Winter circulation in the Sicily and Sardinia straits region

Deep.Sea Research,Vol. 26A, pp. 933 to 954 © Pergamon Press Ltd 1979. Printed in Great Britain

0011-7471/79/0801-0933 $02.00/0

Winter circulation in the Sicily and Sardinia straits region SILVIA GARZOLI* a n d CATHERINE MAILLARDt

(Received 6 January 1978: in revised form 28 December 1978 ; accepted 10 April 1979) Abstract--The hydrology and dynamics of the Sicily and Sardinia straits were studied to determine the general characteristics of the winter circulation. In the Sardinia Straits, the Levantine Intermediate Water divides into two branches parallel to the African and Sardinian coasts. The circulation is characterized by two eddies of opposite sense in the two upper layers (surface and intermediate water). The hypothesis that the eddies are due to a planetary effect is tested using a simple theoretical model. INTRODUCTION AND BACKGROUND

DURING WINTER in the Rhodes--Cyprus area a convective, homogeneous layer of Levantine Water (LW) is formed. This layer, 150 m thick, is characterized by a maximum in temperature (15.70°C) and salinity (39.10% o); the maxima decrease towards the west. LW flows in a cyclonic circulation in the eastern Mediterranean and a branch of this water enters the Adriatic Sea (LAcOMBE, 1977). Near the sea floor, in the Straits of Sicily, LW, which has a local salinity of 38.75%0, flows westward; it invades the western Mediterranean and contributes to the Intermediate Water (IW) layer present in the whole western basin. The course of the Levantine Water (LW) in the Straits of Sicily is mainly controlled by the bottom topography, a picture of which was given by FRASSETTO(1964). The topography is complicated, and near the depth of the LW in the eastern basin, the communication between the eastern and western Mediterranean basins is mainly through two channels, one with a minimum depth of 365 m and whose orientation is towards the northnorthwest [channel B, Fig. l(a)], the other with a minimum depth of 430 m oriented towards the north [channel A, Fig. l(a)]. MORELLI, GANTARand PISANI (1975) have given different depths for the sills, 510 m for the eastern sill (channel A) and 410 m for the western sill (channel B), but the general picture of the topography was similar to that presented by FRASSETTO(1964). Towards the west the water flows to the Straits of Sardinia, a narrow valley with an average maximum depth of about 2000 m [Fig. l(a)]. NIELSEN (1912) and later Wt~ST(1959) and TCHERNIA(1960) showed that the IW follows three principal courses in the western basin: one flows from the Sicily Channel to the Gibraltar Straits close to the North African slope; another flows from the Sicily Channel towards the Tyrrhenian Sea. It appears that after a circuit in that sea, part of it enters the Sardinia Channel; the third flows west and then north along the western Sardinia and Corsica slopes. * Laboratoire d'Ocb,anographie Physique, Mus6um National d'Histoire Naturelle, 43-45 Rue Cuvier, 75231 Paris Cedex 05, France. t Centre Oceanologique de Bretagne, BP 337, 29 273 Brest Cedex, France.

933

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936

SILVIA GARZOLI and CATHERINE MAILLARD

The possibility of a division of the IW into two branches in the Straits of Sardinia is also suggested by a cross section provided by the R.V. Atlantis cruise 263 (Section 12 in MILLER, TCHERNIAand CHARNOCK,1970). Two additional cruises were carried out by the Laboratoire d'OcOanographie Physique du Mus6um: Amalthee 1967 and HydromedH 1968 in the region of the Sicily and Sardinia straits to study the winter circulation. Data from Calypso 1965 and Calypso 1966 cruises (Laboratoire d'Oc6anographie Physique, FacultO des Sciences Paris VI) and an Origny 1963 cruise (Service Hydrographique de la Marine) were also used (MOREL, 1969). In the following we analyze the hydrological and dynamical regimes of the Sicily and Sardinia straits, mainly on the basis of data from the above cruises. The study confirms the splitting of the IW into two branches in the Straits of Sardinia; it shows the existence of gyres that may play a role in the splitting. A theoretical explanation of the gyres is proposed. HYDROLOGICAL AND DYNAMICAL ANALYSIS

