Influence of the Strait of Gibraltar

I I

On the origin of the intermediate double-maxima in T /S profiles from the North Atlantic*

by

\V ALTER ZE~K, Institut für ).leercskundc, Kiel

\\.ith 10 fig-ures and 1 table

Ein Beitrag zmn Ursprung der intermediären Doppelmaxima in Temperatur- und Salzgehalt- profilen aus dem Nordatlantik

Zusam mcnfa SS!i Ji:..:

Kontinuierliche '1'L·mperatur- und Salzgehaltsprofile aus dem NE-Atlantik zeigen h:iufig im Bereich des

~littclmeerwa~~cr:-; eine Doppclma;.;ima-Struktur. Als Erklärung dieser spc7iellen Schichtungsform findet man in der Litnatur zwei Hypothesen: a) Der Aus- strom wird im Golf yon Cidiz durch unterschiedlich beschaffene K:tn:lic in mittlere Tiden des Atlantiks geführt. Aufgruml der resultierenden ungleichen V er- mischung yon ~littclmeerwasser mit Atlantikwasser in diesen Kan:ilcn entstehen unterscheidbare \Vasser- arten, die ent~prcchend ihrer Dichte die Doppel- maxima beim \'erlassen des Golfes Yon Cädiz bilden.

b) Durch gezcitL·ngesteuerte Vermischungsprozesse in der Straße yon Gibraltar werden bereits unter- schiedliche Au~~trom~ \'X'asserarten erzeugt, welche nach Passieren (ks Golfs von Cädiz unterschiedliche Tiefenhorizonte :tufsuchen.

In der vorliegenden Untersuchung wird gezeigt, daß der Au::-strom bereits die Merkmale von diskreten \\-'asscr:utcn besitzt, noch bevor sich ein Einfluß von unterschiedlichen ~lischungsarten in den Kanälen im Golf von Cidiz ausgewirkt haben kann. Folglich werden diese diskreten \\'asser- arten, wclc~ die Doppclmaxima im Atlantik bilden, schon in de~ Straße von Gibraltar angelegt.

Der mediterrane Ausstrom ist starken Schwan- kungen vornehmlich aufgrundvon Gezeitenströmen unterworfen. Die Variabilität der Doppclmaxima läßt sich besser daraus herleiten als auch den unveränderlichen topographischen Eigenschaften der einzelnen Ausstromkanäle.

5• :Meteor A, 16

SuttJJ!Iary

Continuou's tcmperature and salinity profiles from the 1\E-~\tlantic frcquently show a doublc-maxima structurc within thc depth rangc of the ~leditcrranean

outflO\\'. T wo explanations for this special thcrmo- halinc stratification can bc found in literature:

a) The dcnsity of the outflow is varied by different outflow channcls with uncqual mixing properties.

b) In the Strait of Gibraltar two different out- flowing water typcs are produced by tidal currcnts.

In both cases thc different watcr masses spread in different density levcls in thc .:\tlantic.

Data presentcd here indicatc that the bimodal structure must be caused primarily by tidal currcnts at the source. Two-layered outflowing ~leditcrra­

nean water ·was observed evcn bcfore a splitting of the under-currcnt in the Gulf of Cädiz takes place.

The Yariability of the double-maxima can be cxplaincd by varying outflow rates in the source region rather than by a steady influence of thc bottarn topo- graphy.

Introduction

Continuous temperature (1) and salinity (S) profilcs from thc North Atlantic often show a doublc- maxima structurc in the ~lediterrancan Outflow (hcnceforth ~! 0). This spccial form of Ycrtical meso-scale structurc can be obscrved over a wide geographical arca in the Atlantic (e.g., CooPER 1962;

HowE & TAtT 1972; StEDLER & ZE"K 1973).

Similar observations in the Rcd-Sea outflow range

*Contribution number 3498 from thc \'\-'oods Hole Oceanographic Institution

36

haYc been rcportcd by KR.u·sE (1968). Thc time dc:pcndcncc of thc ).[0 structurc has bccn inycsti- gatc:d du ring a tcn-day anchor station 40 km wcst off Cape St. Yincente (Portugal) simultancously by means of bathysonde rccords (ZE:-.K 1971) and hydrocasts (GIESKES et al. 19i0). Time aYeragcd propntics abtairred from these mcasurcmcnts arc summarizcd in tablc 1. Thc rcsults show that thcre cxist two prcdominant depths in thc rangc of thc ).10 whcrc salinity and temperaturc tcnd to bc highcr than in adjaccnt dcpths. Charactcristic dcmity Jcyels (at) for thcse two depths wcre found to bc 27.51 and 27.83.

