Variability of the mediterranean undercurrent in the Gulf of Cadiz

  • View
    213

  • Download
    1

Embed Size (px)

Text of Variability of the mediterranean undercurrent in the Gulf of Cadiz

  • Deep-Sea Research. 1976, Vol. 23. pp. 111 to 121. Pergamon Press. Printed in Great Britain.

    Variability of the Mediterranean undercurrent in the Gulf of CadizS. A. THORPE*

    (Received 12 June 1975)

    Abstract-Measurements of currents, temperatures and salinities were made in March 1970and 1971on the north side of the Gulf of Cadiz in the Mediterranean undercurrent. Observations wereconcentrated in two areas. In the first the undercurrent is free from the sea bed and flows as a freejet although in contact with the continental slope on its northerly side, whilst in the second it is indirect contact with the sea bed and influenced by the topography of a submarine valley. Thesemeasurements demonstrate the mesoscale variability of the undercurrent, and the interruption ofthe flow in the first area by the arrival of denser ncar-bottom water and an eddy-like motion in theoverlying water.

    1. INTRODUCTION a neutrally buoyant float at a depth of aboutIN MARCH of 1970 and 1971 measurements 1400 m in an ernbayed area centred at 36'20'N,of currents, temperatures and salinities were g045'W (the track is shown on Fig. I) suggestmade in the Mediterranean Water on the north that horizontal eddies are important.side of the Gulf of Cadiz (Fig. I) from the Observations of temperature and salinity areR.R.S. Discovery using Aanderaa current meters more plentiful than those of currents. Measure-and Bissett-Berman temperature-salinity-depth ments using T.S.D. probes (for example, ZENK,probes (T.S.D.). HEEZEN and JOHNSON (1969)have 1970; PINGREE, 1972) have revealed rapid varia-reviewed some of the observations of the under- tions in both space and time, but with a strongcurrent in this area. They report several direct tendency for changes to occur along lines ofmeasurements of the current, but these are of constant potential density, so that the temperatureshort duration and give little indication of the and salinity compensate one another. Pingree hasvariability. The most striking indirect evidence emphasized that this may imply that mixing isof strong current is perhaps the variety of sedi- dominated by intrusions rather than by verticalmentary bedforms in the Gulf (KENYON and processes, such as Kelvin-Helmholtz instability.BELDERSON, 1973) and the presence of suspended The area in which layers clearly produced bysediment well off the sea floor (THORPE, 1972). double diffusive processes are found (HOWE andMADELAIN (1970) has described the path taken by TAIT, 1970; WILLIAMS, 1974) lies to the southwestthe undercurrent and has shown that it appears of the Gulf, west of a line joining Rabat to 37N,to be influenced strongly by the Coriolis force and IOoW and we are here concerned with the under-by the submarine topography of the valleys current in a region much nearer to its source in(marked on Fig. I by dashed lines) and the the Straits of Gibraltar. It may be that mixingrocky outcrops on their western sides. The lower processes here are also influenced by doublepart of the undercurrent is there deflected south- diffusive processes, but if so their effect on thewestwards down the continental slope, and speeds mesoscale structure is not obvious.in excess of 1 ms'< have been recorded. Little is The observations reported here are comple-yet known of the temporal variability of the mentary to those made from the Meteor in springundercurrent, although SWALLOW'S intriguing *Institute of Oceanographic Sciences, Wormley,observations (1969) of the almost circular track of Godalming, Surrey, England.

    711

  • 712 s. A. THORPE

    SOw 7W

    N FK..-.----------=~__r_:~-- ----:.--~"~--~=-__I 37

    SOw

    Fig. I. The Gulf of Cadiz. Soundings are in fathoms (I fathom = 1.8288 rn), The dashed lines mark submarine valleyswhich influence the undercurrent. Shown are the positions of single T.S .D. stations (.), repeated T.S.D. stations (*)and moorings (A.) in 1971. The numbers refer to stations mentioned in the text; the letters to moorings. The track ofSWALLOW'S (1969) float is shown by a continuous line with arrows. Submarine contours from MADELAIN (1970, Fig. I).

    1971 (SIEDLER, 1972; ZENK, 1975). They concen-trate in two areas. In the first, ncar 3612'N,g003'W, studied in both 1970 and 1971, theMediterranean Water is no longer in directcontact with the sea bed below but, as we shalldescribe, flows as a wedge against the continentalslope to the north with a mean vertical profilelike a free jet. The effects of its recent contact withthe sea bed are, however, apparent in the shapeof both the salinity and temperature bulge, in therelative stability of the water column above andbelow the core, and in the presence of suspendedsediment (THORPE, 1972).The second area studiedin 1971 is ncar the head of one of the valleysidentified by Madelain at 3617'N, 720'W. Herethe Mediterranean Water, characterized by itshigh salinity and temperature, is still in contactwith the sea bed, and large turbulent downslope

    currents have been recorded. Neither area is'typical' of the Mediterranean undercurrent in theGulf, but both are regions where importantprocesses influence the flow. It is here that theoutflow develops from a continental slope-boundcurrent to a free, though weak, oceanic flowcharacterized by its anomalous temperature andsalinity.