The bottom topography and the positions of the hydrographic stations are given in Fig. l(a) and (b). The core method has been used to trace the path of the IW by studying the distribution of the intermediate maxima of salinity and temperature. We conclude that the IW enters the western basin in the deeper areas of the Straits of Sicily (Figs 2 and 3). In the Straits of Sardinia (about 9°E) and to the west, the maxima of temperature and salinity are along the continental slopes on both the northern and southern sides. Geostrophic velocities and fluxes were estimated by the dynamic method using a reference level of 800 dbar. This level was a compromise between the shallow waters of the Straits of Sicily and the deeper water in the Sardinia Straits.

The Sicilian Ridge Hydrology The characteristics of the IW differ on the two sides of the sill (between latitudes 37~15'N and 37°32'N, longitudes ll°15'E and 11°35'E). Figure 4(a) and (b), along channel A, ~.

Fig. 2.

-

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Maximum of salinity in the core layer of the Intermediate water February--March 1968

(Hydromed II).

Winter circulation in the Sicilyand Sardinia straits region

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- 37"

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Fig. 3. Maximum of temperature in the core layer of the Intermediatewater February March 1968 (HydromedII). display this difference and also show the development of the IW to the north along the lines of the stations used. Water warmer than 14.35°C does not cross the sill: potential temperatures are lower in the north (T < 14.35°C) than on the southern side of the sill (up to 14.68°C); the corresponding salinity and sigrna-0 are 38.74%o and 29.00 in the north. The intermediate layer is deeper, about 300 to 400 m, in the north, i.e. deeper than the LW in the south (150 to 250 m). These changes occur along channel B [Fig. 5(a) and (b)] and the same remarks hold for the intermediate layer. Outside the channels communication between the basins is partially obstructed and the IW flows along the channels to the north.

Dynamics The incoming flow toward the Sicilian Ridge was estimated using the data from the

Calypso 1965 and 1966 cruises. Figure 6 shows the distribution of salinity at the entrance of the Sicilian Ridge [Section I(b), Calypso 1965, Fig. 1(b)]. A two-layer system is evident; it consists of the IW that moves westwards in the western basin and the surface water that moves eastwards. Because of the complicated topography and shallowness the dynamical calculations were not very accurate. An order of magnitude of incoming flux of 106 m 3 s- 1 was deduced from both sections [Section I(a), Calypso 1966 and Section I(b), Calypso 1965]. This value is in good agreement with MOREL'S (1971) estimate from heat and salt balances. The level of flow reversal between surface and IW appears near the isohalines 38.5%o and 38.7%o, which lie close to the estimated boundary between the two water masses.

Connection with the Tyrrhenian Sea Figure 7 shows a cross section along Station VI(a) [Fig. 1(b)] using the Hydromed H data from the Marittimo Islet (west side of Sicily) to the southeast coast of Sardinia. The section shows the IW entering the Tyrrhenian Sea, because near the Sicilian coast, salinity and temperature are higher between Stas 101 and 102. Dynamical analysis of the section (geostrophic velocities and fluxes) was made to

938

S~LVIA GARZOLI a n d CATHERINE MAILLARD

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Distribution of salinity (a) and temperature (b) along channel A, Strait of Sicily (Hydromed II). Sill depth according to FRASSETTO (1964).

939

Winter circulation in the Sicily and Sardinia straits region

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Distribution of salinity (a) and temperature (b) along channel B, Straits of Sardinia [Hydromed H, Section II(b)].

940

S~LV~A GARZOLI a n d CATHERINE MAILLARD

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Fig. 6. Distribution of salinity and direction of flow along Section I(b) (Calypso 65). A positive sign indicates towards the northwest ; a negative sign, to the southeast. The line . . . . corresponds to zero computed geostrophic velocities.

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Distribution of salinity along cross section VI (Hydromed II).

