A Description of the Seasonal Variations of Water Exchange Between the Baltic Proper and the Gulf of Bothnia

MERENTUTKIMUSLAITOKSEN JULKAISU N:o 215 HAVSFORSKNINGSINSTITUTETS SKRIFT

A DESCRIPTION OF THE SEASONAL VARIATIONS OF WATER EXCHANGE BETWEEN THE BALTIC PROPER AND THE GULF OF BOTHNIA

ERKKI PALOSUO

HELSINKI 1964 Helsinki 1964. Val tioneuvos toil kirjapaino Introduction

Numerous studies have been published on possible to study the seasonal variations in the transport and exchange of water in the the deeper parts of the sea, especially as seas around Finland. The first extensive winter observations were almost entirely study was made by Witting (1908). His data, lacking. collected at fixed stations, were mainly As the advective transport of heat proved measurements of currents taken at an- to be of great importance in the calculations chored lightships and tide-gauge readings. of the cooling and freezing of the sea areas, The temperature and salinity values of the the freezing calculations being of the utmost open sea were obtained during four cruises importance to winter navigation, arrange- made in 1904 and 1905—the spring cruises inents were made in 1959 to use Finnish in May and the fall cruises in August — icebreakers for oceanographic studies in September. connection with their regular work. In 1961, Notable among the later studies were the coast-guard cutters also made observations joint Swedish-Finnish cruises to the Åland at oceanographic stations. The latter opera- Sea and the Archipelago Sea in the summers ted mainly in the Åland Sea and in the of 1922 and 1923. One of the main points of southern part of the Bothnian Sea, but also the program was the measurement of cur- in the Bothnian Bay. In 1962, it was rent systems. Palm i (1930) computed the possible to use the research vessel Aranda mean current vectors for the surface water for one early and one late cruise in the and published these as a current chart. The Gulf of Bothnia. This was repeated in 1963. observational data were mainly current Since the data collected are by no means speeds measured on lightships. The Åland simultaneous, only a quantitative survey Sea was again studied by Hela (1958), who is attempted. made a computation of the distribution of In this connection the author wishes to currents on the basis of temperatures and thank the masters and crews of the ice- salinities measured during a special cruise breakers and coast-guard cutters who have in 1956. The Quark was studied by Li-sitzia assisted in the sampling program for their (1946), who compared the currents in the cooperation. The samples were analyzed at Quark with variations in water level and the laboratory of the Institute, under the other factors. direction of Dr. Folke Iforolejf. Therefore As mentioned above, Witting was able to my thanks are directed to him and other perform two cruises in each of the years 1904 colleagues. I wish to thank especially the and 1905, a spring cruise and a fall cruise. Director of the Institute, Prof. Ilmo Hela, During the following years no vessel was for his invaluable advice on the treatment available for marine research, except for too of the data. Mr. Ilklea Noponen and Osnio short a period in summer. This meant that Ranta-alto have helped with the drawing the oceanographic sections could only be of the figures and Mr. Svante Nordström and traversed once a year as a rule, and al- Mrs. J. ll1. Perttunen helped with the trans- most always in summer. Thus it was im- lation and checking of the manuscript. ~ n

:~~_ I'!1I: =__ i

_ i is

hrr - --ii ____ Iii i!- 1_ i.,__r4•,I_ Fig. 1. Contents Page Introduction...... 3 I. The flow of water over the southern sills of the Åland Sea ...... 7 II. The deep current from the Åland Sea to the Bothnian Sea ...... 11 III. The flow of water through the Archipelago Sea ...... 15 IV. The water flow in the upper layers of the Bothnian Sea ...... 17 V. The flow over the Quark to the Bothnian Bay ...... 24

19° 20°

qC knan„;• • alskär Vi ~ Yxsjkärs SiiI k;i

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~ fil dö r • :s ~ 1.I,.i Isj!irn. •i.I ~ Pi _ y • r Isn;; i..~~Ö Susoskn Hiigllrnnl i s e. ian

19° 20°

Fig. 2. The depth contour-s o/ the Ålland ,Sea. Td'hite: 0— 20 to Light grayish: 20— 60 ett Gray: 60-100 Sta Dark gray. 100-200 ni Vety dark gray: more titan 200 ni Black: land I. The flow of water over the southern sills of the Åland Sea

