On the contribution of the Mediterranean Sea outflow to the Norwegian-Greenland Sea

Deep-Sea Research, Vol. 26A, pp. 1199to 1223 ,~'~PergamonPress Ltd 1979. Printedin Great Britain

00l 1-7471/79/1101 1199 $02.00/0

On the contribution of the Mediterranean Sea outflow to the Norwegian-Greenland Sea JOSEPH L. REID* (Received 2 November 1978,- in revisedjbrm 3 July 1979, accepted 3 July 1979) Abstract--In an earlier paper dealing with the mid-depth (t000 m) circulation of the North Atlantic Ocean, the water from the Mediterranean outflow, seen as a high-salinity subsurface layer, was shown to flow northward along the coast of Europe as well as westward. The distribution of the core of high-salinity water has been examined along an isopycnal surface that passes through the core near the source. The isopycnal varies in depth in the North Atlantic in accordance with the general circulation and outcrops at the sea surface in the Labrador and Norwegian Greenland seas. Near 6fin it is shallow enough to extend through the Faroe Shetland Channel. The high salinity of the Mediterranean outflow extends along this isopycnal and contributes substantially to the salinity of the water passing northward into the Norwegian Greenland Sea. It has been supposed previously that it is the upper waters of the Northeastern Atlantic Ocean that pass through this channel and contribute the high salinity of the Norwegian Current. From examination of the temperature, salinity, and oxygen of the various layers, it appears likely to be the Mediterranean core that contributes the characteristics of the Norwegian Current, but with heat exchange through the sea surface, precipitation, and the low-salinity contribution from the Baltic North Sea all uncertain, these characteristics alone do not provide a clear answer. Consideration of the silica field, however, provides a more convincing argument that the deeper water in the depth range of the Mediterranean outflow water provides a major component of the water passing northward through the Faroe Shetland Channel.

INTRODUCTION ALTHOUGH THE MOST o b v i o u s extension of the w a r m high-salinity water from the M e d i t e r r a n e a n is westward across the N o r t h Atlantic O c e a n , as seen in the Meteor Atlas (WOST a n d DEEANT, 1936), there is also a n o r t h w a r d extension along the coast of Europe, in a poleward eastern b o u n d a r y u n d e r c u r r e n t (HELLAND-HANSEN and NANSEN, 1926; REID, 1978). The westward extension has been discussed by WOST (1935), NEEDLER a n d HEATH (1975), RICHARDSON a n d MOONEY (1975), WORTHINGTON (1976), a n d REID (1978). I n the study by Reid, a m a p of the salinity at 1000 m in the N o r t h Atlantic was shown, a n d the n o r t h w a r d extension of highly saline water could be traced to the 1000-m isobath south of the Iceland Scotland Ridge. TAIT (1957), in his investigations of the waters of the F a r o e - S h e t l a n d C h a n n e l from repeated o b s e r v a t i o n s from 1927 to 1952, f o u n d occasional high values of salinity that suggest the presence of waters formed with the M e d i t e r r a n e a n outflow as one c o m p o n e n t . He chose 35.50%0 as the m e a s u r e of M e d i t e r r a n e a n influence a n d found occasional values higher t h a n that. N o values that high are f o u n d in the data set used here to represent the F a r o e Shetland C h a n n e l or the waters to the n o r t h , but, as will be seen when density is considered, the M e d i t e r r a n e a n outflow effect is evident even w i t h o u t such high salinities. * Scripps Institution of Oceanography, La Jolla, CA 92093, U.S.A.

1200

JOSEPH L. REID

EXTENSION

OF THE MEDITERRANEAN

OUTFLOW

The outflow provides a subsurface salinity maximum about 1 km deep over much of the eastern North Atlantic. This is best illustrated in the vertical sections of FUGLISTER'S(1960) atlas. It seems worthwhile to examine the extension of this layer along an isopycnal that lies within the saline core where it is strongest in the eastern North Atlantic, at a depth of about 1150 m there. The reasons for examining the outflow on an isopycnal surface rather than through a core analysis such as WOST (1935) attempted, or along a horizontal surface, are that the influence of the outflow may extend beyond the area showing a vertical maximum in salinity and that the extension may not be as nearly horizontal as lateral (isopycnal). Where isopycnals slope, as they must in a stratified geostrophically-balanced ocean, waters from a particular source may extend to different depths in different areas and may not be traceable along a horizontal surface. By examining the characteristics along some appropriate density parameter (assuming that this is the most nearly conserved of the characteristics), we may hope to follow the extensions of waters from particular sources. Vertical mixing, while apparent and important, may not completely obscure the patterns formed by lateral spreading. The isopycnal chosen varies widely in depth (between 0 and 1300m) in the North Atlantic. Because of the wide range of both salinity and temperature in the area, it is not possible to refer the density calculation to the same pressure everywhere. In the manner of REID and LYNN (1971), the density parameter is the potential density referred to a pressure of 1000 dbar (¢1) where it lies between 500 and 1500 m; where it rises above 500 m in the northern area, it is referred to 0 dbar (ao) of sea pressure. The isopycnal chosen (Fig. l) lies approximately at the depth of the salinity maximum just outside the Mediterranean--between l l 0 0 and 1200m. At higher latitudes it is shallower, with a shape reflecting the geostrophic shear in the north and west (Fig. 2), and it outcrops at the sea surface within the Labrador Sea and the Norwegian Greenland Sea. Within a few hundred kilometres of the strait, the depth-variation of the isopycnal is small and does not suggest that the outflow has any immediate effect upon the circulation. This water, which is a mixture of the Mediterranean water with the near-surface Atlantic water at the Strait of Gibraltar, simply lies at the depth appropriate to its density. From the density field alone, we would not recognize any effect of the Mediterranean beyond the strait. THE CIRCULATION

