The North Atlantic current and its associated eddy field southeast of Flemish Cap

Deep-Sea Research. Vol, 34. No. 7, pp. 1163- HNS. 1987.

0198-t)149/87 $3.t~0 ~- 0.00 © 1987 Pergamon Jourmds Ltd.

Printed in Great Britain.

The North Atlantic Current and its associated eddy field southeast of Flemish Cap W. KRAt SS,* E. FAItRBA(II,* A. ArrSAM.; J. ELKLX", a n d P. KOSKE:!: (Recei~'ed 2 June 1986 in rerixed ./brnl 3(I September 1986; (receiPted 24 Norenzber 19N6)

Abstract--During August 1984 R.V, Arnold Vei,u'r and R.V. Po.~eidon carried out a hwlrographic survey of the North Atlantic Current southeast of Flemish Cap. Satellite-tracked drilting buoys and a Geomagnetic Elcctrokinctograph (GEK) provided direct current information. The data set obtained allows R)r a detailed description of the North Atlantic Current (NAC) between the Newfoundland continental slope and the western flank of the Mid-Atlantic Ridge, During the period of observations the NAC branched at 47°N, 41°W. Nineteen x 111¢' m ~ s i ( 19 Sv) of thc current followed the isobaths of tile continental slope towards the Northwest Corner. the remaining part of the NAC continued directly towards northeast. F r o n / both branches a considerable part of the transport was expelled into rcturn flows, reducing the net Iral/sporl through the boundaries of the observation area to 12 Sv towards the north and 32 Sv towards the cast. The buoy tracks confirm the expelling from the branches. ()bjcctivc analysis of the buoy data shows that the branching is a transient lcaturc. The eddies southeast of the NA(', howe\or, have a lifetime of several months.

1. I N T R O D U ( T I O N Tim G u l f Stream e x t e n s i o n area s o u t h e a s t of the G r a n d B a n k s a n d its c o n t i n u a t i o n into the L a b r a d o r Basin is o n e of the most c o m p l e x regions of the N o r t h Atlantic. Cold a n d less saline waters of the L a b r a d o r C u r r e n t on the G r a n d B a n k s and warm saline waters of the G u l f S t r e a m are s e p a r a t e d in this area by a strong interface, the North A t l a n t i c C u r r e n t ( N A C ) front. T e m p e r a t u r e and salinity differences of 20°C and 4 p . s . u . , respectively, often are o b s e r v e d over distances of < 3 0 n m . C o m p l e x systems of m e a n d e r s , extrusions a n d eddies form c o n f u s i n g p a t t e r n s that change rapidly in time and space. The area is of e x t r e m e i m p o r t a n c e for the E u r o p e a n climate. B e t w e e n the N e w f o u n d land Ridge a n d F l e m i s h Cap the N A C forms, which carries a b o u t 31) Sv of G u l f Stream water and L a b r a d o r C u r r e n t w a t e r t o w a r d s the E u r o p e a n basins b e t w e e n Portugal a n d G r e e n l a n d . T h e heat loss to the a t m o s p h e r e of these water masses is a m a j o r c o n t r i b u tion to the climate of c e n t r a l a n d n o r t h e r n E u o p e . T h e region b e t w e e n the G r a n d B a n k s of N e w f o u n d l a n d a n d the M i d - A t l a n t i c Ridge ( M A R ) has b e e n studied by several investigators. A c c o r d i n g to MANN (1967, 1972), the G u l f S t r e a m b r a n c h e s n e a r the N e w f o u n d l a n d R i d g e , with 30 Sv c o n t i n u i n g towards the east a n d 20 Sv f o r m i n g the N A C along the N e w f o u n d l a n d c o n t i n e n t a l slope. This ::: Institul ffir Mcereskunde an dcr Unixersitfit Kiel, Kiel, F.R.G. '," Institute o1 Thcnnophysics and Elcctrophysics, Academy of Science of the Estonian SSR. Tallin. U .S.S.R. 3 [nstitut tOr Angewandte Physik dcr Uni,,'ersitiit Kicl. Kicl. F.R.G. 1163

1164

w. KRAIISSet

al.

transport further increases to 45 Sv due to the slope waters which join the NAC. This figure was confirmed by DIETRICltet al. (1980) who compiled a map of the current system of the North Atlantic, mainly from data collected during the IGY Polar Front Survey. HiLL et al. (1975) found comparable transport values as MANN (1967) for two sections ~ti the northern boundary of Mann's observation area; however, they found only a weak Slope Current of 5 Sv. They argued that it merges in the NAC further south than deduced by Mann. In a comprehensive study CLAI~KI~, et al. (1980) confirmed thc transport given by Mann. They derived 26 Sv for the NAC. The anticyclonic eddy in the area around 42°N and 42°W proposed by Mann in the separation area transports 18 Sx according to this study and turns out to be a more complex feature than originally assumed.