The exchange of water between the Bal- the Bogskär Deep. It has been impossible to tic proper and the Gulf of Bothnia proceeds make frequent visits to this out-of-the-way on the one hand through the rather large place for oceanographic measurements. Fur- and deep Åland Sea, and on the other hand thermore, old serial observations covering through the numerous but shallow sounds the whole year exist for the Bogskär Deep. and channels of the Archipelago Sea. The Bogskär lighthouse was manned from From the geological point of view, the 1898 until the destruction of the tower in Åland Sea is a tectonic depression of appre- 1914, and the lighthouse crew made depth ciable depth (Fig. 2). It consists of two observations every ten days (Witting, 1908). basins more than 200 in deep, both with The year 1907-08, shown here as an exam- shallower margins and separated by a sill. ple (Figs. 3-4), indicates that the winter The southern basin, called Lagskär deep, is turnover reaches a depth of 80 m, this col- separated from the Northern Baltic by a umn of cold water being of rather low sal- ridge, over which the water depth is 40 m. inity, 6.35-6.50 0/00. In summer the sal- This ridge is partly cut by a deep but inity increases at greater depths, being narrow canyon, the depth of which ranges 7.00-7.25 °/oo at the end of July even at from 150 to 225 m; however, at its southern 50 in and higher than 8.00 °/oo below 70 m, entrance the depth of this canyon is only as was also shown by Simojoki (1946) in 70 m. The only attempt known to the his study of Bogskär data. author at measuring currents in the canyon The saline water mass close to the bottom indicated a rather strong northward current at Bogskär has been sampled during the close to the bottom. recent cruises, and the oxygen concentra- The sill between the two basins of the tion of this water has always proved to be Åland Sea consists of the island of Lågskär low. Thus in 1961 the oxygen concentration and the shallow water to the southwest and at 85 in was 1.73 ml/1, corresponding to 21 east of this island. The deepest place on the per cent of saturation. sill, to the southwest of Flötjan, is about The oceanographic station F 69 of the 80 m.. East of Lågskär the sill is cut by a Lågskär Deep has been visited occasionally very narrow canyon, but most of the water not only by R/VT Aranda, but also by the exchange must occur west of Lågskär be- coast-guard cutters (Fig. 34). The varia- cause of the wider opening. tions are similar to those at Bogskär. The The northern basin of the Åland Sea, the largest fluctuations of salinity take place in Aland Sea proper, is over 200 m deep. At the layer between the surface and the depth Understen the trench grows narrower and of 80-90 m. Below 100 m the salinity in- continues into the Bothnian Sea as a chan- creases abruptly and reaches values higher nel with a sill depth of some 80 m. than 8 0/oo. This water seems to be more or At the southern boundary of the area stu- less stagnant. died lies a lone rock with a lighthouse, Bog- In the Åland Sea the coast-guard cutters skär. Just off this rock is a deep depression, carried out numerous observations at dif- 8

491 F ____

150 ui

t !,Ii'ili! WI! , , ' vI iIIiipIi

III

Fig. 3-4. Temperature and salinity at Bogskcir, in the Northern Baltic in 1906 to 1908. ferent stations in 1962 and 1963 (11Ierenttutk. It is interesting to make a comparison be- J2tlk. No. 208). Since all stations show sim- tween winters of different types. During the ilar features, the annual cycle of the selected extremely mild winter of 1960/61 (Meren- station, FÅ 4, will be analyzed (Fig. 5-6). teLtic. Julle. No. 200) hardly any cold water At this station, too, the annual cycle is remi- appeared in the Aland Sea. The winter of niscent of that at Bogskär. In summer the 1961/62 was colder, and the Åland Sea and surface layer of warm water of low salinity, the Bothnian Sea had an ice cover for a extending to a depth of about 20 m, appar- short period (11Ie•entattk. Julle. No. 206.); ently flows southward most of the time cold deep water was observed for rather (shitting and Pettersson 1925, Hela 1958). a short time only. The winter of 1962/63 The water close to the bottom, presumably (1lierentutk. Julle. No. 213) was very cold; flowing more or less steadily northwards, is even the Northern Baltic had an extensive of rather high salinity. The maxinnan sal- ice cover for a long period. That winter inity is attained at this station in early Sep- the cold deep water of the Åland Sea did tember, i.e., some weeks later than at Bog- not appear until the spring, when the ice skär. cover broke up in connection with the spring In the fall, when the surface water has storms. become cold enough, vertical convection be- The amount of dissolved oxygen (Fig. 35) gins to penetrate deeper and deeper. In De- supports the above view. In the winter at cember 1961 the 6-degree isotherm sank as station rÅ 4 the concentration approaches low as 125 in and the 5-degree isotherm to saturation. This moans that even the deep 195 m, almost to the bottom. At the time water is derived from well mixed surface when the sea ices over, in late January or layers not too far away. In the summer the early February, water of 0°C reaches to a amount of dissolved oxygen decreases in the depth of 70 in and that of 2°C to 120 in. deeper layers, the saturation percentage In the spring, water colder than 2°C is dropping to 80 or 70. It is not impossible found at a greater depth than 120 ni. This that part of this depletion of dissolved oxy- water, with a salinity of about 7.00 gen is caused by advection of deep, less probably comes from the Northern Baltic, oxygenated water from the northernmost where it is found as a nearly homogeneous part of the Baltic proper. layer from the surface to a depth of 50 in.