    2. TilE 1970 OBSERVATIONSThe measurements in 1970 were limited to a

    two-hourly T.S.D. series lasting for 6 days near3612'N, 801'W and to some fragmentary near-bottom measurements of currents using a cameratripod and electromagnetic current meter (THORPE,COLLINS and GAUNT, 1973). The currents weregenerally northwestward with speeds of about4.5 em s-1, but with semi-diurnal tidal oscillations

  • 2HOURLYSALlNI1Y PROFILESAT36'12'N 8'Ol;W28 FEB-5 MAR1970.

    Fig, 2, Two-hourly temperature and salinity profiles within 1,5 km of 36'12'N, gOOI 'w, 28 Feb. to 5 Mar. 1970. Successive profiles have been offset by O.SOC orQ'2rw and gaps appe ar when profiles were not recorded,

    1500

    s...

    r:;'g;~.o..,

    :;.C1>

    ~e:;;5

    ~::Jc:::lCoC1>iie...

    iigS'ETC>

    oc:::;o..,

    QCoN'

    500 E

    :iI-

    1000 fuCl

    _ 1500

    500E

    , ' 1000 :l-e....

    ~1500

    !J-i~__

    2-HOURLY TEM~ERATURE PROFILES'AT36; 12~ N 8' 01' W28 FEB-;-S MAR 1970

    TEMPERATURi 'C10 11' 12 13'14 15 16, iii I J I

    SALINITY, 'Yo .35 2 35 6 360 364 368

    I [ t .r

    E SOO

    ~ 1009e....

    Cl

    E., 500

    :il-e....

    Cl

    -..J-w

  • 28-2 '-2 2-3TIME, (DAYS) 1970

    3-3 4-3 53 6-3-..J-~

    -lP1SS ..---....-.... -- ~ __ = ~::::1i3:9

    ........... ~ :::::::::=::::: =-==== ~ ==--::; - 4:614.S-~ = ~ ~ ~ -= ~:::.g6IJS -...;:;,. -12'5

    12S "'--' - ---------- -12.0__-------------------- -lIS

    115

  • Variability of the Mediterranean undercurrent in the Gulf of Cadiz

    Table 1. Moorings in 1971.

    }[ooring Position Depth No. 01' Laid RecoveredNo. (Ill) Ne t er-a

    A J612.Z,'N S'01.S'," 1.541 5 1216- 9-IU lDDD-29-U:r

    13 J6'12.6NS'02.Z,'V 1539 5 1657- 9-UI D333-29-II:r

    C J616.1 'N S09.0'V 11,.1,2 2125-1S-III 12H-29-n:rD J6'16,O'N 7'22.5'V 921 2 10~7-26-II:r D91S-28-III

    715

    superimposed.The T.S.D. series (shown as profiles in Fig. 2

    and contoured in Fig. 3) revealed the presence ofthe well-known relatively warm, high-salinity coreof the Mediterranean Water centred ncar 1300 m,with a maximum temperature of about l20e andsalinity of 36,6%0' but also the presence of anintermittent salinity and temperature maximumncar 700 m. The method of operation was to lowerthe T.S.D. to as close to the sea bed as was safe (apinger, which changed its frequency when tilted,was hung 2 m below the T.S.D. to indicate theapproach to the bottom) and then to raise it to1000 m and leave it at this depth until it was timeto make a second profile. The instrument wasnormally brought inboard after three profiles, butthe ship's drift occasionally led to large wireangles, or to a position more than a mile from thenominal station position, and the T.S.D. was thenrecovered earlier. The procedure for calibrationfollowed that described by SWALLOW and CASTON(1973).The mean time-averaged profiles ofT and Swith depth show no peaks near 700 m, but thestandard deviation from the mean of both T and Sare large, some three times the average deviationsof O'Ie and 0'03%0 and comparable with thosefound in the thermocline at 200 m, even thoughthe mean gradients at 700 m are small. Thelargest variations of both T and S were near thebottom in the high T, S gradient region at 1470 mwhere the standard deviations reached Iooe and0'25%0' These variations are not, however, reflectedin correspondingly large standard deviations in theill situ density. The average standard deviation indensity was 0014 with a peak of 0050 riear 200 mand the remainder of the values below 200 mlying between 001 and 0025. These observationsare in accordance with Pingree's conclusions that

    variations of T and S at fixed levels are largelysuch as to keep the potential density constant.The variations of density might be produced byvertical motions with an amplitude of about95 m, and this is similar to variations found inother slope areas. There was some evidence ofsemi-diurnal variations in density at fixed levels,but these were not large enough to establish theirphase or vertical distribution with any certainty.

    3. THE 1971 OBSERVATIONSThe observations in 1970, concentrated at one

    position, revealed little of the dynamics of theundercurrent, but in 1971 a number of mooringswere laid carrying Aanderaa current meters withtemperature sensors, recording cvery 5 min", andtwo sections were worked across the outflow inaddition to two two-hourly T.S.D. series. Thepositions of the T.S.D. sections, current metermoorings and T.S.D. series are shown in Fig. I,and other det