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This paper not to be c::..ted without prior reference. to the author.

International Council for the Explo~ation of the Sea

C.M. 1977!c:36

- - .

Hydro~raphy Committee

Variations in the Norwegian coastal current off Arendal during 1975 and 1976

ABSTRACT

by F.E. Dahl

Institute of Geophysics University of Oslo" Norway

Comparison between mean monthly. freshwater discharges to Skagerrak~ surface salinity and computed( geostrophical volume transport through two sections off Arendal indicates a connection between variations in freshwater discharges and volume transports in the Norwegian coastal current in this area.

INTRODUCTION

Based on observations obtained by the International Skagerrak Expedition in summer 1966, Tomczak (1) presented in 19,68 calculations of the exchange o,f different wat.ermasses in Skagerrak (see mapJ, fig. 1). Tomczaks results are based on

calculations of the geostrophic current and are shown in Table 1.

2.

These results indicate that there exist great variations in the volume transports of different watermasses.

Both wind and freshwater discharge from the Baltic and Norwegian coast have been cosidered as driving forces of

, ^{' .}

the Norwegian Coastal Current (NCC). Without entering a

discussion about which of these mechanisms is the most important, this paper intend to show that there is a connection between

variations in the freshwater discharge to Skagerrak and varia- tions of the volume transport of NCC in Skagerrak off Arendal.

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Fig. 1. Map over Skagerrak, showing the Baltic current, the Jutland current ant the Norwegian coastal current.

(After Svansson

1975). .

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Volume transport through the Kristiansand-H8.nstholm section in m s (After Tomczak 1968) ,

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Norwegian Coastal Jutland Norwep:lan Total Mean transport

'rime Current Current 'rrench

, 22.-23.6 1966 -410 +54 +10 -346

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Observations and discussion.

The monthly mean freshwater discharges to ~kagerrak

are shown in table II, which shows significant variations in the freshwatl2r discharp:es. Maximum mean monthly runoff takes place in November, while minimum runoff takes place in September.

There are also secondary maxima in :H'ebruary, May and August.

The variations in the freshwater discharges are causing variations in hydrogranhic conditions. lfig. 2 shows

th~ monthly mean fre~hwater discharges and observed salinity at 2 m depth at a station 8 nautical miles off the Norwegian coast near Arendal. The correlation between the mqnthly mean freshwater discharge and the observed surface salinity is

rather poor. For 1975 the correlation coefficient is found to be -0.54 and for 1976, -0.37. But a closer examination of

figure 2 shows a rather good correlation between the variations in the runoff and the variations in the surface salinity.

Increase in the runoff is usually followed by decreasing surface salinity and vice versa. Since shortterm variations are not

Table 11 Mean freshwater discharge to Skag;errak Net from From the From south-eastern the Baltic 1) river K1ara 2) Norway 3) fJIonth 3 -1 3 -1 3 -1 m s m s m s Jan. 10903 604 560 Feb. 22902 645 -413 Mar. 14861 641 485 Apr. 20949 547 1466 May 22031 447 3659 Jun. 4861 332 3279 Jul. 3700 302 2203 Aug. 26437 392 1830 Sep. 14930 471 1775 Oct. -9223 506 1643 Nov. 32754 566 1350 Dec. 15496 583 859 Mean 15050 503 1627 Total 3 -1 m s 12067 23960 15987 21981 23944 . 8472 6207 - 28659 17176 -7074 34670 16938 17180

1 Hlyrtki 1954 2 )UNESCO 1969 3)To11an 1976 J::;-

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Fig. 2. Monthly mean freshwater discharges to , Skagerrak (qf) and surface salinity (S).

found in the deeper layers off the Norwegian coast in

Skagerrak (see Lj~en and Svansson (2», this indicates that shortterm variations in the hydrbgraphic conditions due to

6 ,

variations in the runoff are restricted to the surface layers, If such variations affect the density gradients, then the

geostrophic volume transport through sections in the area is also affected. In order to investigate such effects,

hydrographic measurements have been carried out in two sections near Arendal since 1975. (Danielssen and Iversen (3)~. The

distance between the sections is 17 nautical miles, and the outermost station at each section is taken 15 n.m, off the coast. From tne data collected in 1975 and 1976, geostroohic volume transport 'through the sections have been computed. The main problem of all such computations is to find the depth of

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no horizontal motion (zero layer). Svansson (4) warns stron~ly a~ainst assuming no motion near the bottom, because this may lead to results which do not agree with observations of other hydrographic propertjes.