2 6464-64

10 1963 I III N \/ vi ,/1I VIII I >/2a ; 0~t~~',li„6 +f2 •110 /

1 1

~8 1 V' I ' 1 1 1 / \I 't / / I II 11 li l

I I Ij 1 ~ /I 11 ' / IIllII 1 1 1{ iY/ / l• !i' / I I \ ~a 1 6 I r I /\1 \ i V Y ' / / 1 1 II~

1 ,

I IC I I \ ~ / I I 11 C/i,; .I 150 `—,, ÅLAND SEA II I,I 'I 1{ >.3 FA 4 I ///. 1

I i // 1 I / i/ / 1 I I '//,I 1, 1/ > p I I 1 <2,, >e2 / ' I , /~ ////1 +~i I I ;4 ~4 a1 ///2 200 - L 1961 1962 1963 0 vm Ix x xl x11 I 11 III Iv v vl vii VIII Ix x xl x11 I II 111 IV v VI vII . vill 600 5251 / 50 55d I 1 { .,— I i `°ill

650 '

, —65 700 l 50 , 1 . 4 '/// i / / ///~ % / / `,-6.50' `6.50

1 I 1 / / 1 ' ,., I / 1 1 , /' 'il2 Av /

/// /

fi i j~ % / 1j / // / k l% ~` /' 4/ 'iii/iä 150 I / ; 700 ' AIAND SEA

/ 7.25 -7.50 1 j / 7.50 //// FÅ4 // , // // I Y/ ”" S 0

/ i / 700// /////// /ir// f9725

200 Fig. 5-6. Temperature and salinity fluctuations at Station Pil 4, in the Aland Sea, isa 1961 to 1963. II. The deep current from the Åland Sea to the Bothnian Sea

As mentioned above, close to the bottom conditions of the Åland Sea (Witting and of the Åland Sea there must exist a north- Pettersson 1925, Lisitzin 1951) the velocity ward current. In a study of the summer of this current was found to be 10 cm sec–'.

1962 Il Ili . Iv . v . vI . VII . VIII . Ix . x . XI . XII +10 +i +2 ,/ i / b '.~+2111:6 1~;;``os\13 1U+ 6 2 >+ 7 f ,1 11 I11, i 1 11111 - y~ 1 1 k II

t ,1'I 11 1111

/,♦~nl 1 1 50 1 // 1~

///, t 11 1 (// ' II /~ I I GRUNDKALLEN >+ ~~'I Y~~~~i ' F33 / ' i f® C

I I II I1 i I ('~ i! 'c.' I 1 11111

1961 1962 0 vm ix x XI XII. I Il Ili IV v vl VII VIII Ix x XI xl1 < 25 •

5 6.00 \ J 50 1 \

1k% 650 1 w6.00, I 1 I ~ / I I~ 1' // GRUNDKALLEN ›zoo 1.6.50' ; F 33

I Z 6.75 6.75 700 7. / 7.

Fig. 7-8. Ten I peratlere (1 5d salinity Ili?etUatio?is at Station F 33, at »Gncnzdkallell1> in 1961 to 1962.

The observations in the canyon at Grund- In the winter of 1961, the turnover of kallen at station F 33 (Fig. 7-8) show an water at Grundkallen seems to have ex- annual cycle similar to that at station FÅ 4 tended to a depth of 60 m. At greater in the Åland Sea (Fig. 5-6). The northward depths low temperatures were observed, but current seems to continue through the the salinity increased with depth. Obvious- whole canyon. It is interesting to observe ly, this higher salinity is connected with a that in 1962 the maximum salinity was at- northward advection from the Åland Sea. tained at F 33 before the middle of Septern- On its arrival at the Bothnian Sea the ber, which is — very roughly — some ten northbound deep water will follow the days later than in the middle of the Åland deepest trench of the sea as a result of its Sea, FÅ 4. This compares well with a cur- relatively high density. This means a flow rent velocity of 10 cin sec-1. towards the northeast and then, starting at

111, `` 1. +1~i i~ 1,1 1111 1 I1111 1 11~15 >+11 III ' `/ ~ ~~`i~ ~fj j~l II \~~` ~ .1

\S////// `\ SEA Ör BOTNN/A " ~ 'ii/' i;` F30 11 ///////Z// 7 o ~1 We

E%»jIl4 4 < •p/Y / moi'

1961 1962 1963 0 VIII IX X XI XII I II III IV V VI VII VIII IX X XI XII I , II III IV V VI VII VIII 5

j 600 50

`~ SEA OF BOTHNIA~ 6. ` _.6.50 j11 __Sneo i// ` ,>,700 / 6.75-7.00 ~, '700 6.75 f I 6.75 6.75 Fig. 9-10. Temperature and salinity /litctuationls at Station F 30, in the Bothnian Sea., in 1961 to 1963.

the latitude of Rauma, almost due north. During the short winter of 1962 (111erentuti . This is seen from the observations at F 30, Julle. No. 206), only a limited amount of west of Rauma (Figs. 9-10). Here an in- cold deep water was formed. This water was crease of the salinity is observed in the found at the bottom of F 30 as deep water deepest parts of the water column in the from May to July. In the severe winter of fall, maximum salinity appearing in Octo- 1963, the whole Åland Sea and the Baltic ber. Advection over the distance from F 33 Sea as well were iced over as early as Feb- to F 30 in one month would correspond to ruary and the ice cover did not melt away a speed of some 4 cm sec -'. This corre- until the end of April (112erentutic. Julle. sponds quite well with other computations No. 213). That year the cold deep water (Voipio, personal communication). did not appear at F 30 until later, at the The cold water of the Northern Baltic, end of May or early June, and was still being always connected with the winter con- there in September, indicating either large vection, penetrates to the greater depths amounts of cold water, or slow advection, with some time lag, for obvious reasons. or both.