Winter circulation in the Sicily and Sardinia straits region

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I T sw : -o.oTx 1o6 m31s J~T IW = O03x 106m33As ~ Totat = - O.04x 106m /s

- SOUTH _ W E S T [ v ] = cm s -I Reference -- 800 db(]rs

Fig. 8. Geostrophic velocities (vg) in cm s- 1 and fluxes (~) in m 3 s - t, from the dynamic method applied to Section VI (HydromedII). The line shows the bottom topography. +, towards northeast; - , towards southwest. evaluate the exchange with the Tyrrhenian Sea (Fig. 8). Positive values mean directions to the northeast (entering the Tyrrhenian) and negative ones to the southwest (comin~ out). A different choice of reference level below 800 dbar results in almost the same values. There is a double inversion in the velocity along the section, for both intermediate and surface waters. Between Stas 101 and 104 the main flow enters the Tyrrhenian Sea. The main flow is outward (to the southwest) between Stas 104 and 106. On considering the exchange of flux, we find that for the surface water (SW), i.e. S < 38.50%o: OSW =

--0.07

× 10 6

m 3 s -~ (outflow to the Tyrrhenian Sea),

0.03

× 10 6

m 3 s-~ (inflow to the Tyrrhenian Sea).

and for the IW: (I)lw =

So, if we estimate the total exchange of flux: Or = ~SW+~IW = - - 0 . 0 4 X 106 m 3 s - t . All three values are negligible with respect to the incoming flux through the Straits of Sicily (106 m 3 s-~ ). Thus there is an equilibrium in the exchanges with the Tyrrhenian Sea for each water mass; the IW flux into the Tyrrhenian Sea from the east is of the same order of magnitude as that out to the west. The same is true for the southwest. To verify this result, a similar analysis was made along Section VI(b) drawn with Origny

942

SILVIA GARZOLI and CATHERINE MAILLARD

data (September 1963) (GARZOLIand MAILLARD,1977). The only difference between Sections VI(a) and (b) is the season: the first corresponds to winter and the second to autumn. Estimated total fluxes along Section 15bis (Origny) are also negligible (~Total = 0.008 × 10 6 m 3 s - l ) .

Wt3ST (1961) compared the characteristics of the IW in winter and in summer and concluded that in summer the IW is slightly less evident than in winter, but otherwise the salinity distribution within the core layer is similar in the two seasons. The isohalines S = 38.50 and 38.65%0 are at nearly the same place in winter and in autumn. We conclude that exchanges of water through the Sicily-Sardinia section are balanced within each layer, both in summer and in winter.

THE SARDINIA STRAITS

Hydrology The hydrological situation in the Straits of Sardinia was similar in the winters of 1967

(Amalthee) and in 1968 (Hydromed II) (GARZOLI and MAILLARD, 1977). The characteristics of the IW mass can be seen in the O-S diagrams (Fig. 9) based on the data provided by Hydromed 1968. The maximum values of potential temperature (between 14.05 and 14.15'~C) and salinity (S < 38.70%0) are lower than on the Sicilian Ridge. The division of the IW into two branches is clearly shown by the hydrology; two examples are given in Figs 10 and 11. Section VII (Fig. 10), in the eastern part of the straits between longitudes 9 ° and 9°10'E, shows that the layer of IW (S > 38.50%0) is almost continuous from north to south in the channel. The second, Section XIV (Fig. 11, near 8°05'E), clearly shows that in the western part of the straits the IW layer has divided into two branches with nearly the same characteristics: the northern branch is parallel to the African shelf at about latitude 37°30'N, near the continental shelf of Sardinia. But the southern branch further subdivides; there is a maximum of salinity at 38°10'N (S = 38.60%0) and another at 37°20'N. Section XVIII, at 7°10'E, shows the same effect, but it is attenuated. More information about this second splitting will be obtained from the dynamical analysis.

