Observations of the mediterranean outflow—II the deep circulation in the vicinity of the gulf of cadiz

Deep-Sea Research, Vol. 26A. pp. 555 to 568 01)l 1-7471/79/0501d)5~ ~,(12.(XF0 rc Pergamon Press Ltd 1979 Printed in Great Brilain

Observations of the Mediterranean outflow--II The deep circulation in the vicinity of

the Gulf of Cadiz

I. AMBAR* and M. R. HOWE+

(Received 29 May 1978: in revised /brm 4 December 1978: accepted 10 December 1978)

Abstract An analysis of the field of mass produced a reasonably coherent pattern of circulation for the Mediterranean water outflow and estimates of the velocity of the undercurrent both before and after its subdivision into separate upper and lower cores were in good agreement with several previous direct current meter measurements. Meanders induced in the flow by the influence of the bottom topography are regarded as the main cause of the variability in the thermohaline and dynamic properties of the current system. A downstream increase in the volume of transport is interpreted in terms of an entrainment parameter that compares favourably with a recent streamtube model for bot tom boundary currents. The analysis also indicates the presence of a deep countercurrent at about 1500m beneath the main outflow in a region between 9 '30'W and 8 3 0 ' W south of Cape St Vincent.

1. I N T R O D U C T I O N

PREVIOUS current meter measurements in the Mediterranean water (MW) outflow west of the Straits of Gibraltar have been restricted at any one time to a particular locality, as were the few attempts to estimate the flow by dynamical computations. Any interpretation of the deep circulation has therefore been obliged to use isolated and non- synoptic observations. There are several that are pertinent to this study, such as those by LACOMBE (1961) and LACOMBE, TCHERNIA, RICHEZ and GAMBERONI (1964), who from repeated direct measurements on a section across the western end of the straits deduced an average MW outflow of 0.9 x 10 6 m 3 s - 1 . MADELAIN (1967) estimated, from dynamical computations, a geostrophic MW flow of less than l0 cm s- ~ at a much greater distance from the source, between 600 and 1500 m along the west coast of Portugal, with a very slow southward movement beneath these layers. Within the Gulf of Cadiz isolated measurements at various locations and times have indicated flow irregular in both direction and intensity. The most comprehensive account to date has been that of MADELAIN (1970), who interpreted the circulation in the form of several streams being directed along various channels and submarine canyons in the region. Some direct measurements by MEINCKE, SIEDLER and ZENK (1975) showed a general northerly flow at five depths between about 200 and 2000 m at a station near the continental slope off Cape St Vincent. Some measurements by ZENK (1975) and THORPE (1976) in the gulf itself are particularly relevant to the results reported here. Previous observations of the temporal

* Laboratorio de Fisica, Instituto Geofisico Infante D. Luis, Faculdade de Ciencias, Universidade de Lisboa, Rue da Escola Politecnica, 58, Lisboa, Portugal.

t Department of Oceanography, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, England.

555

556 I. AtCIBAII a n d M. R. HOWE

variability of the MW outflow have usually been made very near or within the straits. Thorpe, by an analysis of T-S data and moored current meter measurements, showed something of the variability of the MW undercurrent further west when it was still in contact with the sea bed and also at a location where it was moving as an interflow. The extent of the present investigation will enable us to make some interesting comparisons with the results of many of these authors.

In Paper I (AMBAR and HOWE, 1979) of this study, the closely spaced T-S survey through the Gulf of Cadiz and around Cape St Vincent has been described in some detail. The T-S distributions observed during the R.R.S. Challenyer cruise in 1976 and R.R.S. Shackleton cruise 1973 were used to determine the preferred flow path of the MW and estimate the extent and amount of mixing that occurs in this area. The same data will now be used to evaluate the speeds and interpret the intermediate and deep circulation pattern of the MW as it traverses the Gulf of Cadiz. The sections of salinity-temperature-depth (STD) stations (Fig. 1) of the preceding paper will again constitute the basis of the discussion and provide a useful cross-reference. Since the outflow has been shown to be subdivided into separate upper and lower cores, this distinction will be upheld and wherever possible the appropriate flow characteristics will be determined at each level.

