- 56 —
11. New Oceanographic Investigations in the Northern North Sea.
By
Jens Eggvin, Fiskeridirektoratet, Bergen.
Tiased on the Salinity-distribution of surface generally speaking, the features of these charts are water G. B öhnecke1) has constructed charts very alike. A comparison with J ohn B. Tait’s 2) of surface currents in the North Sea. At the end the charts of surface currents based on drift-bottle experi author reproduced two charts showing, schematically, ments shows a good agreement, particularly in respect the surface currents in February and August, and of the southern part.
Salinity mlOfli.
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Ektrsurtd. L inai&srresL inai&srres #/////////////////, '{/ SCOTLAND ’ /i 5' ¥ Figure 1. - 57 — (111, 11) The northern part of the North Sea being subject on the more shallow eastern North Sea plateau the to an important fishery, for herring specially, it will surface current direction may differ from that of be very important, from a fishing point of view, to deeper layers in years when the North Sea water, extend as far as possible our knowledge on movement resting on the Atlantic water below, is widely distri- and exchange of water in deeper layers also and over *) Böhnecke, G.: Salzgehalt und Strömungen der the widest possible area. We may expect to find Nordsee. Veröff. der Inst. f. Meereskunde (N. F.) A. H. variations in the direction of currents at different depths 10. Berlin 1922. especially where different types of water are present 2) Tait, John B.: Surface Drift-Bottle Results in and where depths are great such as in the Norwegian Relation to Temperature, Salinity and Density Distri Deep. From current,-measurements outside Jären in butions in the Northern North Sea. Rapports et Procès- Verbaux des Reunions Vol. LXXXIX No. 8. Copen 1906 H elland-H ansen3) concludes: “In the upper hague 1934. layers down to a depth of about 100 meters there 3) Helland-Hansen, B.: Current-Measurements in is a fairly strong resulting stream from the South Norwegian Fjords, the Norwegian Sea and the North (SW-SE) to North, and there is a weak stream near Sea in 1906 (p. 28). Bergens Museums Årbok 1907. Ber the bottom going in a contrary direction.” But also gen 1908. NOR WAY é M W /M m . shstlhw. BSffäEN ISLAtJDS. ORKNEY /SlAN0\ 'JÆ.REN : E kirs und v / v y /y PtUrhtod Yy///////////////, 'SCOTLAND \ W&WM Figure 2. buted. This happened in the spring of 1936, while and biologic investigations within the northern in the spring of 1935 the North Sea water did not North Sea, from Lindesnes to Peter Head, in south, reach far afield, Atlantic water reaching the surface and from Shetland to Feie, in Norway, going north on the northern North Sea plateau. to Møre. Observations at Møre were made from the An important step towards better knowledge on M/S “Johan Hjort”. A total of 770 temperature and currents in the northern North Sea could be obtained salinity observations were made at 103 stations, in from a net of closely placed hydrographical stations addition: 286 determinations of 02, 277 on pH and with observations at all standard depths from surface 90 plankton hauls were made. From Fig. 2, it will to bottom and from coast to coast, made within the be noticed that sections were placed along each shortest possible time. Until recently no such material degree of latitude and across the waters between has been available. Shetland, Orkney and Peter Head. In the spring, 1936, the Fishery Directorate of The distance between two stations has generally Bergen hired the M/S “Armauer Hansen” from the been 10 nautical miles, exceptionally 20 miles, and “Bergen Museum” and made extensive hydrographic in the western part, across the Norwegian Deep, — 59 — (111, 11) 7—8 miles only. In Fig. 2 stations are indicated by foundation of the cruises and the programme was open circles. prepared by him in collaboration with Mr. J. E ggvin ; With the exception of the northernmost section Dr. R unnstrøm has kindly placed the hydrographical made on May 14th—15th, the programme was com material at the author’s disposal for further treat pleted within 18 days, from April 20th to May 8th. ment. The time interval between stations thus being short, On several occasions H elland-Hansen has stressed the observations are of great importance to the the importance of placing stations as close as possible ; hydrodynamic valuation of the material. A similar pro this has been strongly confirmed by the examination gramme was carried through during spring, 1937. of the present material. Important details for under These investigations are continuations and exten standing exchange of water-masses would have been sions of similar work begun by the Directorate in the lost if the net of stations had been more open. Very spring of 1935 and repeated in September of the probably, not only details but main features also same year. would have been better defined, still, if stations had Through the Investigation Fund of the Fishery been more closely placed. Industries Dr. S. R unnstrøm secured the economic Comparing hydrographical conditions in the North BottomUmjier ature nlA ■V / 5 Un S H fl/A N L /SI A VOS Sumhvryh ORKHCY j ISlAHDS'j ) £/!