On the origin and the spreading of the shallow Mediterranean water core in the Iberian Basin

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Deep-Sea Research I, Vol. 411, No. 11/12, pp. 2167-2177, 1993. 0967J,1637/93 $6.00 + 0.00

On the origin and the spreading of the shallow Mediterranean water core in the Iberian Basin

H A N S - H A R A L D H I N R I C H S E N * a n d M O N I K A R H E I N *

(Received 3 September 1992; in revised form 12 May 1993; accepted 25 May 1993)

Abstract--The origin and the spreading of the shallow M e d i t e r r a n e a n water core (Ms) in the Iberian basin is discussed with a quasi-synoptic hydrographic data set e n h a n c e d by chlorofluoro- m e t h a n e ( C F M ) m e a s u r e m e n t s . Its characteristic density level is found to be q = 27.4. Character- ized by high t e m p e r a t u r e and C F M values, Ms enters the Iberian basin in the region of Cape St Vincent between depths of 500-750 dbar. A heat a n o m a l y of > 1 1 . 8 x 109 J m -2 is chosen as the b o u n d a r y between the presence of Ms and the b a c k g r o u n d field. The core is f o u n d in a tongue-like shape as well as in separate isolated eddies of both cyclonic and anticyclonic circulation. Using the o p t i m u m m u l t i p a r a m e t e r analysis (TOMCZAK and LARGE, 1989, Journal of Geophysical Research, 94, 16141-16149), the North Atlantic Central W a t e r ( N A C W ) , which mixes with the Mediterra- n e a n outflow to form Ms, t u r n e d out to be in the m e a n I°C w a r m e r and 0.11 saltier than in regions with m i n o r M e d i t e r r a n e a n influence. This points to the Gulf of Cadiz as the origin of Ms, where the M e d i t e r r a n e a n outflow is in contact with N A C W of the appropriate characteristic.

I N T R O D U C T I O N

THE mid depths of the Eastern North Atlantic Ocean are the most saline of the major oceanic regimes (W/3sT and DEFANT, 1936). This characteristic is related to the outflow of the Mediterranean Water (MW) released through the Straits of Gibraltar. In the Gulf of Cadiz, the outflow, forming an undercurrent, splits into two major branches, mainly guided by the topography: a warmer, less saline upper core around 800 m (Mu) and a colder, more saline lower one (M1) around 1200 m ( Z E N K , 1970; AMBAR and HOWE, 1979;

RHEIN and HINRICHSEN, 1993). This structure can be traced into the open ocean (Howe and TAIT, 1972; SIEDLER and ZENK, 1973). The hydrographic and tracer signal of the MW- outflow through the Straits of Gibraltar is considerably altered in the Gulf of Cadiz, and entrainment of Atlantic water from shallow horizons affects the upper core, Mu, more than MI (RHEIN and HINRICHSEN, 1993). Moreover, Fll/F12 ratio of the outflow is equal to that of the ambient Atlantic water in the Gulf, so that besides its potential as a mixing parameter, the F l l and F12 distributions carry no "age" information, which can be obtained otherwise under certain circumstances.

In addition to the double maxima structure of MW, a shallow core (Ms) has been observed in the Gulf of Cadiz and off the southern half of the western coast of Portugal (ZENK, 1975; AMBAR and H O W E , 1 9 7 9 ; A M B A R , 1983). This third core was identified at density levels of ot = 27.45 in the vicinity of the Straits of Gibraltar and around at = 27.3

*Institut ffir M e e r e s k u n d e an der Universit/it Kiel, Kiel, G e r m a n y . 2167

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2168 H.-H. HINRICHSEN and M. RHEIN

farther west; it is located between 400 and 700 m. AMBAR (1983) described the evidence of the third, shallower core using C T D , STD and bottle data from cruises of different years.