At least two explanations for the doublc-maxima can bc giYen: a) Thc different outf!ü\v channds of thc ~[() in thc Gulf of Cidiz influence the mixing- of quasi-homogeneaus f..Iediterranean water and 1"\orth Atlantic Central \\'atcr (NAC\V') in different ways which causes slightly different types of :0.!0 (Sw.u1.ow 1969; MADEUI" 1970). b) In the Strait of Gibraltar thc Mediterranean watcr and 0:AC\\r arc mixcd by tidal currents. Thus t\\"0 different :0.[0-types are present e\'cn beforc the water lcaYes thc sill area (SIEDLER 1968).

In both cases thc slightly different outflowing water types reach different dcnsity IC\·cls in the Atlantic wherc they can bc obsen-ed as the double- maxima in T and S profilcs shown in tablc 1.

Table 1 Charactcristic depths of the intermediate double maxima in thc outflow range of thc J\Icdi- tcrrancan \Vater as dcriYcd from thc anchor station 65 (37° OS' N, 9° 53' \V') of ",\[eteor"

cruisc 8 in 1967 according to (a) 61 Bathysondc records (Zr::-;K 1971) and (b) 14 hydrocasts

(GIESKES et aJ. 1970).

Tabelle 1 1\Iittlerc Tiefenlage der intermediären Doppcl- maxim:t im Ausstrombereich des ~Iittclmccr­

wasscrs nach ~Iessunccn auf der Ankerstation 65 (37° 05' !">:, 9' 53' W) \ühcend dcc ",!c- tcor"-Rcise 8 im Jahre 1967 nach 6113athy- sondcnmcssungen (a) bzw. 14 hydrogr:tphi- schen Serien (b).

mean dcpth max. max. Ot

(m) temperature salinity CC) (?~o)

a b a b a b a b

800 700 11.9 12.0 36.15 36.15 27.52 27.50 1170 1250 11.2 11.3 36.39 36.40 27.84 27.83

During "Meteor" cruisc 23 (SIEDLER 1970) numcrous CTD and currcnt mctcr data wcre obtaincd from the Gulf of Gdiz. They allow detailed investigation of the origin of thc double-maxima in thc context of horizontal and vcrtical distri- bution of tempcraturc and salinity in the Gulf area.

The locations of the stations and thc sections discussed here arc summarized in fig. 1.

Influence of the Strait of Gibraltar

Theoretical considcrations (DEF:\ST 1961) anJ obser- vational rcsults (L-\CO~IBE 1972) han: sho\\"11 that thc basic watcr exchange in thc Strait of Gibraltar is modulatcd by surfacc tidal wan:s pas:-.ing the sill.

The pulsation of in/outflow in th~ Strait is couplcd with an intcrnal oscilhtion of the \Trticd thcrmo- halinc stratification which in conjunction \\ ith the varying currcnt profilc, controls thc amounr of salt Icaving the :0.fcditerrancan ~ca during a tidal period.

SrEDLI:R (1968) gavc an cstimatc for thc frClJUency distribution p of the outflow volumc as a function of the outflow salinity S. His modcl assumcs the outflow to bc mixcd totally within a tinw pcriod shortcr than the semidiurnal tidcs .. \ stron.c; \Trtical mixing is esscntially causcd by high curr~·nt shear with a Richardson number Ri = 0(0.1 ::i). The resulting distribution p (S) for thc condition" in the Strait of Gibraltar has t\\"0 prcdomin~lnt pcaks.

This indicatcs that two mixed ~10 t~ pl·~ \\"ith prcferred salinities are produced by tilhl currcnts and arealready present in thc outflow sourcc.