Horizontal circulation The dynamical topographies of the 0-, 200-, and 400-dbar surfaces are shown in Figs 13, 14, and 15 (reference level 800 dbar). The surface circulation is characterized by an anticyclonic eddy centred on 37°57'N, 8°20'E. The mean velocity at the edge of the eddy is 30 cm s 1. This anticyclonic flow also appears at 200 dbar, but it is smaller in extent (velocities are also smaller); a second eddy, cyclonic and less clearly defined, appears at the 200-dbar level at the northern side at about 38°30'N, 8°10'E. A similar circulation pattern was found at 100 and 300dbar. At 400 dbar (IW level) two eddies still appear, but with lower velocities (7.0 cm s- 1). In the surface layer the main flux is towards the east between the two eddies, while at the IW level (400 dbar) the maximum velocities are on the outer sides of the eddies along the slopes. Thus the flow is divided into two lateral branches agreeing with the results obtained from the hydrology. The flow is also shown by the geostrophic velocities in Section XI (Table 1). In addition, similar results come from data provided by the Amalthee 1967 cruise.

Winter circulation in the Sicilyand Sardinia straits region

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0 S diagrams in the Straits of Sardinia

(Hydromed lI).

Quantitative study of velocities at the IW level Dynamical analysis of the meridional sections in the Straits of Sardinia gives us the geostrophic flow at the IW level. The results for both branches are given in Table 2 for the meridional sections. No hydrological stations were occupied near the African Coast on Sections VIII, XIV, and XVI (Hydromed II), so the southern branch is outside of the section studied. Speeds are all between 3.5 and 8.0 cm s-1 with the exception of Section VII(b) (Origny 1963). This may be because the cruises were made during different seasons.

944

SILVIA GARZOLIand CATHERINEMAILLARD

o

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8 Fig. 10. Distribution of salinity along 9°30'E (Section VII, Hydromed IlL

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945

Winter circulation in the Sicily and Sardinia straits region

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Fig. 11. Distribution of salinity along 8°05'E (Section XIV, Hydromed II).

Total flux The flux through the Straits of Sardinia can be estimated at 9°30'E using hydrological Section VII(a) (Hydromed II) and VII(b) (Origny), Fig. l(b) (Table 3). There is a good agreement for the IW flux in both sections, but not for the surface layer. This contradiction may be because of a seasonal variation in surface circulation. Because the sections do not reach the shelves a good estimate of the flux through the west side of the Straits of Sardinia is not available. Near 7°47'E (Section XVI, Fig. 12) the geostrophic fluxes are: ~sw = - 1.75 x 106 m 3 s- 1 (east), ~iw = 0.0. These results can be explained considering Fig. 12 and Table 4: the southern branch does not go through the cross section ; at the IW level (400 dbar) an eddy appears between Stas 22 and 25. For the northern branch, between Stas 17 and 21, the estimated flux is O~w = 0.30 x 106 m 3 s- 1 towards the east. The same value, but with opposite sign, was found between Stas 22 and 25. From the hydrological and dynamical analysis of the Straits of Sardinia we conclude that the winter circulation is related to the existence of two eddies that cover the surface and intermediate layers (to 500 m).

946

SILVIA GARZOLI and CATHERINE MAILLARD

s

0

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Distribution o f salinity a l o n g 7°50'E (Section X V I ,

Hydromed II).

In the surface layer, the main flux between the two eddies is toward the east; at the IW level, however, the flux toward the west divides into two branches outside the two eddies, one along the North African coast and the other along the southern coast of Sardinia. The two eddies are not symmetrical; in both layers the anticyclonic circulation dominates. The imbalance of the values for the net flux on the eastern and western sides of the Sardinia Channel suggests that currents must be important near the coasts. Because of the continuity condition and considering that evaporation and precipitations are negligible, an incoming surface water flux of about 10 6 m a s - 1 must be balanced by an outward flux of IW. The outward flux, however, must be divided into two branches parallel to the coasts. THEORETICAL

INTERPRETATION

OF THE EDDIES

In the previous paragraph, the existence of two large eddies at the IW level, one cyclonic in the northern side of the Straits of Sardinia and the other anticyclonic in the southern side, was established (Figs 13 to 15). Accordingly, the surface flux to the east is along the channel axis and the IW flux to the west is along the northern and southern slopes. The division of the IW into two branches in the Straits of Sardinia may be related to the existence of those two eddies. A possible theoretical explanation of its origin is proposed in this section.