The core appears on a virtual path from the region in the vicinity of the Straits of Gibraltar to around 38°30'N in coastal areas off Portugal. The origin of this core was suggested to be similar to the well established two main cores originating as a subdivision of the upper core caused by topographic effects in the Gulf of Cadiz and around Cape St Vincent (AMBAR and HOWE, 1979). GR/~NDLINGH (1981) reported a solitary event in the Gulf of Cadiz with an increase in t e m p e r a t u r e of I°C for the upper layer of the Mediterranean water level. It coincided with a thickening of the outflow and an increase in volume transport. The origin of this event was traced back into the Straits of Gibraltar, corresponding to anomalous high cross channel water level differences.

Although the third core has been recognized in the past several times, the classical picture of the M W outflow suggests that its existence is quite episodic. The availability of a quasi-synoptic hydrographic data set extended by additional chlorofluoromethane (CFM, compounds F11 and F12) measurements allows the description of its mesoscale property fields within the Iberian Basin. In this paper we discuss the distribution of T,S, and F11 in the density range of Ms as well as the dynamic circulation. F u r t h e r m o r e , the optimum multiparameter analysis

(ToMCZAK

and

LARGE,

1989; HINRICHSEN and

TOMCZAK,

1993) will yield the actual t e m p e r a t u r e and salinity values of N A C W mixed with the MW, defined from observations p e r f o r m e d in the Western Alboran Sea and in the Gulf of Cadiz. A f t e r presenting the appearance of the third core in the Iberian basin by typical profiles and mean distributions in t e m p e r a t u r e and salinity and the CFMs, we will outline the horizontal structure of t e m p e r a t u r e as well as the integrated vertical heat content and the dynamic height field. Finally, the results of the water mass distribution analysis p e r f o r m e d by the optimum multi-parameter analysis (OMP) are discussed.

THE DATA

The observations on which the study is based were obtained on board FS Poseidon during early spring of 1990. T h e majority were executed in the near-shelf area off Portugal, reaching from 35 ° to 40°N and from 9030 ' to l l ° W , although the northern part was more extended, to around 14°W (Fig. 1). From 19 March to 3 April, 61 C T D casts were p e r f o r m e d , mostly down to depths of around 3000 dbar. For 29 of them, CFM measurements are available, additional observation parameters useful for application of the OMP, with 15 stations where the vertical coverage of the CFM values was dense enough to be used in the purpose of this paper. The CFM analysis procedures and the t e m p e r a t u r e and salinity acquisitions are described in RHEIN and HINRICHSEN (1993). The CFM reproduci- bilities amount to + 0.015 pmol kg -1 or + 1.2%, whichever is greater.

OBSERVATIONS AND RESULTS (a) Typical T/S diagrams

Figure 2 shows a typical appearance of the M W tongue in the Iberian Basin during the observation period. The triple maxima structure of St.406 (Fig. 2a) in the T/S domain is mainly attributable to water originating in the Mediterranean Sea. Their extreme values in t e m p e r a t u r e and salinity correspond to at-levels of 27.4, 27.6 and 27.8. The core at the

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The shallow Mediterranean water core in the Iberian Basin 2169

Fig. 1.

. , . . . . ...

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356 :i!i!i!iiii!i!:i:ii!!!!!!~!iii

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38° 398 ~ e~__~-~e367 ~iiiii:ili~:.iii i

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Cruise track Poseidon 172, March/April 1990 showing CTD stations. Black dots represent stations with evidence of the shallower core.

shallowest level is referred to as the "shallower" one, albeit slightly denser than observed by AMBAR (1993) in the Iberian Basin and warmer and fresher than any other maximum.

Station 406 was located within the MW undercurrent (37°30'N, 9°48'W) exhibiting small- scale perturbations probably due to intrusions of colder and fresher water masses and subsequent mixing. At St.422 (located in a Meddy observed at 35°57'N, 9°55'W) the smooth shape of the curvature in the T/S-domain (Fig. 2b) suggests that this Meddy was still less influenced by surrounding water masses, conserving its generation state during the movement away from the assumed source at 7°W (RHEIN and HINRICHSEN, 1993). Again, T/S characteristics yield the typical triple maximum distribution, although the signal of Ms is less pronounced. Maximum values are 13.8°C in temperature and 36.5 in salinity, which are remarkably high compared to the characteristics outside of this Meddy. The CFM profiles in the undercurrent (St.409) and in the Meddy (St.422) are depicted in Fig. 2c and compared to a profile with minor influence of MW (St.398); apparently, the presence of MW and, therefore, also of Ms increase the CFM concentrations and help separate the various water masses in the Iberian basin. CFMs were additionally used in the OMP.

