11
Bottom Topography to the West of the Wyville-Thomson Ridge D.J. Ellett UDC 551.462.2; North Atlantic Summary Soundings collected during research vessel cruises in the region immediately to the west of the Wyville-Thomson Ridge permit the topography to be described in detail, especially with reference to the inflow to the RockallTrough of Norwegian Sea Deep water. Topography directs the densest water towards an isolated deep of over 1700 m depth, but water can only flow from this to the south across a sill at 1300 to 1360 m, or to the northwest above 1110 m. A number of modifications to existing topographic charts of the area are noted. Topographie des Meeresbodens westlich von dem Wyville-Thomson-Riicken (Zusam- menfassung) Mit der Hilfe yon Lotungen, die wfihrend einiger Forschungsreisen gesammelt wurden, ist es m6glich, die Topographie des Meeresbodcns wcstlich des Wyville- Thomson-Rfickens ausffihrlich zu beschreiben, vor allem in Bezug auf das Einfliessen von Ticfcnwasscr aus dcr Norwegischen See in den Rockall-Kanal. Die Topographie lenkt das dichteste Wasser in eine isolierte Tiefenzone mit mchr als 1700 m; jedoch kann Wasser yon dort nur nach Siiden fiber eine Schwelle mit einerTiefe von 1300 bis 1360 m oder nach Nordwesten obcrhalb cincr Tiefe von 1100 m fliessen. Einige Vcrfinderungen an den vorhandenen bathymetrischen Seekarten des Gebietes werdcn vermerkt. Topographie du fond/i I'ouest du Seuil Wyville-Thomson (R6sum6) Il est possible avec des sondages nouveaux a d6crire en d6tail la topographic l'ouest du Senti Wyville-Thomson, surtout ~ l'6gard d'entr6e de l'eau profond de la Mer Norv6gienne au Chenal du Rockall. Ueau plus dense est dirig6 vers une profon- deur plus de 1700 m, mais il ne pouver pas 6chapp6 au sud au dessous de 1300 gt 1360 m, ou au nord-ouest au dessous de 1100 m. Quelques changes au cartes bathy- metrique existant sont indiqud. Introduction The crests of the ocean ridges joining Scotland to Greenland are the source regions for anomalously dense waters which enter the Atlantic at intermediate depths. The Wyville- Thomson Ridge running between northwest Scotland and Faroc Bank provides the smallest of these sources, but one which affects the water mass structure of the Northeast Atlantic adjoining Britain. Overflow of Norwegian Sea Deep water (NSDW) across the ridge was confirmed by E 11 e t t and R o b e r t s [1973], and later assessed from observations made in August-September 1973 during the ICES Overflow '73 experiment at 0.33 • 10~' m3s i

Bottom topography to the west of the Wyville-Thomson Ridge

Embed Size (px)

Citation preview

Bottom Topography to the West of the Wyville-Thomson Ridge

D . J . E l l e t t

UDC 551.462.2; North Atlantic

Summary

Soundings collected during research vessel cruises in the region immediately to the west of the Wyville-Thomson Ridge permit the topography to be described in detail, especially with reference to the inflow to the RockallTrough of Norwegian Sea Deep water. Topography directs the densest water towards an isolated deep of over 1700 m depth, but water can only flow from this to the south across a sill at 1300 to 1360 m, or to the northwest above 1110 m. A number of modifications to existing topographic charts of the area are noted.

Topographie des Meeresbodens westlich von dem Wyville-Thomson-Riicken (Zusam- menfassung)

Mit der Hilfe yon Lotungen, die wfihrend einiger Forschungsreisen gesammelt wurden, ist es m6glich, die Topographie des Meeresbodcns wcstlich des Wyville- Thomson-Rfickens ausffihrlich zu beschreiben, vor allem in Bezug auf das Einfliessen von Ticfcnwasscr aus dcr Norwegischen See in den Rockall-Kanal. Die Topographie lenkt das dichteste Wasser in eine isolierte Tiefenzone mit mchr als 1700 m; jedoch kann Wasser yon dort nur nach Siiden fiber eine Schwelle mit einerTiefe von 1300 bis 1360 m oder nach Nordwesten obcrhalb cincr Tiefe von 1100 m fliessen. Einige Vcrfinderungen an den vorhandenen bathymetrischen Seekarten des Gebietes werdcn vermerkt.