Iyo I IJO~ qwo VIII IX , X XI XII 1 II III IV V VI VII VIII IX X XI XII I II 0

1 1 ' ffi✓ ~I ~ ~ ,~~ I ~ ,~

/ i '~ .' ' NORTHERN QUARK '1 I / /'/ ' / I/i / IIII

100 4 -3 il i/ z ' / / .2l ,5 q 4 I 1/1/

1961 1962 1963 0 Vm Ix x XI xn I u III IV v VI VII Vm Ix x XI XII I II <5. 5 575 5.75'% •'y' 4.00 '\ __i_ " X5.25

X550

50 å 5.75 6.00 •' NORTHERN QUARK _\_. ~ — I F 18 - S %()

100 6.50 d25

Fig. 11-12. Te» iperature and salinity /luetuatio>zs at Station F 18, at »Sydostbrottcn> in the Quark, in 1961 to 1963. 14

In the northern part of the Bothnian Sea, In connection with the autumn turnover, at the latitude of Ulvö and Storkallegrund, in October through December, the coldest only a few winter observations have been water seems disappear from the deeper part made at F 24 for practical reasons. of the water columns. Just south of the Quark observations Summarizing, it may stated that the ob- have been carried out at F18 (Figs. 11-12), servations thus show two different kinds close to the position of the Swedish light- of water flow from the Åland Sea to the ship Sydostbrotten. The opportunities for Bothnian Sea: the saline summer water and actual winter observations were limited. the cold winter water, both along the bot- However, taking into account that the torn. The former is derived from a depth of whole water column is homogeneous and some 50 to 70 m at the latitude of Bogskär rather cold throughout the winter and that and the latter from the surface, perhaps in most of it stays rather cold all summer, the same region. The flow follows the trench possibly indicating a cessation of the north- of the Bothnian Sea from Grundkallen to bound current. This deep current, charac- the latitude of Storkallegrund, where the terized by the cold, relatively saline water, saline summer water is thought to flow must have taken more south another course, westwards towards the area more than presumably towards the west and even 200 m deep off Ulvö. This water rarely. south. Simultaneously, only small changes if ever, seems to penetrate further north- are observed in the salinity and oxygen ward. concentration (Fig. 37). III. The flow of water through the Archipelago Sea

The whole Archipelago Sea is rather shal- what deeper (30 to 35 m) channels, Norra low (Fig. 13), there being only two some- Skiftet and Delet.

28 ') 21 -- --- — — ------

ISOKARI

60. H4 e<~t r 1 4 4~ ~, TURKU

ENKLINGE` . '` k:•

' ~ dry ~:O`

i (lig 1. * • fil rw ;

r,s ~.,• 7i '~• LON ~~~Y~y 1~~J S x ~ lom•'

1I ai

` 8EA'GISKAR.;

Jn •i 71 2)3Q 22 2210'

Pig. 13. The depth contours o/ the Archipelago Sea. White: 0— 20 in Light grayish: 20— 60 ni Gray: 60-100 In Bloc/c: land 16

4.00

5.00

6.00

Fig. 14. The ckaii yes in Ilie dle>isily o/ sXi:jace water at Jungjrntskäv in the A>:chippelago ,Sen, a>id oj deep water oj F 30 in the Bothnian Sea.

Current measurements were made by the observations for a number of depths every Swedish-Finnish joint expedition in the tenth day. Furthermore, the observations summers of 1922 and 1923 (1Pittin.g and at station P 30, in the south of the Bothnian Pettersson 1925). According to the measure- Sea, have been used, although they com- ments analyzed by Paln?fn (1930), the re- prise only scattered observations anade on sultant current of nearly 10 cm see' in R/V Aranda, on the icebreakers, and on the the whole Archipelago runs clue north both coast-guard cutters in 1962. at the surface and at the bottom. Sjöblom The density values computed from tem- (1961) also made some measurements in perature and salinity observations show winter 1953 and the results confirm Pal- that in winter the Jungfruskär water is men's conclusions. heavy enough to sink to depths greater than Oceanographic observations have been 40 m or even 80 in in the Bothnian Sea. In made at the island stations of Ruotsalainen, summer again the Jungfrusk~sr water is so Skiftet, Eniklinge and Jungfruskär (Aferen•- much lighter, primarily because of its high tntll. Julin. No. 201). Since these stations all temperature, that it can appear in the give a consistent picture, Jungfruskär, as Bothnian Sea as surface water only, its flow the site of the most extensive data, has been following the line of the Finnish coast selected for the purpose (Fig. 14). These (Palmen 1930). comprise a set of temperature and salinity IV. The water flow in the upper layers of the Bothnian Sea

