Long-Term Hydrographic Variability Patterns Off the Norwegian Coast and in the Skagerrak
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ICES Marine Science Symposia, 219: 150-159. 2003 Long-term hydrographic variability patterns off the Norwegian coast and in the Skagerrak R. Sætre, J. Aure, and D. S. Danielssen Sætre, R., Aure. J.,and Danielssen, D. S. 2003. Long-term hydrographic variability patterns off the Norwegian coast and in the Skagerrak. - ICES Marine Science Symposia, 219: 150-159. The Norwegian Coastal Oceanographic Observing System consists of observations of temperature and salinity in the surface layer, carried out at fixed positions from coastal liners, and measurements in the whole water column, carried out by local observers. The observations date back to 1935. Additionally, long-term data from the Institute of Marine Research station in Flødevigen on the Skagerrak coast and some selected data from the Torungen-Hirtshals hydrographic section are included. These data have been used to elucidate the long-term hydrographic variability along the Norwegian coast. Four relatively warm winter periods could be identified in the surface layer, culminating around 1950, 1960, 1975, and in 1990-1992. The long-term temperature and salinity trend 1950-1989 is negative along the whole coast. The 1990s, however, are characterized by having the highest mean decadal temperature for the whole period of observations along the southern coast. The importance of the 1990s in the surface layer is gradually reduced northwards. Along the northernmost coast, other decades, such as the 1950s or the 1960s, show higher decadal mean tem perature. Also for the salinity the 1990s show high values along the southern coast, while other high salinity decades dominate further north. The high temperatures and salinities along the southern coast in the 1990s are caused by an increase in the Atlantic inflow in the late 1980s and early 1990s combined with the atmospheric conditions associated with periods of a high level of the North Atlantic Oscillations. Keywords: long-term hydrographic variability, Norwegian coast, Skagerrak. R. Saetre and J. Aure: Institute of Marine Research, PO Box 1870, NO-5817 Bergen, Norway. D. S. Danielssen: Institute of Marine Research, Flødevigen Marine Research Station, NO-4817 His, Norway. Introduction that fluctuations in the hydrographic conditions along the coast might influence the recruitment, Norwegian Coastal Water (NCW) originates prima growth, and distribution of fish stocks. This was used rily from the freshwater outflow from the Baltic and as an argument for establishing the Norwegian the freshwater run-off from Norway. This water Coastal Oceanographic Observing System (NCOOS) mixes with North Sea Water (NSW) and Atlantic consisting both of observations from the surface Water (AW) to form the Norwegian Coastal Cur layer by ships of opportunity and of fixed hydro- rent, which flows northwards along the coast of graphic station carried out by local observers. The Norway as a wedge-shaped low-salinity current observing system was established in the mid-1930s bordered by the Norwegian North Atlantic Current and is still operational. It represents some of the off the central and northern parts of Norway longest continuous oceanographic time-series in the (Figure 1). A description of the characteristic feature world. Data from this observing system have been of this current system was given by Sætre and Ljøen used in a large number of reports and publications for (1972) and in Sætre and Mork (1981). The current various purposes, including highlighting long-term system and water masses in the Skagerrak area have been described by Gustavsson and Stigebrandt variations (e.g. Ljøen and Sætre, 1978; Blindheim (1996) and by Danielssen et a i (1997). et al., 1981; Danielssen et al., 1996). The Norwegian shelf is the spawning and hatching The aim of this contribution is to elucidate the area for several commercially important fish species. long-term hydrographic variability patterns along Early in the previous century it was acknowledged the Norwegian coast and identify possible regional Long-term hydrographic variability patterns 151 O Loppa OVardø 70°N • Skrova 'O Hestmannøy 65°N O Folia Hustadvika Bud O Stad ;0 Sognesjøen 60“N • Utsira ^ FLØDEVIGEN ■ Torungen \ Lista 5 nm i* 30 nm J ® "* ----------------52 nm 0°E 5°E 10°E 15°E 20°E 25"E 30”E Figure 1. Selected stations for the present study along with the persistent currents. (1) Surface layer observations from coastal liners. (2) Fixed hydrographic stations. (3) Norwegian Coastal Current. (4) Norwegian North Atlantic Current. differences with special emphasis on the situation Since 1961 the section has on average been worked in the 1990s. It is mainly confined to the winter out 8-12 times a year. Some data from this section situation, as this season is believed better to reflect are presented as seasonal means as a basis for assess the long-term climatic signals. ing possible biological effects of the variability in the