A shallow core of Mediterranean water off western Portugal

Deep-SeaResearch,Vol.30, No. 6A, pp. 677 to 680, 1983. Printed in Great Britain.

0198-0149/83 $3.00 + 0.00 ~ 1983 PergamonPress Ltd.

NOTE A shallow core of Mediterranean Water off western Portugal I. AMBAR * (Received 20 July 1982; in revisedform 25 October 1982; accepted 15 January 1983) A b s t r a c t - - F r o m an analysis of several sets of hydrological data taken in the Gulf of Cadiz area and off the southern half of the western coast of Portugal, it was possible to identify and establish the continuity of a shallow vein of Mediterranean Water, distinct from the already well-established two main cores. Its thermohaline characteristics, in the form of temperature and salinity maxima in the vertical profiles, allowed its identification at density (or) levels between 27.25 and 27.45 with an equilibrium depth between 400 and 700 m and with a thickness of 50 to 100 m.

INTRODUCTION

THE presence of Mediterranean Water in the North Atlantic Ocean has been linked classically to an intermediate salty and warm layer reaching its equilibrium level at 1000 to 1200 m, corresponding to (~t values of -~27.8. More recent observations with continuous recording systems (STD or C T D ) in and about the Gulf of Cadiz indicate a double structure in thc Mediterranean intrusion, with a second ('upper') core around 800 to 900 m (a, '~ 27.6) in addition to the classicaldeep ('lower')core. Both cores are characterized by salinity(S) and temperature (7")extrema in the verticalprofiles(ZENK, 1970; HOWE and TAIT, 1972; AMSAR, H O W E and ABDULLAH, 1976; AMeAR and HOWE, 1979). More recently, the existence of a third,even shallower but well-defined core between 400 and 600 m has been detected at some C T D and S T D stations in the area of the Gulf of Cadiz (ZENK, 1975a, b; AMBAR, 1977; AMSAR and HOWE, 1979; GROFrDLINOH, 1981). The present study is to trace this third core along the Portuguese upper continental slope. TEMPERATURE-SALINITY ANALYSIS

The Gulf of Cadiz area ZENK (1975a, b) and GRONDUNGH (1981) analyzed data collected during the 1967 and 1971 cruises of the R.V. Meteor in the Gulf of Cadiz and found evidence of a 'shelf branch' of the Mediterranean outflow at depths between 400 and 600 m. Their Stas 44 and 25 (Fig. l) showed T-S extrema for the shallower core (GRONDUNGH, 198 l) corresponding to comparable a t values of about 27.25 (Fig. 2a).

* Grupo de Oeeanografia, Departamento de Fisica, Faculdade de Ci~neias, Universidade de Lisboa, Rua da Escola Polit~cnica 58, 1200 Lisboa, Portugal.

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A similar feature, i.e., a secondary peak in the T - S curve at densities lower than those corresponding to the well-established 'lower' and 'upper' cores, was observed at several stations during the 1976 cruise of the British R.R.S. Challenger (AMsAR, 1977; AMSAR and HOWE, 1979). All the stations were close to the northern upper slope of the Gulf of Cadiz (Fig. 1). The T - S maxima corresponding to the shallow Mediterranean Water core (hereafter called 'shallow' core) is evident in the T - S diagram (Fig. 2a). As a result of the increased mixing with the surrounding Atlantic Water, there is a tendency for a slight general decrease in the density of the shallow core (Fig. 2a) westward from the Strait of Gibraltar (with o t " 27.45) to those further west (where a, approaches 27.3), and also going from inshore to offshore stations. The area off the west coast of Portugal

To investigate the presence of the shallow core along the southern half of the Portuguese west coast, the available T S data from several cruises have been analyzed: the British Shackleton 2/73 (STD data) and the Portuguese HIDROSET 1/78 to 5/78 and OCASO 2/81 (all with bottle stations), conducted by the University of Lisbon. Figure 1 shows the location of the stations in the area where the shallow core was evident in the T - S diagrams, either as a well-defined T - S maximum (e.g., at Sta. 47 of Shackleton 2/73, inset of Fig. 2b) or as an inflexion due to a temperature maximum within the line reflecting mixing between North