Topographie du fond/i I'ouest du Seuil Wyville-Thomson (R6sum6)

Il est possible avec des sondages nouveaux a d6crire en d6tail la topographic l'ouest du Senti Wyville-Thomson, surtout ~ l'6gard d'entr6e de l 'eau profond de la Mer Norv6gienne au Chenal du Rockall. Ueau plus dense est dirig6 vers une profon- deur plus de 1700 m, mais il ne pouver pas 6chapp6 au sud au dessous de 1300 gt 1360 m, ou au nord-ouest au dessous de 1100 m. Quelques changes au cartes bathy- metrique existant sont indiqud.

Introduction

The crests of the ocean ridges joining Scotland to Greenland are the source regions for anomalously dense waters which enter the Atlantic at intermediate depths. The Wyville- Thomson Ridge running between northwest Scotland and Faroc Bank provides the smallest of these sources, but one which affects the water mass structure of the Northeast Atlantic adjoining Britain. Overflow of Norwegian Sea Deep water (NSDW) across the ridge was confirmed by E 11 e t t and R o b e r t s [1973], and later assessed from observations made in Augus t -Sep tember 1973 during the ICES Overflow '73 experiment at 0.33 • 10 ~' m3s i

24 Dt. hydrogr. Z. 41, 1988. H. 1. E 11 e t t, Bottom topography

(E 11 e t t and E d w a r d s [1978]). More significantly, entrainment from overlying layers increased the total near-bottom westward flow to 1.20 • 106 m3s t at this time. Current meter observations by W. J. Gould (in D ic k s o n et al. [1986]) have suggested a seasonal pattern in the overflowing water, with a summer maximum and a winter minimum. The bottom topography is of obvious importance in determining the paths of the dense bottom water as it enters the northern Rockall Trough and efforts have been made to collect new soundings referenced to modern navigation aids during oceanographic work in the area. The results improve upon the topography obtained from older material by E l l e t t and R o b e r t s [1973] and describe the sill depths which the inflowing water must cross after entering the northern Rockall Trough.

Sounding data

Figure 1 shows the area of study in relation to surrounding features and Figure 2 the tracks of surveys made during 1971 to 1980. Profiles collected across the upper trough between the Wyville-Thomson Ridge and Ymir Ridge in 1971 by RV '~ of the Fisheries Laboratory, Lowestoft have been previously described by E 11 e t t and R o b e r t s [1973]. Three further RV "Cirolana" cruises in 1972 to 1973 examined in detail the western approach to the deep, the col in the Ymir Ridge and the topography of the deep. It thus seems appropriate to name the latter the Cirolana Deep. In 1979 on cruises of the Scottish Marine Biological Association (SMBA) RRS "Challenger" obtained soundings across the southern sill and centre of the deep and RRS "Shackleton" ran extensive sounding lines across the east and north of the area, theYmir Ridge col and the southern sill of the Cirolana Deep. RRS "Challenger" during a 1980 SMBA cruise completed the survey of the sill and obtained further material to the southeast of the deep. Sounding lines made by NO "Jean Charcot" in 1973 across the upper trough and to the west of the deep have been published by B e r t h o i s [1974] and provide valuable additional data.

Navigation during the three earliest RV"Cirolana" cruises was by the Decca Navigator system. Coordinates recorded at 10 to 20-minute intervals were plotted on large-scale grids. All subsequent data came from cruises upon which two-component satellite navigation was used. Ten minute dead reckoning positions were recorded together with full details of satellite fixes. The latter were rarely more than 2 hours apart and more usually at about 35 minute intervals. All positions were plotted upon a large scale grid and the dead reckoning positions were adjusted by apportioning the drift error occurring between fixes. Agreement is excellent where sounding lines cross, and comparison of the overlapping portions of two "Cirolana" surveys using the two differing navigation methods suggests that the maximum error which could have occurred between the two systems was less than 0.5 nautical mile.