A very extensive study of the hydrography between the Newfoundland Ridge and Flemish Cap was started by Soviet investigators in 1982 (BARANOV and G1NKUL, I984). Repeated sections in all four seasons confirmed the principal features and showed the high degree of variability in that area, This variability is also evident from moored current meter arrays (FoFoNiWF and HENI)R'C, 1985). The NAC now appears as ~ul intensive current reaching down to at least 200(J m. On the average it closely follows the 4000 m line between the Newfoundland Ridge and Flemish Cap and is identical with the Subarctic front. In most cases it passes the Flemish Cap and continues towards the north into the Northwest Corner (52°N), where it forms a loop and heads towards east. The NAC front turns out to be the southernmost extension of the Subarctic front (Subpolar front). Superimposed on the frontal jet is a broad drift towards the east in the entire are~t between 42°N and the Subarctic front (KRAUSS, 1986). Within that westwind drift narrow intensified current branches are often observed. From 1981 to 1984 a sequence of sections was carried out along the MAR to investigate the NAC over the ridge (MEINCKEet al., in preparation). It can be shown that an average transport to the east of 30 Sv is concentrated in several current branches with about 10 Sv each. The location and intensity of the branches, however, changes from year to year. The variability of the current system, its meanders and its eddy field, has recently been documented by means of satellite-tracked buoys. Several experiments during the last live years in that area (KRAuSS and MEiNCKE, 1982; KRAUSSand K~,sE, 1984; KRAUSS, 1986) confirmed the high variability of the currents between the Grand Banks and the MAR. Meanders and eddies dominate the region and an intensive mixing of water masses occurs. The variability of the current system south of the Subarctic front seems to be closely related to the eddies near the NAC front. Eddy kinetic energy as derived from satellite-tracked buoys shows peak Values along the Gulf Stream (RicHARt)SON, 1983) and the NAC (KRAUSS and K3,s~-, 1984). From hydrographic surveys it is evident that the offshore side of the NAC along the Labrador continental slope is dominated bv elongated high pressure cells (WoRTttlNGTON. 1976; CLARKEet al., 1980; BARANOVand GiNKVL, 1984). As will be shown in the present paper a considerable amount of water leaves the frontal zone at the northern rim of these anticyclones, partially encircles the anticyclones, and becomes part of the eastward drift. Mann's anticyclone near 42°N, 42°W is just one idealized example of these pressure cells. Some relation seems to exist between the length of the high pressure cells at the offshore side of the NAC and the locations of outbursts of cold Labrador Current wateJ

1165

The North Atlantic Current

from the Grand Banks. From satellite images LAVlOLETrE (1983) concluded that three frontal extrusions are very common in that area, which are identical with low pressure troughs on the shelf, and which force the N A C to shift further offshore: (i) a trough over the Newfoundland Ridge which forces the Gulf Stream to flow around this topographic feature; (ii) a trough over the Newfoundland seamounts and (iii) a trough east of Flemish Cap. In 1981 the extrusion of Labrador Current water from Flemish Cap into the deep ocean was documented by one of our satellite-tracked buoys, which became trapped in the Flemish Cap eddy (Ross, 1980) from July to September, before finally entering the deep ocean on the southern edge of a Labrador water extrusion. After crossing the shelf edge it did not follow the 4000 m isobath towards north (the usual path of the NAC), but moved further east towards 42°W before heading northward in water of 19°C with a speed of 1.2 m s-I ( H A R D T K E and MEINCKE, 1984). This is a clear indication that the NAC had shifted considerably towards the east during this event. From 1980 to 1985 infra-red images of NOAA-satellites have been systematically screened to identify the main path of the NAC according to the sea surface temperature (SST). Maximum gradients of SST have been taken as location of the current. During most of the year the area under consideration is cloud-covered, but there are quite a number of clear views available which allow the following classification: (i) The classical situation: The NAC follows the 4000 m isobath into the Northwest Corner with minor extrusions of relatively cold NAC water towards the offshore side of the current. (ii) The branching situation: Only part of the NAC continues into the Northwest Corner; large amounts of water are expelled from the current through the gaps between the high pressure cells on the eastern flank of the current. An example of the latter case is shown in Fig. 1, which was drawn from the SSTdistribution on 26 July 1981. The 12°C isotherm separates the Labrador Current water at 48 °

52 °

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Fig. 1. T e m p e r a t u r e d i s t r i b u t i o n at the sea surface according to N O A A - 7 A V H R R at 26 July 1981 s h o w i n g s i m i l a r features o f the N A C at 48°N, 40°W as d u r i n g the prcsent cruise

[F.C. = Flemish Cap: broken line = branches due to DIETRICII C/a/. (1980). Northwest Corner is located tit 51°N, 44°W].

