Hydrographic and current measurements in the Fram Strait, August 1981 T O R GAMMELSRBD AND BERT RUDELS G a m e l s r o d , T. & Rudels, B. 1983: Hydrographic and current measurements in the Fram Strait, August 1981. Polar Research 1 n . s . , 115-126. The hydrographic (CTD) observations and pendulum current measurements obtained with M / S 'Lance' in August 1981 in the area west and north of Spitsbergen, are presented. The warm and saline northward flow, known as the West Spitsbergen Current, is found to follow the shelf break. In the Fram Strait the hydrographic structure is complicated because of eddies and other transient movements. The current measurements suggest a two-layer structure where the velocity vector rotates in opposite directions in the two layers with a period near the semidiurnal tidal period. T o r Gammelsrfld, Geophysical Insriture, Dio. A . , University of Bergen, 5014 Bergen-Unru., N o r w a y ; Berr Rudels, Norsk Polarinstitulr. Rolfstangcieien 12, 1330 Oslo Lufthavn, N o r w a y ; March 1982 (revised O c t o b e r 1982). Introduction The passage between Svalbard and Greenland, the Fram Strait, is the main connection between the Arctic Sea and the world oceans. It is very significant as regards the heat and mass exchange in the Arctic Ocean, and the large quantity of heat carried north by the ocean current influences the climate of the arctic region as a whole. The main positive heat flux through the passage is provided by Atlantic Water which is transported northward in the West Spitsbergen Current. Almost equally significant in the heat balance is the export of cold polar surface water and ice by the East Greenland Current (Mosby 1962; Vow- inkel & Orvig 1970; Aagaard & Greisman 1975). A careful study of the heat and water balance requires that the measurements be performed across the whole passage, since a substantial part of the northflowing Atlantic Water never enters the Polar Basin proper, but recirculates in the passage (Aagaard & Greisman 1975; SCOR Working Group 58. 1979). In August 1981 the Norwegian Polar Research Institute (NP) arranged a multidisciplinary expe- dition with M / S 'Lance' in the Svalbard waters. The expedition was in two parts. The first cruise was concentrated in the area north and west of Svalbard, while the second was in the northern Barents Sea. This brief preliminary report deals with the physical oceanography programme in the Fram Strait as part of a joint project between NP and the University of Bergen. Severe ice conditions in the Fram Strait pre- vented us from extending the stations into the East Greenland Current. Furthermore, due to damage in the CTD cable, the stations after St. 31 extend only to 1650m. This unfortunately reduced the possibility of studying the exchange of bottom water between the Greenland Sea and the Polar Basin, which is a subject of considerable interest (Aagaard 1981). Direct current measurements were obtained in the deeper parts of the passage, but some impor- tant stations on the continental slope west of Svalbard could not be taken because of poor weather conditions. The positions of the stations are shown in Fig. 1. Methods Vertical profiles (Fig. 2 ) of temperature and salinity were obtained using a Neil Brown CTD sonde lowered at a speed of approximately 1 m/ s. With a sample rate of 30 times per second, this gives a sample approximately every third centimeter. The CTD salinities were standardized against water samples taken with a Rosette sam- pler attached to the sonde. The bottles were trig- gered from the laboratory on board and the CTD values were noted simultaneously from the deck unit. 116 Tor Gammelsrod & Bert Rudels 6 0 0 - 1200 I 8 0 0 2 4 0 0 31100 25.0 25 I 26 0 2 5 . 5 2 1 . 0 2 1 . 5 2 8 . 0 r .;i ( Fig. 2 . CTD station 29. the deepest obtained on this cruise. Temperature (T), salinity ( s ) and density (&) versus depth are shown. Except for a thin surface layer the water column is almost homogeneous Fig. 1. Map showing the station positions. CTD stations are marked with circles and current measurements with triangles. Because of the presence of ice some sections do not follow a straight line. A t Station 25 we had t o replace the CTD sonde after it hit the bottom and was damaged. Both C T D s seem to have worked well, and when obvious erroneous calibration points were removed we got r.m.s. errors of 0.006% and 0 . 