A physical and chemical description of the Mediterranean outflow in the Gulf of Cadiz

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  • A Phys ica l and Chemica l Descr ip t ion of the Med i te r ranean

    Out f low in the Gu l f o f Cad iz

    UDC 551.465.45:551.465.42; Northeast Atlantic, 5Iarsden Square 109

    I. Ambar , M. R. Howe and M. I. Abdu l lah

    Summary The Mediterranean outflow is considered as two separate cores (upper and

    lower) which follow a well defined route between Gibraltar and Cabo SBo Vincente. Their physical and chemical properties are discussed in some detail and used as evidence for treating the upper core as a separate water mass. This core is also shown to suffer a 20 % depletion in ,its concentration as it traverses the Gulf of Cadiz, whereas that of the lower core remains almost constant. In addition there was some indication of winter cooling and sinking of the shelf water which seems to be enhanced by the presence of the more prominent canyons in the area.

    Eine physikalische und ehemische Beschreibung des Mittelmeer-Ausstroms in den Golf yon Cadiz (Zusammenfassung)

    Das Ausstr6men des Mittelmeerwassers geschieht in zwei getrennten (oberen und unteren) Kernmassen, die sich entlang einer deutlich abgegrenzten Route zwischen Gibraltar und Cabo Sgo Vicente ausbreiten. Ihre physikalischen und chemischen Eigenschaften werden ausfiihrlich beschrieben und als Beweis dafiir benutzt, dab man den oberen Kern als getrennte Wassermasse behandeln kann. Es wird auch gezeigt, dab dieser Kern beim Durchzug durch den Golf yon Cadiz in seiner Kon- zentration um 20 % abgeschw~icht wird, wohingegen die untere Kernkonzentration fast konstaut bleibt. AuBerdem gibt es Anzeichen fiir ein Abkiihlen und Absinken des Schelfwassers im Winter, welches durch das Vorhandensein yon Cations in diesem Gebiet verst~rkt zu werden scheint.

    Une description physique et chimique du courant d'gcoulement m6diterran6en dans le Golfe de Cadix (R~sum~)

    Le courant d'6coulement m@diterran~en est consid@r5 comme comportant deux couches distinctes (la sup6rieure et l'inf@Heure), qui suivent une route bien d@finie entre Gibraltar et le Cap Saint-Vincent. Leurs propri6t@s physiques et chimiques font l'objet d'une discussion d4taill@e et sont utilisSes comme t@moins permettant de traiter la couche sup4rieure comme une masse d'eau s@ar@e. On montre 5galement que cette couche subit une d@perdition de concentration de 20 % lors de la travers6e du Golfe de Cadix, alors que celle de la couche inf@rieure reste presque constante. :De plus, il a 6t6 constat@ un certain refroidissement hivernal et un enfoncement de l'eau du plateau continental, qui semble renforc@ par la presence des canyons les plus importants de la rSgion.

    1. Introduction

    I t is now accepted that the outflow of the Mediterranean water from the Straits of Gibraltar can be influenced by several different processes. The seasonal changes that occur have been acknowledged for some time (S chot t [1928]), and recently shorter period fluctua-

  • Ambar et al., Med i te r ranean Out f low in the Gu l f of Cadiz 59

    tions in the flow have been attributed to the variations in atmospheric pressure between the regions to the East and West of the Straits (Lacombe [1961]; Crepon [1965]). There are strong tidal currents superimposed on the flow and Defant [1961] has discussed the likely internal tidal oscillations in the Straits and their effect on the boundary between the inflowing Atlantic water at the surface and the deeper Mediterranean outflow, whereas Boyce [1975] has made a similar study of internal waves. Furthermore, the outflow to the West of Gibraltar may also be influenced by the bottom topography and this has been considered in some detail by Madela in [1970]. Finally an explanation is required for the fact that a core of Mediter- ranean water can be observed in the Atlantic at depths other than the traditional 1200 m !~rel. Several publications have noted the existence of a prominent T-S maximum between 5~0 to 800 m which Howe, Abdu l lah and Deetae [1974] have described in terms of its chemical and physical properties. They suggested that this water originates from a relatively shallow depth within the Straits.

