This paper not t0l! be cited without prior referenee to the author

International Couneil for

Exploration.of the Sea

C. M. 1972/C; 6

Hydrography Committee

•

Intermediate Polar Water in the Denmark -Strait

- "Overflow" August -1971 -

by

Svend-Aage Malmberg

Marine Research Institute

Reykjavlk

Introduetion

In August 1971 a joint Ieelandie-German oeeanographie survey was

made in the Denmark Strait from the r/v "Bjarni Scemundsson".

The purpose of the survey was to investigate the so-ealled "overflow" of

bottom water aeros s the Iceland-Greenland ~idge. This was in. a way a

preliminary investigation for the planned ICES "Overflow" program in 1973,

a program suggested and headed by Prof. Günther Dietrich, Institut für

Meereskunde. University of Kiel.

The work eonsisted of standard hydrographie stations and direet

eurrent measurements from an anehored buoy as weH as from pydrographie

stations. The eurrent measurements were made by the "Institut für

Meereskunde" (M. Stein Land the hydrographie work by the Marine' Research

Institute (Hafrannsoknastofnun) in Reykjavlk.

This paper deals with some preliminary results obtained. from the

hydrographie datCl..

Material

The hydrographie material eonsists of 22 hydrographie stations

(temperature and salinity) whieh are somewhat ineomplete as regards

. ,- -2 . - .-

standard depths and ~pacing and 45 vertical continous recordings from the

sea surface to the bottom (max. depth of about 890 m) by means of XBT 's.

The location of the stations {s shown in Figures 1 and 2. The sec­

tions are numbered I. II. II 1. IV and V. Section I was worked twice and

sections IV and V are not dealt within this paper.

Results

a. Water masses

The observations .. made-in·August;.l97L,show, )the known water ... ~

masses in the Denmark Strait as reported by Dietrich { 1957 L Stefansson

(1962. 1968) and Gade et al. (1965). These are

• Atlantic Water t :> 6°C S'> 35.00%0 ; bt = 27.58

Polar Water t< oOe S< 34. OUo/00 ; ()t < 27.32

Atlantic Intermediate Water tN l.oC . S tV 34.95 0/00 ; 6't N 28.03.'

Arctic Bottom Water t< oOe S = 34.92%0 ; Gt = 28.08

•

Irminger Water (t.:: 3.50-4. OOoC ; S = 34.92% 0) was not observed. pro­

bably due to the norther1y location of the sections.

A distinct temperature and salinity minimum .layer was found in

section I. especially traced by means of ~he XBT recordings (Figs. 3. 4.

7 and 8). Farther north in sections II and III (Figs. 5.6; 9 and 10).

it is not as distinct as in section I. but is though indicated as I. '.-

shown in the; figures~. '. This minimum layer has previously been observed

in the area (Stefansson ,!962. 1967.__ Malmberg 1:963 • .9ade et al. 1965). but

as far as the author knows never illustrated in. such a detail as by the

c~ntinous.XBT recordings here presented.

This intermedia~~ Polar Water with temperatures below 00 and

salinit~es of about 3.4.50-34.55%0 (G'f rv 27 . 76,) ~cCl:lmot develop by" '"

the mixing of the water masses existing in the study area. but must be

formed farther north - probably in an area as far north' as the Greenland

Sea - by the mixing of Polar Water and· Atlantic Intermediat~ Water.

It then nows southwards between these two water masses to the Iceland­

Greenland Ridge. where it has been observed off Kägur (100 m. Stefansson.1962. p. 90).

The oxygen content of about 7 ml/l found in this intermediate water

in September 1963 (Gade et al. 1965. Stefansson 1968) also points to the

fact that a relative1y long time has passed since it was at the sea, surface.

whereas the Polar Water in the .surface layer had values above 8 mI/I.

•

•

- 3 -

When -the intermediate Polar Water crosses the Iceland-Greenland

Ridge it rnoves toward greater depths because of its density. as observed in

the present investigations where it was found down to a depth of 500-:"600"m.

It appears in a thin layer - asound reason why it so seldorn has been

observed by standard depth casts - .which in connection with possibly great

velocities rnay contribute to a rapid mixing 'with the surrounding water

rnasses in the area. This is supported by the fact that it has never as far

as the author knows been found farther south in the Irrninger Sea than: at the

location of seetion I (Gade et a1. 1965. Stefansson 1968).

b. Overflow of Arctic Bottom Water

The results of the investigations made in· August 1971 in the Denmark

-Strait show no overflow of Arctic Bottorn Water across the -Iceland-Greenland

Ridge during the investigational periods (August" 20-23 and 29-30).

The Arctic Bottom Water was only found in the two northernrnost sections

in the Denmark Strait (Seetions II and III. Figs. 5.6.9 and 10)..Le.

off Latrabjarg on August 21-22 and off Kopanes on August 22-23. From

there on it seerned to flow north and eastwards along the Icelandic slope.