Inhomogeneities in the nndcrcurrcnt

In ordcr to study thc strongly focusL·d 1\10 entering thc Gulf of Cidiz a hydrographic scction A (fig. 2) and repeated CTD casts (station CJS) wcrc madc. Thc results show that thc undcrcurrcnt is not a homogencous, highly saline laycr in '"-LTtion A.

A division of the water mass into thrcc types was obtaincd. One would cxpcct a homogeneou:-> layer if thc different \Vater typcs wcre produccd primarily in the outflow channcls bcyond thc po~ition of scction A.

The main part of the 1-.10 with thc highc~t spccd is a rclatiYcly thin layer (1) on thc continental margin.

It is shown in TfS diagrams in fig. 5 whcre rcpcated obscn·ations of station 98 are plottcd and in fig. 6 for thc single station 92.

Both curves also show the lcss salinc and colder layer (2) which was lound ahO\-c (I) in section A. Thc circles in fig. 6 reproduce thc equi- valcnt point cloud from fig. 5 where thc dots werc plotted after cquidistant intcrpolation.

Lcss information about the variability of a near shclf outflowing watcr type (3) in section A is availablc. Since this water type (3) was found as thc sourcc for thc two northcrly outflow channds it is seen in fig. 7 at Station 96 as \vcll as stations 46 and SO.

Its corc layer can be described by

S1

⁼ 36.907-6o and

a1

^~ 27.60.

The properlies of layers (1) and (2) were studied during 25 hours by means of repeated CTD casts.

Thc obtained isothcrms and isohalines are presentcd in fig. 3. Thcir mcan profilcs and Standard dcviations

9" 8'

7'

6' w

PORTUGAL SPAIN

N N

37'~---+~~---~~~=c---+---C~c---+---~37' Cape St. Vicente

9'

Cape S. Maria

___ .. _ ... 046 I I

--~-oso

F1g. 1. Locatinn nf CTD stations and sections A and C m the Culf nf C:'tdiz. The arrows indicate mc~1n outflow dircctions of i11c \lnlitcrr:lnean undercurrent.

stat1on 92/96

AFRICA

7' 6' w

Abb. 1. L:-tgc der Hydrosondenstationen im Golf ...-on C\diz. Die Pfeile geben die mittlere Ausstromrichtung des ~littclmeerwassers an.

96 moonng no: 19101 19104

200,---~~~~~~~~~---i_---, depth

CmJ _ _ _ _ _ _,...--36.0

__.::::=s;;:---.,__

400

600

0 10

Fig. 2. Schem:ltic scction A through the undercurrent at the cntr:lncc to thc Gulf of C.idiz. It shows the three different nutflow watcr types.

SC%oJ

20 30 km

Abb. 2. Schematischer Schnitt durch den Unterstrom beim Eintritt in den Golf yon Cidiz. Drei nrschicdcne Ausstrom-\\'assermassen sind eingezeichnet.

38

Fig-. 3. Isnp\cth di:tgr:tm of rcpcated CTD rccords at station 98 in thc ccnrcr of thc undcrcurrcnt ( secrinn A).

The tri.mg-lcs (T) dcnote the maximum lowcrinl-'; dcpth of thc singlc nbscrvations.

10 15TC°CJ

0 dt>pth 0 01

'

^rrcocJ

CmJ ²⁰⁰ Ir

:::~''

^{Olg 101}

"'

400

""

""'r'~

~,,

600

"" ~

^019.0'

eoo

Fig. 4. :\km tcmperature (T) and salinity (S) profilcs and thcir Standard dcviations (~) from rcpcatcd Station 98.

The inscrts shnw two a\·eragcd tempcraturc profilcs in thc dcpth inten·als of thermistor chains 19101/4. These moorcd units have rccordcd durin~ four wceks.

T C'CJ M23 ST 98

SC'/,.] M23 ST 98 Abb. 3. IsnplcthcnJiagr;lmm von wiederholten llydro- sondcnmcssungcn auf Station 98 inncrlulh des Unter- stromes (Schnitt A). Die Dreiecke (..,) gcbcn die maximale Tiefe der Einzelmessungen an.