    There are therefore several processes which can influence the intensity and characteristics of the outflow as well as its most likely route, and their relative importance has yet tO be determined. The use of TSD systems in this area have provided observations which indicate a complex situation in which water masses are being generated under different circumstances and then mix at different depths to produce considerable variability in the properties in both the horizontal and vertical directions. The t~.t~.S. "Shaeldeton" cruise (2/73) included a study of the region between Cabo Sgo Vieente.and Gibraltar in March 1973, and this provided physical and chemical data which adequately demonstrated the degree of inhomogeneity in the water column. The four sections (Fig. 1) were completed within 5 days although most of the

    375 N

    36 ~

    /~e - - depth contours in fathoms

    ~O0~/~C ente

    Qo77 . % / \ \ \ st2,t \ , , 10C~ < ~ ' ~ , ~, ~ ~, Gibraltar 9~ S ~ 7 ~

    Fig'. 1. Station positions ill sections IV to VII. 9 TSD station. 9 TSD and chemical sampling

    6 ~

    stations (59 to 91) were made within a period of 66 h. The remainder (92 to 100) were delayed due to a storm. The stations were purposely located along the prominent canyons in the area with the intention of observing any unusual effects which might be caused by these topo- graphical features. We shall describe the hydrography of the region at this time of the year ::a terms of the evidence presented in these sections and thereby attempt to explain the events which might occur as the outflow leaves the Straits and moves around the Gulf of Cadiz towards Cabo Sgo Vieente. A TSD system was used and this was supplemented by water samples from standard depths at certain stations (Fig. 1), from which the nutrient salts (nitrate, silicate and phosphate) were determined. The chemical methods of analysis have already been fully described (Howe, Abdu l lah and Deetae [1974]).

  • 60 Deutsche Hydrographische Zeitschrift, Jahrgang 29, 1976, Hef t 2

    2. Temperature, salinity and nutrient profiles

    An inspection of the T, S and nutrient distributions (Figs. 2, 3, 4, 5) will show that the normal Atlantic water column, which is best represented in this area by the offshore stations in each section, can be severely disturbed by the intrusion of the Mediterranean outflow. This main ly occurs near the continental slope between depths of 450 m and 1300 m.

    s to t ion number 69 68 67 66 65 62~ 63 62 61 60 59

    6~176 ~:~

  • Ambar et al., Mediterranean Outflow in the Gulf of Cadiz 61

    The unaffected surface layers between 50 to 400 m show, in general, a regular decrease of T and S with depth, from 15 ~ to 12 ~ and 36.2 ~ o to 35.7 ~ o respectively. This is accompanied by an increase in the values of the nutrient salts (Figs. 2b, 3b, 5b). However,

    20C

    m

    400

    600

    ~ 800

    ~ 1000 1200

    1400

    s tat ion number ) 71 72 7,3 74 75 76 70 71 72 7? 74 75 76

    2oo-

    m

    400-

    600-

    800-

    1000-

    1200-

    1400-

    !I\\ T .. k~--" ~ 1 1 . o - ~

    Fig. 3a. Section V. Distribution of salinity and temperature

    at several inshore stations these layers have also been disturbed, but in this case by an apparent surface sinking process. This is particularly evident in sections V and VII where it seems probable that the effect of winter cooling on the shelf has been encouraged by the presence of a prominent submarine canyon to produce a tongue of downwelled surface water to depths of 400 m or 500 m. Below 400 m the intrusion of the Mediterranean water (MW) can be observed by the relatively high salinities and temperatures and also the correspondingly low

