Climatic variations in the Barents Sea.pdf (1.151Mb)

T I E EFPLC1 OF OCEANOGRAPIIIC CONDITIONS ON DISTRIBUTION AND POPULATION DYN?,IlICS OF COMERCIAL FISil STOCKS I B TIIL BAIIEWS SErl

P r o c ? e d i n y s of clie i h i r d S o o i r i - N o m e p i a n Svmposiuin, Mtirnansk, 26-28 May 1986 E d l i e d by il. Loeng. I i ~ s r i i u t e o f b l a r i n e R e s e a i c i i , B e r g e n , 1 9 8 7 .

Lars blidttun and Harald Loeng Institute of Marine Research

P , O . Box 1870 - Nordnes 5 0 2 4 Bergen

Norwa~r

ABSTRAGT

The eireulation system of the Barents Sea is described. Warm water flows into the sea from west and i s gradually transformed into Artic waters flowing out from the sea, partly a s surface c u r r e n t s , partly a s dense bottom water. The elimatic eonditions of the Barents Sea are determined both b y effects from variation in the inflow and by processes taking place i n the sea itself. The large variations observed in temperature and salinity from standard sections through the inflowing watermasses are examined and possible explanations are discussed.

INTRODUGTION

Although the main purpose of this paper i s a presentation and discussion of the rather large climatic variations observed in the Barents Sea, i t is found worthwhile first to give a short description of the circulation system a s far a s this is Scnown from the literature.

Based on early observations, KNIPOWICH (1905) gave a description of the water masses of the Barents Sea and NANSEN (1906) devized theories on the formation of bottom water in the northern seas including the Barents Sea.

Nansen also believed that dense water formed in the eastern Barents Sea could supply the bottom water of the Arctic Ocean through the channel between Novaya Zemlya and Franz Josef Land as indicated b y Admiral Makaroff's temperature observations in that area. Recently MIDTTUN (1985) has eonfirmed Nansens hypothesis. MOSBY (1938) studied the waters between Svalbard and Franz Josef Land. This area i s mainly dominated b y Atlantic water masses although some outflow from the Barents Sea may take place near Franz Jozef Land.

The majority of the contributions to the oceanography of t h e Barents Sea are presented by seientists from USSR. TANTSIURA (1959) made a comprehensive analyses of the e u r r e n t s in the Barents Sea, AGENEROV (1946) studied a large numbes of current observations. The detailed work by NOVITSKIY ( 1 9 6 1 ) delt w i t h the e u r r e n t s of the northern Barents Sea, SARYNINA (1969) was coneerned with the bottom water of the Bear Island Ghannel, and

KISLUAKOV ( 1 9 6 4 ) studied the conditions at the western inlet to the Barents Sea.

Naturally, the southern part of the Barents Sea has been the most intensively studied a r e a , and where several standard sections have been established in order to investigate variations in the inflowing water masses. Measurements in the Kola section were started as early a s 1900 by d r , N , Knipowich and have been regularly continued since 1 9 2 0 . BOGHKOV (1996) has studied the observed temperature varlations in relation to the solar aetivity. Three seetlons observed by IMR have been analysed b y BLINDHEIM and LOENG (1981) and later b y LOENG and MIDTTUN (1984). The section Fuglrbya- Bj@rn@ya (Bear Island) has been studied by DICKSON and BLINDHEIM (1984).

Varialbllity in the fixed station Nordkapp was analysed b y MIDTTUN (1969), while BLINDHEIM, EOENG and SÆTRE (1981) compared the elimatic variations In Norwe-lan eoastal water with the observatlons from the Kola sections.

BECKSON, MIDTTUN and M U K H I N (19'90) have presented results fyom the hydragraphle work done during the joint O-groups fish s u r v e y s i Barents Sea 1965-69.

Mainly based on the above mentioned literature, LOENG (1987) gave a brief rewiew of the main circulation and water masses of the Barents Sea.

GENERAL DESCRIPTION OF THE Pf-IYSICAE CONDITIONS The c u r r e n t svstems

Fig. 1 , which shows a simplified picture of the surface c u r r e n t system, is based mainly on c u r r e n t maps made b y TANTSIURA (1959) and NOVITSKIY ( 1 9 6 1 ) , Only minor corrections have been made on the basis of some recent observations, The map indicates two main c u r r e n t directions

.