0 0 5 % ~ in salinity. Current measurements were obtained from pendulum current meters (Haamer 1974). These instruments consist of a plow-shaped wing into which a small plexiglass box is attached. Inside the box a compass needle is suspended o n a string and the box is filled with a solution of gelatine, water, sugar and glycol. Current measurements were performed by low- ering a heavy weight (150-200 kg) on a 6 mm nylon rope, which passes through a meter wheel. T h e current meters were attached t o the rope a t predetermined distances from the weight. About 10 t o 15 instruments were used a t each station. Subsurface floats were tied to the rope above the upper current meter and a surface marker was attached. T h e rig was left unattended for about one hour to allow the gelatine solution to stiffen. T h e direction of the current was then determined by the compass needle orientation and the speed by the deviation of the string from the vertical. Hydrographic and current measurements 117 The method has several drawbacks, especially when measurements are made in such cold and deep waters as in the Fram Strait. Because of currents the rope may deviate from the vertical, making the observation depths greater than intended. Because of the time needed to put out the system, there is also the risk that the gelatine solution will gel either during the actual lowering or while the system is approaching an equilibrium with the current field. To minimize these sources of error, the lowering was made at the highest possible speed; a system could be launched to 3000m in 25-30minutes. To increase the time available for lowering the current meters, the gelatine solution was kept as 'soft' as possible. Since it is not possible to determine with cer- tainty which observations have been affected by the factors described above, we will proceed as if all measurements giving sensible results are correct, while keeping in mind that uncertainty increases with increasing depth. In spite of these drawbacks, pendulum measurements do provide a simple and easy method of obtaining an instantaneous picture of the current field at a station. The measurements presented here are the deepest so far made with pendulum current meters. The CTD observations The CTD stations (Fig. 1) define six sections which are presented here ordered from south to north. The southernmost section along 78"N is pre- sented in Fig. 3. Using the classification proposed by Swift & Aagaard (1981), we recognize several typical water masses. Below 800 m the Norwegian Sea Deep Water (NSDW) (t < -.4"C, 34.90%0 < S < 34.94%0) is the dominating water mass (see Fig. 3a and 3b). There is a salinity minimum at around 1000 m where the salinity is slightly below 34.90% This cold water mass (t - -1°C) is usu- ally denoted as Greenland Sea Deep Water (GSDW) and is very dense (Fig. 3c). At the surface we find Polar Water (S < 34.4%0) which in summer may reach temperatures of up to +5"C. The Atlantic Water (AW), defined by t > 3°C and S>34.9%0, is found near the continental slope, sometimes penetrating up onto the shelf. This is demonstrated in Fig. 4. The geostrophic velocity field, which is computed assuming no motion at the maximum observation depth, shows that the Atlantic Water flows northward (Fig. 3d). This flow is known as the West Spitsbergen Current (WSC). On the sections taken farther north, we recog- nize the AW which follows the continental slope (see Figs. 5 , 6 and 7, and 8). The geostrophic calculations also yield a fairly consistent WSC. In the northernmost section (Fig. 8), the AW seems to be colder and less saline than further south. This could indicate that the AW is modified by mixing as it flows northwards, but it is also possible that the current splits up and the main branch is north of St. 76. The water masses in all sections are defined in the description of the section at 78"N. We would like to point out that at the surface on the outer parts of the two northernmost sections there is Polar Water with temperatures below 0°C. Polar