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And Adjacent Seas Rapp. P.-v. Réun. Cons. int. Explor. Mer, 185: 150-161. 1984. Variations of salinity at Dutch lightvessels in the Southern Bight of the North Sea L. Otto Department of Oceanography Royal Netherlands Meteorological Institute (KNMI) P.O. Box 201, 3730 AE De Bilt, The Netherlands Variations of salinity at Dutch lightvessels in the southern North Sea are discussed, considering different time scales. For long-term variations (£ 1 year) observations from a coastal station near Den Helder are included, so the salinity record can be given back to 1860. The long-term variations are parallel for the whole area, including the German Bight, with some exceptions. At tidal periods the effect of the “Texel-Miihle” phenomenon (Dietrich, 1953) plays a rôle at the lightvessels “Texel” and “Terschellingerbank”. At monthly time scales the effect of variations of the flow through the Strait of Dover is found at LV “Noord Hinder”. Because of the various meteorological factors that influence salinity at different time scales the response of this property to climatological variations can be expected to be rather complicated. Les variations de la salinité chez les bateaux-phares neériandais dans la partie méridionale de la Mer du Nord sont discutées, en considérant différentes périodici­ tés. Pour les fluctuations à long terme (> 1 an) les observations de la station côtière de Den Helder y sont ajoutées, de sorte que la série d’observations a été retracée jusqu’à 1860. Les variations à long terme sont parallèles pour toute la région, y inclus la baie Allemande, à part de quelques exceptions. A la période de la marée l’effet du phénomène dit “Texel-Mühle” (Dietrich, 1953) est de conséquence chez les stations “Texel” et “Terschellingerbank”. Aux périodes mensuelles l’effet des variations du courant à travers du Pas de Calais est observé chez la station “NooTd Hinder". A cause des plusieurs éléments météorologiques influençant aux différentes échel­ les de temps la salinité, sa réponse aux variations climatologiques semble être bien compliquée. coastal stations as well, we may refer to van der Hoeven Introduction (1982). At the Dutch lightvessels in the Southern Bight of the A problem, however, is that for nautical reasons the North Sea the salinity has been determined regularly lightvessels have been moved from time to time to dif­ since 1906, albeit with some interruptions during the ferent positions. Moreover after 1948 the two light­ years 1914-1920 and 1939-1948 because of war condi­ vessels “Schouwenbank” and “Maas” were replaced by tions. a lightvessel at an intermediate position: “Goeree” . The data have been published since 1968 in the year­ Depending on the local situation even minor shifts in books of the lightvessels edited by the Royal Nether­ position may mean significant changes in the hydro- lands Meteorological Institute (KNMI)1, and monthly graphic regime, as the horizontal gradients of tempera­ mean values have been published regularly in con­ ture and salinity in the coastal zone are considerable. secutive volumes of Annales Biologiques since 1968. In Figure 1 and Table 1 we indicate the positions of The data constitute a valuable time series, and in the the lightvessels in the relevant period. For those groups following they will be reviewed and discussed. For the of positions where the hydrographic conditions were complete series of monthly means of temperatures and considered sufficiently homogeneous a “combined” salinities at the lightvessels and at a number of Dutch position is indicated. Positions where salinity data are only available for brief periods and that are not dis­ cussed in the following, are not given; for those data the '“Meteorologische en oceanografische waarnemingen verricht aan boord van Nederlandse lichtschepen in de Noordzee”. reader is referred to the report by van der Hoeven KNMI. nr. 141. (1982). 150 Figure 1. Positions of the lightvessels dis­ cussed (see Table 1). G = “Goeree” ; H = “Haaks”; M = "Maas”; Nh = “Noord 50*- Hinder”; S = “Schouwenbank”; T = “Texel”; Tb = “Terschellingerbank”. Den Helder: Position of Den Helder series 30’- (Marsdiep tidal inlet). 54' 54 TbC 30*- TbB TbA 10*- 53c (Den Helder -5 3 40*- 30*- IJmuiden Lowestoft 20*- msterdam' 52° - - 52' Rotterdam 50*- Nh 30*- Oostende Inspection of the mean monthly distribution of the sali­ nity (see ICES, 1962) shows that high-salinity (approx­ The variability of the salinity imately 34-75 units) water enters the Southern Bight of In a study of the temperature and salinity variations at the North Sea through the Strait of Dover, forming a the lightvessels in the German Bight, Becker and band of high salinities in the middle, turning to the Kohnke (1978) discuss the variability at various time northeast at latitude 53°N. The outflow of the rivers scales. They conclude that the salinity variations at time Rhine and Meuse manifests itself in