947

Winter circulation in the Sicily and Sardinia straits region

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Fig. 13. Dynamic topography of the 0-dbar surface relative to the 800-dbar surface. Units are dynamic centimetres.

°

948

SZLVL~ GARZOL1 and CATHERINE MAILLARD

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Dynamic topography of the 200-dbar surface relative to the 800-dbar surface. Units are dynamic centimetres.

949

Winter circulation in the Sicily and Sardinia straits region

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Fig. 15. Dynamic topography of the 400-dbar surface relative to the 800-dbar surface. Units are dynamic centimetres.

950

SILVIA GARZOLI a n d CATHERINE MAILLARD

Table 1.

Geostrophic velocities between the different stations o f Section X I around 8o30 ' (reference level: 800 dbar ). + , towards the west; - , towards the east %

vg

v~

vg

%

vo

51 50

50-49

49-48

48~t7

47 4 6

46-45

45 4 4

32.5 18.0 8.2 8.0 8.0 8.0 8.1 0.0

-- 100.0 --47.0 -- 16.1 --9.5 -8.1 -8.0 -8.1 0.0

-- 32.0 --23.7 -5.1 0.0 0.7 0.7 8.7 0.0

-- 20.0 --21.0 - 13.0 --3.6 -1.1 -0.8 -0.4 0.0

- 1.6 --0.9 --0.9 --1.6 -1.6 -0.9 -0.9 0.0

22.0 21.5 16.0 4.5 1.6 0.8 0.0 0.0

12.0 14.1 13.1 7.5 4.2 2.4 2.9 0.0

z (m) st: 0 100 200 300 400 500 600 800

Table 2.

t,g (cm m

Geostrophic velocities at the l W level,for both north andsouth branches. +, towards, the west .. - , towards the east North branch

South branch

Mean

Section

1)

longitude

Sma,

Depth

9°33 ' 9°40' 9o07 ' 8°35 ' 8 ° 18' 8°04 ' 7o48 '

38.70 38.69 38.70 38.69 38.69 38.70

300 300 400 300 400 400

VII

VIIbis VIII XI XIII XIV XVI

Table 3.

Va + + + + + + +

3.5 1.0 7.0 8.0 4.0 4.0 5.0

S,~a~

Depth

VG

38.70 38.66 38.70 38.70 38.69 -

400 300 400 500 400 -

+ 7.0 + 3.2 + 4.5 + 6.0 -

Estimated fluxes through the Straits o f Sardinia. + , towards the west; - , towards the east

Section

VII

Hydromed II Date ~ s w ( S < 38.5%0) ~ l w ( S > 38.5%0) @rotal(Ref 800 d b a r s )

Feb-March

1968

VIIbis Origny S e p t - O c t 1963

_ 0 . 7 2 x 106 m3 s 1 0.92 x 104 m 3 s - l

0.22 x 106 m 3 s - l 0.95 x 106 m 3 s -

0.20 x 106 m 3 s - 1

1 . 1 6 x 1 0 6 m 3 s -1

-

Winter circulation in the Sicily and Sardinia straits region

951

Table 4. Geostropic velocities between the different stations of Section X VI. +, towards the west; - , towards the east. (Reference level: 800 dbar.) z (m) st: 0 1~ 2~ 3~ 4~ 5~ 6~ 8~

vg 17-32

vg 32-20

vg 20 21

v0 21 22

v~ 22 23

v~ 23 24

v0 (cm s -1) 24 25

5.0 4.3 2.1 5.0 5.0 4.2 3.0 0.0

1.2 -0.4 1.0 -0.4 0.0 -1.0 -0.4 0.0

-24.0 -9.5 -0.6 0.9 0.6 0.9 -0.3 0.0

-7.9 -3.3 -1.8 -1.4 -1.2 -0.9 -0.3 0.0

-35.0 -29.0 -7.0 -0.8 0.4 0.4 0.0 0.0

-1.9 -1.9 -0.4 -1.1 0.0 0.0 0.0 0.0

15.0 13.0 2.2 -1.8 -1.8 -1.4 -0.9 0.0

General equations f o r a steady planetary wave superimposed on an eastern current In this section, the existence of a planetary, stationary eddy, s u p e r i m p o s e d o n a uniform velocity in the eastern direction will be considered. O u r c o o r d i n a t e system is such that the x-axis points east. C o n s e r v a t i o n of potential vorticity for uniform depth b a r o t r o p i c flow o n the beta plane states O (V2tP+fo+fly) Dt

