Definitions

In published works on fronts the authors, as a rule, use various definitions of the terms "frontal zone", "frontal interface", "front", "frontal line". The differences in definitions are related to the concepts of frontal zones accepted by the different authors. Each definition is acceptable and generally applicable to resolve specific problems and describe certain aspects of the investigated phenomenon. Stepanov (1960) noted a connection of the frontal phenomena with convergences in the ocean. Studying the fronts of the Gulf Stream system Baranov (1966, 1972) defined frontal zones on the basis of the concepts of "water masses" as a broad transitional zone between various water masses, which is rather stationary in time and space. Describing the main climatic frontal zones of the World Ocean, Gruzinov (1986) defined them as "quasistationary zones of interaction between waters of various hydrological characteristics having individual ecosystems, which reveal themselves all over the thermocline by maximum horizontal gradients of hydro-logical characteristics and vertical currents". Gruzinov specifies that his work is concerned with "main" or climatic fronts. At the same time Baranov and Gruzinov added that from a climatic point of view, the frontal zone may be considered, as well, as a region of the ocean in which seasonal and interannual transitions of a given front occur. There are also a number of other definitions summarized by Fedorov (1983, 1986). These definitions, which are quite adequate to the problem of the physico-geographical description of the climatic frontal zones, however, do not single out features of their dynamics which produce a significant sharpening of contrasts of the main parameters, and cause the appearance of fronts of various scales (not only climatic) in this zone. These features are taken into account by the definition given by Fedorov (1983, 1986), according to which the frontal zone in the ocean is a zone "in which the spatial gradients of the main thermodynamic characteristics are very high in comparison with the average". This definition is not based on climatology concepts such as "water masses", "thermocline" etc., which require definitions by themselves, and implies the application of some appropriate numerical criterion selected by each investigator. Following Fedorov, the frontal interface will be defined as "a surface within the frontal zone, which coincides with the surface of the maximum gradient of one or several characteristics (temperature, salinity, density, velocity, etc.)". Then, strictly a "front" can be regarded as the result of the intersection of the frontal interface with any given surface, particularly with the free surface of the ocean or with an isopycnal surface (Fedorov 1986).

The North Polar Frontal Zone (NPFZ) in the Norwegian, Greenland and Barents Seas is a complicated oceanic feature, in which processes of all scales are represented. As a whole, NPFZ represents a climatic frontal zone generated by the interaction of two elements of the planetary circulation: relatively warm and salty waters of the Atlantic Ocean, which extend from the South to the North, and colder and fresher waters, which penetrate from Polar areas to the South, formed during general cooling, ice thawing and mixing thawing products with enclosing waters. A rather complicated bottom topography and the coastal line topography of the investigated region result in the division of main streams of waters into separate branches and, therefore, in the existence of the branchy system of permanent currents. The convergence of the currents and their interaction with elements of the topography and the coast line result in the NPFZ to be divided into several frontal zones of smaller scales (100 km). Besides the interaction of sea water with continental drainage waters transported by secondary branches of the general circulation leads to the formation of frontal zones also. Thus the climatic NPFZ is a system of frontal zones with various characteristics. However it is necessary to remember that these frontal zones are "only separate parts of the NPFZ, and, therefore, they must be regarded as climatic".

According to the definition, the gradients of temperature and salinity across the frontal zones should considerably exceed the average climatological gradient. For the region being studied the average climatic values of the horizontal gradients of temperature and salinity are not higher than respectively 0.01°C/km and 0.001%o/km.

A characteristic feature of the majority of frontal zones of this water area is a multifrontal internal structure, i.e. the presence of several fronts (very often of different types). Permanent fronts related to persisting climatic causes, fronts of synoptic or seasonal nature, and also small-scale fronts of local origin are present, justifying the separation made by Fedorov between "frontal zone" and "front".

Frontal zones in the Norwegian and Greenland seas

The description of the characteristics of frontal zones in the Norwegian and Greenland seas was based on the analysis of hydrological data received by the USSR Hydrometeocenter in 1984-1987. The information came from about 10,000 stations. The measurements of vertical profiles of temperature and salinity were made at approximately 30% of stations, only measurements of temperature profiles were made at the others. The resolution of the stations was 10-30 miles.

Frontal Variability Barents Sea
Fig. 2. The position of high gradient zones in temperature field (all observations for 48 months of 1984-1987).

The technique for the determination of the fronts' characteristics was the following: the data for every year were classified according to months, and the position of stations for each month was marked on a map. The vertical profiles of temperature and salinity were analyzed to distinguish pairs or groups of stations on sections, between which high-gradient areas with changes of temperature and/or salinity, considerably exceeding mean climatic ones, were observed. Then it was possible to state that one or more fronts are located between adjacent stations, and that the stations are situated inside or on the rim of the frontal zone. The analysis of the totality of the sections for each month allowed to plot the location of the high-gradient areas on the map as lines dividing waters with different characteristics. In Figs. 2 and 3 one can see the combined maps of the high-gradient zones in temperature and salinity fields at the sea surface for all hydrological seasons altogether made by the superposition of 48 monthly maps. Thus these maps represent a high resolution snapshot of a complete frontal system in the Norwegian, Greenland and Barents seas simultaneously, that is unreachable by satellite monitoring due to cloudiness.

