Variability of the mediterranean undercurrent in the Gulf of Cadiz

  • Published on
    13-Sep-2016

  • View
    213

  • Download
    1

Transcript

  • 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 details are given in Table 1.

    (a) T.S.D. sectionsFigure 4 shows a N-S section at 802'W

    worked on 9th and 10th March between Stas, 7611and 7619 (see Fig. I). A wedge-shaped region ofwater of temperature 13C and salinity 36.4%0 liesagainst the continental slope at a depth of 500 to600 m and another of similar salinity but lowertemperature, 11'5e, is found at greater depths,near 1300 m, and extends south beyond 36IO'N.The former is the upper layer of MediterraneanWater which has recently been investigated byHOWE, ABDULLAH and DEETAE (1974). The latteris the main core of the undercurrent observed in1970.At about 750 m at 36IO'N there is evidenceofa region ofwater of higher salinity and tempera-ture than its surroundings, and it appears to be

  • 716 S. A. THORPE

    8:0M

    500

    E

    :i....

    ll.....

    o

    oN10M

    15

    4

    12

    TEMPERATURE, 'C

    -----= ,,, .

    Fig. 4. T.S.D. section on 802'W. (a) Isotherms at 0'5C intervals.

    ~.of----~

    oo:0M

    500

    E

    ll.~ 1000'" 1------/o

    35.8

    36.2

    35.8

    oNtoM

    36.0

    o;;;M

    SALINITY, %.

    Fig. 4. (b) Isohalines at 0'2%0 intervals below 36%0 and 01 interval above 36%0.

  • Variability of the Mediterranean undercurrent in the Gulf of Cadiz

    '"0 00 N :-

  • 1200

    1600

    E

    :if-a.

    '"o

    183 193 213 22-3 23'3 243150 _"':'50~~'. I -- -----,14.0 14.0-- 14.0'3.0 --13.0-""""' ~ _ - 13.0

    Aoo\-12.0 ~------ '-_12.0------ '2.0 -!AOO-~--._-----'""----,----------

    - -".O--~110 . . ~ ---=-.;>- ,0510'5 . - '0.0__-----....'-- -, ,10.5 -1800

    . ~__ '0.25__~~ 'H, ng80010

    , 0.25-- ~ ~------------~ ~o .@; ~~12.0

    '00- ~----100-~ _.~"::,':::::-- ~ ~,,".5'~,---/--..--- ------.---::::~:::::..:.~.;.....-~ ~ ~. 12.0~ --~'\.=-=--...::: -~ +--.----~-~~ =--"5 -11200'l()S 'IH~ '05~pA' - '=----'"= ~ ~'~ ~1..0

    100 _0;:> ~.: 'y~ .~ _ 100------ '::8, -~~d' v ~ lQ~ ~)' a. 2UJ0 0

    :gm

    Fig. 6, Isotherms at 3612'9'N, g003'S'W in March 1971. Contours are at 1C intervals in the upper layers above 11C, and Q'5Cbelow. The lO'2SoC contouris also shown. The depth of the bottom at the position of the station (see footnote, p. 719) is shown by the hatched line.

  • Variability of the Mediterranean undercurrent in the Gulf of Cadiz 719

    When the series was resumed on 21st, thestructure was found to be less regular, withcolder water ncar the bottom, and during 22ndand 23rd a more fragmented region appeared near1400 m, A dramatic change occurred just beforemidnight on 23rd, when the character of the watercolumn between 900 and 1500 m was abruptlyaltered. The change is clearly shown by the twoT.S. curves of Fig. 7a. Not only was the T.S.structure changed , but there was a significantchange in potential density, with denser waterappearing below 900 m (Fig. 7b). The detailed

    35-6 35-8 360 36-2 - 36--4 366 36-8SALINITY , I..