0

(l)

with fo = m e a n Coriolis p a r a m e t e r fl = f o / R , t o 49o 49o = m e a n latitude, R, = e a r t h ' s radius, where u = - OtP/Oy,

v = + ~tP/~x

defines a stream function that gives the zonal a n d m e r i d i o n a l (y) velocities. F o r steady flow a first integral is VZtP + fo + flY = F(W)

(2)

(SAmT-GUILY, 1961 ; FOFONOF, 1962), where F is a function d e t e r m i n e d by processes that set the potential vorticity of a fluid parcel. We seek a stream function of the form = - Uy + ~ *

(3)

so that W* is a flow s u p e r i m p o s e d o n the z o n a l flow u = - U. If n o streamline o f q ~ is closed a n d if W* ~ 0 at infinity, then F ( - - U y ) = f o + f l Y fixes F. H e n c e V 2 ~ * + fl/Uq/* = 0.

(4)

O n e of m a n y solutions to this v a n i s h i n g at infinity is, in polar c o o r d i n a t e s (r, 0): W* = a sin O J l ( r w / f l / U ) ;

(5)

952

SILVIA GARZOLI a n d CATHERINE MAILLARD

a is an arbitrary constant, so = - U sin O[(r+adl(rx/fl/U)]

(6)

is a possible flow satisfying (1) with J1 being the first kind of Bessel function of order one. The quantity x / ~ gives the scale of the rotating component ; a is an arbitrary constant. For small positive values of a, there are only meanders and no closed eddies. For a > alxf~ [al is a constant for which the equation z+aaJl(z ) has a solution and 2 < al < 3], we get two eddies rotating in the same sense as in the Straits of Sardinia. For a < 0, we have two eddies rotating in the reverse sense.

Application to the Sardinia Straits Equation (6) with a 3 > x / ~

can give a first order fit for the observations. The

theoretical length scale x / ~ can be evaluated in the surface layer: the eastward flux of 106 m 3 s - 1 for the upper 200 m for a channel width of about 200 km between the two continental shelves at the eddies' central longitudes gives a mean current U ~ 0.025 m s - I. So, for the mean latitude of 38 ° : ro = x f ~

-~ 37 kin.

The diameter of the eddies depends on the zeros of the Bessel function Ja and on r o. For the upper value o f r o we have D ~ 140 km, which is slightly larger than the half-width of the channel but of the same order of magnitude. The theoretical speeds for c = a/r o = 5, 8, and 10, respectively, are 8.3, 10.8, and 14.2 cm s - 1. The maximum transverse vertically averaged geostrophic velocities from Figs 13 to 15 are about 20 cm s - 1. Thus theoretical values are rather smaller than the measured values, but the order of magnitude is the same. In Fig. 16 we show the theoretical current pattern (lines qJ = constant) for c = 5.

I

y

c= 5

X

F i g . 16.

s i n O(r + a l l(r/r o)) = constant for c = a/r o = 5 ; a x e s are the dimensionless v a r i a b l e s X = x/ro; Y = y/ro.

