OCEAN FRONTS

TYPICAL LOCATIONS OF PACIFIC AND ATLANTIC OCEAN FRONTS

Aerographers Mate 1 & C
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Figure 9-14.-Seawater transparency of the North Atlantic. First of all, let’s consider the definitions of fronts, eddies, and upwelling. OCEAN FRONTS An ocean front is the interface between two water masses of different physical characteristics. Usually, fronts show strong horizontal gradients of temperature and salinity, with resulting density variation and current shear. Some fronts which have weak horizontal gradients at the surface have strong gradients below the surface. In some cases, gradients are weak at all levels, but variability across the front, as reflected by the shape of the thermal profile, is sufficient to complicate sound transmission. A useful definition for the purpose of naval operations can be stated as: A tactically significant front is any discontinuity in the ocean which significantly alters the pattern of sound transmission and propagation loss. Thus, a rapid change in the depth of the sound channel, a difference in the sonic- layer depth, or a temperature inversion would denote the presence of a front. OCEAN EDDIES An eddy is a rotating parcel of fluid. As such, the eddy concept can be applied to phenomena ranging from momentary vortices in the sea-surface flow to the steady circulation of a basin-wide gyre. For ASW application, however, mesoscale features of 100 to 400 km (55 -215 nmi) are most important. These eddies are rotating masses of water that have broken off from a strong front such as the Gulf Stream. They can be considered circular fronts with water trapped inside having different physical properties from the surrounding water. UPWELLING Surface winds cause vertical water movements. Upwelling can be caused by winds blowing across the ocean surface. Coastal upwelling occurs where prevailing winds blow parallel to the coast. Winds cause surface water to move, but the presence of land or a shallow bottom restricts water movements. When the wind-induced water movement is off-shore, subsurface water flows to the surface near the coast. This slow, 9-15

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