Figure 9-14.-Seawater transparency of the North Atlantic.
First of all, lets consider the definitions of fronts,
eddies, and upwelling.
An ocean front is the interface between two water
masses of different physical characteristics. Usually,
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.
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
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.