/ '■ / / / ■ / y Paftr Figure 4. - 60 — Sea during the springs of 1935 and 1936, a marked bottom temperature was generally 0-5 to 1° lower difference will be noticed in temperatures as also than the 1935 one in the northern part of the North salinities differ. In 1935, Atlantic water (e.g. water Sea. In 1935, bottom temperatures of less than 6-5°C. of more than 35 %0 salinity) penetrated farther into were very limited south of 59° N. Lat. (Fig. 3), in the North Sea than in 1936; a distribution very 1936, however, the major part of the northern North noticeable in the upper 50 m. layer, north of 59° N. Lat. Sea plateau was covered by water colder than 6-5° Salinities of 10 m. depth during spring 1935 and 1936 (Fig. 4) ; certain parts west of southern Norway even are recorded in Figs. 1 and 2, from which it will be show temperatures below 6°. During April, 1935, noticed that the Belt of Atlantic water, in 1935, was bottom temperatures varied between 8-04° and 6-37°, some 196 km. wide along the Feie—Balta section, in 1936 between 7-32° and 5-690. i. e., reaching from Shetland almost to the 3° E. Long. Along the section: Feie—Unst the bottom tempe This distribution may be considered about normal at ratures in spring, 1937, were somewhat lower than 10 m. depth. In 1936, however, Atlantic water reached corresponding temperatures, in 1936, along a line: no more than 82 km. east of Shetland ; south of 59° N. Unst to the W-shelf of the Norwegian Deep (about Lat. there is no marked difference noticeable through 3° E.). Differences recorded reach 1*7° but average out the two years. 0-7°. At the bottom of the Norwegian Deep no At 75 m. depth the Atlantic water-belt was 46 km. difference in temperature is worth recording. wider in 1935 than in 1936, whilst south of 59° N. Lat. Similar conditions occurred along the section : conditions are similar in the two years. Jären—Water Sound, viz., 1937 spring temperatures At the bottom the difference in salinity between were lower than the year before. Difference here the two seasons is hardly noticeable, although 35.30°/00 average about 0-5°, but reach at several localities as reaches slightly farther east in 1935. much as 1-2°. Difference in bottom temperatures in In the upper 50 m., water of salinity lower than the Norwegian Deep were minimal. 35 °/o„ was widely distributed in 1936 and caused an The greatest differences are recorded along the extensive formation of whirls. Half degree isohalines section: Egersund—Peter Head. Between Peter Head (full-drawn lines) and tenths of degree isohalines and the longitude 3° E. the difference in temperature (broken lines) in Fig. 2 indicate an extensive system of April 1937 from April 1936 was minus 0-6 to 0-8° of whirls between Bergen and Alesund westwards to whilst greater differences of l-l0 to 1-7° were recorded the 1° E. Long. Further, the isohalines of Figs. 1 between 2° E. and 5°30' E. Bottom temperatures on and 2 show a well developed whirl between Bergen the Norwegian Deep also were somewhat lower in and Jären in 1935 as well as 1936. 1937. In the above-mentioned work by G. Böhnecke,the It appears that bottom temperatures in the entire author points out that a whirl exists during May off northern North. Sea were low er— in some localities the Norwegian Stavanger coast. This whirl occurs in considerably lower — in April 1937 as compared to the boundary between Atlantic and coastal waters. the previous year. A decline of temperature from The author is of opinion that it is caused by increased April 1935 to April 1936 has been recorded previously, supplies of coastal freshwater forcing the coastal cur this development continued until April 1937. Bottom rent seawards. Closely studying Fig. 2, it will be temperatures of 1935 along the Feie—Unst varied noticed that a similar whirl is indicated near the between 6-51° and 8-04°, in 1937 they ranged from 3° E. Long., off Jären, just on the boundary between 5-40° and 7-35°. Corresponding temperature limits Atlantic and North Sea waters. The figure clearly along the Egersund-section to 3° E. were 6-29—7-05° illustrates that the former whirl between Jären and (1935) and 4-33—5-58° in 1937. Bergen, most strongly developed off Boknfjord Table 1 shows mean temperatures at bottom for (Stavanger), is quite different in character from the various sections in 1935, 1936, and 1937, respectively, latter; it is situated nearer the coast and within the and specified for the Norwegian Deep and the North coastal water; its direction, further, is clock-wise, Sea plateau. From this it will be observed that the contrary to that of the other whirl. It must be diffe decline is greatest in the southern part of the area rently formed, viz., as a reactionary eddy. under discussion, for obvious reasons the greatest de Bottom-temperatures differ considerably during the crease is recorded on the plateau. two years; north of Shetland the temperature is as Lower temperatures of Atlantic water in the North much as 1° lower in 1936, than 1935. The 1936 Sea is no isolated case, in other localities also decreas- Table 1. Section Entire section North Sea plateau Norwegian Deep 1935 1936 1937 1935 1936 1937 1935 1936 1937 Feie— Unst ...... 7-32 6-85 6-51 7-26 6-89 6-26 7-46 6-78 7-00 Slotteröy—Sumburg Head... — 6-40 6-11 — 6-40 5-82 — 6-39 6-55 Jären—Water Sound ...... 6-77 6-21 5-64 6-80 6*30 5-66 6-69 6-02 5-90 Egersund—Peter Head — 6-18 5-26 — 6-22 5-25 6-67 5-98 5-33 — 61 — (111, 11) *<{?ï a X0 < o g £ Eg Z H» O Figure 5. Temperature in 100 m. depth in spring 1936 (full-drawn lines) and in spring 1934 (broken lines). The small circles denote the net of stations. — 62 — ing temperatures during 1936 have been observed. of the section true north of Nordkap than in 1936. 