The deeper trench of the Bothnian Sea, winter water at depths of 15 to 40 in at being more than 100 in deep, begins north F 24 B, F 24, F 23 E and F 22 has a rather of the Åland Archipelago and continues as high oxygen content because of its earlier a sickle-shaped depression curving north contact with the atmosphere during the fall and west and south. The greatest depth of turnover. During the winter the oxygen con- 254 in is found off Ulvö. In the middle of tent has decreased slightly; the saturation the Bothnian Sea a dividing ridge extends percentage was found to be about 80 or 90 from the Bight of Gävle northwards as (Fig.18). On the other hand, the water at 40n Finngrundet and other banks, across the depth at F 23 shows an absolute maximum 62nd latitude at depths of 40 m to 50 m. of oxygen concentration connected with the In connection with the spring and fall secondary temperature maximum. This can cruises of the R/V Aranda in 1962 and be interpreted only as water of r>young age», 1963, special attention was paid to the in- originating from the surface somewhere. fluence of southern waters penetrating (It is interesting to observe that determin- through the Åland Sea and Archipelago Sea ation of the absolute concentration of dis- into the Bothnian Sea. In the following, the solved oxygen seems, for our purpose, to be spring conditions of 1963 will be elucidated, somewhat more useful than the saturation mainly by reference to the section Ulvö- percentage, as the temperature changes of Storkallegrund. the water masses have a greater influence on June 15, 1963, the temperatures of the the degree of oxygen saturation than the bio- uppermost layer, above 15 m, of the section logical effects in the cases presented here.) ,nere already around 8°C, in some places as An overall representation of the tempera- high as 10°C (Fig. 15) as a result of branching ture conditions in the Bothnian Sea shows of surface currents. the effect of the spring warming, especially Water of 0°C was found close to the Swed- in the south (Fig. 19). At the Finnish end of ish coast at depths of 20 to 40 in. This water the section Söderhamn-Rauma the whole is obviously what remains of the cold water water column is already warmer than 2°C to of the previous winter. Mid-sea., further to a depth of almost 60 m. (A similar effect of the east, at F 23, a temperature of 2.40°C warming would have been found close to was measured at 40 m, the temperatures the Finnish coast in the section Brtimö- above and below being 1.50°C and 1.60°C, Kaijakari, too, had the cruise program per- respectively. Closer to the Finnish coast, at mitted sampling at the easternmost end of F 22, another branch of the coldest winter the section.) Near Brämö water somewhat, water was found, with the lowest observed warmer than the surrounding water is found temperature -0.3°C. at 40 in at F 26 h, but not at F 26 g or F 26 i. Some conclusion as to the history of the This may be the same body of water as that relatively warm water at 40 m, F 23, may found at F 23, here moving southward be- be reached by looking into the amount of tween the Swedish coast and the mid-sea dissolved oxygen in it (Fig. 17). The cold ridge.

3 6464-64 is

•0 1.

u1l11l1 1 _nqy..._=__'ee_•=c5: \ v g •• •

'i~~ii

100 120 fTC 140 0= >10°C 9- 10' 160 O= 8- 9' =7- 8° [llIH= 4- 7 180 M=1- 4' ® =05- 1` 200 111111111 =0.25-0.5 = 0 -0.25 220 ® _

Ulvö Storka1le9rund F248 F24

20 SBO' U 40

L \ {{{tP 60

80 6.20

100 640 1 . 120 U 140 D}4ll'! - IIIl 111111 III 0=<5.20 160 =5.20-5.40 IIII 6.6 0=5.40-5.60 llIjIIjt 0=5.60-5.80

180 . II jSV Qllffl1 = 5.80- 6.00 =6.00-6.20 200 - ® = 6.20 6.40 111111111 = 6.40- 6.60 220 = >6.60

Fig s. 15-16. Te» aperature 0)1(1. salinity Oil. the section Utvö Stab(lllegI Ir)zd, o/ the Bothllian Sea., Julle 15, 1963. 19

Ulvö 1963.06.15 Storkatlegrund

100

120

140

160

180

200

220

F24B F24 F23 F23 F22 F

20 / -- --110-- - / / /

40 //// //, /

/// 60 / // / 9o' / / / /// // //~ / / / / /

100 /' / i/ / eo/k/ 120

140 02 70o 160 O = >100 ~, =90-100 180 M =80- 90 ®='70-80 200

220 L Pigs. 17-18. Dissolved oxygen on the section Ulvö - ,Storlrallegrud o/ the Bothnian Sea, June 15, 1963, in mi/l and as saturation perceratage. 20

