One type of interesting phenomenon which makes the dynamics of the World Ocean rather complicated and surprisingly manifold is a “hydrological front.” The term front characterizes regions in which the hydrodynamic fields possess large horizontal gradients. These manifest themselves in the thermohaline characteristics. Fronts in the ocean or in the atmosphere can be defined as regions where background long-term averaged properties of the medium change substantially over a relatively short distance. Such sharp boundaries usually separate two adjacent water masses with different properties. In the World Ocean, however, hydrological fronts can also reach more than 100 km width [233].

We emphasize that a strict standardized definition of the term “oceanic front” does not exist. As a quantitative measure for the systematization of oceanic fronts, the change of any hydrological property across a localized area – say temperature, or salinity, or both – can be used. If such gradients are an order of magnitude larger than similar changes adjacent to this area, then the presence of a front can be identified.

From the viewpoint of water dynamics, the hydrological fronts in the World Ocean can be classified by their influence on the distribution of the density field.