1166

W, KRAUSS eta/.

the surface from the NAC, which continues along this isotherm into the Northwes~ Corner and then turns towards the east and southeast. The 16°C isotherm shows two extrusions towards east, one north of the Newfoundland seamounts (44°30'N, 45°W), the other east of Flemish Cap. This isotherm and all the water having temperatures between 16 and 19°C (the eastern flank of the NAC) turn directly towards the northeast at about 48°N, 42°W and form a separate current branch. The present study is based on a survey whose purpose was to provide more information about this branching by investigating the NAC branches imbedded in a broad drift to the east. These branches temporarily exist between 45°W and the eastern side of the MAR, before they finally disappear by mixing. To achieve a large-scale survey with sufficient horizontal resolution a joint experiment of the Institute of Thermophysics and Electrophysics, Tallin (ESSR), and the Institut for Meereskunde, Kiel, was performed in 1984 with two ships, R.V. Arnold Veimer and R.V. Poseidon. The situation found in the area was similar to that shown in Fig. 1 and exhibits a strong branching of the NAC. =.~ T t l I .

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The area of observations (Fig. 2) extended from the Newfoundland continental slope to the western flank of the MAR. Thirteen sections were carried out perpendicular to the

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Station map of the survey area cast and southeast of Flemish Cap. Large numhcr~, ~rc fl~e section numbers, small numbers ~lrc slation llLIITlbCl's,

The North Athmtic Current

1167

estimated stream direction from 5 to 21 August 1984. R.V. Arnold Veimer worked in the western and R.V. Poseidon in the eastern part of the area. Because of logistic reasons the meridional sections did not extend far enough to the north to include the NAC at the Subarctic front. However, a section carried out simultaneously along the western flank of the M A R from 48°N to 53°N by R.V. Meteor yields further information on the northern part of the observation area. It confirms that this branch is well established. R.V. Arnold Veirner and R.V. Poseidon were equipped with CTDs, GEK-systems and satellite-tracked buoys. On board R.V. Arnold Veimer a Neil Brown CTD was used. The instrument was calibrated on board four times during the cruise by means of it NB1S calibration unit. In situ calibration for conductivity was carried out by means of a General Oceanic rosette sampler with 12 bottles and a Guildline Autosal Laboratory Salinometer. The results of the comparisons with 2/)8 samples suggested a mean conductivity correction of c = 0.011 mS cm -I with a standard deviation of 0.010 mS cm-~. On board R.V. Poseidon two Multisonde CTD-systems were used. Details of the data processing and calibration are presented elsewhere (FAIIRBA¢'If et al., 1985). The accuracy of the final salinity data is, in spite of corrections, not better than 0.02 p.s.u. At some stations the irregular drift of the conductivity probes did not allow for a correction. These stations are not used in the present analysis. The two ships occupied 105 stations with a nominal spacing of 311 nmi. As the purpose of this work was to study meso- to large-scale processes the data set was reduced by vertically averaging over 20 m. The profile depth ranged between 1900 and 2000 m. On both ships a Geomagnetic Electrokinetograph ( G E K ) was towed to provide direct current information. The G E K s used during these observations consisted of modified von Arx type systems (Vos ARX, 1950) built and operated by personnel from the Institut for Angewandte Physik der Universitfit Kiel. The main modifications to the yon Arx system consisted of completely enclosed Ag/AgCl-electrodes with ion exchange membranes or glass-diaphragms providing the necessary electrical contact to the seawater, thus improving the stability of the electrode zero potentials; separately towed electrodes attached to two separate cables at distances of 3(} and 80 m behind the research vessel, thus providing for a simple zero point control by towing both electrodes either at 30 or ~", 80 m instead of changing the ships heading for a certain period of time. The high stability of the electrodes required a zero-check only every 2-3 h. During the cruise 25 satellite-tracked drifting buoys were launched. All buoys had a window shade drogue at 100 m depth. The buoys were manufactured by Hermes Ltd, Halifax, and monitored by the A R G O S system. The original data consist of 4-5 fixes per day with an accuracy of about 0.2 kin. These data were submitted to a probability check and interpolated by a three point Lagrangian scheme to obtain eight equally distant buoy positions per day. 3.

