ICES 1989 PAPER C.M. 1989/C:35 sess.a

HYDROGRAPHIC VARIABILITIES IN THE SURFACE WATER

by

Stig Fonselius SMHI, Oeeanographieal Laboratory, Box 2212, S-403 14 Göteborg, Sweden

ABSTRACT

The eountereloekwise eireulation of the surfaee water in the Skagerrak and the two main eurrents governing this are briefly deseribed. The eHeet of the prevailing winds are briefly diseussed. Results of the Swedish routine ex­ peditions in the Skagerrak from 1987 to the beginning of 1989 are de­ seribed and the transport of nutrients with these eurrents is demonstrated. The site of the Skagerrak front (the salinity front) is shown during different wind situations and the extension and direetion of the eurrent and the Baltie eurrent and the diffieulties in determining these are diseussed. During the winter the eurrents ean easily be traeed by help of their nutrient eontent, but during the summer the surfaee water is stripped of nutrients and only the salinity and temperature ean be used as indieators. It is eoneluded that a denser station network is needed for .a better understanding of the hydrographie situation, but that this requires more expedition time. Due to the fast ehanges of the hydrographie eonditions by the effeets of the winds, it will cause diffieulties to evaluate results aehieved during expeditions lasting several days. The SKAGEX, whieh is planned for 1990 will give an exeellent opportunity for synoptie observations. -- -- -. & ..

2

IritrOdLJctiOn

The surface water in the Skagerrak is governed by two main currents: 1) The Baltic current with its continuation, the Norwegiari coastal currerit, arid 2) The Jutland CurrEmt. Here tne expression "the Baltic current" will be used for the combination of the Baltic current and the NOrWegiari coastal current. .

The Baltic current originates from the Baltic sea. It mixes with water in the Belts and the Kattegat. It generally follows the Swedish coast in the Kattegat and the Skagerrak and then the Norwegian south coast. It has a relatively low salinity, 20-30 PSU. Fresh water is added from the rivers along the Swedish coast in ihe Kattegat. Fresh water is also added from the Glomma and Drams rivers in the Oslo fjord.

The Jutland Current originates from the southern .· It follows the German and Danish west coasts and gets addition of fresh water from the • German rivers. Its salinity is therefore a jittle lower than the salinity of the North sea water, around 31-34 PSU. Outside Hansthölm the Jutland current meets the current coming dir"ectiy from the area between the Orkney arid Shetlaond islands (the Dooley current). The two. currents partly mix and theretore the Jutland current aHen gets a higher salinity and a lower nutrient contemt in ihe Skagerrak. Normally it follows the Danisn co ast in the Skagerrak up to Skagen~ From there it may take different directions depending on the wind conditions. Finally it anyhow joins the Baltic current creating _a counterclockwise circulation of the sudace water in the Skagerrak. Fig~ 1 shows the. surtace current system of the Skagerrak according to Svansson (1975); Richardson et al. (1987a) have described the Jutiand current and its possible transport of nutrients to the Kattegat.

Both currents are very much aepending on the wind conditions. The Baltic current may duririg strang N to E and SE wirids spread over iarge parts of the Skagerrak surface. Dui'ing SW to W winds the Baltic current will be torced against the Swedish. and Norwegian coasts. N to E winds may almost com­ pletely block the Jutland currerit tram entering the Skagerrak. Strong Wand NW winds may entirely prevent the Baltic current from entering the Skager­ rak and everi in same cases the Kattegat, reversing the surface current in the Danish sounds.

The twö currents carry during the winter high concentrations of nutrients, • I' ': . " .. ,~:., '~ '." ( ...:, .' especially nitrate, to the Skagerrak. DurlOg the summer the nutnents are entirely consumed by the phytoplankton. The Baltic current gets its nitrate tram Danish arid .Swedish rivers discharglng in the Kattegatt; from the Öresuna and the Belt sea and trom the Ballic sea. More nitrate is added in the Skagerrak· tram Swedish arid Norwegian rivers. The Jutland current gets its nitrate. tram ttte rivers discharging in the North sea. The nitrate originates mainly tram fertilizers used in agriculture and from car exhausts. 3