Water is also found at the bottom of Raudfjorden (Fig. 8), and is probably produced there during winter. On the two long sections the geostrophic veloc- ities computed show a rather 'wiggly' nature (see Figs. 3d and 6d). This may be due to eddies in geostrophic balance with a radius of about 30 km. However, internal waves may distort the density field to such a degree that the apparent eddy structure is caused by the non-synoptic nature of the sections. This point is discussed further in the next section, where we present the current measurements. Current measurements Within the accuracy of the observed directions (* 30"), the observations revealed thick vertical layers with approximately unidirectional flow. The direction and speed of the current in each of these layers were taken to be those found by vertical averaging over the layers. The resulting simplified current field is shown in plan view in Fig. 9. The surface currents, which are probably dominated by local winds, have not been included. It must be emphasized that this is a non-synoptic picture, and spans a period of 14 days. The current field on the deeper (>2000 m) sta- tions (29-43) has a marked two-layer structure. The thickness of the two layers is 500-1000m. The velocity in a thinner (50-100 m) bottom layer often coincides with that in the lower main layer, but on two stations (29,35) it is in the opposite 118 Tor Gammelsr~d & Bert Rudels I . . . Fig. 3. Contours of (A) tem- perature, ( B ) salinity. (C) den- sity, and (D) geostrophic veloc- ity computed with zero at maximum observation depth based on the CTD section along 78"N defined by Stations 3650. Maximum observation depths are marked with crosses. Hydrographic and current measurements 119 m n 0 0 - N n n " m : , * w I " " I ' " ' I ' ~ " l ' ' ' 15 150 225 300 D l S T I K M ) : SECT 1 ON : S 1 GMA - T . T I M E : 8 { . 8 . 4 : 7 . 2 5 - € 3 3 . 8 . 1: 3 . 4 0 POS: 1 8 . 0 4 ° N 3 . 6 l n W - 78.00'N 1 2 . 0 1 ' E ALONG 78 N 120 T o r Giinimelsrod 6. Bert Rrcdels A 0 Pl '0 + I - z x 0 26 OIST. I N K f l : T E M P E R A l U R E I k OEG. C E L . SECTION ALONG 78 78.13" 8 . 9 9 ' E - 78.1.3'N 1 3 . 4 9 ' E O A T E l JULf 26 - 2 9 . 1 S B 1 I 2 x - P 0 Ff,q 4 . Samc ;I\ Ftg 3 hut b;j,cd o n t h e s e c t i o n along 78"N lion5 > \ b l 4 are ~n the \ame pos- ition as 42-50. N o t e the dlfter- I ' " ' 1 I 0 so 75 100 dutincd h! S t a t i o n c 1-10. Sta- O I S T . I N K t l : 26 PREICTICnL S R L ; N I T Y O R T E i J U L Y ZB - 2 9 . 1961 e m scale$ on t h e a x k SECTION R L O N G 76 78.13'N 8.99'E - 7 8 . 1 3 ' N 1 3 . 4 9 ' E direction. These deep measurements. however. are rather unreliable (see discussion above). The dicections of flon in the two main layers change considcrabl! from station to station. The observed speeds are rather high - 10cm;s - and If east-west sections drawn froin Stations 36- 33 and 3 - 4 3 (Figs. 10. 11) are compared with onl!, a little with d e p t h . the computed geostrophic sections (Figs. 6d. 3d), the agreement is poor. Neither the magnitude of the current nor the vertical shear show any sig- nificant correspondence. T h e current measure- ments give a marked two-layer structure on the southern section (Fig. 11) with a southward mov- ing upper layer above a northward flow in the lower layer. On the northern section (Fig. 10) the Hydrographic and current rneasitrerrients 12 0 5d 15 2d D I S T . I N K M : C , ., I h g 5 As Fig 3 for Stations 11-19 direction of flow changes in both layers from station t o station, suggesting the presence of eddies. Just as internal waves may distort the density field and affect the geostrophic computations, transient motions such as tides may dominate the current field. With comparatively few stations distributed over such a long period (10 days), it is almost impossible to separate time and space variations and identify mean and transient motions. In order to proceed we make the following hypotheses: (a) Semidiurnal tidal motion (M2) is the domi- nant periodic motion in the area. (b) The wavelength of the periodic motion is so large that all stations have the same phase. 