2. F I E L D OF' M A S S : S U R F A C E S O F C O N S T A N T a A N D G E O S T R O P H I C C U R R E N T S

It is reasonable to suppose that after the initial descent down the continental slope from the straits, the MW upper (Mu) and lower (MI) cores will, as they attain their respective equilibrium levels, be subjected to further mixing that will take place mainly along isopycnal surfaces (PINGREE, 1972). So it would be appropriate in establishing the extent and nature of the spread of the MW to use charts of the thermohaline distribution on surfaces of constant trr,. A number of charts were constructed each representing the distribution on a particular density surface that lies in the range corresponding to the core, That is, for Mu several a~ values were considered between 27.50 and 27.65, and similarly for M1 between 27.75 and 27.85. Two such charts, one from each group, are reproduced as typical examples of the isohaline distribution of Mu on cr,, = 27.60 and MI on tr~j = 27.75 (Fig. 1).

By considering these two groups of charts, several features became evident. There is a strong northerly deflection of the isopleths near the straits, in accordance with the bottom contours, until about 7°W, when they then become more directly aligned towards Cape St Vincent. The direction is somewhat disturbed however by a certain degree of meandering, which in fact was coincident with the presence of submarine canyons in the area. In all the charts there is a relatively large decrease in the salinity (and temperature) values towards the offshore stations, which implies little spreading in this direction, with the bulk of the MW being confined to the northern slope. Also, each isohaline shows an increasing southwesterly displacement in the deeper a0 surfaces, which indicates a downward slope in the offshore direction of the surfaces of constant salinity. The distribution of isopleths confirms that Mu is always nearer the shore than M1, and this is denoted in Fig. 1 by the relative position of the 36.4~,, isohaline on the 27.60 tr, surface compared with that on the 27.75 surface. Finally the coherence in the thermohaline properties of each core can be appreciated by noting the extent of the 36.4%o isohaline in both charts. This suggests a fairly uninterrupted flow regime that might also be reasonably steady even though there is evidence of quite large meanders. Consequently a further analysis of the density distribution was made to determine the velocity field.

Observations of the Mediterranean outflow--II 557

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The computation of currents from the field of mass has several well-known limitations that involve an assumption of stationary and frictionless flow against a bottom slope (FoMIN, 1964). Also there are other sources of error such as the distortion due to periodic tidal changes (DEFANT, 1950) and of course there is the usual difficulty of selecting a level of no motion (HIDAKA, 1940; DEFANT, 1941). These considerations must therefore limit the credibility of absolute currents computed by this method.

558 I. AMBAR and M. R. HOWE

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However, not only has it been shown previously in this and in the preceding paper that there is significant coherence in the thermohaline and density distributions in both MW cores, but the dynamic field itself was just as coherent. Therefore, in spite of their limitations, it was thought that the dynamical computations might produce a reasonable estimate of the velocity field. As the MW flow is near the bottom over much of the Gulf of Cadiz a reference level was considered within'the transition layer between the accepted eastward flowing surface current and the westward MW outflow. Therefore a level of no motion between pairs of stations throughout the area was assigned to the upper minimum in the T-S distribution where the North Atlantic Central Water (NACW) is about to be disturbed by the intrusion of the MW. This means that the reference level generally lies within a depth range of 400 to 600m. Similarly, for the stations off the western coast of Portugal, a transition layer between the expected southward flowing NACW surface currents and the northward MW flow was selected. This concept was also adopted by MADEL^IN (1967) when he used a 500-m level of no motion for his dynamical computations off the Iberian peninsula.


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50 to 55. In VI it is 328°T.