IIORIZONTAL

DISTRIBUTIONS

Horizontal maps of temperature, salinity and dynamic height were constructed by objective analysis. An isotropic covariance function f (r) = c~2exp(-r2/R 2) was used, with R = 165 km and cy2 the variance and r the distance between data points. It was assumed that the error variance due to measurement errors and small-scale noise amounts to 15% of total variance of the fields. Only areas where the expected r.m.s, errors in the interpolation are < 5 0 % of the standard deviations of the field were contoured. The applied method is described in detail by HH, LER and KAsE (1983).

1168

W. KRAt/SSet al.

The dynamic topography 10011500 db during August 1984 (Fig. 3a) characterizes the NAC by dynamic height anomalies of 0.6-1.3 dyn m. This concentrated band follows the depth contours from 43°N, 47°W to 47°N, 41°W, where the current splits into two branches: water masses falling into the range from 0.7 to 0.8 dyn m follow the isobath towards the Northwest Corner, those in the range from 0.9 to 1.2 continue to the northeast.

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An elongated high pressure cell, characterized by dynamic anomalies of more than 1.0 dyn m, flanks the current on the offshore side and extends as far east as 35°W. Within that ridge three maxima can be identified; however, they are not intensive enough to appear as contour lines. To the east of the ridge low pressure cells are found. Thus, the dynamic topography of the area is characterized by the gradient field of the branching NAC and its associated eddy field southeast of the current. The dynamic topography reflects the observed hydrographic field. As an example, temperature and salinity distributions at 100 db are depicted in Figs 3b and c. At this depth the NAC is associated with the temperature range of 3-16°C and salinities ranging from 34.6 to 36.1 p.s.u. The high pressure cell at its eastern flank consists of water of 1718°C and salinities of 36.1-36.5 p.s.u. Within that high pressure cell we find the highest temperature and salinity values at that latitude. The high pressure cell southeast of the NAC is the dominating feature in the area of observations. It extends over more than 900 km from the southwest to northeast. Relative maxima within that pressure cell regulate the cross flow transport and therefore play a major role in the mixing of water masses which occurs in that area. 4. SECTIONS

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The North Atlantic Current

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1174

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to show the resultant water mass distribution tit the western slope of the M A R (Fig. 4d). For each section we display t e m p e r a t u r e , salinity and calculated geostrophic v,:locitv (relative to 1900 db). The w e s t e r n m o s t section 5 (Fig. 4a) from Sta. ~7 to~ards t!w continental slope at Flemish Cap (Sta. 34) shows the North Atlantic Current front, with t e m p e r a t u r e and salinity differences of more than 10°C and 3.0 p.s.u, ovc: a distance <~i 30 nmi. The warm, saline pool of water, which is identical with the ridge in the d3;nama~ topography at the offshore flank of the current, reaches down to more than 100{t m~, ,,. vigorous eddy field east of the current continues towards the western flank of the M/\b~. Maximum velocities of 100 cm s ~ are obtained for the upper 100 m: the c~unter currc~3i at the eastern side of the ridge amounts to 4t! e m s After branching at 47°N, 41°W the waters of the eastern llank ol the c u r r c l l l h ~ d northeast. On section 9 (Fig. 4b) t e m p e r a t u r e and salinity differences between the wate~ in the ridge and at the northern side of the current still reach 5::C :~ld I p.~,~ , respectively. A current with maximum speed of (~(J cm s ~ is associated wilh ~i~ir, gradient zone. The ridge extends towards 47°N, 36°W. Most of the water cncilcles this feature and flows back towards southwest. A minor part of it, however, continues towards the c~a>,t.. as seen in section 11 (Fig. 4c). The eddies along this section are less vigorous, and the dominating feature is the mean gradient from north to south, which is related to currclli,~ of < 2 5 c m s ~. This becomes even more evident in the section along the western slope o1 the MAI~ (Fig. 4d). The most intensive eddies are found near the Subarctic front. Their i n t c n s ~ decreases towards south, as has been inferred earlier from drifting buoy dat~.i (Kk.xt ~, and KAsE, 1984). Due to mixing by eddies the distributions along the section south ~.:,i 45°N become more uniform, as is evident by comparing the salinity dislributions ,~I sections 9, ll and 13. Meteor stations on the prolongation of section 1.3 show the typical N A C at tht: Subarctic front (Stas 53-55) and the eddy field south of it. Unfortunately, a large gap exists between the Meteor and Poseidon sections. However, the marked mean inclination of the isotherms and isohalines between the southernmost Meteor station m~d ih~:: northernmost Poseidon station (given as dashed lines in Fig. 4d) indicates a strong, current branch between Stas 44 and 780. This can only be the continuation of that branch which originates east of Flemish Cap. Similarly the branch at the Subarctic front is most likely the continuation of that branch which passes towards the Northwest Corner. 5