0 25 PRRCTICRL S R L I N I T Y r - 50 1 5 O I S T . I N K H : This permits us to use all stations as o n e time series taken at a single station. Tidal motions in broad basins may be described by long gravity waves which are affected by the rotation of the earth. We therefore compute the angular velocity necessary to give the observed change in current direction between the stations. Since we are looking for semidiurnal tidal motions, 360" has been added to the observed change of angle if the time difference between two stations is greater than 12 hours. The angle between the currents in the two layers changes from station to station, indicating a baroclinic velocity field. Assuming that the velocity vector turns clockwise in the upper layer and counter- clockwise in the lower layer, we find that the average angular velocity in the upper layer is - w, , , , , c 7 I50 215 DIST. IN K f l : 15 CEOSTROPHIC VELOCITY I N C M I S E C . SECTION RLOUG 7 9 . 4 78.98.Y l . 5 1 ' Y - 79.67" 10.00'E O R T E I J U L Y 3 1 - RUWST 3 . 1981 Fig 6 A, Fig 3 for Stations 212-33 35"/h and in the lower layer 26'/h. The reverse situation. with the velocity in the upper layer turning anticlockwise and in the lower layer clock- wise. gives the corresponding values. 25"ih (upper) and -23"/h (lower) (see Table 1). This should be compared with W / h . which is the average angular velocity for Mr. It must be admitted that the scatter is considerable. A pos- sible interpretation of these observations is a tide generated baroclinic wave. T h e directions of the orbital motions in the two layers cannot be decided from the data. This is not surprising since 150 D I S T . I N K R : 1 5 PRRC TICAL SRLINITY SECT I O N RLONG 7 9 . 4 1 8 . 9 8 ' N 1 . 5 1 ' U - 7 9 . 5 7 " I O . o O * E D A T E : J U L Y 31 - R U G U S T 3 . 1481 Ja D I S T . IU K f l : - S I F R A - T H E T A . SECTION RLONG 7 9 . 4 7 8 . 9 8 ' N 1 . 5 1 ° k - 7 9 . 6 7 ' N 10.00'E O R T E : JULY 3 1 RUGUST 3 . 1 9 8 1 the mean time lag between the stations (-10 h) is close t o t h e semidiurnal period. T h e currents observed on the continental slope and shelf north a n d west of Svalbard are stronger and their direction corresponds better with those found from the geostrophic computations. O n the western slope the current is generally to the north (Fig. 10) and coincides with the core of saline Atlantic Water entering the Polar Basin (see Fig. 6). T h e east-west component (Fig. 9), on the other hand. shows great variations from station to station. Whether these variations a r e generated Hydrographic and ciirrent measurements 123 0 U, ,. - D - -I* Y) c - 0 - -m v1 YI m y l y l Y)Y) .y1 w v 1 0 T E M P E R A T U R E I N OEG. CEL. PRRiTICRL S R L I N I T V S E C T I O N S N E E R E N B U R 80.33" 9 . 9 8 ' E - 79-63" 1 k . S Z . E S E C T I O N SHEERENBUR 8 0 . 3 3 " 4 . 9 8 ' E ~ 7 4 . 6 3 " I L - 5 2 ' C O R T E : R U W S T 9 - L O . ] g a l D R T E : RUGUST 9 - 1 0 . 1 4 0 1 Y) r - 0 - OlST. I N K n : G E O S T R 0 2 H I C V E L O C I T Y I N C H / S E C . S E C T I O N S H E E R E L B U R a 0 . w ~ 9 . 9 8 ' ~ - 1 9 . 6 3 0 ~ 1 1 . ~ 2 ' ~ O R T E : RUGUST 9 - L O . 1 9 8 1 S I C N R - T H E T R . S E C T I O N SMEERENBUR 80.33" 9 . 9 E ' E - 79.63'N I 1 . S Z ' E D R T E : R U W S T 9 - I D . 1981 Fig. 7. As Fig. 3 for Stations 53-65. A L3 1 0 0 0 1 5 1 . I N fin: T E M P E R R T U R E I N OEC. C E L . SECTION R R U O N E F O 8 0 . 5 8 . N 1 1 29.E ~ 19-67" 1 Z . l l ' E ORlE: RUCUSl 1 0 - 1 1 . 1 4 8 1 I P R R C T I C R L S R L I N I T Y 5 C C T I O I I RRUONEFO 8 0 58.M I 1 29.E ~ 79 61" 1 2 . 1 1 ' € ORlE. RUGUS1 1 0 ~ , I . 1 4 8 1 Fig. 8. As Fig. 3 for Stations 66-76 124 Tor G a n i n i e l s r ~ d & Bert Rudels 81C 8 0 72 70O- l o o O D l o o I hfi % 7 7 - Fig. 9 Pendulum current measurements. vertical layer mcans. see text N O R T H C O N P 7 5 F 'F 73- 0 0 100 11 51 30 29 2 0 2 1 26 ~ , ~ l o o o rn 1000 2 0 0 0 0 k m 50 100 150 200 7 t 81 80° 7 q 0 7 8 O A g . 10. Contours of north com- ponent of current hased o n measurements at Stations 2 6 34. Hydrographic und ciirreiii measurenienis 125 35 s t 30 LO L I L ? L 3 - 200u - - - - N O R T H COMP - - k m 50 100 I 5 0 200 Fig. 11. As Fig. 10 for Stations 35-43. Table I. The change in direction of the currents i n the two layers o n Stations 29-43. For explanation. 