SHORTER CONTRIBUTIONS

S H O R T E R C O N T R I B U T I O N S

On the temperature and salinity structure of the Mediterranean water in the Northeast Atlantic

W. ZENK*

(Received 29 September 1969)

Abstract--Typical results of continuous temperature-salinity measurements from stations west of Gibraltar are presented. Special features of the structure of the Mediterranean Water in this area in relation to the corresponding turbulent mixing and salt-fingering processes are discussed: the two maxima in the vertical profiles, the variation in time of small-scale phenomena, and the step-like structure at the lower boundary of the Mediterranean Water.

INTRODUCTION

As part of a long-time plan of studies of oceanic variability (DIETRICH, 1966), an investigation of the temperature-salinity structure west of Gibraltar was carried out o n R.V. Meteor Cruise No. 8 in winter 1967. Continuous profiles of temperature, electrical conductivity, and pressure were obtained by instruments of the Bathysonde type (SIEDLER, 1968a). The positions of hydrographic stations are given in Fig. 1. Station 65 was a ten-day anchor station. Typical results from a few of these stations will be presented here.

TYPICAL VERTICAL PROFILES AND T-S-RELATIONS

The position of the core layer of the Mediterranean Water in the N o r t h Atlantic has been studied extensively on the basis of Nansen bottle data by numerous authors (Wi3ST, 1936; COOPER, 1967;

MADELAIN, 1967). A n attempt was made in this investigation to study the vertical fine structure of temperature and salinity by continuously recording instruments.

Four examples from stations following the Mediterranean Water from Gibraltar to Cape Finisterre are given in Fig. 2. It can be seen that in the depth range of the Mediterranean Water a strong fine structure with small vertical scales is found similar to former investigations in areas with equivalent hydrographic conditions (SIEDLER, 1969; PINGREE, 1969). This fine structure decreases in amplitude with increasing distance from the strait. The divergence of the main temperature and salinity maxima is a result of the different boundary conditions for the mixing of the two properties near the source (BuBNOV, 1967).

Another characteristic feature is the frequent existence of two main maxima in the temperature and salinity profiles. This type of profile was already shown earlier for a greater distance from Gibraltar by narrowly-spaced Nansen bottle measurements (COOPER et al., 1962, Fig. 90). The mean depths of these two main peaks were found at 750 and 1170 meters from repeated measurements at Sta. 65. It has been shown that these two maxima are a consequence of the tidal mixing processes in the Strait of Gibraltar where a frequency distribution of water types with two preferred temperature and salinity values is generated (SIEDLER, 1968b). These two types of water are denoted by M W 1 and M W 2 in Fig. 3. This water moves down the slope while mixing with the Deep Atlantic Water.

At the depth where it has the same density as the surrounding water it leaves the b o t t o m and spreads more or less horizontally. Figure 3 additionally gives the distribution at Meteor station no. 8-68 in the G u l f of Cadiz and at Crawford Sta. 231 in the Sargasso Sea as examples for areas with Mediter- ranean Water having a small influence. N A C W denotes the N o r t h Atlantic Central Water (SvEaORUP, 1941), at approximately 500 m in such a n area, and the dotted line is obtained from a mean of all

*Institut fOi Meereskunde, 23 Kiel, Niemannsweg 11, Germany.

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profiles o f Meteor Sta. 65. This T-S-diagram displays the fact that the two observed main maxima h.ave their origin in the water types Max 1 and Max 2 which are a mixture of N A C W with MW 1 and MW 2 respectively with the same mixing ratio.

Variabifity in thne

On the anchor station a total number of 61 profiles o f temperature and salinity were obtained in 4-hr-intervals. Samples of these data for the depth range of the core layer are shown in Fig. 4.

Great variations are observed in the total heat and salt content o f this layer as well as in its vertical structure. The double-maximum feature is found during certain time intervals only. Other measure- ments in the neighbourhood o f this station displayed a corresponding horizontal lens-like structure of the water.