These findings are in accordance with former observations. such as

Dietrich' s (1957). who draws attention tb interrnittent overflow of Arctic

Bottorn Wat~r across the Iceland-Greenland Ridge.

The cold "bottorn water l1 along the East-Greenland Slope observed

by means of XBT recordings in seetion Ia. (station 5. Fig. 3) rnay at first

be interpreted as Arctic Bottorn Water. whereas closer analysis' indicate.

that this water is the above-mentioned intermediate Polar Water.

It should be noted that the anchored buoy rnentioned above. anchored

at 400 m depth.; on the East-Greenland Slope (section I. station 4. Fig. 3)

was located in an area where Atlantic Intermediate Water and Polar Water

predorninate but not Arctic Bottom Water. '',i ...::,. ll, : •• ,,'

,.'1';;: :",1'

c. Variability

11 '. ) , . .. '

The distribution of the different water rnasses in the. Denmark Strait

is known to be very irregular. This is clearly shown in the present' •.

investigations. The great changes observed in seetion I (Figs. 3. 4. 7

and 8) during aperiod of only 8- 9 days present an interesting feature of

the August 1971 survey. These ·take place in the Atlantic Water as weH

as in the other water rnasses. Thus. Polar Water has spread -eastwards in

•

•

- 4 -

the' surface layer fromAugust 20-21 to· August 29, and in the bottom

layers the Atlantic Water has been replaced by Atlantic Intermediate Water

and intermediate Polar Water. This last water mass has during the same

period .moved from the East-Greenland Slope farther out into the Strait.

No attempt has beenmade to correlate these findings with· meteorological

factors.

d. Comparison between reversing thermometer temperature observa­

tions and XBT recordings

The accuracy of the XBT recordings (The SIPPICAN CORPORATION·

MARION. l\~ass. U~S.A.) is given as+ O.l oC. Experience shows that it

is necessary to take great care when shooting the XBT for absoulute

accuracy. Diagrams of reversing thermometer observationsver.sus XBT

recordings from seetions I (Fig. 11) seem to be quite acceptable, having

in mind the irregular patterns of the hydrographie conditions' in the study

area and the factors of depth and time of the different measurements as

weH as the measuring units. The correlation in section la is better than

in lb. where the XBT recordin'gs are in general higher than those of the

thermometers. It must be noted that the line drawn in the respective

diagrams is the ideal one. or tOCth = tOCXBT .

Conclusions

The conc1usions.; of this paper can be summarized as follows:

1: During the investigational period (August 20-30. 1971) no overflow

of Arctic Bottom Water was observed across the Iceland-Greenland· Ridge.

2. Direct current measurements of the overflow from an anchored buoy

on the East-Greenland Slope were made in an area dominated by Polar

Water and Atlantic Intermediate Water (section la. station 4. Fig. 3).

3. Great changes in the distribution of the different water masses in

the study area were observed during an interval of only 8- 9 days

(section I, Figs. 3. 4. 7 and 8).

4. Closely spaced XBT recordings can be useful for oceanographic

studies in an area like the Denmark Strait and for studies on overflow of

bottom water gene rally when depths ;do not exceed 800 m.

5. The' main result stated in this paper is the distinct water observed.

at intermediate depths in the Denmark Strait. with temperatures below

OOC and salinities as low as 34.50% 0. This intermediate Polar Water

- 5 -

seems to be part of the general hydrographie eonditionsdn the area and has

only rarely been reported beeause water boHles at standard depths have

failed to diselose it. The intermediate Polar Water· must be formed by

mixing between Polar Water and Atlantie Intermediate Water farther north.

probably in the Greenland .Sea, from where it flows southwards aeross the

Ieeland-Greenland Ridge in thin patterns. It seems to mix rapidly with

the surrounding water masses sinee it has never been observed farther

south than the loeation of seetion I of this paper (Figs. 1 and 2).

· .References

Dietrich, G. 1957

Gade, H. G., S. A. Malmberg,

U. Stefansson 1965

Malmberg, . S. A. 1963

Stefansson, U. 1962

Stefansson J U. 1968

Schichtung und Zircll1ation der Irminger

See im Juni 1955.

Ber. Dtsch. Wiss. Komm. Meeresforsch.

14 J 4.

Report on the 'joint Icelandic Norwegian

Expedition to the Area between Iceland

and Greenland 1963. Preliminary

Results. •

Nato, Subcomm. on Ocgr. Res. Techn.

Rep. 22.

ICNAF - Norwestlant II National Report. ,

Sero No. 1319 Oceanographic Observa­

tions.

North Icelandic Waters.

Rit Fiskideildar, 3.