As the depth of the isopycnal varies so widely in the North Atlantic, the circulation of the water is represented by maps of geopotential anomaly at two depths. In the southeastern area, where the isopycnal lies near 1000m, the geopotential anomaly (steric height) at 1000dbar relative to 2000 dbar is shown (Fig. 2). In the northwestern area, where the isopycnal lies above 500m, the steric height at 0 dbar relative to 1000dbar is shown (Fig. 2). Both these maps of geostrophic shear and the circulation they might imply have been discussed in an earlier paper (REID, 1978), in which I attempted to account for the westward extension of the Mediterranean outflow in terms of the westward return flow south of the North Atlantic Current. The vertical shear in the northern area (Labrador and NorwegianGreenland seas) is greater between 0 and 1000dbar than between 0 and 2000dbar;

On the contribution o f the Mediterranean Sea outflow to the Norwegian Greenland Sea

1201

therefore the surface map is not referred to 2000 dbar, as is the deeper map, but to 1000 dbar. In this case the feature of particular interest is the poleward relative flow along the eastern boundary, both in the subsurface layers of the Atlantic and the surface layer of the Norwegian Sea. On the map of the depth of the isopycnal (Fig. 1 ), this is reflected by a downward slope to the east in that area. NANSEN (1913) proposed on the basis of various vertical sections of temperature and salinity that the subsurface waters just west of Ireland originate farther south, from the northeast Atlantic, instead of from western Atlantic waters carried eastward by the Gulf Stream, as had been commonly believed. He identified the source of their high salinity as the Mediterranean outflow. A northward subsurface flow from Spain was proposed, based upon the downward slope to the east of the isopleths of temperature, salinity, and density below about 200 m. His conclusions are borne out by the more recent data (Figs 1 and 3). THE SALINITY AND OXYGEN

The pattern of salinity (Fig. 3) and temperature (not shown, but nearly identical in pattern) on this isopycnal reveals the strong Mediterranean signal, which extends both westward in the North Atlantic Current--Gulf Stream return flow and northward in an eastern boundary subsurface countercurrent (REID, 1978). The high-salinity water extends northward where this isopycnal rises to less than 500-m depth in the area of the Iceland Scotland Ridge and extends into the Norwegian Greenland Sea and the Norwegian Current beyond 70°N. North of about 35'~N the dissolved oxygen concentration (Fig. 4) has much the same pattern as salinity, but with the highs and lows reversed. Lower oxygen values extend northward along the eastern boundary, with the high salinity, and high values are found in the north and west where the isopycnal lies near the surface and outcrops. The circulation and lateral mixing appear to provide roughly similar patterns in salinity and oxygen. South of 35°N, however, the oxygen pattern is dominated by the eastern subequatorial minimum, as in the Pacific Ocean (REID and MANTYLA, 1978). The lowest values of about 3.8 ml 1-1 on this isopycnal are at about 15~N in the eastern area. The Mediterranean water flowing in contains a little less than 4 ml 1-1 (MILLER, TCHERNIA, CHARNOCK and MCGILL, 1970), much lower than the newly aerated waters of the north and west. It is only slightly different from the subequatorial low, which it reinforces and extends northward. (The line of stations along 16°N has oxygen values so low that they extend the tongue of low oxygen almost to the western boundary. The GEOSECS [Geochemical Ocean Sections Program] data indicate a value near 16°N, 54°W about 0.3 ml 1-1 higher than these values. Perhaps the values on 16°N between 40 and 60°W are lower than they should be, but the values have not been altered here.) A POSSIBLE C O N T R I B U T I O N TO THE N O R W E G I A N G R E E N L A N D SEA

The high salinity of the Norwegian Current and of the deeper waters of the Norwegian and Greenland seas must derive from the Atlantic waters to the south (NANSEN, 1902). The very low salinity of the Arctic Ocean north of 80°N and the Arctic drainage from the northern continents would dominate these seas without a source of saline waters from the south.