T h e o r e t i c a l stream lines ~g = - u

Winter circulation in the Sicily and Sardinia straits region

953

So, for the first order, the variation of the Coriolis parameter with latitude can explain the existence of the double vortex system observed in the Sardinia Straits. This model does not explain the asymmetry of the two eddies; a further study should take into consideration the effect of the topography. One important question is whether the double vortex is stationary or not. Even though the cyclonic and anticyclonic circulations were observed in both 1967 and 1968, the stationary characteristics of the system are not certain. It is possible that amplitude and position of the eddies vary with time. If that is the case, the motion must be associated with non-permanent planetary waves. CONCLUSIONS

The main features of the circulation of the straits of Sicily and Sardinia are (1) In the twolayer system at the entrance of the Straits of Sicily from the south, the estimated fluxes in the opposite directions are about 106 m 3 s- 1. The exchanges with the Tyrrhenian Sea are balanced at each water mass level. (2) While crossing the Straits of Sardinia, the IW divides into two branches flowing along the North African and the Sardinia slopes. (3) The dynamical topography down to 400 dbar shows that the circulation in the Straits of Sicily is dominated by an anticyclonic eddy in the southern part and a cyclonic eddy in the northern part ; the two eddies form an obstacle to the IW flowing to the east, and this can be the reason why the IW divides into two branches. (4) The hypothesis that both eddies have a planetary origin is analyzed in a model. Theoretical velocities and streamlines obtained from this model are of the same order of magnitude as those observed. Acknowledgements--The authors are indebted to PROFESSOR H. LACOMBEfor his most helpful comments and remarks during the preparation of this paper. We thank PROFESSORB. SAIN'r-GUILVfor his helpful suggestions and discussions. Finally, we thank PROFESSOR P. TCHERNIA, Chief Scientist of the Amalthee and Hydromed II cruises supported by DGRST (COMEXO) Grant 66 00-238 and by CNEXO Grant 68 1.

REFERENCES FOEONOFN. P. (1962) Dynamics of ocean currents. In The sea, Vol. 1, M. N. HILL, editor, Interscience Publishers, pp. 323-395. FRASSETTO R. (1964) A study of the turbulent flow and character of the water masses over the Sicilian Ridge in both summer and winter. Rapports et Proc~s-Verbaux, Commission Internationale pour l'Exploration Scientifique de la Met M~diterran~e, lg, 812 815. GARZOL1 S. and C. MAILLARD(1977) Hydrologie et circulation hivernales dans les canaux de Sicile et Sardaigne. Rapport interne, Laboratoire d'Oc~anographie Physique du Museum, Paris (unpublished document). LACOMBEH. (1977) The mesoscale and local response of the Mediterranean to the transfer and exchange of energy across the sea surface. In: Physics of Oceans and Atmosphere, International Centre for Theoretical Physics, Vol. 1, Trieste, Italy, pp. 211-278. MILLER A. R., P. TCHERNIAand H. CHARNOCK(1970) Mediterranean atlas, Vol. 3, Woods Hole Oceanographic Institution, pp. 21 45. MOREL A. (1969) Resultats des observations effectuees en Mer Mediterranbe principalement dans le detroit de Sicile ~i bord du N.O. Calypso. Cahiers Oc~anographiques, 21, 203 244. MORELA. (1971) Caract6res hydrologiques des eaux ~ h a n g ~ s entre le bassin oriental et le bassin occidental de la M~diterran~e. Cahiers Oc~anographiques, 23, 32~342. MORELLIC., C. GANTARand M. PISANI(1975) Bathymetry, gravity and magnetism in the Straits of Sicily and in the Ionian Sea. Bolletino di Geofisica Teorica ed Applicata, 17, 39-58. NmLSENJ. (1912) Hydrography of the Mediterranean and adjacent waters. Report on the Danish Oceanographical Expeditions 1908-1910, Copenhagen, 72-191.

954

SILVIA GARZOLIand CATHERINEMAILLARD

SAINT-GUILY B. (1961) Influence de la variation avec la latitude du param&re de Coriolis sur les mouvements plans d'un fluide parfait. Cahiers Ocbanographiques, 13, 167-175. TCHERNIA P. (1960) Hydrologie d'hiver en M/~diterran6e occidentale. Cahiers Ocbanographiques, 12, 184-198. WOST G. (1959) Remarks on the circulation of the intermediate and deep water masses in the Mediterranean Sea and the methods of their further exploration. Annali lstituto Universitario Navale, Napoli, 28, 3-16. WOST G. (1961) On the vertical circulation of the Mediterranean Sea. Journal of Geophysical Research, 66, 3261-3272.