0

I

"

00157"1 •• /m

005 Ol!sC"/.,J

360 365 370 SC "/ooJ

M23 STATION 9B Abh. 4. ;\littlcrc Temperatur- (T) und Salzgehaltsprofile (S) mit St.mdard.tb\vcichung-en (L) \'On wiederholten !\Ies- sungen auf Station 98. Außerdem sind die gcmittelten Tempcnturprofilc der Thermistorketten 19101/4 einge- zeichnet. Die verankerten Gedtc haben vier \Vochen lang registriert.

T

c•cJ

16

14

12

27.20 27.40 27 60 2780 2800

o,

10~-.-,--~--.-.-.-~~.-,--.-.-.-.-.-.~--.-.-~.-~,-,--.-r-r-r~

35.0 36.0

ri~. 5. T/S dl.l,>.Cram of all n:cords of Station 98. This data from thc ccmer of the undercurrent were inter- pol.ned in 5 ~U>.lr-steps befnre plotting

T [OCJ

16

14

12

10

35.0 360

Fig. 6. T/S diagram of stations 29, 78 and 92. lt shows thc mixing of the main outflow branch between thc gatc way scction A and thc decp Atlantic.

37.0

M23 STATION 98 Abb 5. T-S-Diagramm aller 1Icssungcn auf Station 98.

Die D.1tcn wurden Yor der Eintragung in die Zeichnung auf 5-dbar-lnterYalle interpoliert.

2700 27.20

27.40 27.60 27.80 28 00

o,

oooooooo ST92 --ST78 --ST29

37.0 5 ['/ .. J

Abb, 6. T-S-Diagramm der Stationen 29, 78 und 92.

Die Kurnn zeigen die Vermischung des llauptausstroms zwischen Schnitt A und dem tiefen Atlantik.

40 ^ZENK

350 360 370

Fig-. 7. T/S diagram of stations 29, 46, 50 anJ 96. The curvcs sho\V thc mixing ·within the ncar shclf outflow branches starring at gatc way section A.

arc given in fig. 4. The avcragcd profilcs from two thcrmistor chains are inserted. These wcrc obtaincd by moaring 19 which was situated 9 km northv:cst- ward from thc scction. Thc profilcs arc consistent with thc 25 h CTD mcan valucs. Figure 4 shows thc stablc double structurcd profile in thc rangc of thc 11!0 with laycrs (1) and (2) scparatcd by high1y variable mixing zones (1a) and (2a). Typical valuc for stable laycrs arc S1 = 37.50%0 ,

a1

⁼ 28.25 and

J1

⁼ 36.15%,,

a1

⁼ 27.45 (I'ig. 5).

The prescncc of thrcc instcad of two prcdominant outflow watcr masscs in thc gatc way section A hints that thc mixing proccsscs in thc Strait of Gibraltar arc morc cornplicatcd than assumcd in SIEDLER's modcl. Local effects at thc contincntal shclf may also havc an influcncc on the formation of type (3). BELDERSO>< & KwYO>< (1973) havc found traccs of a southcastward countcr current in sonographs from that area.

Structure of the outflow entering the deep Atlantic

The ?\10 splits into scveral branchcs on its way into thc dcep Atlantic. The cxistcncc of four scpa- rated outflow branches was shown earlicr by thc author (ZEI'<K 1975).

Water from thc highly densc laycr (1) ncar thc bottarn of scction A with thc typical salinity

>37.2%₀ can bc casily traced to its take off point at thc SW cdgc of thc Gulf of cadiz. Somc of this outflowing watcr was dctectcd in station 78 on thc

SC'I .. J

Abb. 7. T-S-Diagramm der Stationen 29, 46, SO and 96.

Die Kun·en 2eigcn die Vermischung innerh::db des schelfnahen Ausstromarmes, die bei Schnitt .-1. heginnt.

SE sidc of thc main outflow scction C. Anothcr filament of thc samc highly salinc and rapidly flowing bottarn layer in scction A follows thc contours of thc south canyon which diddcs immediately to thc wcst of scction A. Thc inter- action of this cffectivc outflow vcin with thc sca floor was dcmonstrated imprcssivcly by KE:\"YO~ &

llALDERSO>i'S (1973) Sonographs.