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It has been supposed generally that it is the Gulf Stream waters or the surface waters from the northeastern Atlantic that provide this salt. WORTHINGTON(1970) proposed that it is the water of about 9°C from the northeastern Atlantic that passes through the FaroeShetland Channel and provides the greatest part of the heat of the Norwegian Sea waters. He noted that because of the Mediterranean outflow the eastern North Atlantic is more saline at all temperatures between 3 and 12°C than the western North Atlantic. Although he excludes water colder than 9°C from the budget, as he proposes that colder water does not pass northward through the channel, there is some reason to believe that somewhat colder waters, still of high salinity, may also enter. The salinity at the sea surface (Fig. 5) is consonant with an inflow of the surface waters from the northeastern Atlantic into the Norwegian Sea between Iceland and Scotland, but the similarity of the salinity pattern in that area to that on the isopycnal (Fig. 3) is remarkable: it suggests the possibility that the Mediterranean outflow waters, moving northward as an eastern boundary undercurrent, may rise near the surface and cross the Iceland Scotland Ridge and contribute substantially to the warm and very saline waters of the Norwegian Current. A large part of the saline surface waters of the North Atlantic Current turn southward before reaching 60°N, into the Bay of Biscay and the Canary Current, and some part turns westward south of Iceland (Figs 2 and 5), yet the surface salinity to the north remains high, decreasing only from 36 to 35%0 between 45 and 70°N, even though there are substantial additions of fresh water from the Baltic and North Sea waters on the east side and from the East Greenland Current on the west side. VERTICAL SECTIONS

To examine the sources of the waters of the Norwegian Current, a set of stations has been selected (Table 1, Fig. 6) that extends from near the Strait of Gibraltar through the Faroe Shetland Channel and on to Spitzbergen, along the extension of high-salinity water northward on the isopycnal (Fig. 3). Although there are many stations available for temperature and salinity, the number with measurements of oxygen and silica was much smaller. Within the Norwegian Current especially, the choice was quite limited, and it was necessary to use observations from different years, seasons, institutions, and equipment to complete the section. Even so, it was not possible to find stations at the most desirable positions. This may account for some of the irregularities seen in the section north of the channel. With these limitations in mind, the coherence seems remarkably good. DENSITY

The density along the section [Fig. 7(a)] is represented by a set of neutral surfaces (PINGREE, 1972; IVERS, 1975). In this case, the neutral surfaces were chosen from the characteristics at the depths sampled at Sta. 253, the deepest station, in order to include the densest water in the south. The depths of each neutral surface at the other stations were determined in the following manner. They are the depths where the water characteristics (temperature, salinity, pressure) have a particular common parameter: these waters, if moved adiabatically to Sta. 253 and placed at the depth of the neutral surface there, would have an in situ density matching the in situ density at that depth at that station. For example, the neutral surface marked 31.89 has a density of 1.03189 g c m - 3, where it lies at 940 m at the defining Sta. 253. It rises abruptly through the Faroe-Shetland Channel and

On the contribution of the Mediterranean Sea outflow to the Norwegian Greenland Sea

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1207

1208

JOSEPH L. REID

Fig. 6.

Positions of the stations used in the vertical section. The shaded area represents depths less than 500 m. The adjacent line is the 1000-m depth contour.

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On the contribution of the Mediterranean Sea outflow to the Norwegian-Greenland Sea

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1214

JOSEPH L. REID

passes through Sta. 55 at 43-m depth. This means that the water characteristics (temperature, salinity, pressure) at 43-m depth at Sta. 55 are such that if this water were moved adiabatically to Sta. 253 it would descend to and lie at 940 m depth there. Thus, each isopleth connects those waters at all the stations that would fit at one of the sampled depths of Sta. 253. These isopleths, then, are the neutral surfaces. The assumption is that because of their common density parameter the waters along such a surface will mix and spread more freely along them than across them, and we may hope to trace paths of such spreading along them. The values of the density parameter at standard depths are shown with the in situ density field of Sta. 253 as the ordinate [Fig. 7(b)]. The density parameter in el and ao on the maps is shown by the dash-dot line on Fig. 7(a) and (b). It lies at about 1100 m at Sta. 071, between the neutral surfaces at about 1000 and 1200 m there and remains between these two neutral surfaces throughout the section. It rises to 400 m north of the channel and outcrops near the northern end. It is nearly horizontal on Fig. 7(b). The in situ density of the deepest sample in the abyssal water at Sta. 253 is 1.04980 g cm 3. This is the deepest neutral surface that can be defined south of the channel on the section. The waters tound below this neutral surface in the channel [below about 600 m: see Fig. 7(b)] and to the north would all be denser than this at that depth and cannot be represented by the set of neutral surfaces chosen from Sta. 253. This means that they cannot be found along the bottom in the south. Any spillover of waters denser than this is mixed in the overflow process with the less dense Atlantic waters and, like the Mediterranean outflow, does not leave a density imprint at great depth in the open Atlantic. SLOPE