Tomczak (1) assumed that the depth of no motion was to be found in the layer(s) where the vertical density gradient had a minimum, and also the horizontal density gradients between neighbouring stations reached a minimum. This method was tried on the present data, but the results were very often meaninf.\less.

Instead a method first proposed by Tully (5) has been used.

This method, which usually gives a depth of no motion just

above or in the upper part of a deep layer with relative uniform salinity, has been found to give good a~reement between direct current measurements and geostrophic calculations in the outer

part of the Oslof.1 ord (Dahl (6)). The method has <:.:.lso the

advantag~ that it is easily pro~rammable for computers. The results of the geostrophic calculations, usin~ the method proposed by Tully, are given in Fig.

3,

which shows the mean value of the geostrophic volume transportthrou~h the two sections above the level of no motion, and observed salinity at 2 m 8 ^n~m. off the coast.

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Fig. 3. Mean geostrophic volume transport (0) and surface salinity (s)

8 •

The correlation coefficient between the ~eostrophic volume transport. and the surface salinity for the observations during 1975 and 1976 is found to be 0.51. Figure 3 shows that in-

creasing surface salinity usually is followed by increasing volume transport and vice versa. Bearing fig. 2 in mind this indicates that increased freshwater discharge to Skagerrak usually is followed by decreased volume transport in the NCC and vice versa.

The immediate consequence of changes in the runoff to fjords and coastal areas is a chanp,e in the density gradients Thorpe (:7/) has shown by experiments that if the Richardson number

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where p is the density, u is the horizontal velocity and g is the acceleration due to gravity, becomes lar~er than 0.2,

then tu~bulence due to horizontal shear flow will not develop.

If the vertical density gradients become too large, then the impact of increased runoff will be restricted to a shallower layer than would be the case if the Ri-number was lower than 0.2, because the density gradients act against development of vertical turbulence.

It is beyond the scope of this paper to discuss

problems associ~ted with vertical transfer of mass and mo~entum

due to turbulence and the geostrophical response to such transfer, but the effect of such phenomenas are important to understand the variations in the volume transport in a coastal current.

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Mean geostrophic volume transport (Q) and ,depth of layer of no motion~(v )= 0).

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4

shows the geostrophic transport (already shown in fig. 3) and the depth ,of the layer of no motion,. This figure shows that incr~ased volume transport usually is ass0ciated with increased depth of the layer of no motio~l and vice versa.

Comparison of figure 4 with figures 2 and 3 indicates that the

10 •

depth of the layer of no motion is connected to the magnitude of the freshwater discharge.

CONCLUSION

-

The variations in the volume transport in the Norwegian Coastal Current off Arendal seem to be dependent

upon the freshwater discharges to Skagerrak in such a way that in- 'creased freshwater discharge will reduce the vertical turbulence

so that the coastal current will be restricted to shallower depths. Decreased freshwater d1schar~es weaken the vertical density gradients, so that the vertical turbulence increase, and the coastal current may extend to ~reater depths.

ACKNOWLEDGEMENT

-

This work is based on hydro graphic observations done by the Biological Station at Fl~devi~en, Norway.

REFERENCES

(1) TOMCZAK, G.

1968:

"Die Wassermassenverteilund und str5mungsverhaltnisse am Westausgang des Skagerraks wahrend den internationalen Ska~errak-Expedition im Sommer 1966~"

Deutsche Hydr. Zeit.

1968.

(2)

^LJ~EN, R. and A; SVANSSON

1972:

"Lon~-term variations of subsurface temperatures in the Skagerrak."

Deep-Sea Res. Vol

9

pp

277-277 1972.

11.

(3)

DANIELSEN, D.S. and S.A. IVERSEN

1976:

"Intern rapport angaende resipientunders~kelsen i Arendalsomradet i 1975.~

Tech.rpt. The Biological Station at Fl~devigen

1976.

(4) SVANSSON, A.

1975:

"Physical and chemical oceanography of the Skagerrak and the Kattegatt."

Report. Fish. Bd. Sweden, Inst. Mar.Res. No. 1,

88

pP.

(5)

TULLY, S.P.

1958:

"On structure, entrainment and transport in estu~rine embayments."

Jour. Mar. Res. Vol

17,

pp

523-535.

(6)

DAHL, F.E.

1977:

"A note on horizontal ~radients in fjords."

Jour. phys. Ocean.

1977

in press.

(7)

THORPE, S.A.

1973:

"Experiments on instability and turbulence in a stratified shear flow."

J. Fluid Mech.

1973

Vol.

61,

part

4;

pp

731-751.