physical environment. From this section three different positions are selected: one at 10 m depth, 5 Material and methods nmi off Torungen in the Norwegian coastal current (Torungen 5 nmi), one at 300 m depth, 30 nmi from During the period 1935 to 1947 Jens Eggvin at the Torungen in the Atlantic water masses in the central Institute of Marine Research (IMR) established a Skagerrak (Torungen 30 nm), and one at 10 m number of fixed hydrographic stations in Norwe depth, 52 nmi from Torungen near the Danish coast gian coastal waters (Eggvin, 1938, 1948). He also in the North Sea water masses (Torungen 52 nmi). initiated a surface layer observation programme Figure 1 shows the location of the stations selected from ships of opportunity. Since 1951, the IMR has for the present study. For the ship of opportunity operated a standard hydrographic section across the programme, regular coastal liners measure the tem central part of the Skagerrak between Torungen on perature at predetermined locations at the intake of the Norwegian side to Hirtshals on the Danish side. cooling water to the engine. Simultaneously, a water 152 R. Sielre et al. sample is taken for analysis of the salinity. The depth and in 1990-1992. Up to the end of the 1980s there of observations is approximately 4 m. The frequency was a negative temperature trend along the whole of observations is usually 8-10 times per month. coast (Table 1). The highest normalized temperature At the fixed hydrographic stations the vertical trend for the period 1950-1989 was -0.9 and -1.2 temperature and salinity profiles are measured for Lista in the extreme south and Vardø in the 2-4 times per month by local observers. Aure and extreme north, respectively. Østensen (1993) present both mean values and long The last period of warm winters started in 1987 term variations from the fixed stations. Since 1919 and culminated at the beginning of the 1990s temperature and salinity have been measured daily (Figure 2). During this period the highest winter in the pipeline through which seawater is pumped temperatures since 1936 were observed along the from various depths at the IMR's Research Station southern and central parts of the Norwegian coast. in Flødevigen. The salinity has been determined by Daily observations from the Torungen lighthouse means of an aerometer (pycnometer). The quality of back to 1867 strongly indicate that the winter of the salinity observations is uncertain and for that 1990 in southern Norway was the warmest in the reason only the temperature observations have been last 130 years (Anon., 1993). Further north the tem used. perature increase in the 1990s was significantly less, The NORWegian ECOlogical Model system and at the stations of Folda, Loppa, and Vardø the (NORWECOM) is a 3-D coupled physical, chemi 1960s show higher values. Comparing the mean cal, biological model system (Skogen et al., 1995). In decadal temperature for the 1990s with the mean for the present study, only results from the physical the period 1940-1989 the normalized deviation is module are presented. The physical forcing variables reduced from 1.28 at Flødevigen to 0.17 at Vardø are 6-hourly hindcast atmospheric pressure fields (Table 2, Figure 4A). The numbers of warm winters provided by the Flindcast Archive of the Norwegian in the 1990s reduce from 8 along the Skagerrak Meteorological Institute (Eide et al., 1985, Reistad coast to 1 at Loppa. Consequently, the effect of the and Iden, 1995), 6-hourly windstress derived from warm 1990s was gradually reduced from south to the pressure fields and freshwater run-off. The mean north. oceanic winter inflow (Jan-Mar) to the North In the surface layer there is also a significant Sea across the whole Orkney-Shetland-Norway negative trend in winter salinity along the whole section was calculated from the daily mean current coast up to the end of the 1980s with the strongest component. salinity reduction at the end of this period (Figure 2, The long-term temperature and salinity trends Table 1). The normalized salinity trend (the salinity along the coast have been normalized by taking trend/standard deviation) up to 1989 was highest the ratio between the long-term change and the along the southern and central coasts (Table 1) with a maximum at Stad of about -1.8. From the end of standard deviation for the same period. Similarly, the 1980s there was again an increase in the salinity the deviation in the mean value between two mean of the surface layer (Figure 2). Just as for the tem periods has been normalized by using the ratio perature, the salinity increase in the 1990s was between the temperature or salinity deviation for the markedly higher along the southern coast. However, two periods and the standard deviation for the first the negative long-term trend before 1990 resulted in period. The terms “warm winter” or “cold winter” the salinity of the surface layer along most of the mean that the temperature is above or below the coast in the 1990s remaining below the long-term long-term mean ±1 standard deviation.