Results

On leaving the straits through section XV the maximum value of 180 cm s-~ appears at 345 m between Stas 83 and 84 (Fig. 2). At depths of 310m (Sta. 83) and 360m (Sta. 84) the velocity is about 100cms -1, similar to the 100 and 50cms -~ at these depths that MAD~LMN (1970) reported for current meter measurements near to Stas 83 and 84. In section XII the outflow is broader and at about 6°50'W the maximum velocity of 46crns -1 at 600m (Fig. 2) is associated with the warm Mu core (see Fig. 2b paper I), which flows inshore. Madelain quoted a direct measurement nearby of 30 to 60cm s- (345°T) at a depth of 580m and more recently ZENK (1975) recorded a mean monthly speed of 68 cm s- ~ (324°T) at a depth of 649 m from a current meter station about 9 km away towards the straits. Major anomalies in the velocity field first appear in the flow regime through section X (Fig. 3a), where the negative values indicate a more complicated flow path for the bot tom core.


For an explanation we need to reconsider the thermohaline distribution in this section which, as described in paper I, has large horizontal T-S gradients (fronts). An offshore meander is suggested. This is depicted in Fig. 3a (inset), which also shows the alignment of the stations in this section. The flow characteristics are represented firstly by the current component of 49 cm s- 1 towards the northwest between Stas 56 and 57, with salinity and temperature values of 37.2%,, and 13°C. The flow turns southward between Stas 54 and 55 with a speed of l l c m s -1 (370",~, 12.8°C) and then finally reverts to its usual direction with a northward component of 28cms -1 between Stas 51 and 52 (36.8" , 12.7°C). The sudden deviation offshore and the increased intermingling with NACW account for the decrease in the temperature and salinity values. ZENK (1975) pointed out that the system of ridges and canyons in this region encourages the deeper layers of MW to turn southward, and it is proposed that this meandering of M1 does recur in sections further west. Zenk's direct current measurements are also of particular interest here. Near (4.6 km) Stas 56 to 57, where the estimated component at 850 m is 49 cm s- 1, he recorded a mean speed of 5 0 c m s - I towards the southwest while at 752, 649, and 557m the westward velocities of 29, 23, and 5 cm s-1 are in good agreement with the geostrophic estimates of 51, 24, and 6 cm s-1, respectively. Another current meter moored at Sta. 55 showed that there was a flow to the southwest between 700 and 1000m with speeds varying from 10 to 44 cm s- 1. This would be consistent with our interpretation of the T-S fronts and the negative values in section X (Fig. 3a) as being due to a southward meander.

At section VI (Fig. 3b) the separation of Mu and MI is obviously complete and the reversal of the current directions due to the implied meandering of MI is again evident. Here however it is clear from the T-S distribution that MI has turned sharply inshore and closely follows the 1300-m contour into a prominent submarine embayment (Fig. 1 paper I). Geostrophic conditions can hardly prevail in these circumstances and so the actual velocity values will not be regarded with any confidence until the more normal westward flow is re-established. In this section the offshore stations are deeper (> 2000 m) than any previously considered and at depths greater than 1400m there appears to be a countercurrent towards the southeast of about 10cms -~. This same counterflow is present in the four sections to the west with values increasing with depth down to 1500 m, which was the limit of the observations, Its maximum computed velocity, 22 cm s-1 is in fact in section III (Fig. 4).

As an indication of the general change in the dynamics of the MW in traversing the Gulf of Cadiz, the maximum velocity for Mu in section III was 16 cm s-1 to the west and this is closely related to the well defined maximum in the T-S distribution at 750m. Similarly related to the T-S properties, but somewhat slower, the Mi core progresses at about 5 cm s-1 above the relatively fast deep countercurrent of 22 cm s-1. The MW intrusion into the open Atlantic is partially intersected by section I (Fig. 4), where the thermohaline distributions show that the main flow of MI will be further offshore and slightly deeper. The Mu velocity remains steady at 20cms -1, whereas the negative (southward) values associated with this near-shore branch of MI suggest an eddy or meander, probably induced by the steep canyon along this section.

A similar result emerged when the 1973 Shackleton data from the west coast of Portugal (Fig. 1 preceding paper) were analysed. For example, in Shackleton section 1 (Fig. 4) there are reversals in the flow directions for both Mu and M1 presumably due to the canyon. The topographical restrictions seem to have produced relatively high speeds in the reversed southward component of MI before it eventually leaves the confines of the

Observations of the Mediterranean outflow II 561

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canyon and continues offshore. It is unlikely that the flow conditions within these canyons would be in geostrophic balance and so the computed velocities cannot be accepted too literally. However, the reversals in direction in these sections may be realistic and will therefore be considered as a local perturbation in the flow path.