In the southern p a r t , the c u r r e n t s are towards 'the e a s t , while the c u r r e n t direction in north is westwards and southwestwards.

The Norwegian Coastal Current flows along the western and northern coast of Norway. Outside, and parallel to the coastal c u r r e n t flows the Norwgian Atlantic Gurrent along the Norwegian continental shelf. Off the coast of northern Norway the Atlantic c u r r e n t splits in two branches, one continuing northwards along the continental slope as the West-Spitsbergen C u r r e n t , and the other entering the Barents Sea along the Bear Island Channel as the Nordkapp Current

.

The southern p a r t of this c u r r e n t continues eastwards together with the Norwegian Coastal Current and proceeds along the Murman coast a s the Murman Current. The northern p a r t of the Nordkapp Current divides along three major routes at about 30'~. One arm t u r n s northwards between Hopen Island and the Great Bank where i t submerges under the lighter Arctic water.

The second branch continues eastwards in the deeper area between the Great Bank and t h e Central Bank a s an intermediate c u r r e n t . The third p a r t t u r n s southeastwards, south of the Central. Bank, and flows parallel to the Murman C u r r e n t , t u r n i n g northeastwards along the axis of the eastern basins. A much less important inflow of Atlantic water to the Barents Sea takes place along the Storfjordrenna between BjGrnØya and Spitsbergen.

The influx rsf Artie water to the Barents Sea takes place along two main routes: firstly, berneen Spitsbergen and Franz Josef Land, and through the opening betkveen Franz Josef Land and Navaya Zemlya (DIGKSQN et al, 1 9 7 0 ) .

The main part of the first mentioned current flows as the East Spitsbergen Current southwards along the coast of Spitsbergen. The current flowing southwestwards south of Franz Josef Land, called the Persey Current, splits north of the Central Bank. Aceording to TANTSIURA (1959); one branch turns southwards to the Central Bank, but this part is probably small. The main part of the Persey Current goes southwestwards along the eastern slope of the Svalbard Bank as the Bear Island Current. The current t u r n s around Bear Island and goes northeastwards around the

S

torfjordrenna.

F i g . l. S u r f a c e c u r r e n t s i n t h e B a r e n t s Sea. A r c t i c c u r r e n t s (-

-

^{3 )}

^,

A t l a n t i c c u r r e n t s (4and C o a s t a l c u r r e n t s ) (.-...b). ( S i m p l i f i e d a f t e r TANTSIURA 1959, NOVITSKIY 1961).

The details of the eurrent system are poorly known. Hydrographie

observations indicate an anticyclonic vortex above some of the bank areas,

such as the Central Bank and probably also the Svalbard Bank. This implies

a lon- resident time of the water masses and possibilities for vertical mixing

during the winter season (NANSEN

1906)

. Current measurements indicate

almost the same current direetion from surfaee to bottom in areas with only one

water mass (BLINDHEIM and

LOENG

1978,

HELLE 1979).

However, in areas

where the Atlantic water submerges the lighter Arctic water, as west and

south of the Great Bank, one must expect different current areetion with

depth, Also in areas wåth outflowing denee bottom water, the current direetion

is probably different from surfaee to bottorn, Therefore,

fig, I ,

onlly to some

extent represents

the

eurrent systems

in

Intermedlafe

and

boitom layers,

Water masses.

Following HELLAND-HANSE# and NANSEN (1909), Atlantic water i s defined by salinity, higher than 35 . O 100. At the entrance to the Barents Sea, the mean salinity and tgmperature in the core in autumn during the period 1966-77 was 35,13 and 6.2 C , respectively (BLINDHEIM and LOENG 1981). Further east in t h e Barents Sea the characteristics change to lower sallnity and ternperature a s shown in Fig. 2,and also clearly demonstrated b y LOENG ans MIDTTUN (1984).

As will be discussed l a t e r , there a r e also great long-term variations in the properties of the Atlantic inflow to the Barents Sea, whickr again may lnfluence t h e properties of t h e locally formed water masses.

The coastal water is characterizeu b y low saliriity ^{( S}

<

34.7 0100) and relatively high temperature ( t

>

3 C ) . This water mass is also most easily traced b y t h e salinity (Flg. 2 1 , The light coastal water spreads out in a wedge form above t h e heavier Atlantic water. The seaward e x t e n h o f thls wedge of Coastal water varies seasorlally and has i t s minimum in t h e winter (SÆTRE and LJOEN 1971),

The Arctic water ( o r Barents Sea winter water) i s during summer mainly found in the intermidiate l a y e r , between 20-150 m, in the northern Barentg Sea.

