front

front.
1. A surface feature of the sea caused by relatively sharp changes in either temperature or salinity. Usually the temperature differences are small, a degree or so, but occasionally, along the boundaries of fast-flowing currents, they can be as high as 5 or 6 °C (9–10 °F). Since the density of sea water is determined by both temperature and salinity, the denser water (cooler or saltier) tends to be sliding beneath the lighter water. So the surface water tends to be converging towards the front, but divergent fronts, where the water tends to be flowing outwards away from the front, can also be formed.

Fronts tend to be maintained dynamically along the edges of current flows, and can be generated by both local and large-scale mechanisms. One of the smaller-scale processes is the effect of a gentle wind blowing over the surface of a calm sea. This generates counter-rotating cylinders of water in the upper few metres that are called Langmuir cells. These cells lie parallel to the wind direction, with a convergent front along one side and a divergent front along the other. Windrows and slicks form along the convergent front, as any floating debris and plankton tends to be accumulated. In coastal waters tidal fronts form which are generated by the ebb and flow of the tide. At shallow depths the friction between the tidal currents and the seabed keeps the overlying water uniformly mixed, but there is a critical depth at which the mixing no longer extends all the way to the surface and here a tidal front develops. So on one side of such a tidal front the water is thermally stratified whereas on the other side the water temperature is uniform top to bottom. At still larger scales, mesoscale eddies and rings that may be tens to hundreds of kilometres across are bounded by meandering fronts, along which there tend to be quite strong local surface currents. Following around the meanders the surface waters may tend to sink (where the water is turning clockwise around the meander in the northern hemisphere), or create an upwelling. So not only does buoyant material tend to accumulate along fronts, but also, where nutrient-rich water is brought to the surface, marine plant productivity is high. Since both phytoplankton and animal plankton tend to accumulate along fronts, they attract predators large and small, including turtles, marlin and even whales. Large currents like the Gulf Stream, which are transporting warm (or cold) water from one region to another, are bounded by a complex of meandering fronts with quite large current shears across them. The breaking away of these meanders from the main flow of the current is the genesis of some of the eddies and rings.

See also antarctic convergence.
2. The forward edge of an advancing mass of cold or warm air.

See marine meteorology.

M. V. Angel