OF THE ISOPYCNAL

The isopycnal shown on Fig. 1 is illustrated on Fig. 7 by the dash dot line, which slopes upward from Sta. 3827, south of the Faroe-Shetland Channel, to Sta. 5040, at the channel entrance. The rise, which allows the deeper Atlantic waters to ascend laterally from 1200 m to less than 400 m and extend through the channel, appears to be the consequence of the vertical geostrophic shear. Just south of the channel the deeper waters are moving westward (IVERS, 1975) and the surface waters eastward (REID, 1978). This shear is not unusually large" between these two stations the relative geostrophic flow at the sea surface relative to 400 dbar is about 1 cm s- 1 and relative to 800 dbar about 5.5 cm s 1 TEMPERATURE,

SALINITY,

AND OXYGEN

Sections of potential temperature, salinity, and dissolved oxygen are plotted, first with depth as the ordinate and second with the density parameter as the ordinate [Figs 8(a), (b); 9(a), (b); 10(a), (b)]. The field of potential temperature [Fig. 8(a)] shows the warmer water (as high as 18~C) in the south and the sub-zero waters of the Norwegian Sea lying above the sill depth, However, no water below 3°C is found at Sta. 3827 ; potential temperature alone does not require the presence of overflow water from the Norwegian Sea at Sta. 3827, though it may exist there as a mixture. The neutral surfaces extend horizontally in Fig. 8(b), and the temperature decreases northward through the channel along each neutral surface. There is water warmer than 9°C in this set of Norwegian Sea stations. The surface density maximum in the channel

On the contribution of the Mediterranean Sea outflow to the Norwegian-Greenland Sea

1215

(Sta. 1458) is emphasized in this presentation [Fig. 8(b)]. Either the upper waters at Sta. 3827 are cooled abruptly as they enter the channel and made a great deal denser, or they do not enter the channel. This figure would suggest that the latter is at least possible-that the 5 to 8°C water in the channel consists of water of that temperature range from the south, rather than water of 8 to 14°C that has been cooled. However, the amounts of mixing with adjacent waters and of heat loss to the atmosphere are not well known, and the figure, while suggestive, is not conclusive. In Fig. 9(a) the stations south of the Faroe Shetland Channel show two salinity maxima. The deeper one, near 1200 m and at a slightly higher value, is from the Mediterranean outflow. The shallower maximum is derived from the great evaporation cell of the central North Atlantic (Fig. 5) and is separated from the deeper maximum by a layer of lower salinity whose characteristics (density, low salinity) suggest its origin to be the upper waters of the Labrador Sea. PINGREE and MORRISON (1973) reached the same conclusions. They presented a vertical section of salinity and stability along a line that is close to this section south of 52'~N but turns westward from there and shows the salinity maximum of the Mediterranean outflow everywhere the section is east of 15°W. They find also from the vertically continuous salinity temperature depth (STD) measurements that there is a layer of higher hydrostatic stability just above the salinity maximum everywhere and another just below it south of 40°N. They propose that the intervening stability minimum represents the Mediterranean outflow water and can be used as a tracer of the deep water (well below the pycnocline). The upper maximum in stability, although not illustrated, is clearly evident in the data for the section presented here, but the deeper stability maximum is only marginally shown by these discrete samples. A single salinity maximum extends into the Norwegian Sea at a depth of about 100 m. From Fig. 9(a) it might appear to originate from the upper maximum. However, when the same values are plotted with the density parameter as the ordinate [Fig. 9(b)], the maximum in the Norwegian Sea might appear to correspond to the deeper maximum. In the case of oxygen concentration (Fig. 10), the strong minimum of the Mediterranean outflow waters near 1000 m does not appear to enter or influence the Norwegian Sea when plotted against depth [Fig. 7(a)] (though the outflow from the Norwegian Sea is clearly demonstrated by the oxygen maximum near 2000 m). But when plotted against the density parameter, there is some suggestion of continuity of the lower-oxygen waters into the Norwegian Sea, above the oxygen-rich deep water. The oxygen maximum from the spillover of Norwegian Sea water does not extend to the bottom at stations south of the channel, even at Sta, 3827, nearest the channel. This is clear evidence that the horizontal density gradients so apparent on Fig. 7(a) are not solely the consequence of cascading along the bottom. These characteristics, however, give no conclusive evidence as to which of the two salinity maxima in the south provide the maximum within the Norwegian Sea. Either must be reduced from more than 36 to less than 35.4°/'00 and cooled to about 6.8°C. This reduction requires the addition of substantial quantities of fresher, colder water, either from the Baltic and North seas or from the area west of the Norwegian Current. At Sta. 3827 the salinity and temperature are 35.48%0 and 10.T'C at the upper salinity maximum and 35.42%0 and 9.4cC at the lower maximum. These values are not very different. Mixing the upper maximum with the coldest water found offthe southwest coast of Norway in winter (5~C, 33.5%°) to achieve the values found inside the Norwegian Sea at the salinity maximum (at Sta. 55: 35.21% o, 6.8TC) requires only 0.16 parts of coastal water to 1 of upper maximum water for salinity but 2.1 parts for

