Front (oceanography)

As it took a long time to combine these data, the obtained front positions gave a time-averaged view showing only the broad-scale structure.

For example, in the Southern Ocean, this led to the definition of five fronts that were all considered to be continuous and circumpolar, reaching to large depths and being strongly influenced by bathymetry.

By continuously measuring sea-surface height (SSH) around the globe, the position of strong currents or jets associated with ocean fronts can be determined at a very high spatial and temporal resolution.

[4][5] This way, short term variability and trends can be analysed and related to other climatological variations, such as El Niño - Southern Oscillation.

When defining fronts on a global scale, often specific values of sea-surface height or temperature are used, resembling the traditional water mass definition.

In this process, several factors play a role, including ocean currents, wind, and Coriolis forces.

[6] Differences in the location and formation processes lead to a wide range of front types.

[8] As these fronts are not in inertial balance, they need a constant source of energy to survive, explaining their relatively short lifetime.

[7] In the shallow seas at the continental shelf, two main types of front can form depending on the processes that play a role.

The westward trade winds then lead to an Ekman transport that moves the surface waters away from the equator in both hemispheres.

[27] In addition, the build-up of water at this region leads to a slight increase in sea level.

The main difference between these two arises by the release of salt during sea ice formation, called brine rejection.

[9] The upper- and lower-layer fronts may be separated where the ocean currents hit perpendicular to the ice, which often occurs for example in bays.

[29] In general, examples of marginal ice zone fronts can be found in the Labrador and Greenland Sea, and in the Southern Ocean.

In addition, the different water masses that meet in this basin are associated with strong gradients in density that reach to great depth and lead to strongly tilted isopycnals surfaces (planes of constant density) that shallow towards the south.

These transport high-density air from a higher elevation downslope under the force of gravity and help maintain the westward current over the shelf and thus the front.

Some frontal types, such as upwelling and convergence fronts, are sites of pronounced exchange between deep and surface ocean and can catalyse the generation of mesoscale eddies and submesoscale filaments.

Some fronts create hotspots of marine biodiversity and biogeochemical processes when they inject macronutrients from an adjacent nutrient-rich water mass into a nutrient-limited and physically stable euphotic zone, enhancing new primary production.

However, strong mixing that occurs at some fronts can provide nutrients to the euphotic zone and enhance productivity.

[18] The surplus of carbon biomass produced on fronts may be exported downwards, feeding deeper pelagic and benthic communities.

Surface estuarine front of the Río de la Plata estuary. The exact location of the color change, implying a change from fresh- to saltwater, depends on winds and currents. Photograph taken by NASA.
The schematic illustration of tidal intrusion front with the flow pattern at the surface
The schematic illustration of the tidal intrusion front with the flow pattern at the surface.
Formation of an axial convergence, longitudinal or shear front. The inflow of ocean water during the flood phase occurs at higher speeds in the centre of the estuary. This leads to gradients in velocity and shear that bring the water towards the centre at the surface and lead to a converging circulation.
Formation of an axial convergence, longitudinal or shear front. The inflow of ocean water during the flood phase occurs at higher speeds in the centre of the estuary. This leads to gradients in velocity and shear that bring the water towards the centre at the surface and lead to a converging circulation.
Mechanism of upwelling near the coast creating upwelling coastal front.
The Pacific white line is an example of a convergent front
The Pacific white line , shown here, is an example of a subtropical convergence front. In this case, the buoyant diatom rhizosolenia is collecting at the front in such large quantities as to be visible from space.
Rough position of the three Southern Ocean fronts related to the Antarctic Circumpolar Current: Southern Antarctic Circumpolar Current Front (SACC), Antarctic Polar Front (PF), and Subantarctic Front (SAF). In addition, the Subtropical convergence Front (STF) is shown.