4 fixed oceanographic stations were established by The transport of Atlantic water, eastwards, in 1936 the Fisheries Directorate along the Norwegian coast was much inferior to that of 1934 and the eastgoing in 1935/36. Observations are made once a fortnight heat-transport correspondingly lower in 1936. This throughout the year. The southern station (61°04'4 N., fully agrees with — probably being the main reason 4°50'5 E.) records colder waters in 1936 than the deep of — the westward move of the comparatively cold water of the previous year, at this locality consisting 3°—4° isotherms in 1936 as compared to 1934. of Atlantic water; this decline was continued in 1937. As already mentioned, a decrease in the spring Thus, the temperature about new-year was 0-9° temperature of Atlantic water was found when a com lower at 175—300 m. depth than a year before. It parison was made of 1936 and 1935 and it has been should be recorded, however, that the decline of deep proved that deep water temperatures have declined water temperatures recorded in March/April 1935/36 in the North Sea. Also salinities have somewhat de was but minimal in 1937, the mean temperature for creased . April at this station being slightly above that of 1936 Studying the distribution of the various types of and below the 1935-mean. These conditions correspond water during the two years, it will be noticed that the well with conditions of deep water in the northern force of Atlantic water has decreased. In 1935 the part of the Norwegian Deep (see table 1) when the area of the vertical section along 60°45' N. Lat., April temperature of 1937 was slightly above that between Shetland and Norway, covered by Atlantic of 1936, but considerably lower than the 1935-mean. water was 15-3 °/0 larger than in 1936. Also the Lofot-stations record decreasing tem In table 2, the distribution of various types of peratures in deeper layers. water is recorded as area-percentages of the three Studying coastal conditions off Norway a decline sections previously mentioned. I, Shetland—Norway Table 2. Sec.. I Sec. II Sec. III Various types of water 1935 1936 1935 1936 ' 1935 1936 Atlantic water ( > 35 °/00) ...... 86-0% 74-6% 78-4% 66-6 «/o 83-3% 66-8 % North Sea water (34 °/00—35 %,,)...... 10-8 - 22-6 - 15-4 - 28-7 - 9-6 - 27-7 - Water ( < 34 °/00)...... 3-2 - 2-8 - 6-2 - 4-7 - 7 1 - 5-5 - in temperature in general will have been observed (60°45' N.), II, Water Sound—Feistein, and III, the ever since 1934. Fig. 5 illustrates temperatures at Egersund section as far as 3° E. 100 m. depth off Tromsø and Finmark and shows a It is a fact that the strength of the North Sea water decrease in temperature from spring, 1934, to spring, has increased at the sacrifice of Atlantic water and 1936, (corresponding observations were not made in that decreases have been recorded on the quantities 1935). Isotherms for 1936 are recorded by fully drawn of less saline water. Within the area between the curves, those of 1934 as broken lines. Temperatures latitudes of the northern point of Shetland and that decrease from 6° off Andenes to 2° at the Varanger- of Scotland (to Norway) the quantity of Atlantic fjord estuary, in 1936. In 1934, the 5°-isothermreached water in spring, 1936, was 1479 milliard ton less than as far east as the Tanafjord (29° E. Long.), in 1936 that of 1935, — a difference corresponding to 14 °/0. it barely reaches east of Torsvåg (20°10' E.). In 1934, Calculations of water-transport and current are water of 3° or less was not recorded west of the SE.- not to be discussed in this paper; it may be said, port of Yarangerfjord and Fiskerhalvoya, in 1936 the though, that the calculations show considerably less relatively cold 3°-water reached as far west as near movement of water masses during April, 1936, when the Honningsvåg (27° E.). compared to those of 1935. The inflow of Atlantic It is worth recording that the major part of the water from the Norwegian Sea to the North Sea is E.-Finmark fishery in 1934 took place as easterly as considerably lower in 1936, thus causing a smaller the Varangerfjord and the Fiskerhalvoya where the quantity of Atlantic water and lower temperatures deep water temperature was about 3°, the fishing of deep water in April 1936 than the year before. was only poor west of the Finmark coast. In 1936, As has already been mentioned, the investigation with lower temperatures and the relatively cold was repeated in 1937, between April 12th and May 7th, 3°-limit reaching no further east than the Honningsvåg, but this year the net of stations has been extended it is a well known fact that a good fishery — the best to 139 oceanographic stations with 1123 observations. for many years past in E.-Finmark waters — took The distance between certain stations was reduced place just as far west as the Honningsvåg. It is an to 5 miles, in the SE. part of the Norwegian Sea the obvious theory that decreasing temperature has been investigations were extended further west, and in the cause of fishing in 1936 taking place so far west north to 64° N. Lat. of the 1934-fishery. In Vol. 2 of ‘"Norges Fiskerier 1936”1) two vertical sections are illustrated showing J) E g g v in , J e n s : Oceanografiske forhold i Nord-Norge that Atlantic water in 1934 occupied a far larger area under Lofot- og Finmarkfisket 1936. Bergen 1937.