(b) Mean distributions

On average the depth range between 500 and 750 dbar represents the upper and lower boundaries for the shallower core as observed during our hydrographic survey. Vertical profiles of T of this depth range (Fig. 3), display both horizontal averages for all stations as well as for those stations, where Ms is evident. The distinguishing features of Ms are indicated first by their differences in T and S from the mean. Maximum differences for temperature were obtained between 625 and 700 dbar (0.8°C), whereas differences for

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(a)

(b)

25.0 0" 6 28.0

T °C

/ ~ j . x " / M u 8 6 5 m /

21". i "1/~ I /i' l ~/I i /

34.5 350 36.5

27.0

28.0

29.0

35.5 36.0 37.0

S

18 T ° C

14

10

2 34.5

25.0 0" 8 26.0

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35.0 35.5 360 36.5 370

S

(c) ⁰

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500

1000

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. J

• [ I I

1.0 1.5 2.0 2.5 3.0

F11 p m o l / k g

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The shallow Mediterranean water core in the Iberian Basin 2171 salinity (0.3) were limited to a smaller depth range (not shown). Figure 3 shows the separation of Ms from the relatively colder and fresher Mu, which is located below Ms.

Decreasing differences d e e p e r than 700 dbar might demonstrate that a shallow intrusion of warm and saline water of Mediterranean origin could be identified, distinct from the well established two main cores. W a t e r of high t e m p e r a t u r e and salinity results in anomalous high heat and salt contents below the deepest layer of N A C W , which is not affected by MW. Modifications of p r o p e r t y contents, although of episodical character, could, how- ever, have consequences for the circulation and mixing processes.

(C) H o r i z o n t a l s t r u c t u r e

Horizontal maps are constructed by an objective analysis m e t h o d (HrLLER and K~sE, 1983) using an isotropic correlation scale of 50 km due to a mean minimum horizontal data distance of around 29 km. Application of this m e t h o d generally results in smoothing of the data; t h e r e f o r e variability in the estimated field is reduced.

The characteristic density level ot for the shallower core was found to be 27.4. E x t r e m e t e m p e r a t u r e values at this level range from 10.8 to 13.1°C (Fig. 4). The geographical distribution emphasizes the role of the M W spreading into the Iberian Basin. Obviously in the vicinity of Cape St Vincent, relatively high temperatures occur, building a s o u t h e a s t - northwestward orientated tongue-like shape. Further continuation of this tongue seems to be diminished towards the northwest by an isolated feature of colder water (<11.4°C) at 37°30'N, 10°30'W, as well as inshore of the tongue, where lower temperatures were found.

Additionally, some more isolated structures of opposite character were resolved, for example, at 36°N, 10°W and 39°N, 13°30'W, both coinciding with areas where Meddies are observed. The distribution in this layer yields separate structures with estimated horizontal scales of ca 100 kin, possibly generated by baroclinic instability of a northward mid depth jet of MW (BECKMANN and K~,sE, 1989). Moreover, intrusions of colder and fresher water found in the southern part of our observation area (10.2°C at 36°30'N, 10°W) suggest that the general southward m o v e m e n t of these water masses could be enhanced by this baroclinic instability. The vertically integrated total heat content between 500 and 750 dbar very well reflects the t e m p e r a t u r e distribution of this layer in the Iberian Basin (Fig.

5). Estimating the additional heat content for this layer, we refer to Fig. 3, where the horizontal averages of temperatures were shown (broken line). Vertical integration of this mean profile amounts to 11.9 x 109 J m -2. Taking into consideration that our data fields were somewhat smoothed by application of the objective analysis method, we choose areas containing more than 11.8 x 10 9 J m - 2 heat as representative for the existence of the shallower core. Its appearance extends to 39°N, 12°W, although an isolated structure (>11.8 x 10 9 J m -2) Occurs at 39°N, 13°30'W. Starting from the 11.8 x 10 9 J m -2 contour line, a positive heat content anomaly of about 0.9 x 1019 J is obtained by horizontal integration. This is about 25% of the heat anomaly of a Meddy observed in the Canary Basin (ARMIet a l . , 1989).