~. T H E I N T E R M E D I A T E A N D D E E P C I R C U L A T I O N

The general coherence in the physical properties has been discussed and the downstream variations have been summarised firstly in the form of the percentage mixing charts for Mu and MI (preceding paper) and then by the isohaline plots on particular a, surfaces (Fig. 1). There are however two features that disturb the general uniformity- the regions of large horizontal frontal gradients in temperature and salinity--(e.g, section X) and those stations with large vertical gradients below about 1400m (Fig. 5). The computations of velocity produced evidence of considerable lateral or vertical shear with an actual reversal in the flow direction. It is therefore proposed that these extreme lateral gradients in the T-S distributions are associated with a meandering in the flow introduced by the topographical contours, and the vertical gradients are indicative of a deep

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countercurrent below 1400 m. With this concept in mind, the velocity field was computed from all the available data and the salient values associated with the Mu and M1 cores were used to construct charts (Fig. 6) of the most likely route of the MW outflow around the Gulf of Cadiz.

In these representations the speeds shown are normal to the sections and usually in a westerly or northwesterly direction. Any reversal in the flow will be accommodated by a meandering effect and the deep countercurrent is included on the MI chart as a separate system. A common combined outflow, with the maximum computed speeds, extends from section XV to the separation zone at section XII. Here the individual routes for Mu and M1 have been plotted and they will provide a more definitive representation of the


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preferred flow paths than the mixing charts that were produced in Paper 1. This is certainly true for Mu which, after section XII, follows a fairly direct course towards Cape St Vincent at speeds that are reasonably consistent. As already established, Mu can be detected mainly at the inshore stations until it eventually turns north along the west coast of Portugal with an appreciable velocity of 20 cm s - 1.

By compar ison the MI flow path is more tor tuous because of the influence of the

}S.R.(A) 26'5 6

564 I. AMBER and M. R. HOWE

canyons and other topographical obstructions. The first significant deviation occurs in section X, with the offshore flow down a canyon at about 7°20'W where it is still in contact with the sea bed. M1 then reverts to a more usual northwesterly direction and thereafter closely follows the 1300m contour. Probably the most interesting comparison to be made here is with some of the observations of THORPE 0976), who investigated the dynamics of the undercurrent at two locations in this area. The first, near 36'~12'N, 8~02'W, is about midway between Sta. 39 and 50 and would in this analysis be regarded as being at the offshore extremity of the main MW flow. The fact that he failed to observe the presence of Mu is consistent with our observations, which place it further north at the inshore Sta. 42 (36°24'N). In addition his description of the large changes in the T-S profiles below 900m, which were also associated with different current speeds and directions, might readily be explained by noting that the observations were made near and between two prominent meanders (Fig. 6). Therefore any adjustment in the curvature of the flow path, which by its meandering will most likely suffer frequent perturbations, would allow the intrusion and replacement of either MW or NACW and so account tor the sudden T-S changes. Indeed, by repeating Sta. 5 in section I a similar effect was observed. Here the MW at both levels was being subjected to a sharp curvature around the cape before encountering a steep canyon. After 12 days there were substantial differences in the T-S values at certain depths, particularly at the upper (650 m) and lower (1200 m) boundaries of the MW layers, where changes of the order of 2 C and 0.7",i,, were recorded. These could most readily be explained by considering an offshore-inshore adjustment in the path of the flow around the cape. Thorpe's other measurements were in the canyon at 7~'20'W near Sta. 56 (section X) where he recorded speeds of about 80 cm s-1 down the canyon in a direction 233°T as well as a significant variation in the relatively high T-S values in the flow. As described previously, the MI bottom current here has been presented in the form of a sharply curved meander towards the southwest, which also agrees with MADELAIN'S 0970) general conclusions. Again it is proposed that the T-S changes can best be explained by the adjustments expected in such a l low path rather than as being due to any significant intermittency in the outflow from the straits.

