogeochemical properties. Usually the gradients associated with the fronts
are referred as to horizontal ones, but it is more correct to assume that
horizontal and vertical gradients in the ocean are often interrelated. Thus the
more general definition of fronts would be: fronts are areas with large
gradients over short distances separating water masses with different
properties (Fig. 2.1). In the open ocean fronts could reach hundred
kilometers in width (Gulfstream is a typical large scale front). In the coastal
areas, estuaries and straits fronts may be several (tents) meter wide. The line
of maximum property gradient is called frontal surface.
The dynamical balance maintaining the fronts includes: (1) mixing
(which is often gradient dependent with decreasing efficiency under stronger
stratification) tending to decrease contrasts in space, and (2) transport, which
brings source waters in the area of mixing (that is convergent flows, Fig. 2. 1),
maintaining the contrasts. Since the mixing (turbulent) is a secondary
property of ocean movements (mixing exists when there is a motion) the
prerequisit for creation of fronts is the convergent motion and thermal (haline)
inhomogeniety.
Temperature and salinity are not the most appropriate tracers to
track the fronts. These two fields affect the density rho=rho (T, S), the latter
is homogenized either by convection in case of vertical instability (in that case
T and S are also homogenized), or by the resulting from pressure gradients
horizontal transport, which tends to reduce the density contrast. Fig. 2.2
clearly shows intrusions of fluorescence along the front measured in the
western Mediterranean Sea. The upper panel (salinity) reveals the position of
the front. However, the fluorescence clearly reveals the structure of the front.
Though the fronts are typical physical phenomena, their relevance to
chemical and biological aspects of oceanography is paramount (Fig. 2.3).
The zones of enhanced contrasts are zones of extremely large
biological productivity (in most cases that is also associated with the
vertical circulation in the frontal zone) attracting wide spectrum of
oceanographic interest of various kind. Satellite derived maps of sea surface
temperature provide quite correct indication of potential fishing grounds.
2.2. Types of fronts
2.2.1 Planetary fronts. The best known and most persistent fronts are the
planetary fronts, which are associated with the planetary heating/cooling and
wind system. The most spectacular one is the Antarctic circumpolar front.
Most planetary fronts are complex baroclinic systems, thus temperature and
salinity distribution shapes to a large extent the distribution of mass and the
circulation in the area of fronts. Density surfaces in the area of Gulfstream
undergo slopes of ~100 m/100km.
2.2.2. Density compensated fronts. The various distribution of heat and
fresh water sources in the ocean makes possible that the effect of
temperature and salt on the density field enhance mutually (Fig. 2.4, upper
panel) or compensate (Fig. 2.4, lower panel). In the first case we deal with
density fronts, in the second with density compensated fronts. Currents in
the density fronts are aligned along density surfaces, but frontal instabilities
may lead to pronounced meandering and ageostrophic transport. In contrast,
the small baroclinic pressure gradients do not affect much the transport
in density-compensated fronts, currents may intersect temperature and
salinity surfaces (the absence of horizontal density gradient makes easy for
the currents to move water particles across the thermal/haline front), creating
thus large horizontal contrasts (intrusions, layering and filaments).
2.2.3. Prograde and retrograde fronts. The angle between frontal
surfaces and bottom determines to a large extent the dynamical balances in
the coastal zone. This angle depends on the distribution of source waters and
the characteristics of circulation. A front in which the density surface slope
upwards towards the coast (Fig. 2. 5) left panels is known as prograde
front. When the density surfaces slop downwards towards the coast the front
is known as retrograde. In the upper left panel the coastal water is denser
than the water on the seaward. The inverse situation is shown in the upper
right panel. The situation shown in the lower right panel is typical for river
plumes intruding the coastal ocean.
2.2.4. Upwelling fronts.
The upwelling fronts form when the thermocline breaks the ocean surface,
which is usually caused by the upward water movement due to
inhomogeniety of wind forcing or specific wind stress in the coastal
zone. Most important upwelling areas are in the zone of trade winds. A
spectacular illustration of dynamic control on the front associated with the
upwelling is given in Fig. 2.6. In the area of Canary current a free floating
buoy was deployed shoreward of the front. The buoy persistently flowed
towards the front, reached it in a few days, and became stagnant in the
frontal area. The conclusion then is that the upwelling front gives the seaward
extension of the upwelling zone.
The convergence there causes the
upwelled (and mixed with surface) water to sink again.