    station number 70 71 72 73 74 75

    , , f f ,

    m

    looo

    12oo

    Fig. 3b. Section V. Distribution of silicate, nitrate an,

    s tat ion number 70 71 72 73 74 75

    , , , i i 3

    , l o

    ~:6 10

    s tat ion number 70 71 72 73 74 75

    I I t r ,

    2OO 0.5

    ~176 1 ~ ~

    phosphate

    nutrient values. This correlation between the distortion of the nutrient profiles in the Atlantic and the high salinities has already been analysed in some detail (Howe, Abdu l lah and Deetae [1974]). The fact that MW of ]ow nutrient content intrudes into the Atlantic water column at depths where the nutrient values are normally much greater, has provided an additional means of identifying its presence. In sections VI and VII (Figs. 4, 5) nearest the

  • 62 Deutsche :[-Iydrographische Zeitschrift, Jahrgang 29, 1976, Hef t 2

    station number 88 87 86 8> 8,4 e? 87 8! 80 79 7,8 7,7

    171 ~ ~35.8~

    4 0 0 " ~ B S Y ~ -

    5.6 ~ . . . .

    a: : . - ' : :5 ' - -

    800-

    1000

    12oo- Section ~ " ' "~k / 1~%~

    1400.

    8- 2 0 0 q ~ 1 4 o ~ __

    I m ~ ~ X _ - - - l ~ . O ~ ~ 1 2 . 5 ~ 400 1 2 0 ~

    .......... ~ ~za2 'e .s~ 8oo- "~ ~'%-Y._-~.~-~--:~.5 ~ ~-~1~0~

    ,ooo8~176 ' . . . . . - " Temperature ~ " : ~ .

    1400. %"::: v4 :'~:~ ::: *"

    Fig. 4. Section VI. Distribution of salinity and temperature

    Straits, there is a continuous layer of MW on the slope extending from about 450 m to 1300 m whereas, in sections IV and V (Figs. 2, 3), the MW has separated into discrete upper and lower cores. Although the salinities are comparable, in all the sections the temperature in the MW above 900 m is consistently about 1 ~ greater than in that below 900 m. There is some indica- t ion of an offshore spreading of both cores in section V but at any depth the greatest values of T and S, together with the lowest nutr ient concentrations, are observed inshore against the cont inental slope. This suggests a preferred flow path from the Straits to the north west, following the contours of the continental slope towards Cabo S~o Vieente.

    Although there is very l i tt le homogeneity in the distr ibut ion of the propert ies in these sections, there are several features which deserve part icular attent ion. In making this more detai led comparison between various water masses at different depths we shall therefore more correctly use the appropr iate values of potent ia l temperature O, and density G o. In section V I I an 'anomalous' mass of MW was observed at the bot tom of stns. 92 (S = 36.88 ~ O = 12.95~ ao = 27.88) and 93 (S = 36.80~ O = 12.71~ (r o = 27.86). The high salinities have produced densities at a depth of about 870 m which are more typ ica l ly observed offshore at 1200 m, as in stn. 98 (S = 36.52 ~ O = 11.63 ~ ao = 27.86). I t is interest ing to speculate on the u l t imate fate of this water. F i rs t ly the cont inuity of the MW down the slope, with its correspondingly high density, would make it difficult for this upper core to sink and displace the water beneath it. Secondly, although no chemical sample was taken in the actual core at a depth of 870 m, the sal in i ty-nutr ient relat ionship at 800 m in stn. 92, together with that at the nearby stns. 91 (600 m) and 95 (800 m) has been shown (Howe, Abdu l lah and Deetae [1974]) to be closely correlated with the sal in i ty-nutr ient charac- terist ics of the upper core of lV[W which was observed in the other sections further to the west. Is it more l ikely then that this upper MW, being so anomously high in sal inity, but unable

  • Ambar et al., Mediterranean Outflow in ~he Gulf of Cadiz 63

    to sink immediately to a more appropriate density level, will lose salt more rapidly than heat due to the ambient conditions at this depth ? Accordingly, does it therefore adjust to values such as those at 780 m in stn. 83 (S = 36.66 ~ 0 = 12.91 ~ av = 27.71) and, consequently, stabilise at the density level at which the discrete upper core of MW is normally observed (section V) ?

