SMHI OCEANOGRAPHY Nr 38, 1990 I ; Norw~y I ! ! I\ '~, .,---- .... i\ I 1 I 1 Il 1 1 I 'j~ /~ - -f1/ \ i\\' , , {_\\ /f __) '.I ,, -v' SKAGERRAK - the gate-way to the N orth Sea SMHIOCEANOGRAPHY Nr 38, 1990 SKAGERRAK - the gateway to the North Sea Stig H Fonselius SMHls tryckeri Norrköping 1990 Contents Page 4 Preface 5 Boundaries and topography 7 Water balance 10 The water's circulation 12 Waves 13 The water's stratification 14 Salinity and temperature distribution 16 Oxygen conditions 17 Nutrient salt conditions 21 The hydrographic conditions in some of the important fiords -Oslo Fiord 23 -Ide Fiord 25 -Gullmars Fiord 28 -Uddevalla Fiords 29 Lit era ture 4 Skagerrak - the gateway to the North Sea Preface The blooming of poisonous algaes, the graphic conditions in the open Skagerrak epidemic of deaths among the seals and have been regarded as oceanic and the the influence of the North Sea have all Skagerrak has not been considered to be given rise to grave concern about the exposed to pollution or eutrophication. increasing risks for the Skagerrak. A summary of the hydrography of the The Skagerrak is the least investigated Skagerrak is hereby presented as an in­ area of all our sea regions. The coastal troduction to the increasing investiga­ regions and particularly the fiords have tion and monitoring of the environmen­ been given much attention but the hydro- tal eon di tions of the Skagerrak 5 Boundaries and topography The Skagerrak connects the N orth Sea to The boundary between the North Sea the Baltic Sea and is regarded as being and the Skagerrak runs from Hanstholm part of the North Sea, whereas the Katte­ on Jutland to Lindesnes on the south gat is understood to be part of the Baltic coast of Norway. The surface area of the and is included in the concept when the Skagerrak is 32 300 km2, the volume 6780 name Baltic Sea is used. Figure 1 shows a km3 and the average depth 210 meters. map of the Skagerrak. Norw y II ''\ -- ' ' ' ,'I•' 'I ( __ \ \ / ) " __ ) ',I I ~ i f I I - ~ I ~/ / ,/ I Fig 1. Map showing the depths af the eastern North Sea and the Skagerrak. 6 The relatively large average depth de­ A typical fiord is relatively deep, but has pends on the Norwegian Trench, which a shallower sill at the mouth. Usually a follows the N orwegian coas t and dee­ river or large stream runs into the heads pens as it nears the Skagerrak. The Nor­ of the fiords. Oslo-, Ide- and Gullrnars­ wegian Trench reaches it' s greates t depth, fiords are typical sill fiords. The Oslo roughly 700 m, about 35 nautical miles Fiord has it's sill in the part of the inner due east of Grimstad. The sill ( the dee­ fiord that is near Dröbak. This sill is 20 m pest connection) between the N orwegian deep. The maximum depth inside the sill Trench and it' s continuation nortwards is 164 rn. In the Ide Fiord the sill depth is to the Norwegian Sea (Atlantic Ocean) is 9 m and the maximum depth inside the 270 m deep and lies outside Utsira. sill is 42 m. The Gullmars Fiord' s sill depth is 38 m and it's maximum inside Eastwards the Norwegian Trench stret­ depth is 119 m. ches almost to the Swedish coast before continuing southwards along this coast untilitleads intotheKattegatbythe Deep Furrow. With the exception of the sill in the Nor­ wegian Trench, one cannot discern a hy­ drographic border between the Skager­ rak and the N orth Sea. The boundary be­ tween the Skagerrak and the Kattegat is usually drawn from the Skaw reef to the Paternoster Lighthouse. However within the Helsinki Commission, this boundary is drawn from the Skaw along the latitu­ de 57°44'8 N. As the first named bounda­ the open sea ry has been used for calculating the surfa­ ce and vol ume of the Skagerrak, this is the one we will use here. Along the Swedish and N orwegian Fig 2. A typical fiord coasts there are many deep fiords that stretcha long way inland. The biggest of these is the Oslo Fiord. The Ide Fiord is the border between Skagerrak is the least investigated of all Norway and Sweden. The most irnpor­ our sea regions. The coastal regions and tant Swedish fiord is the Gullmars Fiord. particularly the fiords have been given South of this fiord isa whole system of much attention but the hydrographic fiords between the mainland and the conditions in the open Skagerrak have been islands of Orust and Tjörn. This fiord regarded as oceanic and the Skagerrak has system is called the Uddevalla fiords. In not been though t to be a tar get for poll u tion reality they are not true fiords but rather or eutrophication. a system of semi-enclosed bights connec­ ted to each other by means of narrow sounds. They are however comrnonly called fiords. 