    Fig. 7. Details of the water structure at 3612.9'N,g0Q3 '5 '\V before and during the changes on 23rd. Profile Iwas made between 2100 and 2119, and profile II between2307 and 2331. (a) T-S diagrams. Profile II is the con-

    tinuous line.

    related to the wedge-shaped upper layer lyingagainst the slope at 500 to 600 rn, The presence ofthis region and the intermittency observed in 1970at similar depths, as shown in Fig. 2, suggest thatnear 36IO'N 'blobs' of water are found whichhave originated in and possibly detached from thewater to the north. The slope of the lines ofconstant potential density is consistent with anincreasing westward flow as depth increases inalmost the whole section below 500 m. There exists,however, a reversal of gradient near the bottomncar 2645'N, suggesting the possibility of a weakeastward flow, although the apparent reversal maywell be due to tidal effect or the influences of localtopography. The water of greatest salinity is near3645'N.

    The salinity distribution in the section fromSta. 7619 to 7629 (Fig. 5) shows again the tendencyfor the high salinity water to be on the northernside of the Gulf. The slope of the 361%0 isosalineis about tarr? (3'5 X 10-3) between Stas. 8720 and7626. The deepest part of this section is at mooringD where a valley crosses the section at right angles.This is one of the valleys down which' Madelainhas suggested the undercurrent is steered beforeturning again northwest to form the 1300 m corefurther west. The highest salinities and largestisosaline slopes are found here.

    (b) T.S.D. series near moorings A and B-Figure 6 shows the isotherms at 3612'9'N,

    8035'W from 18th to 24th from a two-hourlyT.S.D. series carried out as in the previous year.The phase of the tide was very similar to thatduring the 1970 series, both beginning about 2days before neap tides.

    The series was interrupted on 19th by north-west winds in excess of 15 m S-I. During 18th and19th no remarkable changes occurred, andalthough the temperature maximum ncar 1300 mwas present, it was neither so extensive nor solarge as was found in 1970. Fragmented regionsof 1O5C water were present at 1200 m andpersist for 4 to 5 h. There was little sign of thepersistent secondary maximum nea~ 700 m seenthe previous year.

    13-0

    12-0

    wcc::>

    < 100""wC>.

    ~~ 90

    80

    70

    27-4 27-6POTENTIAL DENSITY

    21-8

    1 2 '~

    28 -0

    All stations were within I mile of this position. Fig. 7. (b) Potential density versus depth.

  • 720

    600

    800

    S. A. THORPE

    E 1000

    :r:Ii:

    ~1200

    1400

    100

    Fig. 7 (c) T and S against depth, profile II, taken from an expanded scale direct read-out. The depth scale is notcompletely linear owing to slight variations in winch speed. The well-known salinity 'spikes' resulting from rapid

    temperature changes have been smoothed by hand.

    temperature and salinity structure of profile IIafter beginning of the change is shown in Fig. Tc.This shows the very layered nature of the waterstructure with abrupt changes of T and S in thevertical. These again support the notion ofhorizontal intrusion, rather than dynamical (ordiffusive) vertical mixing processes, as the valuesof T and S are not well-matched simultaneouslyin the vertical. Four quite distinct regions withuniform properties ('layers') can be seen near1200 m with average thickness of 124 m. Theregion near 1400 m contains rapid small-scalefluctuations and, as we shall see in the next section,is a region of large mean shear.

    (c) Current meter observations, moorings A, Band C

    Three moorings were laid near the position ofthe T.S.D. series (see Table I). Moorings A and Bwithin I km of one another each carried five

    current meters. Mooring C, which was out for theshortest time, carried a single current meter. Cwas located 12 km bearing 310cT from B,approximately along the direction perpendicularto the mean slope, but was separated from theother two moorings by a ridge with crest runningE-W at about 1460 rn. The buoyancy on allmoorings was below 210 m.

    The mean currents and temperatures at themoorings are given in Table 2. Currents at alldepths are northwestward, roughly following thetopographic contours. The currents and tempera-tures at 1388 and 1398 m on mooring A and at1402 m on mooring B are fairly well correlatcd*and the two moorings have been taken together

    * The mean 2-day correlation coefficients of temperatureand currents in directions 050 and 3201' are 0.643, 0.735and 0.714, respectively, for 1398m on A and 1402m on B,and are 0.882, 0.883 and 0.870, respectively, for 1388 and1398m on mooring A.