I) I R E C T

('URRENI

MEASUI?,EMENTS

Direct current measurements were carried out during the experiment by means od satellite-tracked drifting buoys and a towed GEK. Twenty-five buoys, drogued in 100 m depth, were deployed in the area. The trajectories for the time 6-16 and 16-26 August (in Figs 5a,b) confirm the pattern of the dynamic topography: there is a well-developed eastward flowing current at the northern edge of the elongated pressure cell and a return flow at its southern flank. South of 45°N the situation is more complicated. Most of thc buoys in that area were deployed in the central part of the high pressure celt and showed little movement during the time of the expedition. The buoy tracks give additional information on the time variability of the current field There are various examples shown in Fig. 5 where buoys crossed the tracks of other ones,

Thc North Athmtic Current

1175

when they passed by the same location after some time. One example of very rapid changes is observed in the area at 44°N and 44°W. There 12 buoys were launched along sections 1,2 and 3 within 4 days. Three of them, the most westerly ones, followed nicely the N A C up to 35°W, where they recirculated to the southwest as expected by the dynamic topography. Two others, launched in the high pressure cell, passed to the eastern side of the pressure cell due to the relative m a x i m u m in the dynamic topography (Fig. 3a). The rest of the buoys were launched at the eastern flank of the cell and drifted

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towards south. The situation near 45°N. 44°W changed within 10 days (Fig. 5b). After passing around the northeastern part of the high pressure cell, buoys crossed the previous location of the NAC at 45°N. 45°W heading towards the continental shelf. Obviously, the NAC at this later time had shifted back further to the west. It appears that no buoy followed the continental slope towards the Northwest Corner directly after deployment. Only after circulating around the high pressure cell one buoy headed further north (Fig. 5c). This indicates the intermittant character of the observed branching. In Fig. 10 we will display for the northern part of the area a series of 4-day averages of drift velocities and the associated stream functions obtained by objective analysis: the mean value of the drift velocity i~ the area has been subtracted from the figures for each period. The overall mean amounts to tt = 3.3 cm s-j. v = -0.6 cm s ~. Further details are given in Section 8. On the first map of the series the velocity vectors confirm the current band along the Newfoundland continental slope southeast of Flemish Cap, the branching and the extended high pressure cell. At 36°W all of these buoys turned to the south. Three of them encircled the high pressure cell, one only continued towards the east. Following the main stream the current speed was subject to importanl variations. This holds for the current and the counter current. The areas of intensive currents are closely related to strong gradients of the pressure cell. having diameters of about 100 km. We further mention the a s y m m e t r y o f t h e c u r r e n t speedwhichreached 100 cm s-~ on thenorthernflank and only 50 cm s-~ on the southern flank of the ridge. Unlike the buoy observations. G E K data give current information simultaneous in time with the geostrophiV velocities estimated from the hydrographic section. Furthermore they give quasi-continuous information along the track. However, one must keep in

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mind that the G E K records not simply a current in a given depth, but a conductivitx. weighted depth integral of current in the entire water column (SAnFORD, 1971 ). The G E K observations presented here (Fig. 6) also support the main features of the described current system. The observed voltage signals reached maximum values up to 2.5 inV. Using the vertical component of the earth's magnetic field in this region, the voltages have been converted to flow velocities according to the basic equation fo~ motionally induced voltages (yon ARx, 1950) with a k-factor of unity. As can be seen from Fig. 6 there were strong eastward directed current components with velocities of (l.8-1.1 m s-' in the area from 46°N to 47°N and 40°W to 44°W (north ot the high pressure celt). The return flow south of the cell existed from 45°N to 46°N and 37°W to 44°W with velocities of ().5-().8 m s-~. The westward directed flow vectors at 47.5°N and 42°W, which also are shown in the geostrophic flow scheme, can be interpreted as track-transverse components of the northwesterly directed branch of the NAC. South of 45°N, between 36°W and 41°W. eastward directed current vectors are shown again in accordance with Fig. 7. In comparing Figs 6 and 7 with the dynamic topography of Fig. 3a one has to bear in mind that the arrows in Figs 6 and 7 are actually current components perpendicular to the hydrographic sections. t~. ( ; E ( ) S T R O P I I I ( '

('AI.('UI.