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On the contribution of the Mediterranean Sea outflow to the Norwegian Greenland Sea

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On the contribution of the Mediterranean Sea outflow to the Norwegian-Greenland Sea

1219

temperature. The extra cooling needed if only 0.16 parts of the coastal water are used could be achieved through loss through the sea surface in about 10 months if the upper 200 m are cooled at the rate of 75 × 10-3 cal c m - 2 year-1 as calculated by WORTHINGTON(1970). This is not unreasonable, but the deeper salinity maximum is a little cooler. If it rose to the surface and were exposed to the atmosphere, it would require only seven instead of 10 months of cooling. While this calculation favors the deeper maximum as a source of the saline waters of the Norwegian Current, the difference is too small to be significant. Because the oxygen minimum in the Norwegian Sea is at a higher value than that to the south and lies immediately beneath the upper layer, it cannot be taken as convincing evidence of the continuity of the minimum from outside the Sea. SILICA

On a map of silica on the isopycnal chosen to examine the outflow (Fig. 11 ), we find the highest values in the southeast, roughly associated with the area of minimum oxygen (Fig. 4). The Mediterranean contribution itself is of lower concentration, less than 10 units (SCHINK, 1967 ; MILLERet ai., 1970; ROETHER,GIESKESand HUSSELS, 1974). The effect of the Mediterranean low-silica concentration is to decrease the high silica values of the waters from the eastern tropical zone as they pass northward and mix with the outflow, However, the concentrations are still high in the poleward flow along the eastern boundary, and relatively high values extend with the high salinity (Fig. 3) into the Norwegian Current. On the vertical section [-Fig. 12(a)], the silica concentration does not increase downward monotonically at every station. In the south the injection of lower-silica water from the Mediterranean (ScHINK, 1967) is seen at about 800 m, but this effect is not strong enough to be seen farther north. Just south of the channel a silica minimum extends southward near 1500 m, corresponding to the oxygen maximum of the Norwegian Sea overflow water. North of the channel a weak maximum is seen at Stas 61 and 55, with an underlying minimum and an increase downward to values as high as 14 units. The values at the nearsurface maximum are somewhat higher than the values at that depth south of the channel, though the Norwegian-Greenland Sea is not recognized as a source of high silica concentration. The deeper waters of the Norwegian-Greenland Sea have a maximum silica content of about 14 units in the southeastern area (GEOSECS Sta. 19); in the western and northern area, they are less than 11. The lowest concentrations in the abyssal waters were found southwest of Spitzbergen at G E O S E C S Sta. 17, at the lowest temperature and salinity and at the highest density, presumably in the region of deepest overturn. The silica maximum seen at or near the neutral surface at Stas 3862 and 3827 in the south and at Sta. 61 in the north, and the underlying minimum in the north, can be detected in various other stations in the northeastern part of the Norwegian Sea. The silica maximum also appears at G E O S E C S Stas 18 and 19 and at several of the other stations near 71°N in the east. Unfortunately there are no silica observations along the line of this section north of 71:N. It appears, then, that the silica maximum in the Norwegian Current derives from the higher values introduced from the Atlantic. Later, following convection in the central Norwegian-Greenland Sea and spreading, these Atlantic waters may contribute to the silica concentrations in the deeper water of the Norwegian-Greenland Sea, including the relatively high values found in the waters beneath the silica minimum in the area of the Norwegian Current.

1220

JOSEPHL. REID

When plotted against the density parameter [Fig. 12(b)], the extension of the higher silica values into the Norwegian Current seems clear: there is nothing in the upper waters to the south that can provide the high silica values in the north. In particular, the density range of the upper salinity maximum has no silica greater than 5 units, while the lower maximum has values above 12 units and the deeper waters have still higher values. It seems clear that it is the deeper, denser waters--below even the Labrador Sea layer that provides the intervening salinity minimum between the two maxima--that are the source of the high silica concentrations found in the silica maximum from just north of the channel to about 71°N. This maximum is separated from the deeper high values by a silica minimum and cannot be derived directly from below. A map of the silica at the sea surface (Fig. 13) has been prepared from the data set selected for the silica on the isopycnal (Fig. 11). The shallower areas where the isopycnal does not extend were excluded in the selection. Thus, the map does not represent the continental shelves, which in some areas may have concentrations over 5 ~tg-at. 1-1 (for example, the northwestern coast of Africa (FR1EBERTSHAUSER, CODISPOTI, BISHOP, FRIEDERICHand WESTHAGEN,1975) and the southern part of the North Sea (JOHNSTONand JONES, 1965). The map (Fig. 13) shows no values high enough anywhere in the eastern Atlantic to provide those of the Norwegian Current. The highest values are found above the Iceland-Scotland Ridge, where the surface density shows a local maximum (Fig. 7 and BOHNECKE,1936), reflecting the rising of the denser waters and local vertical mixing above the ridge. High values are also found near the outcrops of the isopycnal (Fig. l) in the Labrador Sea and between Iceland and Greenland. However, the surface values of the open ocean of the North Atlantic are all much too low to provide the high concentrations found within the Norwegian Current.