  • (a) .500I ~ - ------~- i ....110

    ~ ~~ -V v ~ -v-'~ M / --V I A . ~ _ (\ I Ioc;JC

    I i ---/".../ > ~ ' 10'5 'V ' , / ~~M v: 'V 1- ~J: ~CF\J ~ 10-5 \V ,

  • Variability of the Mediterranean undercurrent in the Gulf of Cadiz 721

    Table 2. Mean, currents and temperatures from moorings A, Band C. The final line, marked with anasterisk, are the mean valuesfrom A for the time that C was laid.

    Mooring No , Depth spee~1 Direction Temperature[m] em 'T 'c

    A 510 0.64 )1) 11.29

    B 760 2.93 )46 10.9)A 924 4.26 342 10.)6B 1077 6.625 )31 10.78A 1199 7.30) )19 10.96B 1303 4.21 )20 10.57C 1)84 9.19 )22 9.72A 1)88 1.)7 )06 9.82A 1398 0.85 )11 974B 1402 1.09 )21 9. 68B 1480 1.01 340 8.25

    A" 1388 2.72 )05 9.)9

    in assessing the mean flow and in constructingFig. 8. Moorings A and B show a jet-like flowwith a maximum speed of 73 ern S-l at 1199 m.The mean Richardson number of the flow above1100 m, computed using mean potential densityfrom the T.S.D. series, is about 103 whilst belowit is about 102 ; the flow is thus less stable tovertical mixing processes below the core thanabove.

    Figure 8 shows the contours of temperatureand currents, and has been constructed using alinear interpolation between the meters, afteraveraging over intervals of 2 h. The temperaturecontours reproduce the change already notednear midnight on 23rd. It appears to be theonly such change which occurred whilst themoorings were in position. By 28th, the earliertemperature structure has been re-established.

    The current contours are shown for com-ponents approximately parallel to and normal tothe slope. The currents have a fair degree ofvertical correlation. The influence of the semi-diurnal tide can be clearly seen in some parts ofthe record and there is some evidence (for examplein the period 26th to 28th) of baroclinic effects.The jet-like structure is best developed between15th and 18th when speeds exceeded 20 em S-l attimes. The changes in temperature and salinityon 23rd were preceded by a change in the current

    regime near noon of 22nd, when the currents,first near the sea bed and later in the upper partof the water column, began to flow off the slopeand towards the south. The progressive vectordiagram, Fig. 9, shows the change very clearly,and the appearance of an eddy-like motion,particularly in the region above the core.

    The progressive vector diagram for the currentmeter at 1384m on mooring C is shown in Fig. 9b.Before 22nd the currents were weak, less thanthose at 1402m on mooring B, and dominated bythe tides. On the 22nd the currents set to thenorthwest and thereafter increased, reachingspeeds in excess of 30 em S-l on 28th. The meanspeed from 23rd to 28th is 137cm S-l in direction322T.The current meter at 1402m on mooring Bshowed a similar development although themaximum speeds were much less and the mean isonly 66 em S-l in direction 309T. The tempera-tures at the current meters were similar if a timedelay for advection is accounted for, and itappears that the flow was intensified in crossingthe ridge between the two moorings.

    Figure 10 shows the hodograph of the meancurrents during the periods of the two T.S.D.stations shown in Fig. 6. The main change whichhas occurred is the acceleration towards the westof the water between 1300 and 1400 m. In bothhodographs the water near 1000 m is moving

  • 722 S. A. THORPE

    1191m10

    1303 m

    13

    28

    ""'-t 25~P,480m

    13

    10

    2Z. 16 19"-~22. 13 1402 m

    924 m

    28

    (, 10 2O--;O~0-5oSCALE. km

    13

    760'!'10

    13

    13

    2310

    16

    16

    ~2822 1928 2522

    101077 m

    PROGRESSIVE VECTOR DIAGRAMSMEDITERRANEAN OUTFLOW

    Fig. 9. (a) Progressive vector diagrams computed from hourly means of currents from moorings A and B. Thecrosses mark the midnight positions.

    ,

    D 20 30~.ALE. km

    Fig. 9. (b). The same for mooring C.

