Harald Loeng, Svein Sundby and 0ivind 0stensen Institute of Marine Research P.O. Box 1870 Nordnes 5024 Bergen, Norway

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1 nternational Council for the Exploration of the Sea C.M. 1989/C:19 Hydrography Committee DRFTNG ARooS BUOYS N THE BARENTS SEA by Harald Loeng, Svein Sundby and 0ivind 0stensen nstitute of Marine Research P.O. Box 1870 Nordnes 5024 Bergen, Norway ABSTRACT During the summers of 1986 and 1988, six and ten Argos satellite-tracked drifting buoys respectively, were released in the western Barents Sea. The buoys were equipped with a 11 m 2 sail at 30 m depth. The buoys were released during spring or early summer, and were drifting for periods of months. The aim of the study is to map the general surface current pattern and especia~ly to study the mesoscale current features important for the transport and dispersion of plankon organisms and passively drifting pelagic fish larvae. The present paper presents the tracks of the buoys. The vortex features appearing from the tracks are discussed and compared to other information of the circulation pattern. NTRODUCTON As apart of early juveniles surveys in the southwestern Barents Sea and as an attempt of mapping the general circulation conditions in the western Barents Sea, drifting satellite-tracked Argos buoys have been periodically released during spring and early summer since n 1986 and in 1988 a larger number of buoys (six and ten respectively) were released. These experiments enabled us jointly to analyse the upper layer circulation in the western Barents Sea. Emphasis is laid on the vortex structure appearing from the buoy tracks, since the vortex structure, in particular, plays an important role,for the transport and dispersion of pelagic fish larvae and juveniles in the southern part, and for the identification of important regions for bottom water formation in the nortern parts.

2 2.. Sundby (1984) has shown that the drifting tracks of buoys are strongly influenced by the bottom topography, especially above bank areas and troughs. The most dominant features in the investigated area, are the banks of Troms0 flaket and Nordkappbanken and the through of ng0ydjupet (Fig. 1). The inflow of Atlantic water takes place through the broad Bear sland Channel ' 34' 73 72,.\ Ị,,,,,...400, \,.. ',-... '\ '.. 3~~ \...'... ~..--:' :'".300'-"'''-,.,",:.' ".,' '---.,:::) (.. ~..." ~:>... ~:~ ~:::~:"..,-,-,:..: J )NORDKAPP BANK v (', \... i r'" _ ~<~" tt i.. NGB y- "" :. '..' \;,.".? DJUPf! -'.. - :' ' Fig. 1. Main features of the bottom topograpy in the southwestern Barents Sea. MATERAL The buoys were constructed to withstand severe stranding which frequently occurs to buoys released along the coast of Norway. The design of the buay system is shawn in Fig 2. The battery package. the transmitter and the antenna were placed in a cylindrical PVC housing. The top plate of the housing was made fram 'stainless steel. The housing fits inta a spherical buoy which has a diameter of 90 cm. The buoys are made from Divinyceil material which has very good shock absorbing properties. The buays were attached to a sail at 30 m depth. The sail was 240 cm across and the heigth was 450 cm. which correspond to 10.8 m 2 Ta reduce heave of the sail by wave action, 4 trawl floats are attached to the rope at 4 m intervals near the buoy.

3 3 Fig. 2. The design of the cm --~" buoy system The buoys were positioned on the average 10 times per day, which partly revealed the semidiurnal tidal loops. For 1988 all positions have been used in order to plot the drifting tracks, while only one position a day is used in the plots from The time of drifting for each buoy is shown in Table 1. t varied from 0.5 to 5 months. n 1986, one buoy, and in 1988, two buoys, ran aground or were picked up by fishermen, but they were all released again at a new position a few days later RESULTS Fig. 3 shows the vertical distribution of a t in the upper 100 mof the water column in Atlantic and Coastal waters during summer f we neglect the numerical values, the development of at-profiles were almost the same in both water masses. n early May the upper m was wel mixed. The stratification of the watermasses usually starts to develop during May andin the beginning of July, the water masses were wel stratified. This stratification continues to develop until the last half of August. n early September the stratification starts to break down, and there was a rather weak stratifi~ cation in the upper 30 m in the Coastal water at that time (Fig. 3). One month later the whole upper 40 m was wel mixed. Even if the water masses change their t-s characteristics north and eastwards, the vertical profile of a t does not change much. The development shown in Fig. 3 is therefore representative for most of the areas where the buoys drifted in 1988, and also in 1986.