2.2.5.Shelf break fronts. Pronounced differences between properties of
coastal and open sea waters are
due to either local fluxes or
heating/cooling of shallow coastal zone. Thus specific coastal water
mass is formed, which is separated often from the ocean interior by sharp
front (Fig. 2. 7). In case of density front the current tends to align to
density surfaces. The coastal line guides the transport along the coast
therefore the front adjusts to the local topography. In case of the
retrograde front shown along the eastern U.S. coast (Rhode Island coast, Fig.
2. 7) we have winter homogenization characterized by low temperature and
salinity coastwards. There is however no full compensation of the effects of
temperature and salinity on the density field and the front is determined as
salinity dominated. The sea surface is higher in the coastal zone (where the
density is lower) and the current has the cost on its right. The topography has
the major control in positioning the fronts of this type, and they are usually
arrested over the shelf break. From here we have the name of that front. Its
width is given by the baroclinic Rossby radius of deformation, and in the
case shown above is about 20km.
An oposite situation could also exist when the shelf water is denser
than the ocean water. In that case slope convection tends to bring denser
water deeper down the continental slope, until the density of sinking water
reaches its neutral level, that is its density becomes equal to the density of
ocean water. An example is given below for the Bass Strait, a shallow region
between Tasmania and mainland Australia (Fig. 2.8) with two sills at its
eastern and western boundaries. The small depth of the coastal sea results in
smaller thermal inertia and lower winter temperatures than in the surrounding
ocean. A sharp shelf break front is thus produced along the eastern edge of
Bass Strait (Fig. 2.9). The coastal water has density of ~0.4 larger than
density of the ocean, which triggers slope convection (right panel of Fig. 2.9)
reaching ~400m (Bass Strait Water Cascade). The northward extension of
gravity flow is consistent with the left deflection of motion in the southern
hemisphere.
2.2.5. Shallow sea fronts. These fronts owe their existence to specific
mixing properties of shallow seas in the presence of moderate to strong
tidal currents. A classical balance known from the theory of upper mixed
layer assumes that heat input from solar radiation tends to stratify the
water column. The wind mixing works in the opposite direction, tending
thus to deepen the well mixed surface layer. Similar balance exist in the shelf
area, however the mixing due to tides takes over. In the areas where the
mixing power of tidal currents is insufficient to homogenize the whole water
column we observe vertical stratification. Elsewhere the water properties are
uniform in vertical direction. The energy required to completely mix the
water column increases with increasing the depth. Tidal currents,
however decrease with increasing depth, thus beyond some isobath the
water column is not anymore homogeneous. This effect has a
pronounced signature at sea surface and is well revealed in the temperature
distribution. The sketch in Fig. 2.10, as well the plot on the right displaying the
situation in the Irish Sea visualize the above arguments.
2.2.6. Fronts in estuaries
Plume fronts are observed in ocean areas where fresh water from the river
mouth discharges into the ocean (Fig. 2.11). The water in the plume is well
mixed and the contrast with ocean water is pronounced along the plume
periphery. These phenomena are described by the theory of buoyant plumes,
assuming that the entrainment of ocean water in the plume is controlled by
dynamics and stratification (the mixing is dependent on the Richardson
number). The large density contrast at the bottom of plume shields the
modified river water from the oceanic water. The plume grows until fresh
water input from the river gets in balance with the mixing at plumes boundary
(the surface of latter also increases with growing plume).
Estuarine fronts run parallel to the banks of estuaries at some distance
(Fig. 2.10). Dynamically they are similar to the shallow sea fronts: in the
estuarine fronts vertical stability is due to positive buoyancy by the fresh
water source, while again the vertical mixing is of tidal origin. Thus tidal
mixing is weaker in the core of estuary (because of the larger depth) and the
vertical stratification there is essentially two layer. The halocline in proximity
to the banks weakens or completely dissapears. The estuarine front is
located where the upper halocline first appears. Since the dominating
direction in the estuaries is associated with the shore the estuarine front
tends to follow the shore. It is well developed during ebb and flood period and
weakens or disappears during slack current period.
Figures.
Fig. 2.1
Fig. 2.2
Fig. 2.3
http://seawifs.gsfc.nasa.gov/SEAWIFS/CZCS_DATA/global_full.html
Fig. 2.4
Fig. 2.5
Fig. 2.6
Fig. 2.7
Fig. 2.8
Fig. 2.9
Fig. 2.9a
Fig. 2.10
Fig. 2.11