  • Variability of the Mediterranean unuercurrent in the Gulf of Cadiz 723

    Fig. 10. Hodograph of currents from moorings A and B during the times of profiles I and II of Fig. 6. Profile II isthe continuous line.

    quite rapidly towards the southeast, and there is alarge shear between this water and that below itmoving to the west or northwest. There is alsoconsiderable shear below 1400m. The Richardsonnumbers of the flowhave been calculated using thedifferencesbetween the currents at different levelsand on the potential density differences fromFig. 7b. Above 1077 m these exceed 19 (andexceed 120 between 924 and 1606 m). Valuesbetween 4 and 5 are found between 1077 and1303 m. A remarkable decrease has occurredbetween the two stations at the 1303 to 1402levelwith the Richardson number falling from over144to below 5. These 'layer' Richardson numbersare still large compared to those values below 025at which Kelvin-Helmholtz instability may occur.However, as STEWART (1969: see also TURNER,1973, p. 320) has remarked, the layer Richardsonnumber is an upper bound of the minimumgradient Richardson number. The presence ofsmall layer Richardson numbers increases theprobability of there being turbulent patches on asmall scale, and it seems likely that the regionnear 1400 m of Fig. 7c is one such patch.

    The power spectra of currents and tempera-tures at these moorings are dominated by a peaknear the semi-diurnal frequency with decay athigher frequencies with approximately a - 19power law for current and - 1'7 for temperature.

    (d) T.S.D. series nearmooring DFigure 11 shows the temperature profiles and

    isotherms at 3617'5'N, 7204'W* from a two-hourly T.S.D. series. This position is near thetop of the valley and ridge crossed by the sectionon Fig. 5. Below 700 m there is a mass of warmerwater which is fairly uniform in temperature,although slightly increasing both in depth and intime during the period of observation. Thetemperature increases rapidly and irregularlynear 650 m, on the whole more abruptly than atthe previous station (compare with Fig. 2).

    It is interesting to compare a typical profile ofthis series, made on 25th with those taken at36149'N, 7213'W and at 3611"'5'N, 726'O'W(Sta. 7636, Fig. 1) on 17th. The latter is to thesouthwest of the others, further down the valley.The temperature and salinity profiles for thesethree stations are shown in Fig. 12 and marked I,II and III, respectively. Between 300 and 550 mthe three profiles are similar, and each has a fairlyuniform layer in contact with the sea bed. Thesalinities and temperatures of the layers in II andIII are similar but the layer in III is shallowerthan in II. A considerable increase in salinity andin the thickness of the Mediterranean Wateroccurred at the head of the valley in the eight days

    "'All stations were within I mile of this position.

  • (a)Temperature, "C

    J' ',2 ',3 ' \4 ',5 \6 2HOURLYTEMPERATURE PROFILESAT 36' 17.5' N. 7' 20.4'W

    ~

    .r: ~~II.UJ I J iJJl1 J!Jj / fl fU)}JE.,oo: @ ~~'~/~~ .31-~~~t~~~\\~tfl~~~~Ti\l~

    tOO E400

    J600 ~j'8OO ~\000

    Fig. 11. Tem peratures at 3617'5 'N, 7204 'W. (a) Temperature profiles made every 2 h am) successively offset by half a degree.

    (b)ISOTHERMSAT36'1 7.5'N. 7" 20.4/W. 1971

    VJ?>

    ~o:;

    ~

    24 .~ 25 3 26.3 27.3 28 .3

    200

    E400

    :r: sool-c,w 800Cl

    '000

    "t-=====-~~ E

    ]

    600 'I.'

    800 J,000

    F ig. 11. (b) The isotherms at O'5e intervals above !Joe and O'25e intervals below. The bottom is shown as a hatche d line.

  • Variability of the Mediterranean undercurrent in the Gulf of Cadiz 725

    11

    TEMPERATURE, "c

    12 13 14 15

    400

    E:i 600Ii:

    ~BOO

    1000

    Fig. 12. Profiles of temperature and salinity at (I) 36177'N, 7"20'7'W at 1853 to 1911on 25th, (II) 36149'N, 721'3'Wat 1238 to 1255 on 17th and (III) 3611'5'N, 726'O'W at 1425 to 1444 on 17th.

    between I and II, with a corresponding increase ofpotential density in the deepest layer from2779 to 2797.