UMEÅ

SEA OF BOTHNIA 1963. VI. 10-14 VAASA ULVÖ ,ae` FZ z3

'KASKINEN STORKAI(EQRUND" i lls

BR,4MÖ KAIJAKARI MÄNTYLUOTO

SÖDERHAMN LIII = 12-10°C F32 F. RAUMA

lo = 9-8° ° 20 —:= 8-7

30 - = 7-4°

40 m = 4-1°

50 ® = 1-05,

60 ® = 0,5—Q25°

70 = Q25-0°

100

110

120

Fig. 19. Temperature distribution in Otte Bothnian Sea, June 10-14, 1963. 21

UMEÅ

SEA OF BOTHNIA VAASA 1963 VI. 10-14 ULVÖ F24b/ F27 23E F22 c

9 / F21 F21 BGOa KASKINEN 9 ~ ~ ~ S10RKALLEGRUND

BRÄMÖ I 9 b G G ~i~ ~g~' KAIJAKARI MÄNTYLUOTO

so / X 80.

SÖDERNAMN w RAUMA

02 m//( 9.50-10.00

=9.00-9.50

8.50 - 9.00

=8.00-8.50

2 = 7.50-8.00

L] = <7.50

Fig. 20. Amounts of dissolved, oxygeia, in nnl/l in the Bothnian Sea, June 10-14,1963. 22

A study of the oxygen concentration in Storkallegrund and Ulvö around the north- the Bothnian Sea (Fig. 20) shows the ern end of the mid-sea ridge, and then fi- amounts of dissolved oxygen to be higher nally southwards on the Swedish side of the near the Finnish coast than near the Swed- Bothnian Sea. ish coast, notwithstanding the lower tem- The summer development then leads to perature in the latter region. This, too, is the situation found in early fall, which is consistent with the hypothesis of a body of indicated by the oceanographic observa- water coming from the south in spring. tions of August 28 to 31 (Fig. 21). The sur- Thus, at a depth of 30 to 40 in a relatively face layer, extending to a depth of about warm current seems to flow in spring, fol- 25 m, is as 15° to 16°C. The lower end of the lowing the course of the deeper part of the summer thermoclino reaches temperatures Bothnian Sea, turning at the latitude of as low as 2° to 3°C, except at the northern

HUSUM 2 r i RÖNNSKAR

SEA OF BOTHNIA

1963. VIII. 28-31 ULVÖh":' II

~z,azuil KASKLNEN ~~ I~1 fi`~~%~ r -~ I ~ ~/ S TORKA LIEGRUND • Illll."~~~~`(~~~~{~I7*?-.. -:.1 -____ Ila

BRA I RA IJAKARI MANT YL UOTO

= >10'C SÖDERHAMN = 10-9" F3 F31[ "F k PRAUMA = 9-g. 10 'iii'= 8 - 7* 20 ni 30 40 50 60 70 [~ fN7 =025-0° 80 90

100 110 120

Pig. 21. Temperature distribution in the Bothhtiaaa Sea, Awgzust 28-31, 1963. 23 end of the Bothnian Sea. The section south in summer turns around and returns Husum-Rönnskär still crosses a body of southward from the Storkallegrrid-U1vö. rather cold water. This is another indication Thus the general circulation pattern is the that the warmer water coming from the same in early fall as in spring. V. The flow over the Quark to the Bothnian Bay

The Bothnian Bay consists of a single A still narrower channel passes through basin, reaching depths of 100 to 120 in the Vaasa sherries to Ritgrund. Strong cur- northeast of Bjuröklubb. The Bothnian Bay rents are frequently observed at its nar- is separated from the Bothnian Sea by the rowest points; however, its narrowness and Quark, a set of openings in an elevation, a small depth of about 10 in do not allow the large part of which rises above the sea sur- passage of large water masses. The main face. The main opening, in the middle of the exchange of water between the Bothnian Quark but often called the East Quark, has Sea and the Bothnian Bay takes place a sill depth of only 10 to 15 in at Nordvalen. through the Quark proper, — through the The Western Quark, between the island of two or three deeper channels and through Holmö and the Swedish mainland, has a the many wide openings with a mean depth deep channel with a depth of about 30 ra, of only a few meters. but measurements have so far not disclosed To start with, the exchange of water any appreciable exchange of water through through the Quark was studied by com- this sound. paring the density of water at the light.-

2.00 Qt

3.00

4.00

.Pig. 22. The changes ina the tleasitg of surface water at »Snaipaia> in the Quaric awl of deep water at »Helsinagkallann> in the south of the Bothnian Bay. 25 ships Snipan and Helsingkallan (Fig. 22). ready reversed and the surface water of The comparison was made for the year 1925, Snipan cannot any longer sink to the hot- as the lightship Helsingkallan was with- tom after passing the Quark. This situation drawn shortly afterwards. To eliminate holds until the observations on September freak phenomena at the surface, the tem- 21. Thus the deeper part of the Bothnian perature and salinity values observed at the Bay is deprived of the sinking warmer water depth of 5 m by Snipan were chosen to from the south at a much earlier date than represent the surface layer and the density the Bothnian Sea, in which the water from was computed from these. At Helsingkallan the Archipelago area sinks until the end of the depth of 30 m close to the bottom was May. chosen. An extensive investigation of the early The result shows the surface water at spring conditions in the Bothnian Bay was Snipan to have a greater density than the planned for 1962, when the cruise with the bottom water at Helsingkallan during the R/V Aranda was scheduled to cover the winter until May 1. On the next observation Bothnian Bay during the first days of May day, May 11, the density difference is al- shortly after the break-up of the ice.