a strip of low- scales > 1 year are identical for the whole German salinity water (down to < 30 units) stretching along the Bight. For the Dutch lightvessels it appears justified to Netherlands coast to the northeast. assume a similar synchronous variability at the longer The lightvessels, except “Noord Hinder” , are situ­ time scales. On the other hand, as will be shown, local ated in the transition area between those two water conditions influence the variability at shorter time masses. “Noord Hinder” lies close to the axis of the scales. Strait of Dover inflow. The mean monthly values as published in Annales Fluctuations of salinity at longer time scales have Biologiques are in the intermediate time range, and it is been studied by Schott (1966) and by Dickson (1971). thought useful to pay special attention to the signifi­ The former considers the variations of the run-off as the cance of their variability. So we will discuss the vari­ direct cause of these fluctuations. The latter suggests ability at, successively that variations in the inflow of Atlantic water may also be a significant factor, either by variations in the a) longer time scales ( ^ 1 year) strength of the inflowing current or by variations in its b) shorter time scales (< 1 month) salinity, while changes in the precipitation-evaporation c) intermediate time scales. budget may be significant as well. 151 Table 1. Positions of lightvessels discussed in the text. Position “Combined” position “Noord Hinder” ........................................... ................. 1906-1914 51°35-4'N 2°36-6'E 51°37'N 2°35'E 1920-1928 51°35-4'N 2°36-6'E 1928-1937 51°38-4'N 2°33-8'E 1937-1939 51°39'N 2°34'E 1953-1972 51°39'N 2°34'E 1972-1981 51°39'N 2°34'E “Schouwenbank” ......................................... ................. 1912-1916 51°47-1'N 3°27-4'E “Goeree” A 1950-1952 51°52-3'N 3°34-5'E 51°52-5'N 3°35'E 1952-1953 51°52-7'N 3°35-4'E “Goeree" B .................................................... ................. 1953-1955 51°55'N 3°40'E 1955-1962 51°55-8'N 3°39-7'E 51°55-5'N 3°39-5'E 1962-1971 51°55'N 3°39'E “ Maas” ............................................................. ................. 1912-1915 52°1-6'N 3°53-5'E “Haaks” ........................................................... ................. 1906-1914 52°57-8'N 4°18-3'E 52°58'N 4°19'E 1922-1939 52°57-8'N 4°19-2'E “Texel” A ...................................................... ................. 1947-1951 53°5'N 4°31-5'E 53°6'N 4°31'E 1951-1954 53°7'N 4°30'E “Texel" B ......................................................................... 1954-1971 53°1-5'N 4°21-5’E “Texel” C ......................................................................... 1971-1975 53°00'N 4°23'E “Terschellingerbank” A ............................................... 1906-1914 53°27'N 4°51-6’E 1916-1917 53°27'N 4°52'E 53°27'N 4°51-5'E 1923-1925 53°27'N 4°51-5'E “Terschellingerbank” B ................................................. 1925-1928 53°29'N 4°51-5'E 1928-1930 53°27-2'N 4°47-4'E 53°27'N 4°47-5'E 1931-1938 53°27'N 4°46-7'E 1938-1939 53°27'N 4°47-5'E “Terschellingerbank” C ................................ ................. 1951-1954 53°29-7'N 5°7-6'E 1954-1956 53°29'N 5°7'E 1956-1965 53°29'N 5°8'E 53°29'N 5°7-5'E 1965-1970 53°29-5'N 5°8-5'E At very short time scales the effect of the run-off is nity determination, but during sampling and transport largely filtered out in the estuaries and the near-coastal of the samples from the lightvessel to the laboratory zone. Variations in the advection (including that by tidal there is always a chance of some mishap. In practice currents) play an important rôle. therefore extreme values have been discarded, except At the intermediate (monthly) scales variations in when there was reasonable evidence that they were both run-off and advection (by residual currents) affect real. However, judgement in a positive or negative the salinity. sense always has a subjective element. This brings us to the variations of the residual cur­ With this in mind we may look at the distribution rent. These variations are caused primarily by the varia­ curve given in Figure 2. It should be noted that the tions of the windstress over the North Sea and adjacent frequency distribution is different for different tidal regions, as follows from observations (e.g. Ramster et phases (see “Salinity variations at shorter time scales”). al., 1976) and from numerical models (e.g. Prandle, 1978 a). Horizontal density gradients are of minor im­ portance in determining the residual circulation (Pran­ dle, 1978 a). Salinity variations at longer time scales Before continuing, some notes on the salinity varia­ The turn-over time of water in the Southern Bight is of tion in a statistical sense: The salinity in the sea varies the order of 0-1-0-2 years (1-216 months) (ICES, between a maximum value (for the North Sea some­ 1983).
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