Resuits of expeditionS

Frorn the beginning of 1987 the Oceanographical Laboratory of SMHI in Göteborg has begun to carry out 5 expeditions annually in the Skagerrak with the R/V ARGOS. Fig. 2a-d shows surface maps of the Skagerrak demonstrating the distribution of saliriity, temperaturei nitrate and· phosphate and fig; 2e-h shows vertical cross secti.ons through the mouth of the Skagerrak from OX0Y in to Hanstholm in Denmcük with the same parameters in March 1987. In the salinity seciion (Fig. 2e) 'Ne can see the Baltic current to the left at the Norwegian coast, going out from the Skagerrak. The salinity is in the cora below 29 PSU, but the current extends almost . t6 the Danish side. At the Danish coast we can see the Jutland current with a salinity beiow 34 psiJ entering the Skagerrak. The temperatLire secHon (Fig. 2f) shows th,ät the Saltic water has a very low temperature, around 00 in the' core. Also the Jutland current is cold, below . 10 . At station 7 in the section (Fig. 2e and f) Skagerrak deep water with a salinity of 35 PSU and a temperature above 60 extends up to the surface. The nitrate seetion (Fig. 2h) shows very low nitrate concentrations in the Baliic water and high concentrations (up to 10Jlmol/i) in the Jutland current. in the deep water the nitrate concentraiicin is between 8 to 9 ,J.mol/1. The phosphate seetion (Fig. 2g) shows the same featllres. Ollring the first day of the expedition when the eastern part of the expedition was conducted, the wind was E to NE with an average force of 8, m/s. Therefore the Baltic cLirrent was forced out from the Swedish coast emd the Skagerrak front (the Salinity front) extemded almost in a, northerly direction fram Skagen (fig. 2a). The wind changedthen to NW forcing the Baltic current out fram the Norwegian coast to the middle of the Skagerrak; TheJutiand current had been blocked arid was conceritrated to the Jammer Bight. The R/V DANA worked in the Jutland cLirrent area 24 Febru,ary to 2 March 1987 (Richardson et al. 1987b). They report 0.6 Jlmol/I phosphate along the Danish coast up to Skagen and 10 Jlmol/I nitrate up to .

In the, begirining of April (fig. 3a-d arid 3e-h)) we find almost the same hydrographic conditions. The surface ternperatLire is now more homogenaus and warmer. But the nitrate concentration in the Jutlarid current outside Hanstholm Is extremely high, up to 30 Jlrnol/1. The concentration in the . Baltic current is rnuch lower, below 3 Jlfnol/1. The phosphate values are low, below 0.4 Jlmol/I in the Baitic current and around 0.6 ,J.moill in the Jutland current. During most part of the expedition we had strang E winds between 8 , ',.... I" • • ~., .. .,. ". '. " ..., \. • '.. and16 m/s. Thls obvlously prevented the Jutland current from entenng the Skagerrak; Oniy a small part could slip up along the Danish coast to Skagen; The extremely high nitrate values are the best indicators for the Jutland current. ~ small dome with 34 PSU water is visible in the central part of the Skagerrak. The Skagerrak front goes fram Skagen towards N.

The next expedition was carried out in june 1987 (fig. 4a-d arid 4e-h). Strang ....

4

NE winds had spread the Saltlc water over the whole ~~rface of the" Skager­ rak. A salinity front (probably the Skagerrak front) could be observed at the section OX0y-Hansthohn. The Jutland current was absent in the Skagerrak. During the summer period His nutrierit values are, generally very iow due to the primary production of plankton algae. The nitrate values vere low ovar ihe whole surface.' Close to the Danish coast nitrate values between 1 and 2 J,imolll could be found. Also the phosphate values vere somewhat elevated there, above 0.1 J,imolll.

In August (fig. 5a;.d and 5e-h) the wind was st the beginning SE; but changed at the Hanstholm-Oxoy section 10 SW to. W; Because the stations in the area of the Jutland currerit were run the first day with SE winds the Jutland current can only be deteeted to ihe Jammer bight. The effect of the wind change 10 W had still not forced the" Baltic current water back from the ceritral Skagerräk, when the stations there were taken. The nitrate values were high at the Danish coast, but very lew in the ether parts of the surface water. The phosphate values ware verY,low. The Skagerrak front extended from Skagen to NNE probäbly because the west wind häd ceased when the stations in the eastern part of the Skagerrak were taken. The core of ths Baltic curren1 extended. along the NOrWegian coast.

Fig. 6a-d and 6e-h show ttie conditions in november 1987. During the seccind day of the expedition' N winds with a force of 5 m/s blew out the Baltic current water from the Norwegian coast. The dcüne with higher saline up­ welling is cleariy visible in the middle of ttle' Skagerrak but the salinity is' only 32 PSU. The Jutland current can oe distinguished along the Danish coast due to its high, content of nutrlents The Baltic current can clearly be seen along tha Swedish and Norwegian coasts. The nitrate content of nie Baltic current is oetween 3 arid 4. J,imolll. In The Jutiand current the nitrate content is around 8-9 J,imolll. The phosphate concentration is arourid 0.5 J,imol/I in ths Baltic currerit arid around ,1.1, j.Üri6/1 in the Jutland current. Ttle Skagerrak front goes from Skageri towards NNE.