    This upper core is easily identifiable in sections IV and V where it is notable for its relatively high temperature, with such values as O : 13.54 ~ (S = 36.54 ~ ~o = 27.49, D = 636 m) at stn. 66 and similarly at stn. 71 (O = 13.48 ~ S = 36.52 ~ ao = 27.49, D - 793 m). The identification is also facil itated by the presence, as explained above, of low nutrient concentrations between 600 and 800 m. In contrast the lower core in these sections was more saline with the greatest values at stn. 63 (S = 36.64 ~ O = 12.78 ~ av = 27.73, D = 1000 m) and stn. 73 (S = 36.59 ~ O = 12.09 ~ ao = 27.82, D = 1270 m).

    stotion number 89 90 91 92 93 94 95 96 97 98 99 100

    .~36.2~ 2OOrn ~ ~ ' ~s6~

    3 5 8 ~ 600 "::.'.::....... 66 3 9 . 6 ~

    ":;.. 6-8 36.3 1000

    1200 SectionZZ ~" .~\ Salin,ty %~ "'::~\

    "::.. 360 ~ 1400 "::~:5.8--

    / 200{~>._ ~ 14.o

    4oot : : % ~ ~ ~ ~-~ . o ~

    800 ::"~k-f-'-12~

    "".'... \~ \ r ~-

    Tempe roture ~ ~":'~} / ~10.5 "-:. 10.0

    ~4oo : :~/~ - ~ ~

    Fig. 5a. Section VII. Distribution of salinity and temperature

    3. O-S diagram

    There is some indication from the general distribution of the water mass characteristics ~hat there could be a significant difference in the mixing rate of the upper and lower cores of the MW, with the depletion of salt and heat being dependent on the ambient conditions at each level. An attempt to quantify this was made by applying to the O-S diagrams the graphic method of the "triangle of mixing" and thereby estimating the percentage of different water masses that were present (Mamayev [1975]). The construction of such a triangle is determined by the choice of the thermohaline indices (O, S) to represent the original water masses. Since

  • 64 Deutsche Hydrographisehe Zeitsehrift, Jahrgang 29, 1976, Heft 2

    previous evidence (1974), based on the temperature values and the salinity-nutrient relation- ship, suggests a separate source of MW for the upper core, two triangles of mixing were constructed and applied to the MW either above 900 m (Mu) or below 900 m (ML).

    M u t r iang le It has been concluded that the M u water originates from a depth of about 135 m in the

    Mediterranean, passes through the Straits with little mixing, and then sinks to 600 to 800 m in the Atlantic. This result, together with that obtained by constructing the triangles of

    !

    s to t ion numbe#

    89 90 91 92 93 94 95 96 97 98

    . . . . 2.5 200 " - - . . - - l~5 m ?!~

    - - - z5 600 ' ---8,5

    5.5 - -40 .5 800

    1O00

    1200

    1400

    s ta t ion number

    2(70m ~ ) ! 40o

    _~ 600 800 ~0

    1000

    120o 1400

    station number 89 90 91 92 93 94 95 96 ~7 9B

    I I , i I I I i I - ' _ _ _ _03

    -435 200- - 43.7

    1400 Sect ion ALLL ~2~

    5b. Section VII. Distribution of silicate, nitrate and phosphate

    mixing around the upper core by projecting the tangents on the O-S diagrams to intersect at a likely source (S tockman [1946]), indicate a reasonable value for the M u index of O = 14.00 ~ S = 37.42 ~ o. The mixing with the Atlantic central water will then occur between depths of 200 and 900 m for which there is well documented O-S data (e.g. Fug- l is ter [1960]) to support these present observations. The M, index was therefore combined with values of 14.90~ 36.13 ~ 0 and 8.40 ~ 35.00 ~ D respectively, to c...

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