While the distribution of properties of Ms is relevant for water mass tracing, the dynamic

Fig. 2. Typical property distributions in the Iberian Basin: (a) T/S diagram at St. 406 located within the MW undercurrent; (b) T/S diagram at St. 422 located to a Meddy; (c) F11 profiles of St. 398 (minor MW influence), St.

409 (MW undercurrent), and St. 422 (Meddy C).

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2172 H.-H. H1NRICHSEN and M. RHEIN 11.0

500

p d b a r

600 - -

700 - -

11.5 12.0 T °C 12.5

t 1 I I I r I J I T i

/ i i

I

J / / / /

! I I

% I

I

I I

Fig. 3. Mean temperature profile between 500 and 750 dbar. Broken line refers to the arithmetic mean of all stations, whereas the full line represents only the mean of those stations, which are

highly influenced by MW,

1 5 ° Fig. 4.

1 4 ° 1 2 ° 1 0 ° W 8 °

Temperature at the shallower core level (o t = 27.4 kg m-3).

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The shallow Mediterranean water core in the Iberian Basin 2173

1 5 ° 1 4 ° 1 2 ° 1 0 ° W 8 °

Fig. 5. Vertically integrated total heat content [J m -2 × 109] between 500 and 750 dbar.

height field is more relevant for the dynamical structures of the circulation. The circulation in the shallower core layer resulting from the geostrophic mass transport stream function field of 500/750 dbar with respect to a reasonable level of no motion at 2000 dbar is shown in Fig. 6. The dynamical situation does not support the picture of a tongue-like spreading of the MW into the Iberian Basin. On the contrary, the mesoscale eddy field reveals separated anticyclonic and cyclonic features with mass transports between 0.8 and 2.2 Sv.

The marked features A, B and C are of anticyclonic rotation, all of them corresponding to typical Meddy structures (K.g.SE et al., 1989), described below. A similar spreading pattern is observed for Mu (HINRICHSEN et al., 1993). The circulation pattern for D is of quite different character. Whereas the property distributions (Figs 4 and 5) yield anomalously high contents of warm and saline water between 500 and 750 dbar, for which we might expect anticyclonic circulation, the relations here are reversed. A cell of strong cyclonic rotation of about 2.2 Sv appears, resulting from the influence of the dynamical structures of the deeper layers, which are characterized by the absence of the two well established main cores and the lateral influence of water from higher latitudes. This suggests a similar explanation of the dynamical circulation to a so-called "Anti Meddy", observed in the North Atlantic around 47°N, 20°W

(SCHAUER,

^1989).

M I X T U R E O F N A C W I N M S

Water masses are physical properties of finite volume; mathematically they can be described by a linear functional relationship between their characteristics (e.g. temperature, salinity and CFMs). For our purpose we attempt to clarify the origin of Ms, described as a mixture between the original outflowing MW released through the Straits of Gibraltar and the central water masses of the northern Atlantic, using the OMP with T, S and F11 as

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Fig. 6.

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Oeostrophic mass transport (Sv) between 500 and 750 dbar, level of no motion at 2000 dbar.

Table 1. Water type definition. [The characteristics o f the water masses are obtained by linear regression o fo u r data set o f the Iberian Basin and the Gulf o f Cadiz ( RHEIN and HINmCHSEN (1993)]

Parameter NACWu NACWI MW NADW

Temperature (°C) 18.00 I0.00 13.35 4.00

Salinity 36.497 35.400 38.400 35.130

F l l (pmol kg -1) 3.05 1.19 1.40 0.10

parameters. Note that the characteristics of central water masses are rather well described by a straight line between two property combinations. Linear regression of our observed data results in the water type definition in Table 1. The applied m e t h o d solves the set of mixing equations through minimization of the least squares residuals (for details see TOMCZAK a n d LARGE, 1989).