The other important result of this analysis of the field of mass is the evidence of a deep countercurrent beneath M1 in the offshore stations south of Cape St Vincent (Fig. 6). In establishing the general coherence of the T-S and velocity fields, and accepting that the dynamical computations produce values that compare favourably with the reported direct current measurements, the existence of this countercurrent deserves some credibility. The speeds of 6, 22, 14, 10, and l l c m s -1 from section lI to VI, respectively, are always associated with a prominent discontinuity in the thermohaline profiles at about 1400m (Fig. 5 ), which in itself implies that there might be a large vertical current shear. So far as we are aware there is no other report of such a countercurrent, although as a possible consequence of this flow there may be some connection with another interesting measurement further offshore where SWALLOW (1969) observed an eddy current at 1400 m with speeds of 14cm s - 1 However ZENR (1975) did not observe such a counter flow at a current meter station near section V. In view of our discussion of the lateral displacements that affect the MW flow in this area, this deep current may also suffer similar offshore perturbations in its flow path. Although its apparent existence is an interesting product of this analysis, confirmation must await further direct measurements.

To complete the geostrophic computations and provide a comparison with the dynamics of the flow in other regions, the data from the sections along the west coast of

Observations of the Mediterranean outf low--II 565

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Table 1. Transport ¢?f volume (106m3s- l ) .~)r the M W outflow at various sections in the Gul['o]" Cadiz.

Section Transport

XV 1.51 XIV 1.75 XIII 1.05 XII 2.60 X 2.74 IX 2.38 II 2.60 I 1.20

Portugal and the southwest area (Fig. 1, preceding paper) were also considered. After the effect of the canyon at 37°N, branches of Mu and M1 proceed northward, parallel to and between the 1000- and 1500-m contours with speeds of 29 and 5 cm s-1, respectively (Fig. 7). As indicated previously the M1 mainstream is probably further offshore. There is a final impedance due to yet another canyon along section I, where the speeds associated with the curvature are probably exaggerated, before the MW can progress into the North Atlantic with speeds between 7 and 4 cm s-~. In contrast the MW in the southwest area was associated with a complicated system of sluggish eddies where the velocities did not exceed 5 cm s- 1.

Finally, estimates were made of the transport of volume of the MW outflow as it progresses downstream from the straits (Table 1). The transport through each pair of


stations in the section was calculated by assuming a mean upper level of no motion determined by considering the minima in the T-S characteristics as described, and for the case of a sloping bottom the velocity at the lower common level was then assumed to be constant to the bottom. The value of 1 .51xl06m3s -1 through section XV can be compared with the classical value of 1.65 × 10 6 m 3 S-1 (DEFANT, 1961) but more critically with recent estimates based on direct current measurements. In a similar section LACOMBE et al. (1964) computed a transport of 0.66 to 1.38 x |06m 3 s -1 and LACOMBE (1971) estimated 0.72 to 1.57× 106m3s-1. The range in values was due to tidal effects and different seasonal conditions. Although only sections XV and XIV probably intersect the entire MW outflow, and considering that there will be some loss around the inshore and offshore extremities of the other sections, it is obvious that there is an increase in transport downstream to section X. The apparently low value for section XIII is probably due to the small angle of intersection and the use of velocities that represent a small component of the flow.

The divergence off Cape St Vincent is evident in the reduced transport through section I due mainly, as previously discussed, to the lower core following a more westerly offshore path around the cape. If the increase in transport is due to entrainment of adjacent water, the mass continuity equation can be used to quantify it:

~(UA) - E U ,

~x

where E is the entrainment length, x is in the direction of the flow, U is the mean velocity, and A is the cross-sectional area of the MW. Using the transport volumes from Table 1 for sections XV to XIV, E is 0.02 km and between sections XIV and XII, E is 0.05 km, which represent entrainment values near the source. Beyond these sections the two cores have separated.