ATI()NS

Various authors have given geostrophic mass transport for the area of consideration Because of technical reasons, 2000 db often is used as reference level. It is evident thin calculations using a fixed reference level are not the most realistic approach, although this method does allow for the determination of the significant features of the current field and their relation to other calculations for the North Atlantic. In the present case we have calculated the currents relative to 1900 db because some profiles did not reach 2000 db after processing. An estimation of the related difference by means of station.s reaching deep enough showed that the currents are modified by some tenths of a c m s J and the mass transport by some ppt. To aw)id any speculations on the reliability of geostrophic calculations in the near-surface layers, the mass transport was only calculated up to 40 db below the surface. For an estimate of the reliability of the geostrophic currents, the computed currents in the level of drifting buoys ( 100 db) are shown in Fig. 7. The main structure of currents appears as in the direct current measurements and agrees remarkably well with Figs 6 and 10a. In the areas of strong currents the directly measured and the geostrophic currents agree within 10%. As this corresponds to the geophysical noise level, the discrepancies for small current speeds are not surprising. The agreement between the directly measured and the geostrophic currents gives some confidence in the derived baroclinic mass transports. A representation of the calculated transports from section to section and across the boundaries of the survey area is given in Fig. 8. Each arrow represents the cumulative transport across that part of the section for which the currem direction remains the same. The curvature of the arrows is drawn according to the dynamic topography. Cumulative transports of <5 Sv are not shown in the figure for clarity. The NAC can be followed with a mean transport of 35 Sv. This transport increases near the maxima of the high pressure cell to more than 40 Sv ~md decreases between them to <20 Sv. The current at the southern flank of the ridge yields ~

1179

The North Atlantic Currellt

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return flow of 25 Sv, ranging from 15 to 30 Sv. The net flow to the northeast across section 3 amounts to 25 Sv. This transport splits at 47°N, 41°W into two branches: one follows the continental slope towards the Northwest Corner with 19 Sv. Part of this northward transport is c o m p e n s a t e d by flow to the south. The remaining northward transport across the northern boundary at about 47°N amounts to 12 Sv. The other branch crosses the Newfoundland Basin to the east with a transport varying between 18 and 40 Sv. The overall transport out of the area towards the east between 41°N and 53°N amounts to 32 Sv. Current branches with similar water-mass properties but smaller in transports can be found further to the east over the M A R , with 7 and 12 Sv, respectively (ME1NCKE et al., in preparation). As the southern branch leaves the area through the gap between the Meteor and Poseidon sections, it does not appear as a remarkable geostrophic current, but it can be detected in the mass transport. 7. W A T E R M A S S E S W I T H I N T H E N A C A N D W A T E R M A S S T R A N S F O R M A T I O N

The area under investigation covers the transition zone where subtropical water masses carried by the N A C are mixed with water masses of subpolar origin. In the most southwesterly section 1 (Fig. 2) a salinity m a x i m u m of more than 36.35 p.s.u, is found in 100 m depth with a t e m p e r a t u r e of 17.5°C. This corresponds to the 18°C water, as defined by WRIOnT and WORTmNGTON (1970). It presents the high salinity limit of Central Atlantic Water of ISELIN(1936), which according to HARVEY (1982), is the Western North Atlantic W a t e r ( W N A W ) with a comparable lower salinity as its counterpart in the eastern North Atlantic. The deeper part of the W N A W includes water with t e m p e r a t u r e s as low as 4°C and salinities of 34.99 p.s.u. It further includes the Western North Atlantic

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The North Atlantic Current

1181

Central Water ( W N A C W ) and part of the Western Atlantic Subarctic Intermediate Water (WASIW), according to EMERY and MEINCKE (1987). They restrict the W N A C W to temperatures higher than 7°C. This water mass extends vertically to 900 m depth and represents the main inflow from the southeast into the survey area. At the western border of our observational area the influence of the Labrador Current is noticeable due to low salinity, cold water. This water can be classified according to LEE (1968) as Canadian Arctic Water (CAW) with -1.75°C and 33.20 p.s.u. Our sections do not extend far enough on the Grand Banks to obtain pure CAW, which is consistent with the fact, that we do not reach the southward flow of the Labrador Current in the geostrophic calculations. HILL et al. (1975) found the Labrador Current inshore of 2000 m water depth. However, remarkable intrusions (sections 3 and 5) of cold, low saline water in the northeastward flowing current indicate the intensive interaction across the frontal zone between the two currents. The water masses in the area are depicted as T - S diagrams (Fig. 9). Four stations are plotted in Fig. 9a: Stas 17 and 21 of section 3 represent the waters of the southeastern high pressure cell and the water closest to the Labrador Current, respectively. At 2000 m depth, the water masses of the section are nearly identical. On the Labrador side salinity decreases with decreasing depth. The surface layer is warmed from about 0°C at 100 m to more than 7°C at the surface (Sta. 21). Within the N A C strong mixing occurs between the two water masses, mainly by interleaving, as is obvious from Stas 18 and 19 in Fig. 9a. That part of the NAC, which follows the isobaths towards the Northwest Corner, consists of water masses highly influenced by Labrador Current water (Fig. 9b). On the northern flank of the high pressure ridge (Stas 716-724, Fig. 9c) the water masses of the NAC are dominated by W N A C W . A clear influence of subpolar waters was found only at Sta. 724. That part of this current carries 14 Sv. As stated in the previous section, similar current branches are found again west of the M A R (MEINCKE et al., in preparation). The current branches observed in that area seem to be directly related to the (time-dependent) branching of the NAC east of Flemish Cap. 8. T I M E D E P E N D E N C Y OF T I l E E D D Y F I E L D