Soundings from RV "Cirolana" were obtained with a Marconi Fishgraph II echo-soun- der. Corrections were determined and applied for phasing error and transmission depth, and the sounding velocity of the machine was carefully checked upon each cruise. RRS "Chal- lenger" and RRS "Shackleton" used Mk III Precision Echo-Sounders of Institute of Oceanographic Sciences design ( T u c k e r and S t u b b s [1962]), sounding at a constant 1500 m s -~, and with adjustment made for transmission depth.

Application of M a t t h e w s ' [1939] tables to the correction of soundings in this area presented difficulties, as three of Matthews' regions adjoin here and correction would have given rise to small but false discontinuities. In July 1972 comparisons of soundings with depths obtained simultaneously from unprotected thermometers above a bottom-locating pinger gave a mean vertical sounding velocity of 1497 m s 1 for six positions in depths of 450 to 1630 m from the Cirolana Deep to the crest of theWyville-Thomson Ridge, and this value has therefore been used tO correct all new soundings.

Depths were read from the original records at 2-minute intervals and additionally at any maxima and minima in the intervening period. After correction as detailed above they were plotted upon a grid of scale 1 : 92,600 at 60 ~ N (2 cm = 1 nautical mile), together with the

Dt. hydrogr. Z. 41, 1988. H . i . E 11 e t t, B o t t o m topography 25

~ \ m l ..

0 : : I " ~ r II

~ ~ . ~ ~ '%_

o~,)" / ; [ J I I al I I I t t t

~ / j i r z

'/ r / ', ; ~ " .';4"', ;,,~-

/ t r / . ~ r - ,t/J~

k j 0 ~ '

0

~ z (D

N

Od

O

-/ O

(M

/

< . r

7 -

/

r

O �9

O,I

- r'-

- %

< \

\

\ o o

/

~- o

tO

G',

O �9 m - ~

~ - r

~

"XJ r , - �9

~ o

.-~ "<2

~ e 4 �84

26 Dt. hydrogr. Z. 41, 1988. H. 1. E I I e t t, B o t t o m topography

Q

0 07

o~176

.~..' o.."

. 6LI~ H3

.'/i _~ . i I

�9 . / l# :~ o I I ~..

~ ~ ' ~ I l " ~ i ~ ~ ~ ~- ID

A.,"I~ ...... " .... ~'7 / ,v~ I ~ i,- ..... ,

I i i - t l " ~ ............ I .' J I I I " 1 i / ~i ~ / ~ o - ~ I l i .+i

_=..~.._7_.L. _<_../-.+ ..... ~ i_ I -;<,>~ ..... ~!'..t 6 , / ~ c:.,- / i ~..;~'i <:~.., .,. ~ ~ ~.,-":~>o

�9 ,"<'>/' ) / ~ I ~,, , ...,i o+ ~, I

/ ~ I I �9 ,." / ( <,,i ,

" ~ i l "'"', ~ I/'_# I~,

S ~

oj "'~. ,, ~. . -I-

~-.~.~o" 3' ~, ~ =I b,

"~"/ I 1 .....

~ I ~-I

I I . . Q ! i t " •

I I I

.2 ... . . . . . . . . I

0

O z 0

�9

~= "eZl ~

0

0

Dt. hydrogr. Z. 41, 1988. H. 1. E 11 e t t, Bottom topography 27

CO

/ // /,.

// #

�9 . .. ""

.- ~i~-.. / \

~..~ Q .~ ' > - ,

I'-...'~.......~.. / ~('.. X /

/ ~ ;.~. �9 I ' , "

/ % i / ":.

/'! / /

/ ) �9

1 ,''.. I ," .......

,~ ? J " I

I?' " /I . //

�9 I

L..

/ ,

: . . .: /i

~

d r

" o

~2

~/? "r e "

G

.=_~ > ' E

eqJ

d~ L~

o z / lad

28 Dt. hydrogr. Z. 41, 1988. H. 1. E 1 l e t t, B o t t o m topography

. , .

~, P!!'~ / ;11 I /

.,..

. . . . ]. 4 . . L .L . --~_J/ /..>

~ - ~ ~ . . " . . . ;

.'R'x... . ?"i ' :':" -"_'_:

�9 .." \ ~ - , --

. .~\ { If .. ,< ; - < . ~-

~ i \ .... . . 7 ~ ........ ~ ...... <:

\ ' k \ \ , .