Fig. 3. The position of high gradient zones in salinity field (all observations for 48 months of 1984-1987).

It follows from them that the location of the high-gradient zones has a large space and time variability, generally related to the existence inside the frontal zones of several local fronts and/or a significant intra-seasonal variability of their position. The simultaneously observed high-gradient sites do not provide a narrow and continuous picture. It is possible to distinguish only rather broad areas (the frontal zones), in which the main changes of properties (i.e. sharpening of horizontal gradients) between water masses take place.

Various parts of the NPFZ considerably differ by their characteristics. So it is expedient to distinguish its individual parts (or frontal zones of smaller scale) due to different types of water interactions, bottom and coastal topography, features of quasi-permanent currents, etc. Based on the analysis of wide scientific publications, satellite and in-situ data in the Norwegian and Greenland seas it was possible to distinguish the following nine parts of the climatic NPFZ and adjacent frontal zones differing from each other in main characteristics, and to describe them in detail in (Kostianoy et al. 2004):

1. The Norwegian Current Frontal Zone (NCFZ) is conditioned by the interaction of the Norwegian Current with the East Icelandic Current in the Norwegian and Lofoten Basins, and the Norwegian Plateau. There is a great amount of work devoted to the general hydrology of the Norwegian Sea, however there is very little information published on the local fronts relating to the NCFZ.

2. The Coastal Frontal Zone of the Norwegian Sea (CFZNS) is formed by the interaction between water of the Norwegian Coastal Current (to the south of 65° N it is called the Baltic Current) with water of the western branch of the Norwegian Current and the eastern branch of the North Atlantic Current (to the south of the Faeroe-Shetland Strait).

3. The Iceland-Faeroe Frontal Zone (IFFZ) is formed by the interaction of the North Atlantic Current water and the East Icelandic Current water, stretching from the Iceland coast along the Iceland-Faeroe Ridge approximately up to 6264° N, 3-5° W. The IFFZ was repeatedly investigated from the beginning of the 1960s.

4. The East Greenland Frontal Zone (EGFZ) is the zone of interaction of the East Greenland Current and Irminger Current, stretching along the continental slope of Greenland.

5. The Iceland Coastal Frontal Zone (ICFZ) is the zone of interaction of the Coastal Iceland branch of the Irminger Current with waters of the East Greenland and East Icelandic Currents, located in the region of the northern and eastern shelf break of Iceland. Research on this zone have been carried out very intensively in the 1950s and 1960s in the support of fishing.

6. The Jan Mayen Frontal Zone (JMFZ) is the zone of interaction of the East Icelandic Current with the East Greenland Current, passing almost meridionally between 6° and 9° W from the Iceland-Faroe Ridge along the Jan Mayen Ridge approximately up to 70° N.

50°W 40° 30° 20° 10" 0° 10" 20" 30° 40°E

50°W 40° 30° 20° 10" 0° 10" 20" 30° 40°E

Fig. 4. The pattern of the main currents and frontal zones (FZ) in the Norwegian and Greenland seas. The Roman numerals designate the frontal zones and the Arabic - the currents:

- I - Norwegian Current FZ, II - Coastal FZ of the Norwegian Sea, III - Iceland-Faeroe FZ, IV - Jan Mayen FZ, V - East Greenland FZ, VI - Iceland Coastal FZ, VII - Mohn Ridge FZ, VIII - Northern Greenland Sea FZ, IX - West Spitsbergen FZ.

- 1 - North Atlantic Current, 2 - Norwegian Current, 3 - Irminger Current, 4 - East Icelandic Current, 5 - East Greenland Current (5a - Jan Mayen branch), 6 - Norwegian Coastal Current, 7 - Baltic Current, 8 - West Spitsbergen Current.

Fig. 4. The pattern of the main currents and frontal zones (FZ) in the Norwegian and Greenland seas. The Roman numerals designate the frontal zones and the Arabic - the currents:

- I - Norwegian Current FZ, II - Coastal FZ of the Norwegian Sea, III - Iceland-Faeroe FZ, IV - Jan Mayen FZ, V - East Greenland FZ, VI - Iceland Coastal FZ, VII - Mohn Ridge FZ, VIII - Northern Greenland Sea FZ, IX - West Spitsbergen FZ.

- 1 - North Atlantic Current, 2 - Norwegian Current, 3 - Irminger Current, 4 - East Icelandic Current, 5 - East Greenland Current (5a - Jan Mayen branch), 6 - Norwegian Coastal Current, 7 - Baltic Current, 8 - West Spitsbergen Current.