DISCUSSION

The result of this examination of the characteristics is positive evidence for the entry into the Norwegian Sea of waters which in the open Atlantic lie at depths much greater than the sill depth of the Faroe Shetland Channel. While the temperature, salinity, and dissolved oxygen patterns are consonant with such an entry of waters with characteristics of the Mediterranean outflow, they do not preclude the possibility that it is only the upper waters, which are also saline and somewhat warmer, that enter the channel. The silica pattern, however, requires the deeper Atlantic waters as a source for the higher values entering the Norwegian Sea. While a contribution of some characteristics from the upper layers through vertical mixing with the deeper waters cannot be excluded, the contribution from the denser waters, with their high silica, is necessary. A conclusion is that the saline waters of the Mediterranean outflow a-s well as the highersilica waters from farther south contribute importantly to the waters of the Norwegian Current and hence the Mediterranean outflow helps to maintain the high salinity of the Norwegian Sea. This is important because without this source of high-salinity water the Norwegian Greenland Sea might not provide the denser waters that fill the Arctic Basin and contribute a major component of the North Atlantic Deep Water. At present the Norwegian Greenland Sea has a source of cold, low-salinity water from the Arctic (less than 33%0, colder than -1.5°C) and a source of warm, saline water from the Atlantic (maximum values 35.25%0, 4°C). A cooled mixture of these waters produces the dense

On the contribution of the Mediterranean Sea outflow to the Norwegian Greenland Sea

1221

abyssal waters of the sea (about 1.02811 in potential density, 34.92%o and - 1 . 3 ° C in potential temperature). The warm, saline waters of the Norwegian Current flow into the Barents Sea and turn westward and southward near Spitzbergen (HELLAND-HANSENand NANSEN, 1926). In the shallow Barents Sea, the runoff is reduced in winter and the saline waters from the Norwegian Current are cooled with little dilution. They become denser as they turn and move westward and southward south of Spitzbergen, and the densest surface waters are found between Spitzbergen and Iceland (DIETRICH, 1969 ; REID and LYNN, 1971). In this area they overturn and achieve the final characteristics (about 34.92%o in salinity, potential temperature about - 1.25°C) of the Norwegian Greenland Sea bottom water. This forms the dense layer that mixes with the upper waters and overflows across the sills into the Atlantic (WORTHINGTONand WRIGHT,1970; REID and LYNN,1971). The overflow from the Denmark Strait is at 60°N at about 3000 m with characteristics about 34.89%o, 1.0~'C in potential temperature, and 27.98 in potential density. In Fig. 3 there is shown an extension of the saline tongue westward to the southwest of Iceland, toward the Irminger and Labrador seas. This westward flow of saline water was also noted by WORTHINGTON(1976). It is possible that it is this water, with its high salinity from the Mediterranean outflow, that contributes to the high density and deep overturn within the Labrador Sea. This suggests an intriguing relation between geostrophic flow and thermohaline processes. In a geostrophically balanced flow, the pressure field is defined. If the ocean is stratified, the density field is largely set up to be in balance with the flow. Isopycnals that lie deep within an anticyclonic gyre may extend to shallow depths or outcrop within an adjacent cyclonic gyre. The idea of a lateral extension of surface characteristics along such isopycnals from the upper layers to greater depths has been a long-standing concept (MONTGOMERY,1938). The converse, that mid-depth characteristics in the central ocean may outcrop in high latitudes, is implicit in this concept. However, most studies have dealt with lateral extensions of vertical extrema of salinity, and most salinity extrema originate directly from the sea surface. The intermediate-depth salinity minima in the lower latitudes of the Pacific Ocean, which derive laterally from the colder, less saline high-latitude surface waters, are an example (REID, 1965). But in the case of the Mediterranean outflow, an extremum in salinity is injected at mid-depth in the central North Atlantic. As the high-salinity water spreads and flows laterally, some of it extends into higher latitudes and lies, because of the geostrophically related density field, at successively shallower depths, eventually outcropping. Though its original high salinity and temperature have been decreased by mixing with adjacent waters, the salinity is still higher than the ambient salinity within the Norwegian Greenland Sea. and with cooling it can reach, because of its high salinity, a density at least as great as the abyssal waters there and overturn can occur. Likewise, the part of this high-salinity water that extends westward south of Iceland and outcrops within the Labrador Sea arrives with a salinity higher than the bordering waters. When cooling takes place, the higher salinity allows it to become dense enough to overturn to great depths, providing part of the source of convective overturn within the Labrador Sea. With this concept in mind we may speculate that the Mediterranean outflow is a contributor to all three of the layers that WOST (1935) called North Atlantic Deep Water. It is clear that it forms the Upper Deep W a t e r - - t h e great salinity maximum seen on the vertical sections in FUGLISTER'S (1960) atlas and discussed earlier by Wt2ST (1935). If it