~ < " ":~ .v . . . . d . . . - , / . . . . ~ "" i / . ,-"

~-<v...( ! q G " ~

[

, . / . . :" / "

/ i

/ %-

\ .I . . . . . " . . . . .

. . . . . . . /t ] ! ~

' / i / / ! �9

. . . . . . i @,'i / ' : / I : - -

< . . ~ . - .~_J / ! ~ g b : / :

~ ~- 4.-'2--<.. i

7'<' '

�9 \ - .

: \

\ < , �9 .\ �9 �9 / / N\ -

�9 .'~ . 4 - ;

< '" " ' - ? . c . , :C} /

.id; ..... '

b

{ o "m

b

h

s

f, r

8

~ 8 ~ b

~a8 m m

"-a o

U ~

~E Z

I :a

v q

&

o Z

t l D

Dt. hydrogr. Z. 41, 1988. H. 1. E 11 e t t, Bottom topography 29

material collected for the previous study (E 11 e t t and R o b e r t s [1973] from GEBCO charts and RRS "Discovery" cruises. Soundings from these sources and the NO "Jean Charcot" 1973 cruise were corrected by Matthews' tables and it seems probable that regions 7 and 8 were used west of the Wyville-Thomson Ridge. By comparison with the material corrected to a sounding speed of 1497 m s -1, these data would appear shallower by 11 and 7 metres respectively in a depth of 1000 m. For the present purposes this is significant only in areas of low relief, where differences of this order should be regarded with caution. Figures 3a and b show contours drawn from the large-scale plots. Also marked are the sounding positions and the depths of all maxima and minima along the sounding lines. Some older data have been removed where there is obvious disagreement with new material.

Discussion

E 11 e t t and R o b e r t s [1973] showed that although dense water from the Norwegian Sea could cross most of the northern half of the Wyville-Thomson Ridge, descending water was likely to collect initially within a trough of depth about 900 m between the main ridge and a spur, the Ymir Ridge (Figures 1 and 3a). A deep but narrow gulley between the northwestern end of theYmir Ridge and the southern flank of Faroe Bank offered the most obvious route for overflow water to enter the Cirolana Deep, and from thence to the northern RockallTrough. Problems which could not be resolved from the existing soundings were the possibility of a second channel out of the basin at the mid-point of theYmir Ridge, where the crest-line descended to an unknown depth; the maximum depth of the sill to the south of the Cirolana Deep; and the possible existence of channels which could provide an exit from the deep at its northwestern side. The new soundings have enabled these points to be clarified, and demonstrate additional features of interest.

a F e a t u r e s of t h e u p p e r t r o u g h

The first detailed soundings crossing the southwestern foot of the Wyville-Thomson Ridge suggested the existence of a 'moat' along the margin of the trough ( E l l e t t and R o b e r t s [1973]) which could have led descending overflow water westwards rather than allowing it to reach directly the bottom of the trough between the Wyvil!e-Thomson and Ymir Ridges. In fact, the new material does not show this as a single continuous feature, but indicates the existence of at least three separate channels descending into the trough from the flanks of the ridge (Fig. 3 a).

The central part of the trough forms a basin with depths of about 940 m, and has a western sill in 8 ~ 30' N at 900 m. Approximate calculation shows that about 3 km 3 of dense water could be stored here. Although this is a relatively small quantity, the existence of the basin and its ability to retain some overflow water must promote mixing and entrainment from the overlying Atlantic water at comparatively shallow depths, and will also prolong these processes after direct overflow across the Wyville-Thomson Ridge has ceased.

b T h e co l i n t h e Y m i r R i d g e

The crest-line of the Ymir Ridge is broken between 8 ~ 15' and 8 ~ 35' W and previous sparse soundings suggested a second possible exit channel from the central trough for overflow water. Although some ambiguity remains as to the precise depth of the col in the vicinity of 8 ~ 20' W, if errors in the positioning of adjacent sounding lines are less than 0.5 nautical mile, as discussed earlier, examination of the contoured soundings of Fig. 3 a indicates a probable maximum depth of about 750 m. If greater navigational inaccuracies should exist, the deepest channel would be at 844 m. In either case, observations in recent years (e. g., E 11 e t t and E d w a r d s [1978]) show that water containing a high proportion of NSDW is unlikely to flow out from the basin at these levels, and subsequent observations along the southwestern foot of the Ymir Ridge offer no evidence of such events.