Tbc TfS diagram in fig. 6 shows clcarly thc relationship bctwccn station 92 ( onc example from scction .A) and Station 78. Thc continuation of the main outflow hranch down into thc C ppcr Dccp .Atlantic \Yatcr is demonstratcd by comparing thcsc Stations with thc sample station 29 south of Cape St. Vincentc. At station 29 thc ?\10 has found its density lcvcl of Gt = 27.85. This is similar to thc observations on the anchor stations wcst of Cape St, Vincentc (sec table 1).

In thc samc way thc mixing of thc lowcr saline layer (2) found in fig. 5 and rcproduccd in fig. 6, can bc studied. It mixes downwards whcre it contributes to thc formation of the upper maximum which was found in both stations 78 and 29.

In a similar way water type (3) of section A can bc traced along thc contincntal margin. Thc mixing between this watcr type and NAC\V Ieads dircctly t9 the T

fS

rclations of stations from thc shclf branch (station 46) and thc intermediate hranch (station 50) shown in fig. 7. Outflowing watcr from this origin has a dcnsity Gt = 27.55 when it rcaches the deep Atlantic ncar Cape St. Vinccntc which corresponds to the samc Ievel found for thc upper

L_____

_{' ~- --}

-- ,, -==---

^~

"

~OG

--- _"

-=--=--c ~-=-~ ^~

-

--

~ 2-::::J=

,,

/

...:=::,,,=--=

-~ "=~

I I

1000 6 30 GMT

L

I, 5 1971

~

11) 30

..

,

-~

1230 14 30 16 30 1B 30 1030

'----

"

- - r - - 22 30

'"

⁵ s ¹⁹⁷¹

'" "'

^{6 30 736}

T [°CJ M23 5T 67

•

lo~~ ot co~ducl,v•ty o•ob~

:::t:;;;;... __ _::..;::-;;::::;;_~; __ :_~-~-:O:~=-::::.,;:_:; ^__ =-~:;_~;~~--~~~~~~-b~~~~~--~;~~~::_::==--~~=.--~,;:-.;;'_:_4-!t:==J~~~-c?;;;;:;;~::--=:;~-~-~-~~~~~;;;,;;;;;~:;_~

~···~

---~

• •

• • • •

• •

--~--,...

•

,---,--~-r-~··

• • • • • •

_T

ß }J 10 30 12 30 14 30 1630

hg. 8. ls1 )plcth diagram of tcmpcrature and salinity from n:pe:ued erD rccords at Station 67.

ma:ximum in thc repcatcd profilcs from thc }.letcor anchor station in 1967 (tablc 1). For com- parison thc 'f/S curvc from station 29 with its typical bchavior for the NE Atlantic is sccn in fig. 7.

Becausc of thc geographical location, howcvcr, it is clear that this station is not influcnccd by the northcrn outflow branchcs.

Further cvidcncc for thc formation of thc double structurc comcs from data on the i'J\V' sidc of scction C. Thc remarkablc diffcrcncc bctwccn this sidc and thc SE sidc lies in thc uncqual lcvcls of thc maximum spceds. Thc gcostrophic currcnt profilc which fits thc rccordcd currcnt meter data indicatcs that thc N\V outflow fccds the dcpth Ievel of the

1B30 10J0 22 30

'"

^no

^'" '" "'

5 519'/'l

5['/ .. J M23 5T 67 Abb. 8. ]soplcthendiagramm der Temperatur und des Salzgehaltes \"on wiederholten I IyJrosondcnmessungcn auf Station 67.

uppcr maximum. Rcpcatcd CTD obscrvations wcrc uscd to study the Yariability of this filamcnt.

Figurc 8 and fig. 9 show isoplcth diagrams for tcmpcraturc and salinity and thcir mcan Yalucs in·

cluding standard dcviations.

Thc high variability found in thc dcpth intcrval 600-700 m indicates thc rather atypical situation during thc observation. From thc inscrt in fig. 10 onc recognizcs that thc data werc takcn during an c:xtraon.linary flow of warm ~[cditcrrancan \Vater.

1-Iowcvcr, thc bimodal structurc pcrsists cvcn during this singular cvcnt of unusually high l\[editcrrancan watcr flow as fig. 10 shows. The uppcr maximum associatcd with spceds

>

40 ern/sec was obscrvcd in

42

10

dtpt~-rr-,-rio~~~====~:"~~~~~~;;/"-"---~;---~r-~c==---r- 'm'

200

400

36

Fig. 9 . ..\Ican tcmpcraturc (T) anJ salinity (S) profilcs and thcir st.md:ml dcvi:J.tions (l:) from rcpcatcd Station 67. The inscrt shows thc geostrophic current profilc at station 67 togcthcr with mcan spccd :1rrows rccordcd by moaring 22.