4. DISCUSSION

The general situation where the MW leaves the Straits of Gibraltar and sinks to an equilibrium level at 1200m in the Gulf of Cadiz is well known. These results suggest a separation at about 6°40'W where an upper core, with a speed of 46 cm s-~, follows the 800-m contour while remaining at most places in contact with the continental slope. It appears to turn northward sharply around Cape St Vincent with a speed of 15 to 20 crn s-~. There was some offshore spreading, mainly west of 8°W, associated with the prominent canyon at 8°30'W. It was only here that Madelain detected this warm upper core. He assumed that it had originated as a subdivision of the main common stream by being directed along channels in the bottom topography and flowing around the 500-m contour before eventually spreading offshore down the canyon. The lower core remains in bottom contact until about 8°W, where it attains its equilibrium level and proceeds as a free oceanic flow. Its offshore course originates at about 7°20'W, where the flow path is drastically disturbed by the bottom topography, which then continues to influence its dynamics by inducing prominent meanders along the route. The high speed of 180 cm s- t

at the source is reduced to about 36 cm s-~ at the separation zone and then, apart from local variations that can be quite substantial within the meanders, the velocities decrease to an average of 20 cm s- ~ in a rather divergent flow around Cape St Vincent. As the T-S distributions and field of mass analysis both show a high degree of coherence it is proposed that the reports of variability in the undercurrent, particularly west of 7°W,


might be explained more readily by variations in the scale and extent of the meanders, rather than by any sudden interruption in the downstream flow properties due to tidal or other mixing processes in the straits. The consequent interaction with, and replacement of, MW by NACW would then account for the large thermohaline changes and apparent discontinuities that occur at certain locations in this region. A further consequence of this interaction between the water masses produces an increase in the downstream transport with an estimated entrainment parameter near the source of 0.02 to 0.05 km.

Applying a streamtube model to the MW outflow, SMITH (1975) concluded that the bottom friction was the dominant factor in the dynamics of the flow near the source, but turbulent entrainment would become more important further downstream. His optimum entrainment parameter, which agreed best with the average flow properties of the lower core, was E = 0.05 km. Smith recognised the limitations of the data he used (MADELAIN,

1970) but even so there is some agreement with the magnitude and the increase in the entrainment value predicted by the streamtube model.

The other new result is the possibility of a counterflow beneath the lower core, which was systematically deduced in the sections between 9°30 , and 8°30'W. The same feature also appears in the 1973 Shackleton data. An interesting similarity therefore arises between this oceanographic situation and certain aspects of the Gulf Stream.

An undercurrent, in contact with a continental slope, experiences a downstream increase in transport and is subjected to meanders, while a countercurrent exists beneath the offshore edge of the flow. Although the Gulf Stream counterflow awaits a complete theoretical explanation, HOLLAND (1973) investigated the effects of variable bottom topography and was able to reproduce in his model the downstream increase in transport and the countercurrent. If the present results can firstly be confirmed by direct measurements, it might then be feasible to apply similar theoretical concepts to explain the dynamics of the deep circulation in this region.

Acknowledgements We are indebted to our colleagues at Liverpool University who took part in the work at sea and to the crews of R.R.S. Shackleton and Challenger. The data were processed by MRS A. WOODHOUSE. The study was supported by a N.E.R.C. Grant GR3/2643 and ISABEL AMBAR was the holder of a N.A.T.O. Research Scholarship.

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Wasserbewegungen im atlantischen Ozean. Meteor.-Werk, 6, LieJg. 5, 191-260. DEFANT A. (1950) Reality and illusion in oceanographic surveys. Journal of Marine Research, 9, 120--138. DEFANT A. (1961) Physical oceanography, Vol. I. Pergamon Press, 729 pp. FOMIN L. M. (1964) "/he dynamic method in oceanography. Elsevier, 212 pp. HIDAKA K. ( | 940) Absolute evaluation of oceanic currents in dynamical calculations. Proceedings of the Imperial

Academy ~f Tokyo, 15. HOLLAND W. R. (1973) Baroi=linic and topographic influences on the transport in western boundary currents.

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geotechnique de Tanger au 1/25000. Notes M. Serv. Geol. Maroc. No. 222, I l 1-146. MADELAIN F. (1967) Calculs dynamiques au large de la Peninsula Iberique. Cahiers ocbanographiques, 19,

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Observations of the mediterranean outflow—II the deep circulation in the vicinity of the gulf of cadiz