BAY OF BOTHNIA 1962 VI. 18-20

BJURÖKLUBB _ >100C Elli = SIKEÅ Uj = 9-8° 10 'iii' = 8 - 7° 20 ®= 7 -4° 30 40 ® = 4 - 2° 50 ®= 2 - 1° 60 ® = I -05 70 ® = 05-025 80 =025-0° 90 ®= 50°

L'ig. 23. Temiaperatnie di$tiibutioIi in the Bodanian Bay, Jwne 18-20, 1902.

4 6464-64 26

foc 1962 09.12 0 SIKEA S5 S4 S3 S2 Si MÄSSKÄR

100

0 S o0

0 SIKEA 55 S4 MÄSSKÄR I I \ I I I~~ I

Fit's. 24-25. Temperature and salinity on the section Sikeå — Dlässlcär o/ the Bocknian. Bay, September 9, 1962. 27

02 1962 09. 12 0 SIKENS S4 S3 S2 S1 MÄSSKÄR

50 02 mi/l ]= <7.50 in =7.50-8.00 = 8.00-8.50 [[] = 8.50 —9.00 100 =9.00-9.50

02~o0 O SIKENS S4 53 52 Si MÅSSKÅR

02 %o >105 = 100 —105 0= 95-100 100 ® 90 — 95

Figs. 26-27, Aviounts o/ dissolved oxygen, itii m111 and as saturation percentage, in the section Sikeå — Dlässkär o/ the Bothnian Bay, September 9, 1962. 28

Unfortunately, the cruise was delayed and readily identified in the section Sikeå- the waters had warmed up appreciably by Mässkär (Figs. 24-25). At station S 3 the the time Aranda reached the area in June temperature at 50 in was 9. 1°C and at 40 in (Fig. 23). The temperature of the surface 3.0°C. The cold water at intermediate water was close to 10°C and the cold winter depths may have come through the Quark water in the depths had apparently been in spring. The warm water close to the bot- replaced by warmer, saline water from the tom cannot have been many days old, to south. judge from the well-defined interface be- The Aranda cruise of fall 1962 was accom- tween the water masses. Obviously, the rel- panied by persistent strong southwesterly atively deep water, with high temperatures, winds which drove warm, rather saline wa- contains a smaller amount of dissolved oxy- ter over the sill of the Quark. This water was gen than the cold spring water (Figs. 26-27).

BAY OF BOTHNIA 1962. IX. 13-15 lom- FINNKLIPPA å C-.'- ; I , KEMI

2 zl RQNNSKAR • R3 KC-LM! ~R4 RS ç1J -TTRR RAAHE

BJURÖKL UBB

b/ 7 BB OHTAKARI SIKEA

sa ' = iz-10°C s 1 ç 1 _ UMEÅ ' = 9 - s PIETARSAARI M= e - 7°

:•F17 :• F16 F15 ~ ® = 4 - 1' lo— RITGRUND zo _ 30 = 0,25- 0° 40 ---- —•

Pig. 28. Temperature distribution in the Bothnian Bay, September 13-15, 1962. 29

Farther northward in the Bothnian Bay to 1963.09.01-02 (Fig. 28) the fresh intrusion is found at a 55 54 53 52 depth of 40-60 m at station B 5 of the ° section Bjuröklubb-Ohtakari, where it is closer to the Finnish coast. The same water is not found at any station of the section Röm7skär-Raahe. Either the Varm water had not had enough time to penetrate so 50 far northward or it had floraed westward and then southward along the Swedish coast in a kind of cyclonic eddy. The latter as- sumption is supported by other observations. The fall cruise of 1963 was made later 10o - than usual, in September. This time, too, a warm water mass had been driven over the sill. It was readily identified on the section t0 1963.09.05 Sikeå-Mässkär, but was not found farther Sikeå 55 54 53 5 northward, at least not on the section Rönn- skär-Raahe or Finnklippan-Kelmi. Because a small storm arose during the cruise, it was decided to rerun the section Sikeå-Mässkär (Fig. 29). Now the water had already be- O = 0 9' come warmer on the Finnish side, which can = 9-B° only have been due to an intrusion of warm- = 8 - 7' er water from the south. At the same time l colder water had arrived at the Swedish end fill = 2- 1' ® = i -0w- of the section. This could only have come ii =oso-O2s from the north, from the region of Bjurö- 025-ö klubb. The waters of the Bothnian Bay thus 100 seem to circulate counterclockwise, at least in the fall, in a wind-driven gyre (Palmen 1930). 4 to The annual march of the oceanographic conditions of the Bothnian Bay may be further studied by a scrutiny of the measurements at a single station. Thus at station B 5 of the section Bjuröklubb-Ohtakari the whole water column cools off in winter (Figs. 30-31). However, the spring obser- 50 vations made on board the R/V Aranda point to a body of deep water below 100 m in the middle of the Bothnian Bay which does not cool below 1° or 2°C. At the station R 6 of the section Rönn- 100'— skär-Raahe (Figs. 32-33) the water near Fig. 29. Changes oj temperature on the section Si/ceå the bottom shows only a very minor annual — .Mässkär of the Bothnian Bay, between September range of temperature. 2 and September 5, 1963.