Dissolved nutrients J oxygen and water

masses in the Northern Irminger Sea.

Deep-Sea Research, 15.

o

•

SURVEY I20-23/8 1971

2•

Figures 1-2

•

Location of Stations

SURVEY2o XBT ST._60 29-30/8 1971

39

•0 •

00

46m: .. Ib

0 0 ••0 0

0

1l: •Jd 0

310

300 250

r----------------- --

"OVERFLOW" DENMARK STRAfT

t·C- XBT20-2118,1971

0ST.2 3 4 5 12

m '0-g- '>=8-

100 7--,-

200

300

700

600

1500

400

800

•

•BOn.m. 50 o

Figure 3 Temperature distribution in Section Ia - XBT.

100

200

300

400

~oo

600 -

100-

800

70n.m.

"OVERFLOW" DENMARK STRAIT

tOC-XBT

29/8,1971

o

Figure 4 Temperature distribution in Section Ib - XBT

-~-9°­

"___--8°---

"OVERFLOW" DEN MARK STRAIT

tOC-XBT

,21-22/8, 1971

o,--",;:,,;~r-,-~~r----=-r----T:""';~--r~r-----;r--__-=-1;:..3--....,

m

100

200

(!)

1°u.a::u.«0..,

m300

u.«..Ja::Wt-:1:-«(f)..J

1°

400

600

500

e'

o700'----L.------l..------J-----..L-----l-- --l__--.J

50n.m.

Figure 5: Temperature distribution in Section I I - XBT

fOC-XBT

22-23/8, 1971

"OVERFLOW" DEN MARK STRAIT

_____- 8" -------

I"

>I"

----I"~

- 0"

<-I·

;~-I.----------0"

mOr-s;;..T.;;"·120~~~/~.;22;1~~/~.~g2~2§===~~~~~=~;::lJtF~~:::::::::::=o""'--T=---=--...;2=r9;"'-'-----:--------=71~_-9.·---__

100

200

500

300

600

400

100n.m..

Figure 6: Temperature distribution in Section I I I - XBT

50 o

500 .

600

500

600

sr. 2 4 6 sr 2 300 4 60 0m ___I. 2" m -31.

"--32.0

~7~~33.0 --

100 J457~100 34.7 •

6°

'" 5·-".

~:34.7 34.8 34.9

) .200 200 -

2"-

-........,..300 300 >35.0

e400 400

700 700"OVERFLOW" DEN MARK SlRAIT "OVERFLOW" DENMARK STRAITe loe 5°/00

20/6,1971 20/6,1971

800 800

20n.m. 10 o 20n.m. o

Figure 7 Temperature and salinity distribution in Section Ia

•

...........::::::=:::::::34.9

J4.8

\. ~50

~~34.7~,:

34.8

" , .. ,. ' .. .. .. .. ..>34.95 ' .. ..,

Or---=.::.:....=.:::..-------:;e:...-:::::;;;oo---:;<.::--::::>'.;.~-...,

m

800"OVERFLOW" DENMARK STRAIT

5 0/0.

29/8,1971

500 -

600

700

100

300

400 -

200

•

•

•

/........-/.

t·c29/8,1971

~OO

700

·OVERFLOW· DENMARK STRAIT800

600

0Sr. 39

m -2·

<2·•

100

>J.)•

200

/.

300

e400

o 30n.m. o

Figure 8 Temperature and salinity distribution in Section Ib

Figure 9 :

...u.1l:......0'"111........Ja:WI­" ....fI)...J

8° •-

,oe21-22/8, 1971

"OVERFLOW" DENMARK STRAIT

/~:../

O..-....:s~T.!:... .!.;19~ .--__!.:17~_o:::::::-;;;;~~=--,,.y..::.......;::-_,_~13:..-_....,

m~<) ~ ,.

600

100

500

200

300

400

400

500

600

' ..o ..,,• .34.95 \

\\\\

o

13

Cl... a:... ...0-'

Ul... .....Ja:WI­r· ...U) ...J

"OVERFLOW" DENMARK STRAIT

21-2218, 1971

Temperatureand salinitydistributionin Section II

o700'I----I ..l- --..l. ..l- --l .l..-_--l

50n.m.

200

____/0_---- ""

300

400

500

600

"OVERFLOW" DEN MARK STRAIT

,·e22-23/8, 1971

IOOnm. 50 o

o

S%o

22-23/8, 1971

"OVERFLOW· DENMARK STRAIT

-------

" ... ,"

"",34.95\\

\\

\\

--34.0'-------

3~)34.5

500

300 . ;;.----

400

100

e200

Figure 10 Temperature and silinity distribution in Section I I I

5o-2 '--........._-.L._-L-_...I-_.&..---IL....----I._--'-_........_-'--_"-----I'---~

-2