As mentioned in Section 5 in connection with the trajectories of the satellite-tracked buoys, the vigorous eddy field and the associated branching of the NAC are a timedependent phenomena. Repeated hydrographic sections on a seasonal base yield mass transports which vary by about 50% (BARANOV and GINKUL, 1984). Some insight into this variability may be obtained from the satellite-tracked buoys which stayed in the area for several months, slowly migrating towards the east due to the superposed mean drift of u = 3.3 cm s-l, v = -0.6 cm s-l from August until November. Figure 10 displays the time evolution of the eddy field east of Flemish Cap. Because of the large inhomogeneity of the mean current in the area, only the northernmost buoys have been used for an objective analysis of the current field. The area 0 < x < 800 km, 0 < y < 500 km covers the range from 44°N to 48°N and 35°W to 45°W. The left lower corner corresponds to 44°N, 45°W in each figure. Fifteen buoys have been used to compute 4-day averages of the current vectors. The data have been grouped into overlapping 3 week periods, shifted by 10 days. The sequence starts with the period from 7 to 27 August 1984 and terminates at 5-23 November 1984. The mean value for each

1182

W. KRAUSSet al.

period has been subtracted as required by the analysis. As in horizontal maps an isotropic covariance function was used. As the buoys drift freely, each 3 week period gives ~ different number and distribution of data. The arrows show which features are supported by data and which ones are merely results of the objective analysis. During the time of the hydrographic survey (7-22 August) the elongated pressure celt dominates the area. During the second half of August this cell breaks off into two independent cells, indicated by the arrows in the center of the figure. During the following months only the eastern part of the area is well covered with buoys, They sho~ that the eastern anticyclonic eddy persists over several months and remains approximately at the same position. We conclude from these figures that the branching of the NAC

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The North Atlantic Current

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is an intermittent p h e n o m e n o n but that the eddies at its eastern flank have a lifetime of several months.

9. DISCUSSION The N A C in the area east and southeast of Flemish Cap has been investigated by a detailed survey, with a horizontal resolution of about 50 km. In August 1984 the mass transport to the northeast obtained by geostrophic calculations relative to 1900 db amounts to 29 Sv on a section reaching from the northeastern edge of Flemish Cap to 41°N 37°W. On a section along the western flank of the M A R between 53°N and 41°N we observed 32 Sv towards east. This is in the range of earlier findings. The correspondence between the geostrophic currents and the currents obtained from 25 drifting buoys gives confidence in the obtained current pattern. Observations in August 1984 displayed branching of the N A C at 47°N, 41°W. This is clearly seen in the temperature and salinity distributions at 100 db depth and the corresponding dynamic topography (Figs 3a-c). Whether or not the westernmost part of the N A C reaches the Northwest Corner cannot be decided. But less than half of the

1184

w . KRAUSSet al.