1222

JOSEPH L. REID

contributes the high salinity of the Norwegian-Greenland Sea, then it triggers the formation of the Lower North Atlantic Deep Water (Wt3sT, 1935; REID and LYNN, 1971). And if it contributes also to the Labrador Sea overturn, then it contributes to the Middle North Atlantic Deep Water as well and is a critical component of the whole North Atlantic Deep Water array. Following Wt2sT's (1935) discussion one step farther, it is the highly saline North Atlantic Deep Water that penetrates into the low-salinity waters of the Weddell Sea where it is cooled further and enriched with brine to provide the Antarctic Bottom Water. To the extent that this series of speculations is correct, the exchange between the Atlantic Ocean and the Mediterranean Sea is of some importance: it would dominate the thermohaline circulation of the World Ocean as we understand it today. It is interesting to consider what might have been different in periods when the Strait of Gibraltar was closed, either by tectonic effects or by lowering of sea level. Without the Mediterranean source, the density field of the central North Atlantic might not be different: the direct effect (Fig. I) is not obvious. Water of that density is present both north and south of this source, but it is colder and less saline (Fig. 3). If the Atlantic waters entering the Norwegian Current were of the same density as at present but less saline, then the deep Norwegian Greenland Sea water would be less saline and less dense, even at freezing temperature, than at present. Without the Mediterranean source (Fig. 3), the waters of this density range might derive their characteristics from the south and have salinities and temperatures less than 34.8%o and 4°C. Such waters could not produce or maintain the high salinity and density of the deep Norwegian Sea. At present, the incoming waters of about 35.25%o produce an abyssal product of about 34.920/00. If the incoming water were 34.8%0, a similar decrease by mixing with the Arctic surface waters would produce an abyssal layer of about 34.5%0, with a maximum potential density of about 27.78. Even if this product spilled back into the Atlantic without mixing with the overlying waters, it would not sink to 2000 m. If it were altered as much by mixing as are the present waters, its potential density might be about 27.6, as found at depths above 1200m in the North Atlantic. Such a reduction of the salinity of the waters flowing northward into the Norwegian Greenland Sea would certainly preclude the formation of abyssal waters of such extreme density there and eliminate the deepest of Wt3sx's (1935) three components of North Atlantic Deep Water. If all three of Wi)st's components of the North Atlantic Deep Water originate from the Mediterranean outflow, and this outflow were cut off, then the deep saline layer of the North Atlantic might disappear, and the present proposed formation of Antarctic Bottom Water could not take place. Without a continuous supply from this source, the deep and abyssal layers would become less dense. The circumstances that form dense waters in the Norwegian-Greenland, Weddell, and Ross seas would be altered. Where would the larger quantities of dense abyssal waters be formed ? That is, where would the density difference between the surface and abyssal depths first disappear?

Acknowledgemems--I wish to thank Dr ROBERT S. ARTHUR and Dr MIZUKI TSUCHIYA for reading and commenting upon the manuscript and Dr DAVID SCHINKand Dr WOLFGANGROETHER for making available the Trident and Meteor data used in the figures. This paper represents one of the results of research supported by the Office of Naval Research, the National Science Foundation, and the Marine Life Research Program, the Scripps Institution's component of the California Cooperative Fisheries Investigations, a project sponsored by the Marine Research Committee of the State of California.

On the contribution of the Mediterranean Sea outflow to the Norwegian Greenland Sea