30 Dt. hydrogr. Z. 41, 1988. H. 1. E 11 e t t, Bottom topography

c T h e s o u t h e r n s i l l o f t h e C i r o l a n a D e e p

To the south of the deep a broad sill, about 18 km across, rises to depths of about 1300 m (Fig. 3b). East-west sounding runs in this region show a narrow, deeper channel of 1400 m depth extending south from the western side of the deep which may be continuous with a somewhat wider depression of 1360 m existing south of the sill. It seems probable that this latter depth is the deepest level at which water of overflow origin can enter the Rockall Trough from the Cirolana Deep, its descent being thereafter unrestricted by major topog- raphical constraints in its southward flow.

d C h a n n e l s t o t h e n o r t h w e s t o f t h e C i r o l a n a D e e p

The surveys show the deep to have a greater extent than deduced from previously existing soundings ( E l l e t t and R o b e r t s [1973]), and a maximum depth of at least 1727 m. Erroneously posit ioned shallow soundings in latitude 60 ~ 20 'N had given the deep an east-west configuration in the earlier paper and the bathymetric chart of R o b e r t s , H u n t e r and L a w s o n [1979], but the RV "Cirolana" data show that the northwesterly trend of theYmir Ridge is continued by the southwestern flank of Faroe Bank (Fig. 3b). The NO "Jean Charcot" soundings (B e r t h o i s [1974]) cross a channel of depth 1172 m at the northern extremity of the deep, but sounding lines across the southern and southeastern foot of Bill Bailey's Bank did not encounter any extension of this feature, giving a sill of somewhat less than 1100 m for any water circulating westwards around the northern wall of the Rockall Trough.

e B a n k s t o t h e s o u t h w e s t o f t h e d e e p

Although the bathymetric chart of R o b e r t s , H u n t e r and L a u g h t o n [1979] shows an extensive area shallower than 1000 m to the southwest of the deep, the soundings presented here greatly reduce the extent of this to the east of 10 ~ W(Fig. 3 b). South and west of the bank, which rises to at least 830 m in 9~ W, topography is irregular, with narrow valleys or clefts. Typically these valleys or troughs are 90 m deeper than their surroundings and 2 km wide, but the distance for which they continue is unknown. Fig. 4 shows an echo-sounding transect in this vicinity, and at least four other valleys have been observed from RV "Cirolana" echo-traces, although with insufficient position data for incorporation into Fig. 3 b.

f T h e s o u t h w e s t e r n f l a n k o f t h e W y v i l l e - T h o m s o n R i d g e

Although this work has concentrated upon clarifying the topography of the area adjoin- ing the northern half of the Wyville-Thomson Ridge, the southwestern flank of the ridge has not been ignored. Though no evidence of NSDWoverf low has been obtained here, a general examination of the topography between the Hebridean Shelf and the Ymir Ridge has been made on passages across the area and confirms that bot tom features are generally much smoother than immediately to the north. Fig. 5 shows the form of a section in this vicinity, the most interesting relief being the bot tom waves of 10 to 24 m height and 0.5 to 0.75 km spacing observed at the deepest point, iust to the east of station G9. Abrupt changes in bot tom form have, however, been encountered at the junction of the Wyville-Thomson Ridge with the Hebridean Shelf, where the echo-trace of Fig. 6 may indicate a minor spur ofthe ridge proiecting southwestwards.

Dr. hydrogr. Z. 41, 1988. H. 1. E 11 e t t, Bottom topography 31

lOOO rrl

n 11oo

I 12o0

T i m @ . ,,,

0 30 6 0 m i n 9 0 I 1 l I I I I I I I I t i t I I ~ ~ I t I

0 5 1 O k r a . I i , , , i . . . . I

Fig. 4 Sounding profile to the southwest of the Cirolana Deep (W-X, Fig. i)

5 0 O

1 0 0 0

G 1 3 G 9 G 6 0 I I I I I I I I

m

YMIR %r ............ ~ / SCOTTISH RIDGE SLOPE

0 10 2 0 k m I i i J I

Fig. 5 Sounding profile between the Hebridean Shelf and theYmir Ridge (G6 G13, Fig. 1)