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Fig. 3. Temperature-salinity diagram indicating the mixing of Me ]North Atlantic Central Water. The following abbreviations are used:

N A C W -- North Atlantic Centra] Water at approximate]), 500 m dept ]VDV ], ~ 2 = main t),pes of outflowLng Mcdite~ancan Water in th M a x 1, M a x 2 = p r o d u c t o f mixture o f N A C W with M W 1 a n d M W :

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S T E P - L I K E S T R U C T U R E A N D S A L T - F I N G E R I N G

Characteristic step-like structures of temperature and salinity were found at the lower boundary of the Mediterranean Water in certain areas. Later after the measurements, information was obtained that similar steps had been observed by British oceanographers in 1966 in the Madeira area (TAIT and HOWE, 1968). A n example of the Meteor-data is given in Fig. 5. The dots denote depth levels where salinity was computed from digitized data, the lines are drawn according to analog data.

A step-structure was observed at 8 stations. The depth range with steps is found between 1300 and 1545 meters, with an average number of 7'6 steps per profile. The mean thickness of the homo- geneous layers was 21-6 meters similar to the results of Tait and Howe. The mean temperature and salinity differences A~F and ± S across the interfaces and the corresponding standard deviations were found as: A-T - 0.37 ± 0.19~C, f S = 0.069 - 0035!!~o. It has been shown earlier (TURNER, 1967) that such a step-like structure should be expected if salt-fingering occurs at the lower boundary of the warm and haline Mediterranean Water. The vertical transport coefficients Ks (see TURNER, 1967) calculated from the Meteor-data yield values of Ks ~ 7 crn 2 sec ~ as compared with K~s. ~ 5 cnr e sec obtained by Tait and Howe in the Madeira area. The lifetime T of the salt-fingers is found to be T ~ 4 days and is therefore shorter than in the Madeira area where T = 6 days was calculated.

Acknowledgements--The results described here would not have been possible without the cxtensive support of Prof. Dr. G. DIETRICH, Prof. Dr. G. S1EDLER, the Marine Physics Branch of the lnstitut ffir Meereskunde o f Kiel University, and the crew of the R.V. Meteor. The work was supported by the Deutsche Forschungsgemeinschaft, Bad Godesberg, Germany. Cruise reports and detailed presenta- tion of these and further results will be published in: Meteor-Forschungsergebnisse, Reihe A, Berlin.

R E F E R E N C E S

BUBNOV B. A. (1967) Vertical turbulent exchange and transformation of Mediterranean Water i~r the Atlantic. Oceanology, 7, 456-460.

COOPER L. H. N., P. G. W. JONES and A. J. LEE (1962) Hydrograpbical conditions along the 14 W meridian south-west of Ireland, July 1960. Ann. biol., 14, 73-76.

COOPER L. H. N. (1967) Stratification in the deep ocean. Sci. Pros., OaJ~, 55, 73-90.

DIETPOCI~ G. (1966) Ver~nderlichkeit im Ozean. Kieler Meeresforsch., 22, 139-144.

MADELAIN F. (1967) Etude hydrologique au large de la p6ninsule Ib6rique. Cah. Ocdanogr., 19, 125-136.

SIEDLER G. (1968a) In situ measurements and automatic recordings of conductivity, temperature and pressure. Paper Nr. 25 in: Proceedings of the LE.R.E. Conference on Electronic Engineering in Oceanography, Southampton, 12-15 Sept. 1966, Supplement Vol. London: Institution of Electronic and Radio Engineers, 5 pp.

SIEDLER G. (1968b)Die H~iufigkeitsverteilung von Wasserarten h~q Ausstromgebiet von Meeresstrassen.

Kieler Meeresforsch., 24, 59-65.

SIEDLER G. (1969) O n the fine structure of density and current distribution and its short-time varia- tions in different areas. Ptvgress in Oceanography, 5, 81-94.

PINGREE R. D. (1969) Small-scale structure of temperature and salinity near Station Cavall. Deep- Sea Res., 16, 275-295.

SVERDRUP H. U,, M. W. JOHNSON and R. H. FLEMING (1942) The Oceans, pp. 668-671. New York:

Prentice-Hall.

TMT R. J. and M. R. H o w e (1968) Some observations of the thermohaline stratification in the deep ocean. Deep-Sea Res., 15, 275-280.

TURNER J. S. (1967) Salt fingers across a density interface. Deep-Sea Res., 14, 599-611.

WOST G. (1936) Die Stratosph~ire des Atlantischen Ozeans. Wiss. Ergebn. dr. atlant. Exped. ~ Meteor ', VI, 1. Tell, 2. Lfg., 221.