5ee text. Station A I @upper A 0 upper B upper H lower A @ lower ti lower h degrees degrees o/h degrees degrees o/h Anticvclonic rotation in upper 29 260 30 10 210 31 1 n 230 32 8 15 33 6 0 34 4 295 35 20 200 38 30 260 40 9 230 41 5 135 42 5 185 43 6 1 50 laycr -310 -20 - 145 -345 295 -(265 + 360) -(60 + 2.360) - 330 - 265 - 5 0 - 325 -31 0 -2.0 -18.1 -57.5 -73.8 -31.3 -26.0 -36.7 -53.0 -10 0 -54.2 160 325 1 61) 260 245 315 315 305 340 50 10 t 195 t 165 t 260 - I5 +(290 t 360) + ( @ t 2.300) + 10 +325 + ? Y O t 40 t 19.5 t I h . 5 t 3 2 . 5 t 2 . 5 +27.0 i 2 4 . 0 r 1 . l t 65.1) + 5 8 . 0 + ( I 7 __ __ -35 x 25.3 ~~ ~~ Station A t Bupper AH upper @ upper 0 lower A 8 lower R lower h degrees degrees o ‘h degrees degrees oi h Cyclonic 29 30 31 32 33 34 35 38 40 41 42 43 rotation i n upper layer 260 10 210 10 230 8 15 6 0 4 195 20 200 30 260 9 230 5 135 5 1x5 6 150 50 340 215 15 65 (95 + 360) (300 t 720) 30 95 310 35 5.1 34.0 26.9 2.5 16.2 21.8 34.0 3.3 19.0 62.0 5.8 21.1 - 160 325 160 260 245 315 315 305 340 50 10 - 165 - 195 - 100 -345 -(70 t 360) - (0.720) 350 35 70 320 -16 5 - 1Y.5 - 12.5 - 5 7 . 5 -17 Y -24 0 -38 9 -7 0 -14.0 -53.3 -26 1 __ 126 Tor Gammelsrad & Bert Rudels EAST COMP 53 51 56 5 8 I , , , , , , I I I , 1 , I I T ’ ’ 0 k m 2 5 50 7 5 Fig. 12. Contours of east component of current based on measurements at Stations S 5 8 . locally by the bottom topography, or are due to transient motions, cannot be decided from so few data. On the north-south sections north of Svalbard (Figs. 12, 13) we find a relatively strong (10- lScm/s) and persistent flow towards the east. These observations show that the Atlantic Water present on the shelf moves east north of Svalbard as it enters the Polar Basin. This agrees with the classical observations made by Nansen (1915), Sverdrup (1933), and Mosby (1938). Summary Our measurements show a complicated structure in the waters and motions in the Fram Strait. It c t 500 i I ! 1---1-- I ’ ’ I ’ 4 0 k m 2 5 5 0 Fig 13 As Fig. 12 for Stations 71-75 is a region where 4-5 different water masses meet and mix and perhaps new water masses form. The simple inflow-outflow current pattern is compli- cated by baroclinic tides and probably by strong eddy motions. To reasonably resolve the structure in the passage would require a much greater effort than the present one. The warm and saline West Spitsbergen Current is revealed both by CTD measurements and direct current measurements. It follows the shelf break, turning east north of Svalbard. Acknowledgements. - We express our gratitude to the crew on board the M/S ‘Lance’ for their great help and interest in our work. Bruce Hackett is acknowledged for his critical review of an early version of the manuscnpt. Torgny Vinje organized our participation on the cruise and Arne Foldvik contributed with stimulating suggestions. One of us (BR) was supported by the Nordic Council of Physical Oceanography. References Aagaard, K. 1981: On the deep circulation in the Arctic Ocean. Deep Sea Res. 28A. 251-268. Aagaard, K. & Greisman, P. 1975: Towards new mass and heat budgets for the Arctic Ocean. J . Geophys. Res. 80, 3821- 3827. Haamer, J. 1974: Current measurements with gelatine pen- dulum. Vanen No. 1 . Mosby, H. 1938: Svalbard Waters. Geofysiske Publikmpner XI1 ( 4 ) . Mosby, H. 1962: Water, salt and heat balance of the North Polar Sea and of the Norwegian Sea. Geophys. Publ. 24, 28!+313. Nansen, F. 1915: Spitsbergen Waters. Oceanographic obser- vations during the cruise of the ‘Veslem~y’ to Spitsbergen-in 1912. Vid. Selsk. Skr. I . Matem.-Naturuit. KI. 1915 No. 2. Christiania. SCOR Working Group 58, 1979: The Arctic Ocean Heat Budget. Rep. No. 5 2 , Geophysical Institute, Div. A , Univ. of Bergen, Norway. Sverdrup, H. U . 1933: Oceanography. Scientific results of the ‘Nautilus’ expedition 1931. Part 11. Pupers in Phys. Ocean. and Met. I1 (1). Cambridge, Mass. Swift, J . H . & Aagaard, K. 1981: Seasonal transitionsand water mass formation in the Iceland and Greenland Seas. Deep Sea Res. 28, 1107-1130. Vowinkel, E. & Orvig, S. 1979: The climate of the North Polar basin. In Orvig, S. (ed.): Climates of the polar regions. World Survey of Climatology 14. Elsevier. 370 pp.