1223

REFERENCES BOHNECKE G. (1936) Atlas zu: Temperatur, Salzgehalt und Dichte an der Oberflache des atlantischen Ozeans. Wissenschafiliche Ergebnisse der deutschen atlantischen Expedition "Meteor", 5, 74 Beilagen. DIETRICH G. (1969) Atlas of the hydrography of the northern North Atlantic Ocean. Conseil International pour I'Exploration de la Mer--Service Hydrographique, Charlottenlund Slot, Denmark, 144 pp. FRIEBERTSHAUSERM. A., L. A. CODISPOTI, D. D. BISHOP, G. E. FRIEDERICHand A. A. WESTHAGEN(1975) JOINT I--Hydrographic Station Data, R/V Atlantis H Cruise 82. Coastal Upwelling Ecosystems Analysis, IDOL, Data Report 18, 243 pp. EUGLISTER F. C. (1960) Atlantic Ocean atlas of temperature and salinity profiles and data from the International Geophysical Year of 1957-1958. Woods Hole Oceanographic Institution Atlas Series, 1, 209 pp. HELLAND-HANSEN B. and F. NANSEN (1926) The eastern North Atlantic. Geofysiske Publikasjoner, 4, 1-76. IVERS W. D. (1975) The deep circulation in the northern North Atlantic, with especial reference to the Labrador Sea. Ph.D. Dissertation, University of California, 179 pp. JOHNSTON R. and P. G. W. JONES (1965) Inorganic nutrients in the North Sea. Serial Atlas of the Marine Environment, Folio 11, 3 pp., 10 plates. New York, American Geographical Society. MILLER A. R., P. TCHERNIA, H. CHARNOCK and D. A. McGILL (1970) Mediterranean Sea atlas of temperature, salinity, oxygen profiles and data from cruises of R.V. Atlantis and R.V. Chain (pp. 1-93, l 11-190) with distribution of nutrient chemical properties (pp. 95-110). Woods Hole Oceanographic Institution Atlas Series, 3, 190 pp. MONTGOMERY R. B. (1938) Circulation in the upper layers of southern North Atlantic deduced with use of isentropic analysis. Papers in Physical Oceanography and Meteorology, 65, 55 pp. NANSEN F. (1902) The oceanography of the North Polar Basin. The Norwegian North Polar Expedition, 1893 1896, Scientific Results, Vol. III, No. IX, FRIDTJOF NANSEN, editor, Longmans, Green & Co., 427 pp. NANSEN F. (1913) The waters of the north-eastern North Atlantic. Investigations made during the cruise of the "Fridtjof", of the Norwegian Royal Navy, in July 1910. Internationale Revue der gesamten Hydrobiologie und Hydrographie, Bd. IV, Hydrographische Supplemente, Zweite Serie, 1 q39. NEEDLER G. T. and R. A. HEATH(1975) Diffusion coefficients calculated from the Mediterranean salinity anomaly in the North Atlantic Ocean. Journal of Physical Oceanography, 5, 173-182. PINGREE R. D. (1972) Mixing in the deep stratified ocean. Deep-Sea Research, 19, 549-561. PINGREE R. D. and G. K. MORRISON (1973) The relationship between stability and source waters for a section in the Northeast Atlantic. Journal of Physical Oceanography, 3, 280-285. REID J. L. JR (1965) Intermediate waters of the Pacific Ocean. Johns Hopkins Oceanographic Studies, 2, 85 pp. REID J. L. (1978) On the mid-depth circulation and salinity field in the North Atlantic Ocean. Journal of Geophysical Research, 83, 5063 5067. REID J. L. and R. J. LYNN 0971) On the influence of the Norwegian-Greenland and Weddell seas upon the bottom waters of the Indian and Pacific oceans. Deep-Sea Research, 18, 1063-1088. REID J. L. and A. W. MANTYLA (1978) On the mid-depth circulation of the North Pacific Ocean. Journal of Physical Oceanography, 8, 946-951. RICHARDSON P. L. and K. MOONEY (1975) The Mediterranean Outflow--A simple advection-diffusion model. Journal of Physical Oceanography. 5, 476-482. ROETHER W., J. M. GmS~ES and W. HUSSELS(1974) Hydrography of a transatlantic section from Portugal to the Newfoundland Basin. "Meteor" Forschungsergebnisse, Reihe A, No. 14, 13 32. SCHINK D. R. (1967) Budget for dissolved silica in the Mediterranean Sea. Geochimica et Cosmochimica Acta, 31, 987-999. SVERDRUP H. U., M. W. JOHNSON and R. H. FLEMING (1942) The oceans, their physics, chemistry, and general biology. Prentice Hall, 1087 pp. TAIT J. B. (1957) Hydrography of the Faroe-Shetland Channel 1927-1952. Marine Research, 2, 309 pp. WORTHINGTON L. V. (1970) The Norwegian Sea as a mediterranean basin. Deep-Sea Research, 17, 77-84. WORTHINGTON L. V. (1976) On the North Atlantic circulation. Johns Hopkins Oceanographic Studies, 6, 110 pp. WORTHINGTON L. V. and W. R. WRIGHT (1970) North Atlantic atlas of potential temperature and salinity and oxygen profiles from the Erika Dan cruise of 1962. Woods Hole Oceanographic Institution Atlas Series, 2, 24 pp., 58 plates. WOST G. (1935) Die Stratosph/ire. WissenschaftlicheErgebnisse der deutschen atlantischen Expedition "Meteor", 6, 109-288. W/2ST G. and A. DEFANT (1936) Atlas zur Schichtung und Zirkulation des Atlantischen Ozeans. Schnitte und Karten yon Temperatur, Salzgehalt und Dichte. Wissenschaftliche Ergebnisse der deutschen atlantischen Expedition "Meteor", 6 (Atlas), 103 pp.