The core is usually found between 30-60 m with temperature below -1.5 C and salinity between 34.4-34.6 0 1 0 3 In the horizontal map (Fig. 2 ) most of the water with temperature below O C i s Arctic water.

F i g . 2. D i s t r i b u t i o n o f t e m p e r a t u r e ( l e f t ) and s a l i n i t y ( r i g h t ) a t 100 m d e p t h , autiimn 1984.

The area between the Atlantic and t h e Arctic water masses i s called t h e Polar front. In this area Arctic and Atlantic water mix. In the area west of the Central Bank, t h e Polar front is s h a r p and follows typical features of the bottom topography. In the eastern Barents Sea t h e front area is less distinct, and a mixed water mass cover great areas.

Bottom water of different kinds may be formed at various places in the Barents Sea, Bense bottom water is formed through rejeetions of brine during ice freezing and Is a more or less regula- pkrenomenorm, partieularly on the shelf of Novaya Zemlya, but sornellmes also on the "albard Bank (MIDTTUN 1 9 8 5 ) -

Bottom water with a somewhat lower salinity is formed on t h e C e n t r d Bank during the winter season, The bottom water of the Bear Island eihannel, however, is formed in the frontal zone (Polar front area) during the period of vertical winter circulation on "se southeastern slope of the Svalbard Bank (OARYNINA 1969). This bottom water may h a v e ' t e m p e r a t u r e s higher than 1 C .

The winter and summer situations differ most clearly in the vertical s t r u e t u r e of the water masses, During winter, vertieal mixing takes place all over t h e Barents Sea, Over shallow bank areas, conveetion may reaeh down to bottom and contribute to bottom water fornation a s already mentloned. In the deeper a r e a s , the water masses may be homogeneous down to more than 200 m , In the Ice-eovered areas, the temperature will be homogeneous

,

whlle a salinity gradlent will maåntaln a weak vertleal stabllity, Ice-freezång wlth rejectlon of brimle wlll bi-eak down the salinity gradlent,

Sea ice condltions.

The variation in the position of the iee e d g e , based on satellite Pmages from a pesiod of 10 years,(1971-1980), is shown in Fig, 3 for the months February, April, J u n e , August, October and December

.

The figure shows considerable variations in ice extension, which may take place from year to year. Some months, especially in summer and autumn, these variations may exceed 500 km, In spite of t h i s , the seasonal variations of the sea ice extension i s , in i t s broad features, similar from one year to another with maximum and minimum extension in March-May and August-September

,

respectively (LOENG 1979, LOENG and VINJE 1979). The formation of ice usually s t a r t s in late September o r i n October, and the ice border moves rapidly southwards to the Polar front d u r i n g November and December. The melting of ice s t a r t s in May-June, but in the beginning the melting i s v e r y slow. The retreat of the ice border i s usually most rapid in July and in early August.

Fig, 4 shows the variations of maximum ice coverage in the years 1979-1985.

In the vicinity of Bear Island, there Is alrnost no variations from one year to an other. In t h e eastern Barents Sea, however, the variations a r e considerable.

A s seen in Figs 3 and 4 , t h e year-to-year variations may be considerable, Some authors have proposed eyclic ~ a ~ i a t i o n s in the sea ice condltions of 3-5 years (KISSLER 1934, LUNDE 1965). Long-term variations, not neeessarily of cyclic n a t u r e , a r e also well known from other marginal aaras of the Arctic, e . g . , on the west coast of Greenland (DUNBAR 1972),

CLIMATIC VARIATIONS General description

Climatic variations can be recorded in sections crosslng the inflowingo water masses. At the entrance, between Fu$Øya and BJØrnØya (along 19 30'1E), temperature and salinit2 have been okserved eaeh autumn since 1964, while t h e Kola-section (along 33 30'E)

,

with exceptions of "E0 periods

,

1906-1920 and 1941-1944, has been observed b y the Russians back to 1900.

Z Fisken.

MINIMUM EXTENSION M A X I M U M EXTENSIOM

F i g , 3 , S o u t h e r n l i m i t o f s e a i c e a t t h e end of t h e months F e b r u a r y , A p r i l , J u n e , August, O c t o b e r and December d u r i n g t h e p e r i o d 1971-1980 ( a f t e r V I N J E 1 9 8 3 ) .