7 Water balance It is difficult to find information on the The yearly precipitation is reported to be evaporation from the Skagerrak' s surface 700 mm (Grimås and Svansson 1985). This in the available literature. In the follo­ represents about 23 km3• The influx of wing section I have therefore tried to river water has been calculated by Svans­ estimate the evaporation by using the son (1975). information obtained from the Norwe­ Table 1 shows the average water dischar­ gian, Danish and Swedish hydrological ge of the various rivers. coastal stations. TABLE 1 Rivers running out into the Skagerrak Catchment area km2 Water discharge m3 /s Örekilsälven 1327 21.0 Diverse Swedish rivers 1543 24.0 Tista 1550 '23.7 Glomma 41284 708.0 Mosseelv 690 10.7 Dramselv 16020 313.0 Numedalslågen 5513 115.7 Skienselv 9975 298.0 Toke 1168 33.0 Vegårdselv 491 15.6 Nidelv 3907 124.5 Tordalselv 1700 63.1 Otra 3539 149.0 Mandalselv 1746 87.0 Diverse Norwegian rivers 11717 249.0 Total 102870 2245.3 Supply of river water to the Skagerrak 1911-1950: 2245.3 m3 /s = 71 km3 /year Table 2 shows the water balance of the Skagerrak TABLE2 ~ km3 /y__ear Removal km3 /y__ear Fresh water From Kattegat 515 Evaporation 13 Precipitation 23 River water 71 Total 609 13 The contribution of fresh water to the North Sea is thus: 609 - 13 = 596 km3 /year 8 It is usual to call the whole system for the " Bal tic curren t" and this is the term I will P = Precipitation E = Evaporation· be using throughout. R = River water supplement U = Outflow of water I = lnflux of water (U-l)k = Net transport from the Kattegat. The Baltic current lies above a very large (U-S)s = Net transport from the Skagerrak 3 Ucr =Totaltransport from Kattegat and Skagerrak respectively. water transport of at least 15 000 km / ler= Total transport to the Skagerrak from the North Sea year that mainly comes from three direc­ tions: a) From the English Channel, as a 13 23 surface current sweeping up the West p coast of Denmark. This is called the "Jut­ Us , Us, 15500 2500 land current". b)From the Orkney /Shet­ .. ---- ---- u ~ u -:;; ~ .... bO lands area, streaming more or less east­ ,B QJ .2l .... I U - I 15 bO I U - IIK -:;; 0 .. wards, and c) From the outer part of the 596 ..: 514. 6 ~ z <fl QJ QJ QJ I ~ N orwegian Trench in the form of a deep ~ ~ f-< f-< f-< 2000 current. As a rough guide one can say I Be IS 000 that the greater part of the water that flows into the North Sea temporarily oc­ 71 cupies the Skagerrak. Fig 4 shows the surface currents of the Fig 3. Box diagram showing the water balance in Skagerrak according to Svansson (1972). the Skagerrak Fig 3 shows the water balance of the Ska­ gerrak in the form of a box diagram. The net gain of fresh water from the Kattegat is 514.6 km3 /year. Naturally this is blen­ ded with salt water and orginates partly from the Baltic and partly from rivers and precipitation in the Kattegat. A deep current of Skagerrak water to the Kattegat is blended with brackish water, which in the surface layer flows out from the Baltic. A surface current of about 2500 km3 /year of Baltic (brackish) water from the Kattegat is thus formed. This current, called the Baltic current, continues north­ wards along the Swedish coast until it joins the Norwegian Coastal current off Norway. The latter current is reinforced with fresh water, mostly from Glomma Fig 4. Map showing the surface currents in the and Dramselv, before continuing out to Skagerrak (Svansson 1975). the Atlantic as a rather narrow coastal stream. 9 The total transported amount is thus 15 Fig 5 shows a cross section of the Skager­ 000 km3 /year influx from the North Sea rak from Norway to Denmark which plus 2500 km3 /year influx from the Kat­ demonstrates the condition of the cur­ tegat, of which about 2000 km3 /year re­ rents (Rodhe 1987). We can see the Baltic turn, making the total outflow of 15 500 current flowing out along the N orwegian km3 /year. coast as a surface current. An outflow is The tumover time for the Skagerrak' s also to be found under the 270 m level. water has been calculated as being a few The influx of the Jutland current along weeks in areas of strong currents and the Danish coast, with a maximum velo­ somewhat longer in coastal areas or in the city on the surface and under the 270 m water at a lower depth than 270 m in the level indicates an anti-clockwise circula­ Norwegian Trench (Grimås and Svansson tion, both on the surface and in the depths.