T C°CJ 18

16

14

12

6T

05

mooring no.:022101 depth: 635m

I I I

11.96

ooc

bMIM~btf1\h--+--+-~<'Y'I -0 5

365 5("/ .. J

M23 STATION 67 Abb. 9. ,\Iittlcre Temperatur- {T) und S;tlzgdnltsprofile (S) mit St:mdan.bbweichungcn (~)von wicdcrh(J!ten '1\lcs- sungcn auf Station 67. Das eingezeichnete gcostrophischc Stromprofil von StJ.tion 67 stimmt mit den (;c~chwindig­

keitsvcktoren von Verankerung 22 übcrcin.

27.00 27.20 27.40

10,_-r-r-r-r-r-r-r-.-.-,~~---..-.-.-.-.-.-.-~

35.0 36.0

Fig. 10. T/S diagram of all rccords of station 67. Thc data wcrc madc cquidistant

p.p

= 5 dbar) bcfore plotting. Thc inscrt sho·ws the extraordinary high tempcraturc during the observational time of station 67.

37.0

s c

'/.,J M 23 STATION 67

Abb. 10. T-S-Diagramm aller i\fcssungen auf Station 67.

Vor dem Zeichnen wurden die Daten iiquidistant inter- poliert (j,p = 5 dbar). Die Zusatzzeichnung zeigt die außergewöhnlich hohe Temperatur, die w:ihrcnd der Beobachtungszeit auf Station 67 herrschte.

thc dcnsit\" k\·el a~ = 27.58. The sccondary ma:xi- mum wirh irs lo\\·er spccJ <30 cm,'scc reachcs a Uensity a~ = 27.75 . . \lthough thcsc two prcdomi- nant dcnsity lc\·ds do not coincidc e:xanh· with the prcfcrrcd it:Ycls giYcn in tahle 1, thcy are an c:xampk of thc ,-ariability of the double maxima.

Thc origin of the \\-ater masses found in fig. R-10 on thc ::\\\. sidc of section C is difficult to explain uniLJucly. Prohabl~· it is a mi:xturc of all threc \YatL·r ryres found at the gate ,,·ay section :\.

This mi:xin~ takes rlacc \\'hilc traYcling- down to scction C. l3\ Yolumetric anah·sis (ZE:'.-K 1975) thc ncar-shclf part alone can contrihute only a mmor part of this outflow type.

Conclusions

From thc uhscn·ations it is eYidcnt that rhc

~[cditerrancan undercurrcnt ts alrcady structurcd when it cntcrs thc Gulf of Cidiz. Tidal currents in the Strait nf Gibraltar haxc a strnng cffcct in thc forrnation nf preferrcd outflowing watcr types.

According to thcir different densitics thcsc watcr typcs are distrihutcJ into se\·eral outflow channek

\X'hcn thc\· ka\T thc Gulf they are ahlc to form

Referenccs

CooJ>t:.H, I.. !I. II., P. G. W. )o"'r:s & A. j, LFE (1962);

Hydrogr.1rhical conditiorls alon~ thc 14~ \V mcridian South-\\-'cst of IreLmd, July 1%0. - Ann. Bin\. 14:

73-76.

CRF.PO"\", ,\J. (1 965): Influcnce de b prc<;sion atmo-

spheriquc sur le nive:-tu moyen dc h ~ICditerranCe

Occicknt:tle ct sur le flux

a

travers de DCrroit dc Gibr:-tlt:u. Cah. Oe. 17: 15-32.

DHANT, :\. (1()01): Physica\ Oceanography, \'ol. I. - Perg:\tll( Hl Press, Oxford.

Gu:sn:s,

J

_\1., _1. .\lnJ\'CKJ, & A. \\;'E....;K (1970): llrdro- graphischc und chemische Beob:lchtungen auf einer Ankersenion im östlichen ~ordatlantischen Ozean. --

".\letenr" r:orsch.-l·:rgehn.A,H: 1-11.

