VI. 



The exchange of water between the N. Sea and the Baltic through Cattegat and the Great Belt. 



Regarding this subject I must refer to former publications. x ) 

 The circulation of waters through the Great Belt, which 'is the chief 

 channel by which the ocean-water finds its way into the Baltic, can 

 be represented by the following, diagram. 



This figure represents a longitudinal section through the Great 



Belt, with 3 stations; one at Revsnaes in the northern entrance of 

 the Belt, another at Langeland at its southern egress, and the third 

 at Korsor, in the middle. The salt under-current enters the Baltic 

 nearest to the bottom of the sound and introduces water with a 

 salinity of 30 °/ 00 and 32 °/ 00 , the limits of which are shown 



Stare Belt 



24/7- 29/7 ID 07 



St. Lakeland 



St Korsor 



St. 



o i 3 TLaxLt.nriLe» 



Fig. 16. 



by the isohalines. The fresher surface stratum is the Baltic water 

 which streams out from the Baltic, its lower limit being represented 

 by the isohaline for 16 and 20 °/ 00 salt. Between the surface current 

 and the under-current there exists a limiting-stratum, in which 

 the two other water-strata are mixed. We see that the in-going and 

 out-coming kinds of water rest on each other like two wedges, turned 

 with their sharp ends towards each other. 



Korsor 





U 



{ 



B 



~ — — 





N -< € 



Fig. 17. 



€ s | 



The pressure of the salt water-masses from the Ocean drives 

 in the lower wedge, and the pressure of all the river-water that is 

 collected in the Baltic presses out the upper water-wedge. The re- 

 sult is an outward flowing surface-current (the Baltic stream) and 

 an in-going under-current. It was first believed that these currents 

 were continuous and that the one was dependent on the other, so 

 that a greater out-current at the surface necessitated a more rapid 

 inflow of salt water below, in order to preserve the balance in the 

 exchange of water between the Baltic and the Ocean. A. W. Cro- 

 nander, however, made some observations at the light-ships in the 

 Cattegat, which showed that both currents were not always active 

 at the same time. Then the opinion was adopted that it would be 

 the result of the wind and the barometrical pressure over the North 

 Sea if the lower wedge was pressed inwards more powerfully than 

 the upper wedge was pressed outwards: for occasions are imaginable 

 when the under wedge is pressed inwards with such force that its 

 salt-water masses dam up the surface-current, so that the latter ab- 

 solutely cannot make its way out. The reverse would be the case 

 if the water-pressure, or the wind- and barometrical pressure, were 

 strongest over the Baltic; the out-going water-wedge in the Great 

 Belt then being able to fill the whole of the Belt all the way to the 

 bottom and prevent the entrance of the under-current. In a word, 

 the exchange of water would, it was thought, depend on temporary 

 atmospherical causes, such as wind, rainfall, atmospheric pressure 

 etc. This is the opinion generally adopted at present. 



But in July, 1907, the Swedish Hydrographical Commission 

 made observations at the named stations in the Great Belt and, 

 with the help of modern apparatus, studied the movements of the 

 limiting-stratum lying between the surface-current and the under- 

 current. It was found that its movement was not always the same, 

 but that it was pulsatory or periodic and it was also discovered that 

 the movement proceeded from the lower wedge, which was pressed 

 inward more powerfully every 12th hour of the day and regressed 

 once in between, i. e., also at intervals of 12 hours. The limiting- 



1 O. Pefctersson. Stromstudier vid Ostersjons portar. Haffce III af Svenska 



surface between the currents, consequently did not lie exactly 

 straight, or, more correctly speaking, at a level slope from north to 

 south, but went in waves as shown by the following schematic 

 illustration. 



St.Langeland St.Korsdr St.Eevsnses 



Fig. 18. 



This was, of course, clearly a tidal-phenomenon and a very unex- 

 pected one for, as is well known, the tide ceases to ,be felt at the 

 embouchure of the Skagerak, no ebb and flood being noticeable at 

 Lindesnas. Inside Lindesnas the tide is slightly noticeable, amounting 

 on the Bohuslan coast to about 3—4 decimetres while in the Belt 

 itself, at Korsor, it is about iy 2 Danish foot at the surface. We 

 found, however that the tide-waves in the limiting-stratum at Korsor, 

 18 — 20 metres below the surface, were about 10 times as high or 

 about 3- metres. This showed that the ebb and flood of the North 

 Sea, which seems almost to disappear in the Skagerak and Catte- 

 gat, still exist in the lower limiting-stratum, although it is lessened 

 towards the surface by the upper stratum of light Baltic water, which 

 acts as a moderator on the waves. 



If this moderating surface-stratum did not exist, i. e. if the 

 Baltic were a lake (as during the Ancylus period) and the Great Belt 

 formed the innermost bay of the North Sea and the Cattegat, with 

 homogenous water of oceanic origin from bottom to surface, the al- 

 ternations of the tidewater along the shores of the Belt would pro- 

 bably be considerable. The low shores of the Danish Islands would 

 then, perhaps, have the appearance of a marshland defended from 

 the waves by enormous dams, as at Walcheren and on the Frie- 

 sian coast of the North Sea, or else of a »drowned-land », such as 

 exists off the west coast of Schleswig. It was, therefore, surprising 

 to feel the pulse of the sea beat so strongly in the depths of the 

 Great Belt. But the tidal wave which rushes in through the Belt 

 at a depth of 18 — 20 metres, does not cross the exceedingly uneven 

 bottom of the strait undisturbed and so enter the Baltic. Each 

 tide-wave drives like a cascade of salt water over the threshholds of 

 the Belts and sinks down into the deep channel of the Baltic south 



rografisk Biologiska Kommissionens Skrifter. 



19 — 