7. The Mohn Ridge Frontal Zone (MRFZ) is located in the region of the Mohn Ridge, and is a zone of interaction of waters of the Jan Mayen branch of the East Greenland Current with waters of the western branch of the Norwegian Current limited in the south by 70° N, and in the north by 73-74° N.

8. The Northern Greenland Sea Frontal Zone (NGSFZ) is formed as a result of the interaction of water of Atlantic origin, penetrating through the Greenland basin, with Arctic water. It is located in the area of the Greenland basin, to the north of the Mohn Ridge. This region has been rather well investigated due to the MIZEX experiments and other programs.

9. The West Spitsbergen Frontal Zone (WSFZ) is located along the western edge of the Spitsbergen shelf and is caused by the interaction of water of Atlantic origin with water penetrating from the Barents Sea shelf off the Spitsbergen Archipelago.

The pattern of the main currents and the location of the main frontal zones in the Norwegian and Greenland Seas are presented in Fig. 4.

Earlier there were few attempts to distinguish different parts of the NPFZ in the Norwegian, Greenland and Barents seas. However such systematization was made either not on the whole water area or in a very detailed way, or only on the temperature field.

First of all, one shall consider V.K. Agenorov's work (Agenorov 1947). For the first time, a new method of selection of water masses and detection of the position of the frontal zones based on the analysis of fields of gradients of a number of hydrological characteristics and of vorticity developed by V.K. Agenorov (1944) was used. Based on the data of the temperature field in the Barents Sea in summer months, maps of main water masses with distinctly marked frontal zones and front positions at 25, 50, 100 and 200 m depth for July-September were drawn.

Kolesnikov (1962, 1967) was the first to systematize frontal zones in the Norwegian Sea. He revealed and studied separately the frontal zones of the Mohn Ridge, the Jan Mayen, the Iceland-Faeroe and the frontal zone of the Norwegian Current (Helholand frontal zone as called by Kolesnikov [1967]). The analysis was made basing on the materials of monthly micro surveys, which were carried out by the BaltNIRO and PINRO, and also by other Soviet and foreign expeditions from 1951 to 1960. The position of frontal zones was determined according to V.K. Agenorov's method. It was showed that the system of the fronts in the Norwegian Sea is connected with specific rises of bottom topography - the Mohn Ridge, Jan Mayen Ridge, Iceland-Faeroe Ridge, Thomson Ridge, and the eastern slope of the Norwegian Basin.

On the basis of the 2,300 AXBT surveys of the temperature field in the Norwegian Sea, Smart (1984) identified the Iceland-Faeroe, Jan Mayen, Iceland Coastal and Norwegian Current frontal zones. Johannessen (1986) mentioned the Iceland-Faeroe, East Greenland, Norwegian Coastal, Polar Oceanic and Barents Sea Polar fronts. Kuznetsov et al. (1986) identified frontal zones corresponding to the Jan Mayen, Iceland-Faeroe, Norwegian Coastal Currents and the frontal zone of the Norwegian Current. Korablev (1987) processed the hydrological information with the resolution of 60 miles over the main part of the Norwegian and Greenland seas and identified the Iceland-Faeroe, Jan Mayen, Norwegian Current, Mohn Ridge and Northern Greenland frontal zones. This work was generalized in (Alekseev and Nikolaev 1987; Nikolaev and Alekseev 1989; Alekseev and Bogorodskiy 1994). A more complete history of the research of oceanic fronts in the Subarctic seas can be found in (Kostianoy et al. 2004).

Unfortunately, all the classifications of the frontal zones were made only on the basis of the temperature field. Apparently, this did not allow to identify precisely the frontal zones, strongly and steadily expressed in the salinity field and weakly or unstably in the temperature field (with modification of a magnitude and alternation of a sign of cross-frontal drop of thermohaline characteristics in different seasons). Besides insufficient time and space resolution of the collected data did not allow to describe seasonal variability of the spatial and thermohaline characteristics of the frontal zones and the structure of fronts inside them.

Rodionov and Kostianoy (1998) and Kostianoy et al. (2004), on the basis of the joint analysis of data from the Russian Hydrometeocenter archive for 1984-1987 and other published information made a systematization of frontal zones in the Norwegian, Greenland and Barents seas and described their characteristics for all seasons. The main characteristics were determined on the basis of the temperature and salinity vertical profiles at both sides of each frontal zone. The width of the frontal zones, the characteristic drop in temperature, salinity, and relative density across the frontal zones, as well as a set of other supplementary parameters for the near-surface and deep layers were calculated. Besides, on the basis of the hydrological data published in 1900-1992, the characteristics of the local fronts inside the frontal zones were evaluated by graphic analysis of the hydrological sections shown in historical publications. For this special analysis, the sections with a resolution between stations of no more than 20 km were used (Rodionov and Kostianoy 1998; Kostianoy et al. 2004).

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