4 4. Table 1. Argos drifters used in 1986 and Argos-D Released in Released Recovered Number position of days Argos 1580 N 71 00' E 18 00' Argos 1576 N 71 15' E 19 00' Argos 1575 N 71 40' E 18 30' Argos 1575 N 71 00' E 26 10' Argos 1578 N 72 05' E 19 20' Argos 1558 N 72 30' E 19 20' Argos 1583 N 73 15' E 18 58' Argos 1577 N 70 27' E 17 08' Argos 1580 N 70 27' E 17 40' Argos 1583 N 72 35' E 20 00' Argos 1584 N 72 25' E 20 00' Argos 1587 N 72 15' E 20 00' Argos 1587 N 72 15' E 23 00' Argos 1584 N 72 15' E 27 00' Argos 1558 N 73 00' E 23 00' Argos 1576 N 74 57' E 28 32' Argos 1586 N 75 15' E 32 30' o :: :J:... Q. c ,6,8 27.0,2.t., , \ 1 \ : \ : ". '" ", 10 MAY-- 8 JULY SEPT...",. S OCT " '\.\,,... ~, , '.", \ \... \ \ " \ \ '" \ "\.' \ \ \ \ \', \ \: \,.., \., ~, :: Q.... :J:... o c , ,4,6.8 28,0 "... " \,, \.1 \', '. ~ \'\ '.\,.~ \ Co \'. \ \: : \ l: rṫ, t r, :,: 1=, : li J COASTAL WATER ATlANTC WATER Fig.3. Vertical distribution of 0t in the upper 100 m of the Coastal and Atlantic waters in 1988.

5 5 The drifting tracks of all buoys in 1986 are shown in Fig. 4. n Fig. 5 the southwestern part of the area is enlarged. and the bottom contaurs are indicated in order to show the influence of the bottom topography on the circulation pattern. n 1986 two buoys were released at the western shelf of. 'Troms0flaket. After they had drifted in small eddies for a coup~e'of weeks. they parted. One was carried by the Coastal current across Troms0flaket. while the other went northwards and followed the depth contaurs round the bank area. Along the northeastern slope of Troms0flaket~ all buoys in 1986 and 1988 drifted southeast. in direction S0r0ya~ From here the current patterns follow the coastline eastwards. Same places there seems to be a weak on-share,component. since one buoy both in 1986 and 1988 ran aground. This is also observed from other drifting experiments (surface drifters) in the same area (Steinbakke 1978). The northernmost buoy drifting along the northeastern slope of the bank of Troms0flaket. was caught by a counterclockwise eddy above a trough. called ng0ydypet. The main eddy seems to consists of several small eddies which kept the buoy there for about two months. Farther east. north of Nordkyn. there seems to be an other eddy of the same kind (Fig. 4). -'" \... \:~\.~... _.~. ~ "..'.,. t.....:. "., "'ll!. ~ ~... ~..."... :1"1\ "'i ~... f.._ '0, ,_".. ~ """0"'"....~ ".. ~ ' : Ja 11' Fig. 4. Drifting tracks of all six bouys in 1986~ The cirle indicates the.start point. while the flag indicates the end point of the drift.

6 !Wo buoys were released approximately in the middle of the Bear sland Channel, where it is assumed to be an inflow of.atlanticwater, in the entire water column. The results werea bit confusing, with one buoy drifting almost straight east, and the other just drifting all around in the area. The distance between the start and end points for the latter over aperiod of three months, was less than 100 km, which correspond to a mean velocity of, about 1 cm s-l. The results from 1988 to some extent confirmed the results from 1986~ and in addition gave more new information on the current pattern' (Fig. 6). Buoys 1 and 2 were released at the western slope of Troms0flaket. As in 1986 one buoy crossed the bank area with the Coastal'current, while the othe~ followed the northern slope where it for a couple of weeks'was caught by an eddy. At about 24 E they reunited, and both drif~ers were caught by several eddies where they stayed for some while. probably at the border'between the Coastal and the Atlantic watermasses. Drifters 3 and 4 were released in the deepest part of the Bear sland Channel where the current was expected to go east or northeast.'both buoys drifted with low speed during their two months of drifting. Buoy No. 3 drifted 16',... '0-1S',",, ", ". : ị,. ~.. :t: := ; ~:~ " \.,~ "" '., '.. "'.:',. "'. ",,, "... \. '.... '\ \ i ~ \, i, i.," \ ~./.: ~\ :. ~; " ". '.,.\...\.,/':,.i Fig. 5. Enlarged picture of drifting pattern inthe southwestern Barents Sea in The bottom contours are also indicated.

7 t ,,_..,...:...-- ~ Q ;JA,;::..::s-~~~ =_..,...--,;,.::~::::;.,-----_-:- ~ T J : t-----o;,..::li,...;;lo"c;;;;..lwo,_-=-..,.....;::a.;::::- """"!"'" Fig. 6. Drifting tracks of 10 Argos-buoys in [J indicates start point and A the end point.