32 Dt. hydrogr. Z. 41, 1988. H. 1. E 11 e t t, Bottom topography

o I

m i m ~ ,,- 3 0 ra in

I I I I I I I I I I I

6o I I

5OO

u n c o r ~ m

C~

1 0 0 0

if)

o

z ~

tO

U

Fig. 6 Sounding profile at the junction of theWyville-Thomson Ridge with the Hebridean Shelf ( Y - Z , Fig. 1)

Dt. hydrogr. Z. 41, 1988. H. 1. E 11 c t t, Bottom topography 33

Conclusions

Systematic echo-sounding surveys using modcrn position-fixing equipment have allowed the major topographical features immediately to the west of the Wyville-Thomson Ridge to be dcscribed with sufficient detail to assist oceanographic surveys in the area. Dense water which either crosses the low central section of the ridge-crest at about 600 m or the northern part of the ridge at 500 to 550 m can collect in a trough between the Wyville-Thomson and Ymir Ridges at depths of 900 to 940 m, To the southwest, the Ymir Ridge is a barrier with a maximum crest depth of 844 m, or more probably 750 m. From this trough between ridges overflow water can pass by a narrow descending channel to the Cirolana Deep, which has a maximum depth of over 1700 m. The main outlet thereafter is across a broad sill to the south of the deep at about 1300 m, although a somewhat deeper channel at 1360 m may cut this sill. An arm of the deep extends northwestwards but does not present an exit for water below 1100 m. These complex paths must enhance the quantity of Atlantic water which becomes entrained into the intermediate and deep levels of the RockallTrough.

Topography to the southwest of the Cirolana Deep is irregular and dissected by narrow valleys. Between the Ymir Ridge and the Hebridean Shelf relief is smoother except at the meeting-point of the Wyville-Thomson Ridge and the Scottish continental shelf, where more abrupt features have been observed.

A c k n o w l e d g e m e n t s

Thanks are due to the Masters and officers of RV"Cirolana" , RRS "Challenger" and RRS "Shackleton" for their assistance and careful navigation. The interesting topography of this area was first drawn to my attention by Dr. J. Ulrich some years ago in correspondence following his description of the bottom topography of Rosemary Bank and the banks immediately to the west of Faroe Bank (U 1 r i c h [1964]).

References

B e r t h o i s , L., 1974: Mission Faegas, carte bathymetrique. Bull. Inst. Geol. Bassin Aquitaine, 16, ;193-195.

E l l e t t , D. J. andA. E d w a r d s , 1978: A volume transport estimate for Norwegian Sea overflow across the Wyville-Thomson Ridge. ICES C.M. 1978 C:19. 12 pp.

E l l e t t , D. J. and D. G. R o b e r t s , 1973: The overflow of Norwegian Sea deep water across theWyville-Thomson Ridge. Deep-Sea Res. 20,819-835.

D i c k s o n , R. R.,W. J. G o u l d , C. G r i f - f i t h s , K. J. M e d l e r andE. M. G m i t - r o w i c z , 1986: Seasonality in currents of the Rockall Channel. Proc. Roy. Soc. Edinb. 88 B, 103-125.

Matthews, D. J., 1939: Tables of the velocity of sound in pure water and seawater. Hydro- graphic Department, Admiralty, H.D. 282, 52 pp.

R o b e r t s , D. G . , R M. H u n t e r andA. S. L a u g h t o n , 1979: Bathymetry of the northeast Atlantic: continental margin around the British Isles. Deep-Sea Res. 26, 417-428.

T u c k e r , M. J. and A. R. S t u b b s , i962: Underwater acoustics as a tool in oceanog- raphy. Underwater Acoustics, Ed. V. M. A1- bers, NewYork: Plenum Press, 301-319.

U l r i c h , J., 1964: Zur Topographie der Rosemary-Bank. Kieler Meeresforsch. 20, 95-100.

Eingegangen am 10. Mai 1988

Angenommen am 2. August 1988

Anschrift des Verfassers: D. J. Ellett, Scottish Marine Biological Association, RO. Box 3, Oban, Argyll; PA34 4AD, Scotland.