A shallow core of Mediterranean Water off western Portugal

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Fig. 2. Scatter plots of the temperature-salinity (T-S) characteristics of the maximumcorresponding to the shallow core of Mediterranean Water: (a) in the Gulf of Cadiz, with data from Challenger 13/76 (AMnAR, 1977) and Meteor 1971 (GRONDLINGH,1981) cruises; (b) off the west coast of Portugal with data from Shackleton 2/73, HIDROSETsand OCASO 2/81 cruises. The inset shows two typicalexamples of T-S curves where the shallow core is evident. The symbols arc the same as those used to identify the different cruises in Fig. 1. Atlantic Central Water and layers of Mediterranean origin (e.g., at Sta. 16 of OCASO 2/81, also represented in the inset of Fig. 2b). From the Shackleton STD data, it was possible to detect the shallow core in 14 out of a total of 16 stations occupied between the 500 and the 2000-m bathymetric contours; the thickness of the core was generally 50 to 100 m. The bottle stations from the other cruises had a vertical resolution of 50 m (OCASO) or 100 m (HIDROSET) in the depth range of interest and it is not surprising that the feature could be identified in only a few cases in these cruises. The main features evident in the analysis of the thermohaline characteristics of the shallow core in the area can be summarized as follows: (a) the density (or) values extend from 27.45 to 27.25 (see Fig. 2b); (b) the shallow core is parallel to the main bathymetric contours, offshore of the 500-m contour (see Fig. 1), and its equilibrium level lies generally between 500 and 700 m; (c) the core seems to 'feel' the canyons in the sense that it approaches the shore under their influence, as shown in the distribution of stations of Fig. 1 along the axes of the main canyons of Setfbal and Sao Vicente. Low nutrient contents have been reported for the main Mediterranean Water intrusions in the Atlantic (CosTE, 1969; HOWE, ABDULLAHand DEETAE, 1974) and, if this is also the case in the shallow core, it probably influences the chemistry and biological productivity of the

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waters of the Portuguese upper continental slope. There is also some evidence of upwarping of the upper layers of Mediterranean Water near the shelf break in the area during summer, in association with coastal upwelling (FIOzA, 1982) with probably biological implications. CONCLUDING REMARKS

Comparison of the different sets of data demonstrates that a shallow vein of Mediterranean Water, distinct from well-establisbed two main cores, can be identified from the vicinity (near 60 40'W) of the Strait of Gibraltar to about 38°30'N off western Portugal, following a path along the upper continental slope of the Iberian Peninsula. The feature may possibly result, as was suggested by AMBAR and HOWE (1979), from a subdivision of the upper core, probably due to the influence of bottom topography in the Gulf of Cadiz. Interesting questions are how far northward the shallow vein maintains its identity, what are the details of its time and space scales, and what is its influence, both through its probable low nutrient content and its advective effects, on the chemistry, biology, and quality of the Portuguese coastal waters. Some of these matters are currently being investigated under a research project of the University of Lisbon. Acknowledgements--This work was partially supported by Junta Nacional de Investigaq~o Cientifica e Tecnoibgica under research contract No. 314.81.56. The author wishes to thank Dr. M. HOWE of the Oceanography Department of the University of Liverpool, U.K., for making available part of the data. REFERENCES AMEAR I. (1977) A thermohaline analysis of the outflow of the Mediterranean water in the vicinity of the Gulf of Cadiz. Ph.D. Thesis, Department of Oceanography, University of Liverpool, 190 pp. AMBAR I. and M. R. HOWE (1979) Observations of the Mediterranean outtlow--l. Mixing in the Mediterranean outflow. Deep-Sea Research, 26, 535-554. AMBAR L, M. R. HOWE and M. I. ABDULLAH(1976) A physical and chemical description of the Mediterranean outflow in the Gulf of Cadiz. Deutsche Hydrographische ZeitschrOrt, 29, 58--68. COSTE B. (1969) Echanges de sels nutritifs dissous entre la mar M~diterran~e et l'Oc~an Atlantique. Cahiers Oc~anographiques, 21, 943-963. FIOZA A. (1982) The Portuguese coastal upweLling system. In: Present problems of oceanography in Portugal, proceedings of a seminar conducted under the sponsorship of the NATO Special Programme Panel on Marine Sciences in Lisbon, 19-20 November 1980; Junta Nacional de Investigaf~o Cientiflca e Tecnol6gica, Lisbon, pp. 45-71. GRONDLINGH M. (1981) On the observation of a solitary event in the Mediterranean outflow west of Gibraltar. Meteor Forschungsergebntsse A/B, 23, 15--46. HOWE M. R. and R. I. TAIT (1972) The role of temperature inversions in the mixing processes of the deep ocean. Deep-Sea Research, 19, 781-791. HOWE M.R., M. L ABDULLAHand S. DEETAE (1974) An interpretation of the double T-S maxima in the Mediterranean outflow using chemical tracers. Journal of Marine Research, 32, 377-386. ZENK W. (1970) On the temperature and salinity structure of the Mediterranean water in the northeast Atlantic. Deep-Sea Research, 17, 627--632. ZENK W. (1975a) On the origin of the intermediate double maxima in T--S profiles from the North Atlantic. Meteor Forschungsergebnisse A, 16, 34--43. ZENK W. (1975b) Some current and temperature observations in the Mediterranean outflow west of Gibraltar. Meteor Forschungsergebnisse A, 15, 20--48.