!IowE, ,\1. R. & R. I. 'l'.'\rr (1972): Thc role of tcmperaturc in\·crsions in the mixing processes of the dcep occ:1n. -- lkcp-Sca-Res. 19: 781--791.

KENY0:---.1, ~. II. & R.ll. Bani'.H'iON (1973): ßedforms of thc \lcJiterrancan undercurrcnt obscrved with side-scan somr. -- Sedimentary Geology 9: 77-99.

KRAL'Sr., G. (1%R): Struktur und Verteilung des Wassers aus dem Roten .\leer im Nordwesten des Indischen (h:eans. ..~leteor" I·'(Jrsch.-1 :rgebn. A, 4: 77 -9H.

LAcm.uw,ll. (1972): Lcs pri:1cipaux proh\Cmcs rclatifs aux ondcs internes d'aprCs \es exposC> annoncCs. In:

Physical variability in thc North Atlantic, Proc.

Symp. Dublin 1969 (Ld. A.

J.

Ln: & 1-1. C11AR!'o:OCK).

- Rapp, er procCs-\·crhaux des rCunions, 162: 45--52.

~harp double ma:xima in the tcmpcrature and :->alinity profilcs of rhe _-\tbntic.

Rcsidcs mi:xing on the way into thc :\tlantic thesc structure~ are influcnccd hy seasonal cffccts and mctcorological factnrs in thc sourcc arca which Ieads to thc Yariability in thc sharpncss nf thc double maxima. For imtancc air prcssurc diffcrcnccs betwcen thc Gulf of Cidiz and thc .-\lboran ~ca

can haYe a major influcncc on thc currcnt in thc Straits (C:Rüo...: 196:1) and consC(]Uently on thc :'\10.

Therdorc the origin of thc double ma:xima is primarily rclatcd to modubtions in thc rate of mi:xing: at thc source rcgion. Thc influcncc of thc topography in thc Gulf of (:iJiz sccms to hc of secondary importancc on thc Formation of thc double rnaxima.

.:·1cknoll'lu~~llll'JIIs

This work has hccn surporteJ hr thc Deutsche Forschungsgemcinschaft, Ronn-Bad Godeshcrg, hy proYiding R/\' "~lctcor" and thc computcr facilitics of 1-.:icl L'ni\·cr~ity . . \dditional computcr time was madc aYailahlc hy ;\, ~11l.LER, \\'oo&• Hole. Thc hclp of G. SIEDLER as cruisc coordinator is greatfully acknowlcdgcd.

.\1.'\0f'L:\I:-;", I-'. (1970): lnflucnce d~ Lt topographi~ du fond sur l'Ccoulcment \l~ditcrran~cn entre lc D~troit

de Gihr.litar et Je Cap Saint-\'incent. · Cah. Oe. 22:

43- 62.

SJFDLER, C. (1%R): Die lbufigkeitsnrteilung von

\\'asscrartcn im Ausstrombereich von .\fcercsstraflcn.

- Kielcr \Ic<:resfnrsch. 24: 59 65.

(1972): ~ordost-Atbntik-Expeidition 1971. - "\lc- tcor" Forsch.-Frgebn. A, 10: 79--95.

SIEDLFR, G. & \\'. ZE:--.:K (1973): \'ariahility nf thc thcrmohaline ,<;tairc:t"e. -- ~aturc Phvs. Sc. 244,

(131): 11-12. .

SwALLOW, J. C. (1969): ;\ dcep edlly off Cape St. \'in- ccnt. -- Dcep-Sea Res., Suppl. to Vo!. 16: 2~5-295.

ZFl"K, W. (1970): On temrcrature and salinity structure of the ,\Teditcrranean \Vater in thc l\:orthcast Atlan- tic. - Dccp-Sea Res., 17: 627-632.

(1971): Zur Schichtung des \Iittclmccrwasscrs west- lich von (;ihraltar. - ":-..Teteor" l'or~ch.-Ergchn. A, 9:

1-30.

(1975): On the .\lcditcrrancan outflow \\CSt of Gihraltar. ".\lctcor" Forsch.-Frgchn. A, 16:

23-34.

Rcceivcd July 5, 1974