    (e) Current meter observations, mooring DThis mooring was laid near the head of the

    submarine valley and recovered after 46 h. Thecurrent meters were 21 and 45 m above the seabed. The upper recorded a mean current of798 em S-1 (standard deviation of 5-min sampleswas 102 em S-I) and little variation was recordedfrom the mean direction 233T. The strength ofthe current fell from just over 100 em S-1 to about70 em S-1 during the period of measurement.The mean temperature was 1299C (standarddeviation 0'07q and increased slightly. Thelower meter recorded a mean of 689 em S-1(standard deviation 95 em S-I) in direction235T. The current fell from about 77 to 62 em S-I.The mean temperature was 1296C (standarddeviation O07q. The mooring was anchored bya tripod carrying a camera which photographeda weighted flap. The inclination of the flap gave ameasure of the current. The mean current was600 em S-1 in direction 229"T at a height of18 m from the bottom during an 8h period ofoperation (THORPE, COLLINS and GAUNT, 1973).

    The current at all these meters is directedalmost exactly down the slope of the valley, and

    the continuity of properties of the water down thevalley appears to be a consequence of the down-slope of the ncar-bottom water. If we take80 em S-1 to be a typical speed of the undercurrentdown the valley, and its depth to be 300 rn, theFroude number based on the difference inpotential' density between 600 and 700 m ofprofile I, Fig. 12, is found to be 0,63, and theflow is therefore subcritical (TURNER, 1973, p. 65).Internal hydraulic jumps arc not expected.

    The power spectra of the currents at 12 mmeasured by the e.m. log has a slope close to- 17 in the frequency range 10-2 to 10-1 Hz,but those of the Aanderaa current meters have aslope near - 10 in the frequency range 015 to3 cycles h-1

    4. DISCUSSIONThe most interesting feature of these observa-

    tions is the disturbance which interrupted thegenerally northwesterly flow at moorings A and Bon 22nd March. The changes may be linked to theconsiderable increase. in salinity and potentialdensity which occurred near the site of mooring Dbetween 17th and 24th, which perhaps led toincreased flows and an adjustment of the flowfurther west. The changes began at mooring Aand B on 22nd, with a flow off the slope with acomponent near 1200 m in direction 140,

  • 726 S. A. THORPE

    opposite to the direction of mean flow at thisdepth. Near mid-day on 23rd the current below1250 m veered rapidly to a westward directionand increased in strength, and shortly beforemidnight the sudden changes illustrated in Fig. 7saw the appearance of denser water in the lowerpart of the water column. The arrival of this wateris so abrupt as to suggest a front or hydraulicjump-like structure. The subsequent adjustmentof the water column shown in Figs. 8 and 9 givesthe appearance of an eddying motion. This isparticularly suggested by the progressive vectordiagrams of Fig. 9 at levels above 1000 m. By27th the hodograph of the mean 24-h currentshas a regular spiral structure, rotating anti-clockwise from 075T at 510 m to 3200Tat 1480m.The size of the eddying motion above 1000 m isdifficult to judge, but if the progressive vectordiagrams may be taken as a rough indication ofscale, the eddy may well have extended for over20 km at the 924 m level, and thus be comparablein size with that observed further west bySWALLOW (1969).

    The cause of these changes is uncertain. Apartfrom the changes near the head of the valleymentioned already, it is possible that the strongnorthwesterly winds on 19th and 20th had aneffect although, if so, even stronger easterly windswhich blew on lIth and 12th had none. A furtherpossibility is that the changes are the long-rangeresult of a modulation of the strength of theoutflow through the Straits of Gibraltar, causedeither by the spring-neap tidal cycle or byforcing through an imbalance of air pressureacross the Straits. SMITH (1973) has described howthe path of the outflow may vary with the netflux of Mediterranean water through the Straits,and recently long-term current meter measure-ments have been made in the Straits to determinethe nature of the variability there, that is of the'inlet' conditions for the Mediterranean Water inthe Gulf. Whether the changes observed at themoorings are simply an adjustment to a new flowcondition or perhaps a manifestation of a baro-clinic instability is not clear.

