Introduction

Frontal zones and fronts are natural boundaries in the ocean. Drastic changes in the properties of oceanic waters - evidently of frontal origin - like sharp interfaces between warm and cold water masses or changes of current direction, were known to seamen since at least the 15th century. Phenomenological studies of surface effects of oceanic fronts relate to the middle of the 19th century. A great contribution to the understanding of the physical nature of fronts was made by the pioneer studies of the Japanese oceanographer Michitaka Uda in 1938 (Uda 1938). However the intense investigation of fronts only began in the 1970s as a consequence of the accumulation of numerous observations in the ocean, their analysis, the development of theoretical hydrodynamic concepts and the wide use of new oceanographic equipment and methods (especially remote sensing) which made it possible to measure oceanic properties with high space and time resolution. There was a gradual reconsideration of the views of fronts as rather static and almost impenetrable boundaries separating water masses, widely accepted by the traditional descriptive oceanography before the 1970s. An approach viewing the fronts as a physical phenomenon with a complex inner dynamics possessing self-supporting characteristics received more and more recognition among scientists. The fronts were then regarded as one important unit in the chain of the energy transfer from scale to scale (the "energy cascade") from the elements of the global oceanic circulation down to small-scale phenomena. Besides permanent frontal zones of a climatic nature, including large oceanic currents of the Gulf Stream type, a variety of fronts exists in the ocean associated with coastal currents, gyres, eddies, upwellings, intrusions in the intermediate waters, river discharges into coastal zones, etc. Frontal instabilities, in their turn, give rise to the formation of eddies and jets with their own frontogenetic mechanisms and lifetime from a day to 2 or 3 years which produce cross-frontal water exchange and horizontal mixing in the neighbouring waters (Zatsepin and Kostianoy 1994; Ginzburg and Kostianoy 2002). So, oceanic fronts are multiscale in both space and time. Besides, various phenomena and processes are associated with fronts, such as high biological productivity and abundant fishing, anomalies in conditions of sound propagation, anomalies of wind waves, high velocity of jet currents, sharp changes of sea color, intense vertical movements, local weather conditions, etc. Large-scale fronts have important effects on the weather and also on the climate.

An important contribution to the study of oceanic fronts was made by Professor Konstantin N. Fedorov (corresponding member of the USSR Academy of Sciences). His fundamental work "The Physical Nature and Structure of Oceanic Fronts" published in its Russian edition in 1983 and by Springer-Verlag in 1986 remains a basic reference for all oceanographers involved in the study of fronts, it contains a summary of all the data collected in the beginning of the 1980s (Fedorov 1983, 1986).

The possibilities of the traditional descriptive approach to the problem of the frontal zones are not exhausted yet. Many regions of the World Ocean have not been actually explored, even within the limits of this approach, or the information about them remains fragmentary and not systematized. The investigation of the Norwegian, Greenland, Barents, and Bering seas (Fig. 1) has been going on since the end of 19th century. A great amount of field observations has been collected (the Subarctic seas are among the best explored regions of the World Ocean).

Subarctic Basehttp://www.lib.utexas.edu/maps/islands_oceans_ poles/arctic_region_2000.jpg)."/>
Fig. 1. The Arctic Ocean and Subarctic Seas (http://www.lib.utexas.edu/maps/islands_oceans_ poles/arctic_region_2000.jpg).

Many descriptions of their background hydrology are available but, unfortunately, the information about the characteristics of the fronts remains very fragmentary. Still, the existing observations constitute a rich data base to construct a useful depiction of these fronts - which, within the water area of the Norwegian, Greenland and Barents seas are part of the climatic North Polar Frontal Zone (denoted NPFZ in the following) - even within the limits of the traditional hydrological approach.

This review presents the systematization and brief description of accumulated knowledge on oceanic fronts in the Norwegian, Greenland, Barents, and Bering seas, and it is partly based on the book by V.B. Rodionov and A.G. Kostianoy "Oceanic Fronts of the North-European Basin Seas" published in Russian edition in 1998 (Rodionov and Kostianoy 1998) and "Physical Oceanography of Frontal Zones in the Subarctic Seas" by A.G. Kostianoy, J.C.J. Nihoul and V.B. Rodionov published in Elsevier in 2004 (Kostianoy et al. 2004). This work was based on the numerous observational data, collected by the authors during special sea experiments directed to the investigation of physical processes and phenomena inside certain parts of the NPFZ and in the northern part of the Bering Sea, on archive data of the USSR Hydrometeocenter and other research institutions, as well as on a wide scientific literature published in Russian and Western editions. The books contain general information on the oceanic fronts of the Subarctic seas, brief history of their investigation, state of the knowledge, as well as detailed description of the thermohaline structure of all frontal zones in the Norwegian, Greenland, Barents, and Bering seas and of neighbouring fronts of Arctic and coastal origins. Special attention was given to the study of the multifrontal character of the NPFZ and of peculiarities of its internal structure at different sections, to the description of diverse oceanic features observed in the NPFZ, as well as to some characteristics of the horizontal and vertical fine structure of hydrophysical fields in the NPFZ. Observations were completed by the results of the numerical modeling of the northern Bering Sea where an extensive survey was carried out for 5 years in the scope of the NSF ISHTAR Program (Walsh et al. 1989). The main features of the northern Bering Sea's summer ecohydrodynamics were investigated with the help of three-dimensional direct and inverse models developed at the GeoHydro-dynamics and Environment Research Laboratory (GHER) of the University of Liege, Belgium (Nihoul 1986, 1993).

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