MIDTTUl81

-

e t - al. (1 98%) using the Kola-sectian

,

caleulated manlhly mean temperatures on _the basis o f the perisel 1921-l980 and anomalies from those means have _been caleulated for the whole perlod up LO 1985 (Fi",5),

F i g , 4 . Maximum i c e c o v e r a g e i n t h e B a r e n t s Sea d u r i n g w i n t e r i n t h e p e r i o d 1979-1985.

Fig. 5 shows that the years up to 1906 were cold. According to SETERSDAL and LOENG (19841, almost the whole missing period, 1907-1920, also wac cold.

After some yeam with hlgher temperatures in the be@nning of the 1 9 2 0 ' ~ ~ the years up to 1930 had lawer temperatures than normal. The longest period o f a warna regime was between 9930-1939, with maximum in 1 9 3 8 . The years afåer 1945 ase eharasterized by flzactuations of duratlon 3-5 y e a r s , These periods

i

^-20

Fig. 5 , Temperature anomalies in the Kola section for the period 1900-1985 (continuous line) and ice index for the Barents Sea (dotted line).

esincide w i t h cyelie variatisns in sea lee @snd%lsns, as proposed by K I S S L E R ( 1 9 3 4 ) and LUNDE ( % 9 6 5 ) *

A

perlod of 11 y e a r s , which coineides wlth the

solar aetivity eyele, has also been suggested botli for temperature and ice eonditions (BQCHKOV 19761,

During the 1990's, large variations have been observed in elimatie eonditions in the Barents Sea. The period 1970-1976 was 'warm

,

while the seeond half of the decade was chareterized by lovs temperatures (Fig, 5 1 , Through the beglnnings of the 1 9 8 0 ' ~ ~ temperatures were increasing, An lee index for t h e perisel after 1970 shows similar t r e n d s , in&cating a elose rela"lonship between variations in sea temperature and iee eonditions (Fig, 5

1 ,

The sallnity also shows g ~ e a i variatlons after 1970, Fig, 6 shows temperature and salinity variatlons between 50 and 200 m at a seetion between FuglBya and Bj@rn@ya (BE%P;6DHEIM and EBENG 1 9 8 1 , LOENG and M I D T T U N 19841, The temperature varlations a r e egraak to those in the Kola-seetlon, The most striking deature in the salinåty eurve is the deerease, whieh started In 1969 and lasted until 1979, The minimum in 19'68-19'19 is characterized by- t h e lowest salinåty valeaes eve% observed In khis section, and t h e salinity 06 the Atlantic ånflow w a s belovs 35-0 o 1oo. Since then the salinity has been increasing towards the mean values for the period 1966-77, @ven by BLINDHEIM and LOENG (198%).

F i g , 6, Mean t e m p e r a t u r e ( c o n t i n u o u s l i n e ) and mean s a l i n i t y (broken l i n e ) of t h e A t l a n t i c i n f l o w t o t h e B a r e n t s Sea i n autumn d u r i n g t h e p e r i o d 1964-1985,

Already in 1909 HELLAND-HANSEN and NANSEN (1 909) suggested that elimatie variations in the Barents Sea probably a r e of adveetlve nature. They observed a time lag of ane year between Lofoten and the Kola-seetion.

LBENG, NAKKEN and RAKNES (1983) showed t h a t temperature changes in the eastern part wlll most often oeeur about one year later than in the western p a r t . Also in the Norwe@an Sea, cIimated varlatlons seem lo be due to adveetlve proeesses in the Atlantic Inflow (BLINDHEIM 19871,

A s shown b y the majorlty of the other contributåones to thic Symposium, the r a t h e r large variations observed in the physieal environment of the Barents Sea exereise great effeets on biologeal eonditions in the large fish stoeks of the sea. Pt is therefore felt important to diseuss pssslble explainations and t r y to understand the physleal eauses behind the climatie variations observed in the inflowång watermasses, The variations a r e deseribed b y "te lon- series of tempesature observatisns in the Koia seetion csossing the main braneh of the Murman "urrent (Fig 1). The temperature i s alternating wlth warm and

eold periods in suecession with lenghts of 3-5 years (Fig, E)). Similar variations are observed in other seetions f u r t h e r west in t h e Barents Sea, and a r e also refleeted in the lee coverage, It can be seen from Figs, 3 and 4 that the ice coverage in winter has the greatest variations over the Central Bank and in the eastern p a r t of the Barents Sea,