    The shape of the mean salinity and temperatureversus depth profiles near mooring A are very

    different from those found. further west (e.g.HOWE and TAIT, 1970), where the larger gradientsoccur above the salinity or temperature maximum.The asymmetry here results from the recentexposure of the Bottom Mediterranean Water tothe underlying North Atlantic Water as it leavesthe shelf. The asymmetry further west is theresult of diffusive processes, probably dominatedfirst by shear instability and later by doublediffusive convection. There is no indication oflayers growing below the core at this position,although the strong variability in properties maygive rise to large horizontal gradients of tempera-ture and salinity which may later serve to triggerhorizontal layering (THORPE, HUIT and SOULSBY,1969; TURNER and CHEN, 1974).

    The size of the patches of 1O5C found near1200 m in 1971 may be estimated from the meanflow. They are about 15 km in horizontal extentin the northwesterly direction, and are about40 m deep. Their cause is unknown.

    More work needs to be done on the effect oftopographic features on the undercurrent. Therelative influence of topography, stratification andbottom friction in this area is not clear.

    Acknowledgements-i-l: thank the Master and crew of theR.R.S. Discovery for making these observations possible.

    REFERENCESHEEZEN B. C. and G. L. JOHNSON (1969) Mediterra-

    nean undercurrent and microphysiography westof Gibraltar. Bulletin de l'Tnstitut oceanographiquede Monaco, 67(1382), 95 pp.

    HOWE M. R., M. I. ABDULLAH and S. DEETAE (1974)An interpretation of the double T-S maxima inthe Mediterranean outflow using chemical tracers.Journal ofMarine Research, 32, 317-386.

    HOWE M. R. and R. 1. TArT (1970) Further observa-tions of thermohaline stratification in the deepocean. Deep-Sea Research, 17,963-972.

    KENYON N. H. and R. H. BELDERSON (1973) Bedforms of the Mediterranean undercurrent observedwith side-scan sonar. Sedimentary Geology, 9,77-99.

    MADELAIN F. (1970) Influence de fa topographie dufond sur I'ecoulement rnediterraneen entre IeDetroit de Gibraltar et Ie Cap Saint-Vincent.Cahiers oceanographiques, 22(1), 43-61.

    PINGREE R. D. (1972) Mixing in the deep stratifiedocean. Deep-Sea Research, 19, 549-561.

    SIEDLER G. (1972) Nordost-Atfantik-Expedition 1971.Meteor Forschungserbegnisse,A; 10,79-95.

  • Variabilityof the Mediterranean undercurrent in the Gulf of Cadiz 727

    SMrrHP. C. (1973) The dynamics of bottom boundarycurrents in the ocean. Ph.D. thesis, MassachusettsInstitute of Technology-Woods Hole Oceano-graphic Institution.

    STEWART R. W. (1969) Turbulence and waves in astratified atmosphere. Radio Science, 4,1269-1278.

    SWALLOW J. C. (1969) A deep eddy off Cape St.Vincent. Deep-Sea Research, Supplement 16,285-295.

    SWALLOW J. C. and G. F. CASTON (1973) The pre-conditioning phase of MEDOC 1969-1. Observa-tions. Deep-Sea Research, 20, 429-448.

    THORPE S. A. (1972) A sediment cloud below theMediterranean outflow. Nature, London, 239,326-327.

    THORPE S. A., E. P. COLLINS and D. I. GAUNT (1973)An electromagnetic current meter to measureturbulent fluctuations near the ocean floor. Deep-Sea Research, 20, 933-938.

    THORPE S. A., P. K. Htrrr and R. SOULSBY (1969)The effect of horizontal gradients in thermohalineconvection. Journal of Fluid Mechanics, 38,375-400.

    TURNER J. S. (1973) Buoyancy effects on fluids, Cam-bridge University Press.

    TURNER J. S. and C. F. CHEN (1974) Two-dimensionaleffects in double-diffusive convection. Journal ofFluid Mechanics, 63, 577-592.

    WILLIAMS A. J. (1974) Salt fingering in the Mediter-ranean outflow. EOS, 55, 297.

    ZENK W. (1970) On the temperature and salinitystructure of the Mediterranean Water in thenortheast Atlantic. Deep-Sea Research, 17, 627-632.

    ZENK W. (1975) On the Mediterranean outflow westof Gibraltar. Meteor Forschungsergebnisse, A,(16), 23-34.

Recommended

View more >