current's estimated transport is available to feed a northward branch during the survey period. However, in contrast to D~ETmCH et al. (1980) no permanent branching of the NAC can be confirmed. The satellite-tracked buoys which stayed in that area for several months indicate a changing vigorous eddy field. The NAC transports about 30 Sv towards the east, but only the Subarctic front seems to be a permanent feature. The branches between the Azores and the Subarctic front are transient phenomena related to the eddy field between the MAR and the Grand Banks. The NAC enters the area of observation from the southwest as a pool of relative warm and saline subtropical water into a cooler and fresher environment. Consequently opposite density gradients are maintained at both sides of the intruding water mass. The gradients to the northwest are identified by the presence of Labrador Current and Subarctic Intermediate waters. The high pressure cells at the eastern flank of the NAC vary in time and space. Their origin is not well understood. Most likely they represent the pressure maximum in a Munk-like circulation pattern (MUNK, 1950), which decomposes into cells according to the shape of the western boundary. We deduce from the present survey that the detailed shape of these high pressure cells is important for the actual current pattern in the area west of the MAR. Part of the NAC is expelled at the northern rim of these anticyclonic pressure centers and forms current branches within the broad westwind drift. They often extend beyond the MAR. The present data set does not allow us to follow the northern branch of the NA(L Consequently, we cannot prove that it is identical to the current branch over the MAR. However, this hypothesis is very likely. REFERENCES BARANOV E. 1. and V. G. GINKUI. (1984) Dynamics of waters of the Newfoundland energy activc z o n e Meteorologiya di Gidrologiya, 12, 78-84. CLARKt:~R. A., H. W. HILt,, R. F. RI£1NI(IERand B. A. WARRI,ZN(1980) Current syslcm south and cast of the Grand Banks of Newfoundland. Journal oJ"Physical Oceanography, 10, 25-65. DIETRICH G., K. KALLE, W. KRAUSSand G. SIEDLER (1980) General oceanography, 2nd edn. John Wiley, New York, 626 pp. EMERY W. J. and J. MEINCKE (1986) Global water masses: summary and review. Oceanologica Acta, 9, 383-392. FAIIRBACtl E., W. KRAUSS,J. MEIN('KEand A. SY (1985) Nordatlantik '84~-Data Report. Bericht des Instit,te~ J~ir Meereskunde, Kiel, 146, 811 pp. FOI-ONOH- N. P. and R. M. H}~NDRY (!985) Current variability near thc southeast Newfoundland Ridge Joun~al of Physical Oceanography, 15, 963-984. HARDTKE P. G. and J. Mt-INCKE (1984) Kinematic intcrpretation of infrared surface pattern in the North Atlantic. Oceanologica Acta, 7, 373-378. HARVI-Y J. (1982) 0-S relationship and water masses in the castcrn North Atlantic. De¢?~-Sea Research, 29. 1(}21-1{)33. HH,t. H. W., P, G, W. JONL~S, J. W. RAMSTER and A. R. FOLKARJ) (1975) The current system casl ol Newfoundland Grand Bank. ICNAF Spec. Publ., 1(1. pp. 41-55. Hn,L[~R W. and R. H. KASE (1983) Objective analysis of hydrographic data from mcsoscalc surveys. Berichte des Institute far Meereskunde, Kiel, I la, 78 pp. ISH,~N C. O'D. (1936) A study of the circulation of the wcstcrn North Atlantic. Paper,~ in Physical Oceanography and Meteorology, 4, I01 pp,, Woods Hole Oceanographic Institution, Woods Hole, Mass. KRAUSSW. (1986) The North Atlantic Current. Journal of Geophysical Research ~fthe Oceans, 91,5(161-5074. KRAuss W. and J. MEINCKf-:(1982) Drifting buoy trajectorms m the North Athmtic Current. Nature, 296, 737-740. KRAUSS W. and R. KAsv (1984) Mean circulation ;rod cddy kinetic energy in the castcrn North Atlantic. Journal of Geophysical Research, 89, 34{}7--3415.

The North Atlantic Current

1185

LAVIOI.E'rr[~ P. E. (1983) The Grand Banks Experiment: A satellite/aircraft/ship experiment to explore the ability of specialized radars to dcline ocean fronts. Norda Report, 49, 1-116. LH~ A. J. (1968) NORTHWESTATLANT Surveys: Physical Oceanography. ICNAF Spcc. Publ. 7, part 1, pp. 31-54. MANN C. R. (1967) The termination of the Gulf Stream and the beginning of the North Atlantic Current. Deep-Sea Research, 14, 337-359. MANN C. R. (1972) A review of the branching of the Gulf Stream System. Proceedi,gs o]'the Royal Society of Edinbargh, Set. B. Biol. Sci., 72, 341-349. MUNK W. H. (19511) On the wind-driven ocean circulation. Jottrmd of Meteorology, 7, 79-93. RI('HARDSON P. L. (19831 Eddy kinetic energy in the North Atlantic from surface drifters. Jo,rmd ~/" Geophysical Research, 88, 4355-4367. Ross C. K. (198(I) Observations of drifting buoy trajectories over Flemish Cap. NAFO SCR DOC, 80, N 199, 12 pp. SANFOrD T. B. (19711 Motionally induced electric and magnetic liclds in the sea. Jo,rnal ~/ Geophysical Research, 76, 347(3-3492. YON ARX W. S. (1950) An electromagnetic method for measuring the velocities of ocean currents from a ship under way. Papers in Physical Ocea,ography attd Meteorology, Woods Hole, I !, 62 pp. WORTItlN(;TON L. V. (19621 Evidence for a two gyrc circulation system in the North Atlantic. De¢7~-Sea Research, 9, 51-67. WORTIIIN(ITON L. V. (1976) On the North Atlantic circulation. Oceattographic Stttdies, 6, 1-110. The John Hopkins University, Baltimore, MD. Wrl~;trr W. R. and L. V. WORTIHNG'~ON (19711) The water masses of the North Atlantic ocean: A volumetric census of temperature and salinity. Scr. Atlas. Mar. Environ., Amcr. Gcogr. Soc. Folio 19, 8 pp. + 57 plates.