In January 1988 we again' had winds from E and SE. The Jutland current was riow extencling deep into the Skagerrak.' It was forced towards N and NE from Skagen (fig. 7a-Ci arid 7e:..f). The nitrate'content was 9 J,imol/I NE of Skägen and the phosphate content. was there above 1 J,irrlOl/1. The dome with Nor'th sea water' (abave 33 PSU) extends irito tt1e eastern part of the Skagerrak. The, Skagerrak front extends from Skagen towards NNE. During the April crulse (fig. Ba-d arid 8e-h) the wind was rioftheriy with wind forces up to 12 rn/s. The Skagerrak front extended from Skagen towards Göteborg. The Baltic current almost ceased arid the Jutland current was also. forced back. O~ly a very narrow strip exta~ded up to Skageri very close to the Danish coast. Low saline, water from the mouth of tha Oslo fjord covered the rest of the Skagerrak surface. 5

in May the wind was southerly until we begun the Oxoy-Hanstholm section cit the Norwegian coast. Low saline water (20-28 PSU) was covering the northern and central Skagerrak (fig. 9a-d arid ge-h). The wind then, shifted to NE to E, allowing the Jutland current to enter the Skagerrak at Hanstholm. Almost no nutrients were found in the surface water due to the plankton production, witn the exception of the station outside Hanstnolm.

When the Skagerrak program was run in August (tig. 10a-d and 10e-h) we had strang southerly winds the first day fo~cing the Skagerrak front towafds N-NE tram Skagen.· The second day the wind vas weak and varying. The third day when the Oxoy-Hanstholm sectiori was run the wind changed to SE. It is not possible to find the jutland current. The nutrient concentrations were still very low and could not give any indication of the presence of the Jutland current.

• The pragram was 'again run in November 1988 (fig. 11a-d and 11e-h). NOw the Skagerrak surface was to large parts covered by relatlvely high salinewater (30-34 PSU) in a very thin layer. The weatner canditions were bad with strang W winds inostly up to 12-14 m/s. The Skagerrak front extended from Skagen to NNE, but outside "Göteborg tne saiinity was still above 28 PSU and outside Lysekil 31.9. The salinity in the Jammer bigh! was above 34 PSU and no traces of the Jutland current could be detected. _One would expect eievated nutrient values in tne Jutland current, but at the station c10sest to the Danish coast at the Oxoy-Hanstholm section somewhat lower nitrate values could be seen. Along the Norwegian coast water witti a salinity somewhat below 30 PSU could be detected. Obviously the strang W wind prevented the Baltic current, fram entering the ,Skagerrak. The water outside the Norviegian coast consists of remains of the Baltic current, slightly • diluted with low saiine water fram the Norvitegian rivers. Fram 1989 we have results fram the expedition 27 February to 2 March. This expedition (fig. 12a-d and 12e-h) shows a clear Jutland current along' the Danish coast up to Skagen. The surface salinlty is between 31 and 32 PSU: The current turns to the north tram Skagen. The phosphate values are somewhat higher along the Danish coast than in the rest of the Skagerrak. The most interesting result is the nitrate distribution. Now we find extremely high nitrate vaiues (abave 20 Jlmol/l) in the Jutland current up to Skagen., The Oxoy-Hanstholm section, however, does not show such elevated nitrate values at the Danish coast. Obviously, the current go es closer to the share or it is a pulsating curre.nt, which has been suggested by same scientists. During the Oxoy-Hanstholm section the wind was SSE 11 m/s and the current at the stations in the Jammer bight was running paralleli to the coast. going in to the Skagerrak with an estimated speed of 2 knots.. CI ase to the Norwegian coast we can see the Baltic current with nitrate values araund 12-16 JlmollJ. 6

Conclusions

The results of the expeditions are not easily interpreted. Generally it seems that wirids from N-E tend to spread the brackish water o~ the Baltic eurrent over the surfaee ot, the Skagerrak, preventing the Jutland eurrent tram entering the area. Winds tram S-W tend to force the Baltic current against the S~edish eoast, allowing the Jutland current to enter the Skagerrak. N and W winds will also weaken the Baltie eurrent and in eases wiui strang and persistent winds even reverse it in the Kattegat. The atm6spherie pressure over the North sea and the Baltie sea also play an important. role by torcing water out and in to the Baltie sea (Svansson 1975). Another important faCtor for the surface salinity conditlons in the Skagerrak is the supply of fresh water from the rivers during different seasons (Svansson loe cit). But for a belier uriderstanding we need more data. A denser station network js obviously needed. The difficulty is that then also more tirne is needed. The changes in the hydrographie conditions occur very fast due t6 the • strong irifluence of the wirid on the current system. \Ne often encountered completely different condiiiolis eit the beginning ~md the end of an expedition. Synoptic rrieasurements using several ships, would certainly give a much more ciear picture of the condi- tions. The plarined SKAGEX in 1990 will give us an exeellent opportunity for synoptie measurements.