1961 1962 1963 0 viu ix x x1 x11 I II III W V VI v11 v 11 Ix x XI XII I.i u

i iii \\ I ll i„ 1I liibl~, / 1 \\\ \ >f10 111ay11l 1 // 111 I,( II 1 \' \\S \ I V 1 1 1 / / 11 1 III I,Y Al 1 \ 1 11 11 ii

', B'? AY OF BOTHNIA ' \ 11 1'i f n i r ;1 ~` 'I ~ ~, 8 5

r 11 I if o ~1 t C 50 1 °

Iii [1/ ill I ::f>7. li `'1 `' 1 11, 1 11 I 1 111 V'//// rll,111 1 1 1 v / .

1961 1962 1963 0 vm Ix x x1 xl1 I I it m 1v v VI v11 v 11 1x x XI X II II 3.75 <350 I 3.75 1 `-' 1 ~\

\ 1

`.\ ' \ 3.75 `\ ' —' - BAY OF BOTHNIA `I - BS 50 S%o

>4 5\4.00 4f70 >4.25 ue\4.Q0 Fig. 30-31. Temperatine and salinity fluctuations at Station B 5, in the Bothniay. Bay, 1961 to 1963.

1961 1962 1963 0 VIII ix x X1 XIS I i 11 111 Iv V VI VII VIII ix x XI XII I i 11 <3.

400 BAY OF BQTHA'/A 50 ' R6

i 4.00

1961 1962 1963 0 vill Ix x x1 x11 l 11 Ili Iv v vi VII vill Ix x x XII 1 11

2 o /i+o'i' ,'~ BAY OF 80THNIA 50

Fig. 32-33. Temperature and salinity /lvctuations at Station R 6, in the northern Bothnian Bci.y, 1961 to 1963. 31

Fig. 34-37. T7o:titaI distribution oj te)nperatvre, salinity and the dissolved oxygen, at va>iovs times, at Stations F 69 in the Lägskär Deep; at PÅ 4 in the Åland Sea; at F 30 in the Bot/inian Sea and at F 18 in the Quark. References

GRANQVIST, GUNNAR, 1936: Zur Kenntnis der Temperatur und des Salzegehaltes des Baltischen Meeres and den Klisten Finnlands. Fennia 65. Helsinki. HELA, IL➢IO, 1958: A hydrological survey of the waters in the Åland Sea. Geophysica 6: 3-4. Helsinki. LISITZIN, EUGENIE, 1946: The relations between wind, current and water level in the Gulf of Bothnia. Soc. Sc. Fennica, XIII. 6. Helsinki. —»— 1951: A brief report on the scientific results of the hydrological expedition to the Archipelago and Åland Sea in the year 1922, Fennia 73, No. 4. Helsinki. MERENTUTIK. JULIO. No. 200: Icewinter 1960/61 along the Finnish coast. Helsinki 1961. —»— No. 201: Temperature and salinity at the fixed Finnish stations 1957-1959. Helsinki 1962. —»— No. 206: Ice winter 1961/62 along the Finnish coast. Helsinki 1962. —»— No. 208: The Gulf of Bothnia in winter time. I. Data from winter cruises 1959-63. Helsinki 1964. —»— No. 213: Ice winter 1962/63 along the Finnish coast. Helsinki 1964. PALMMIN, ERIK, 1930: Untersuelmngen fiber Ilie Strömungen in elen Finnland umgebenden Meeren. Soc. Sc. Fennica V. 12. Helsinki. SlnuOaoiu, HEIKKI, 1946: On the temperature and salinity of the sea in the vicinity of the Bogskär lighthouse in the Northern Baltic. Sec. Sc. Fennica, XIII. 7. Helsinki. SJönLOis, VEIKKO, 1961: Wanclerungen des Strömlings (Clupea Harengus L.) in einigen Schären- und Hochseegebieten der Nördlichen Ostsee. Vanamo, 23, No. 1. Helsinki. WITTING, ROLF und PETTERSSON, HANS, 1925: Thalassologische Beobachtungen im Alanclsmeer und Schdrenmeer in Juli 1922 und Juli 1923. Merentutk. Julk. N:o 30. Helsinki. WITTING, ROLF, 1908: Untersuehungen zur Kenntnis der Wasserbewegungen und der Wasserumsetzung in den Finnland umbenden Meeren, I. Finnliinclische Hyclro- graphische Untersuchungen. No. 2. Helsinki.