8 8. northeastwards, and was several" times caught by small eddies. The other buoy returned to the same place as it was released after a roundtrip. Buoy 5 was released close to the western slope of the Ceritral Bank, near the frontal zone between the Atlantic water and the cooler arid less saline bank water. n the beginning, the buoy was drifting in the frontal area, but after a while the buoy was caught by the Atlantic water and transported northwards. Buoys 6 and 7 were released close to each other at the northeastern slope of Tromsoflaket, but showed quite different drifting patterns. Like three buoys in 1986, No. 6 was caught by the current that follows the slope of the bank~ "'" ""' 0' and ran aground after only 19 days. The other buoy drifted first east to 30 E, and then turned northwards. Drifters 8 and 9 were also released in the same area, but at a different tim~ (Table 1). No. 8 w~s dropped in the same area as the buoy which did not move much in 1986, but showed a quite different behavior as it drifted east. No. 9 came into the Coastal current.,the most striking feature was that both of them were caught by counterclockwise eddies fit the same longitude (28 0 E). in which they stayed for about one month before they continued east.the period was almost the same for both bouys, only with a small' difference in the start and. ending time. Buoy 10 which drifted between these two buoys passed the same longitude without being caught by an eddy. DSCUSSON The results in some areas confirmed earlier information about the current pattern, but the tracks revealed a much more complex circulation pattern than known from earlier investigations (Ljoen 1962, Sundby 197~.Loeng and Sundb~~ 1986, Midttun and Loeng 1987). The strong influence of the bottom topography is clearly demonstrated (Fig. 5) in spite of the influence being less pronounced during the period when the water masses is stratified (June September), as pointed out by Sundby (1984). The most predominant effects on the current conditions of the bottom topography are the clockwice circulation above the bank of Trom~oflfiket and the' counterclockwice circulation above the trough ng0ydjupet. These "features are earlier decribed by Bjorke and Sundby (1984) by means of hydrographical observations and distribution of fish larvae. The relatively stable southeasterly current along the northeastern slope of Troms0flaket is also aconsequence of the bottom topography. The results showed a strong current (50-80 cm ~-1) almost parallel to the coastline in areas close to theshore~ At a few localities'this current seems to have a component towards shore. Earlier experiments with drifting envelopes at the surface, showed that the onshore component i8 concentrated to a few localities (Steinbakke 1978). n the rest of the Barents Sea'the mean current was rather weak (1-20 cm s-lj and had a direction towards east or northeast.

9 .- 9 The vortices generated from the bottom topography features seemed to be stationary, while others, in particular those in the Bear sland Channel, were less permanent and may have been generated by shear processes between the inflowing Atlaritic water and the outflowing Arctic water. Also in the mixing area between the Coastal and Atlantic watermasses, there seem to be several small semi-permanent eddies. One striking feature is that buoys which occasionally are in the same position {but not necessary at, the same time}. usually continuein quite different tracks. At the bank of Troms0flaket the buoys are either following the' depth contours round the bank area, or they travel by the Coastal current straight acr~ss. However, they all have one thing in common, as'they are caught by small temporaryeddies for a'period from a few days to a couple of weeks. Also in the more central part of the Bear sland Channel different behaviour is observed. n 1986 the two buoys in the area passed the same position, but had later quite different tracks {Fig. 4}. Also in 1988, several bouys crossing the same positions continued in different directions {Fig. 6}~ t may be questioned which depth the observed current patterns represent. n spite of a relatively large sail at 30 m, it is obvious that the observed tracks are influenced by the effect of wind. The wind will have a direct ", effect on the buoy, and an indirect effect through the surface current. What we may conclude, is that the results indicate the mean movement of the water masses in the upper 30 m, which is the most important depth layer for transport of fish larvae {Bjprke and Sundby 1984}~ REFERENCES Bj0rke, H. and Sundby, S Distribution and abundance of post larval Northeast arctic cod and haddock. PP72-98 in God0,O.R. and Tilseth,S. {eds}. Proceedings from the symposium on Reproduction and recruitment of Arctic cod. Leningrad September nstitute of Marine Research~ Bergen. Lj0en, R The waters of the western and northern coast of Norway in July-August FiskDir. Skr. Sero HavUnders. 13{2}: 1-39 Loeng, H.' and Sundby, S~ Hydrography and climatology of the Barents Sea and coastal waters of northern Norway. pp in Alton,M. {compiler} Proceedings of a workshop on comparative biology, assessment and management of Gadoids from the North Pacific and Atlantic oceans. U.S. Natl. Mar. Fish. Serv., Northwest and alaska Fish. Cent., Seattle, WA.

10 .. 1O~ Midttun, L. and Loeng, H Climatic variations in the Barents Sea. pp in Loeng, H. (ed.) The effect of oceanographic conditions on distribution and population dynamics of commercial fish stocks in the Barents Sea, Proceedings of the third Soviet-Norwegian symposium, Murmansk, May 1986, ntitute of Marine Research, Bergen. Steinbakke, P Drivkortfors0k pa TromS0flaket. lku-rapport P : 1-49 (in Norwegian). Sundby, S Oceanographic conditions in the regions Malangsgrunnen Fug10ybanken-Troms0flaket. Fisken og Havet 1976(1): 1-53 (in Norwegian, abstract in English). Sundby, S nfluence of bottom topography on the circulation at the ~ continental shelf off northern Norway. FiskDir. Skr. Sero HavUnders., 17: