I This paper not to be cited without orior re ference to the author

Transcription

1 I This paper not to be cited without orior re ference to the author International Council for the Exploration of the Sea C.. M. 1984/C 17 Hydrography Comn1ittee Formation heavy bottom water in the Barents Sea by,,. :,. :- ~.. - Lars Midttun Institute of Marine Research P.O. Box 187, 511 Bergen-Nordnes Norway Abstract Formation of heavy bottom water through rejection of brine during ide freezing is a regular phenomenon in th~ Barents Sea, particular on the Novaya Zemlya shelf, but also observed in other shallow localities of the sea.... Water with temperature around -1 C and salinity above is found moving eastwards on the southern side-of the Novaya Zemlya-Franz Josef Land Channel which conununicates with the Polar Sea. The relative high salinity is a result-of admixture from heavy water formed during the freezing proc~ss..,'. '

2 - 2 and (1983) in discussing formation the since the a third of suggest saline to of water of the Arctic Ocean (Polar Sea) main sources are deep water from Seas through the Fram Straith But deep waters are not exceeding is required to obtain ~he salinity water which highere The authors Barents Sea is the likely source of dense ocean This view is supported from in the different watermasses Also lower to sees two high enough freezing the adj (1981) the formation of the so-called the Ocean with salinities higher than to be augmented by a salt source probably Atlantic water or on the shelvese Aagaard s, one is cooling of Atlantic water with, the other is brine rejection during the e The latter has to occur in selected areas on ~ As an example Aagaard refers to an obseropen shelf in the Chukchi Sea near bottom as high as exceptional ing temperature, but regard this as However, high formed by ice zing are not exceptional occurrences in Barents Sea.. Already Nansen (196} took observations of the famous Russian scientists dr~ Knipowitch, dr. Breitfuss and Admiral Makarow (Knipowich 1951 with the results from Arnundsens expedition with 11 Gj u and his own observations on the nfram" expedition (N'ansen, 1961 in forming his theories on the formation of bottom waters of the Northern Seas, including both the Barents Sea and the Polar Sea (Arctic Ocean) is examining the observations available from the makes a point of, the heavy water formed particu larly in shelf, Barents Sea, notably on the Novaya Zemlya Wolleb~ck the Norwegian vessel

3 ""'... _,...,"""~~ in ition 71 48'N, 49 38'E observed water with = (t =.8 C, S = 35 17}. Nansen concluded heavy water could only be formed by i.ce freezing and brine e Nansen further believe that heavy water formed in Barents Sea can supply the bottom water of the Polar Sea through the deep channel between Novaya Zemlya and Franz Josef Land. Makarovs observations of low,temperatures here (Knipowich 195} supported this suggestion, but Makarov s salinity samples were not trustworthy enough to make a firm statement :f.n the matter.. A similar is also the basis for the bottom water formation in the Antarctic as first suggested by Mosby (1934). Mosby (1967) an instructive description of the process which are both to southern and northern oceans. Since the question of heavy water formation and the processes of deep production in the northern seas have again been brought into focus, it might be of interest to look at some newer observations from the Barents Sea of heavy water formation which can confirm the early observations and the conclusions drawn by Nansen (1961. The formation of heavy water can be regarded as a process which takes in t"i.~o steps.. First the density is increased from cooling of water. This is a process which gradually becomes slower as the temperature is decreasing towards the freezing point,.. The step starts when formation of ice begins and brine is rejected to the water, whereby the density is increased because of the increasing salinity. Now the process of heavy water formation much faster. A rough calculation shows that the at unit energy loss {calories} becomes about ten times higher after ice formation has started.

4 4 Al will s <the In the water now formed and depres how the heavy water The during summer winter This cor re Barents to socalled in winter and melts from melting water a is the remains with last winters quality salini ty.. amount of which have delution from melting of 2 meter ice is most cornmon, but values more than 2 m are often observed in the Barents Sea Next and renew continue area the than rally until of This the fast summer surface layer winter situation and then eventually more ice year In this northern netto s from the sea through means a heat loss in winter in summer The ice thickness therefore genethe same water mass from winter to winter, is where the isolating effect water from higher heat losso the marginal zone being of heavy water since high production minimal isolating conditions, i.e. If in addition the surface layer and the ice is the production locality the water will be maximum@ The process can be from surface water to

5 = 5 According (196) the water is formed on the Novaya Zemlya Bank. Since 1971 this area has some times been visited by Norwegian vessels "G... O.. Sars" and 11 Johan Hjort 11 during their autumn cruises in the Barents Sea.. In Table I are listed selected examples of temperature and salinity observations the near bottom layer, normally about 1 metres the bottom. We shall look closer at some of these examples.. In August Johan Hjort.. worked a short section along longitude 54 E between 'N and 76 [N.. The location c.an be seen in Fig.. 2 no.. 1).. Temper a tu re salinity and density ( <J t }_ are shown in Fiq. 3. The observations are from four stations with Nansen water bottles at standard depths taken to 1 m above bottom and with 3 miles between the stations. On the bank area in the northern temperature layer has of the section bottom water with freezing 35.9 are found. The upper 5 meter deluted by ice melting and somewhat warmed during the summer season.. The mean salinity in the layer from surface to bottom 34 "75 at the northernmost station.. I.f the content of salt in sea ice is 5 /oo, melting of 158 cm ice would be enough to change the mean salinity from to in the whole 125 m deep layer.. In the deep depression in 75 N the bottom salinity as high as 35e24 and temperature Cm This water has obviously been formed on a shallow bank area and drained down into the hollow where it was observed. In this hollow, between 74 and 76 N outside the west coast of Novaya Zemlya, heavy water has been observed several times (Tab.. I, example no 3, 4, 5, 8, 9, 1 )_. The highest density was observed with CTD by 11 Johan Hjort" in 1981: Q.t = (Tab.. I example noq 91 But also further south on the Novaya Zemlya coast heavy water originating from ice-freezing has occasionally been observedo Thus, in the area near the above-mentioned "Heimdal 11 from 19 (Nansen 196), njohan Hjort" observed in 1979 bottom water with salinity and temperature below o8 C (Example 6 and 7 of Tab I}.

6 6 a Barents bar.tk area near The mas into the over the ins more 1979 \vhen ea us a Barents Sea westwards sion of the eastern 3 meter deepo In Fig. 4 from the Novaya Zemlya Bank in in and the Fig 2} Both water originating from the with warmer and along the slope.. inflow from west is regime in the water In s in the north-running deep water current from the sinking heavy bank water will, even if the production of such at normal The area beyond 5 water about 25 water of in 1979 no t extent eastward from heavy Barents Sea Polar Sea could not be shown this over a ridge with silldepth probably in posit about 77 45'N, E. In 1974 Fig meters to was covered during a survey of from bottom are shown in area itself is by water of salinity near freezing At depths around 3 of Novaya Zemlya and Franz this salinity and In saline water is with " worked section along the 6 E and Franz Land.. Cold and on southern side of the channel and sal around 35a (Fig. 6}

7 This moving through the channel Sea.. Also on Bank area heavy water formed through brine rejection observed Thus, in G.. O. Sars" observed in position 77 15'N, l E at 16 metres and 1 metres above bottom t ~9 C, S = 35G25 and ot = 28e4. The heavy water will sink diagrams f the S w. ~z..,.-... ""'-'~ bottom'watere and gradually mix with warmer and illustrated in Fig. 7, showing T-Sat different depths along the track The formation show during fenomenon served in not intend to analyse the deep water Barents Sea in full scale. The purpose is to of heavy water through rejection of brine zing in this area a more or less regular on the Novaya Zemlya shelf, but also obshallow localities. It is further shown that water with temperature around -1 C and salinity above is found moving eastwards on the southern side of the Novaya Zemlya-Franz Josef Land Channel which communicates with the Sea. The relative high salinity is a result of from heavy water formed during the freezing process..

8 References Aagaard, K On circulatlon in the Arctic Ocean Res e a rch 28' A, Knipowitsch, N Hydrologische Untersuchungen.im Europaischen Eismeer.. Annal en der Hydrogra phie und Mari t imen Meteorologie, Mosby, H The waters of the Atlantic Antarctic Ocean.. Det Norske Viden:.. Akad..,. ~.. Res.., Norweg i an Antarctic Exp ed.., 1927_;1.928, 1 (ll}_.. Mosby, H., Bunnvannsdannelse i havet.. The Nan sen memorial lecture October 11th '1'9'6 5.. Uni versi tetsforlaget, Oslo, Nansen, F Northern Waters: Captain Roald Amundsen's oceanographic observations in the Arctic seas in 191" Vit en sk. -:s etsk.. Skr..,.!_, JYT:a th.. -Nat u rv.. Kl.., no.. 3, 14~.., Swift, J.. H., Takahashi, T. and Livingston, H... D The Contribution of the Greenland and Barents Seas to the Deep Water of the Arctic Ocean. Jourrtal of Geophysical Rese a rch, '88, '59'81-59'86..

9 Table 1 Examples of bottom water observations west of Novaya Zemlya. Example Inst~y- Obs. no. Station ment Date Position Depth S /oo 1 G.O. Sars st.51 STD 28 Aug 'N 56 'E loo Johan Hjort st.537 NC 21 Aug 'N 54 'E' G.O. Sars st.824 STD 5 Oct 'N 53 4'E G.O. Sars st.85 STD 6 Oct 'N 54 'E Johan Hjort st.l13 CTD 2 Sep 'N 51 'E Johan Hjort st.l283 CTD 12 Sep 'N 5 'E Johan Hjort st.l284 CTD 12 Sep 'N 51 'E Johan Hjort st.l59 CTD 1 Sep 'N 52 'E Johan Hjort st 161 CTD 2 Sep 'N 54 'E Johan Hjort' st.l62 CTD 2 Sep 'N 52 'E (m) t c x)std: Bissett-Berman System NC: Nansen cast CTD: Neil Brown System

10 Fig. l. Illustration of heavy water formation.

11 SO" 2 ~ 3 m Fig. 2. Location of the sections and hydrographic stations.

12 1 2 Cl) LIJ :: 1- LIJ ~ ~ 1 :I: l- a.. LIJ Cl 2 ~27.1 I , ~---28,---- ~28,1~ 1 --~ ,~ dt ~ AUG ~ 28,4 Fig. 3. Section 1. "Johan Hjort" August

13 "JOHAN HJORT" "G.. SARS" LAT. N i'!."jo'?s :~o 7!1 11' LONG E so!i2' L ST. NO (1) 9!'1 1 I LONG.E!i2" LAT. N 78 n 76 S\N+O_. 1 9,_11_9..._1_, !17..,_-+ ---r~ /-1,5 /--1 9,_6 ~ ,==== ) ,6 lll w 1 a: I W ::E: ~ :X: 1-.. ~ 2 lll w a:: 1- w ::E: ~ :X: 1- a.. w Cl /"'~ / //],:~ / " ' V 282' dt ~. 'JOHAN HJORT" 2-22 SEP.19 "G.. SARS" 25 SEP ~28 o't "G.O.SARS" 25 SEP Fig. 4. Section 2 and 3 "G.O. Sars" and "Johan Hjort" September l979.

14 so ss 6o 6 so ~~~~~~-L~--L-~~-~~-+--~~-L~--~~~ r~r~~--l-~-l-~l-~-l~.. ~~_l~l_~_l_l~ 79" -,99 34, ,1 34,, ,8-1,!8 2~1 2{7 34,87 28,9 28, ~16 34,9 34,9 34,88 28,1 2a.9 28~8 78" 1/. -1,18 34,86 34,84-1,66-1,13 34,95 34~ I -1~35 2~ ,;9 34,82 2 ~ 28,4-1,;6-1.[ ,88 34,82-1,6 34, n.. 2s.o3 28,7 28,5 28,8 -,~2 34,97-1,5 28, ,2 -,~ ,. -1.pt -,~4 3~ ~1 2~8 15 1f8 35~,J 34,97 2a.1o 2,17 2a.1o '14,92 34,9l 34,911 34,95 34,94 28,15 28,1 28,16 28, , ,15 28,13 77" 1,.6-1,55 76" ~.95 34:9s -1.f1 34,98. -1,63 35,4-1,77 35,2-1,48 -,81 -,73-1,57-1,23-1, ,98 34~97 34~99 35,3 34:9 34:98 28'.15 28,15 2,17 2,14 28~15 'l8}2 28,9 28,17. 34,97 3f 1~ 1~2 26,16-1,21-1,8-1.:6-1,36 -t.p 1!7 9 34,9 35,9 35,4 35, 28, , ,19 -J.84-1~ ,7 3S,o7 ~ ,27 28~ i1 2e:22 2a.17 1~ " 73" -1,5 34,93. -1,84, 35,11. -1,77 35,4..1! 34, ,13 211~ ~. 28, ,94 Fig. 5. "G.O. Sars" 1-15 October Left: Temperature and salinity of the bottom layer. Right: crt and observation depth.

15 LAT N 8" 79 LONG E 6' ST. NO LAT N 79' 78' 77' 76' L ST NO ftpo/) 1 L_L~ ~~ JU ) -1 > -1 2 toe 7~ 3 t C 78' 77' I ~ 2 2 S /oo 3 S%o , Fig FRANZ JOSEPH LAND - NOVAYA ZEMLIA LONG. E. 6 "G. O.SARS" 29 SEP. - 3 OCT Section 4 between Novaya Zemlya and Franz Josef Land along 6 Ee Left: "G.O. Sars" 28 August Rigth: "G.O. Sars" 29 September-3 October 1973.

16 ~----~~--~~-, r--r , Fig. 7. A: B: c. "G.O. Sars" St. 85, August 1981 in 77 15'N 19 34'E -16 m. ' "G.O. Sars" St. 799, August 1981 in 76 2'N 19 59'E, -245 m. "G.O. Sars" St. 659, September 1977 in 75 5'N 14 53'E, -363 m.