The variations may be hypothetically explained as caused b y similar varia"cons in the property of the inflowing water, that i s , the e u r r e n t system of constant velocity and volume is bringlng in a watermass with ehanging temperature and salinity. Bul the varåatians may a s well be a result of variallons in the e u r r e n t system itself, Sinee both temperature and salinity increase in countereurrent direetions, high veloeity should result in high temperature and s d å n i Q y , low veloeily ln low temperature and salinily, In the last case t h e varåatlon of the current system has to be explained, Agarln, the variation in the e u r r e n t activity csuld be forced upon the sea from outslde, bul may a s we%l be a esu ult of proeesses taking pliace in the Barenes Sea itself,

Water of high density is forrned during the winter a s a result of eooling and iee formation and drained out from the sea as bottom c u r r e n t s , The process of lee formatlon is also the source to formation of the light surface water brought s u t b y the Arctic e u r r e n t s , The proeess i s deseribed

In

detail b y MIDTTUN (1985) and can be regarded a s a separator transferring salt from surfaee water to deep water and in this way gradually building up dense bottom water and light surfaee water, The bottom water forms bottom e u r r e n t s along the righthand side of channels leading out from the Barents Sea to the Norwegian Sea through the Bear Island Channel and to the Polar Sea through the Novaya Zemlya-Franz Josef Land Channel, The water volume, whieh in this way leaves t h e sea, has to be replaeed b y inflowing water from west, The aetivity in building up dense bottom water may vary from one year to another followed b y variations in the outflow wlth corresponding change in t h e inflow a s reaction. After a great inflow i t may take more than one year to again build up the required conditions to initate the next dense water outflow.

To some degree the r a t e of dense water formation will depend on the salinity of t h e inflowing water since density is a funetion of salinåty,

Fig 7 shows that the areas with high density vaater formations a r e located over the Central Bank and on the Novaya Zemlya Bank, This is the same area where the winter ice coverage changes (Fig, $1, Fig, 8 shows the effect of the large water exchanges b e h e e n 1982 and 1983, The Inflow and outflow from t h e Norwedan Sea to the Barents Sea has been measured In the section between FuglØya and BjØrnØya from a series of anchored e u r r e n t metres. The results a r e presented in Fig. 9 , Transport ealeulations indieate about 3 SV, in and around 1 Sv out through the section. This would require another outflowing c u r r e n t , most likely located in the ehannel between Novaya Zemlya and Franz Josef Land, Outflow of high density water in this area has been reeently described b y MIDTTUN (1985),

The above eonsiderations lead u s to formulate the following tentative eonelusions :

1. The meehanisms behind t h e climatie variatlons in the Barents Sea can be deseribed as the result of dense water formation d u r i n g winter eooling and ice formation. This process transforms the inffowing water mass into two outflowing waters, viz. battom water of high density and surface water of low density.

2, The transforming p-ocess i s time dependent on " t e property and quantity of the iaaG1-t~. A f t e r a. pergael of high influ-, the transforma$le~nn requires

F i g , 7 , i)epth of t h e g s u r f a c e s 28-0 and 28-1 i n 1.977, D o t t e d l i n e s i n d i c a t e a r e a l i r n i t a t i o n s . t

Fig. 8. Temperature i n t h e m e r i d i o n a l s e c t i o n o f 4 5 ' ~ i n September E982 a n d 1983.

LAT NORTH 7;" 73" 72" 71" 70"

BJØRNØYA , F U G L Ø Y A

-

¹

Fig. 9. Mean current component (cmos ) through the FuglØya-BjØrnØya section from measurements at mooring stations in September-October 1978.

The observation depths are indicated by i. (BLINDHEIM, personal comrnunication)

more than a one year's winter eooling to build up the bottom water density high enough to inltiate a new outflow,

3 . The bottom water outflow takes plaee mainly in the northern Bear Island

Channel and along the southern p a r t of the Novaya Zemlya - Franz Josef Land Channel.

4 , To confirm this hypothesis direct eurrent observations in the above mentioned areas of outflow a r e strongly recomniended.

ACKNOWLEDGEMENTS

We wish to express our appreeiation to M r . H , Kismul for preparing the figures and to M s . I , Byrkjedal and I . Meland for typing the manuseript,

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