References

Carlberg, S. 1987a. Rapport frän oceanogratisk expedition i Skagerrak, mars 1987. Expeditiorisrapp. SMHI, Oeeanogr. Lab. Göteborg (Stericil). • Expedition Reports, SMf-U Oceanogr. Lab. Göteborg (fv1imeo). Carlberg, S. 1987b. Rapport trän oceanografisk expedition med miljökvali• tetsövervakning (PMK) i Skagerrak, 86husläris fjordar, Katte- gatt, Öresund, Östersjön oeh Bottniska vikEm med U/F Argos 25 maj-17 juni 1987. Ibid.

Fyrherg, L,. 1987a. Ra.pport träri. oeeanogratisk expedition Kattegatt oeh Skagerrak, april 1987. Ibid.

Fyrberg, L. 1987b. Report ot the oeeanographie expedition with R/V Argos in the Skagerrak, the Kattegat arid the Swedish fjords, 17 - 21 August 1987. Ibid. 7

Fyrberg,. L. 1987c. Rapport frän oceanografisk expedition med miljökvalitetsövervakning, PMK, Östersjön, Öresund och Västerhavet 2 - 19 november 1987. Ibid.

Fyrberg, L. 1988a.. Rapport frän oceanografisk expedition med U/F Argos i Östersjön, Kattegatt och Skagerrak 18-27 januari 1988. Ibid.

Fyrberg, L. 1988b. Rapport frän oceanografisk expedition med miljökvalitetsövervakning (PMK) i Skagerrak, Kattegatt och Östersjön med U/F Argos. 'maj-juni 1988. Ibid.

Fyrberg, L. 1988c. Report of the oceanographic expedition in the Skagerrak, the Kattegat and the Baltic proper, August 1988. Ibid.

• Fyrberg, L. 1988d. Report of the oceanographic expedition in the Gulf of Bothnia, the Baltic proper, the Kattegat and the Skagerrak, November 1988. Ibid.

Fyrberg, Land J. Szaron. 1989. Report of the oceanographic cruise with R/V Argos in the Skagerrak and the Kattegat, 27 February - 2 March 1989. Ibid. - Richardson, K., T. Jacobsen and O. Vagn Olsen. 1987a. The Jutland Current: A mechanism for transporting nutrients to the Kattegat. Proc.15th Conference of the Baltic Oceanographers, Copenhagen1986 Vol. 11.

Richardson, K. and O. Vagn Olsen. 1987b.· Winter nutrient concentrations and primary production in the eastern North sea. ICES CM1987/C:23

• Svansson, A. 1972. Canal Models of Sea Level and Salinity Variations in the Baltic and Adjacent Waters. Fishery Board of Sweden, Series Hydrography, Report No. 26.

Svansson, A. 1975. Physical and chemical oceanography of the Skagerrak and the Kattegat. Fishery Board of Sweden Inst. Marine Res. Rep. No 1.. 8

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Fig. 1 Map showing the surface currents in the Skagerrak. (Svansson 1975). 9

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h Fig. 2. Surface maps of the Skagerrak and the Okso­ Hanstholm section showing salinity, temperature, phosphate and nitrate distribution in March 1987. 10

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_ r NO, -~ "'"01/1 -S~:=T umo1/1 g h Fig. 4. Surface maps of the Skagerrak and the Oksö• Hanstholm section showlng sallnity, temperature, phosphate and nitrate distribution in June 1987. 12

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h Fig. 5. Surface maps of the Skaberrak and the Oksö• IIanstholm section showlng sal1nlty, temperature, phosphate and nitrate distribution in August 1987 13

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Fig. 6. Surf'ace mapl'; of the Skagerrak anrl the Okl';Ö• Hanstholm section showing salinity, te~perature, phosphate and nitrate distribution in November 1987. 14

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Fig. 7. Surface maps of the Skagerrak and the Oksö• Hanstholm section showing salinity, temperature, phosphate and nitrate distribution in January 1988, 15 I -- --1 !

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Figo 8. Surface maps of the Skagerrak and the Oksö-: Hanstholm section showing salin1ty;-temperature, phosphate and nitrate distribution in April 1988. 16

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FigQ 12. Surface maps of the Skagerrak and the Oksö• Hanstholm section showing salinity, temperature, phosphate and nitrate distribution in February-March 1989.