The analysis presented here is based on observations of 46 stations, where the existence of the shallower core is represented by t e m p e r a t u r e and salinity pairs only. F u r t h e r m o r e , 15 stations with additional chlorofluoromethane observations were available. Figure 7 shows the fraction of M W at at = 27.4. The analysis was able to reproduce all observations with the defined e n d m e m b e r s of M W and N A C W (Table 1), although weak variability is present. The fraction of the Mediterranean e n d m e m b e r varies from 0.08 to 0.23 on at = 27.4 (Fig. 7) with the maximum found in the center of Meddy C. The contribution of M W to the northwestward oriented tongue does not exceed 0.17, whereas the fraction (0.23)

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T h e s h a l l o w M e d i t e r r a n e a n w a t e r c o r e in t h e I b e r i a n B a s i n 2175

Fig. 7.

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° . =

1 5 ° 1 4 ° 12 ° 1 0 ° W 8 ~

F r a c t i o n of M e d i t e r r a n e a n W a t e r at (7 t = 27.4 k g m -3 (for w a t e r m a s s d e f i n i t i o n see T a b l e 1).

Fig. 8.

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S a l i n i t y o f a d m i x e d N o r t h A t l a n t i c C e n t r a l W a t e r at a t = 27.4 k g m 3.

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2176 H.-H. H1NRICHSEN and M. RHEIN

for M e d d y C, a n d its high F I 1 c o n c e n t r a t i o n s s u p p o r t s t h e s t a t e m e n t t h a t its g e n e r a t i o n was a r o u n d 7 ° W i n t h e G u l f of C a d i z (RHEIN a n d HINRICHSEN, 1993). T h e N A C W , which m i x e s with t h e M W at t h e a t = 27.4 layer, has t e m p e r a t u r e s of 11.9 _+ 0.5°C a n d salinities of 35.66 + 0.07, w h e r e t h e s h a l l o w core is p r e s e n t ( h e a t c o n t e n t > 1 1 . 8 × 1 0 9 j m 2) (Fig. 8). I n r e g i o n s with m i n o r i n f l u e n c e o f Ms, t h e a d m i x e d N A C W is c o o l e r (11.09 + 0.4°C) a n d f r e s h e r (35.55 _+ 0.05); these v a l u e s m a t c h closely to the i n t e r m e d i a t e s a l i n i t y m i n i m a v a l u e s of N A C W . F i g u r e 8 r e p r e s e n t s t h e s a l i n i t y of a d m i x e d w a t e r types of N A C W at at = 27.4. T h e p r e s e n c e of Ms, h o w e v e r , p o i n t s to m i x i n g with N A C W o r i g i n a t i n g in t h e m e a n f r o m a b o u t 350 m d e p t h , a n d in t h e a r e a of M e d d y C, e v e n N A C W f r o m 200 m d e p t h with a s a l i n i t y of 35.92 a n d a t e m p e r a t u r e of 13.8°C is f o u n d to b e the m i x i n g e n d m e m b e r . T h e m a j o r p a r t of Ms r e p r e s e n t s the m i x i n g with w a t e r types of r e l a t i v e l y high t e m p e r a t u r e s a n d salinities, which m a i n l y o c c u r i n t h e G u l f of C a d i z , w h e r e the M e d i t e r r a n e a n o u t f l o w is in c o n t a c t with N A C W of t h a t c h a r a c t e r . T h e h i g h e r t e m p e r a t u r e s a n d salinities of N A C W a d m i x e d in the p r e s e n c e of Ms i n d i c a t e s t h a t a m a j o r p a r t of t h e m i x i n g occurs in the G u l f o f C a d i z , w h e r e t h e M e d i t e r r a n e a n o u t f l o w is in c o n t a c t with N A C W of that c h a r a c t e r i s t i c . T h e cold a n d fresh w a t e r i n t r u s i o n at 36°30'N, 10°W was c o r r e l a t e d to m i x t u r e s of N A C W , b e l o w t h e typical i n t e r m e d i a t e salinity m i n i m u m l a y e r of the I b e r i a n B a s i n ,

Acknowledgements--We are grateful to the officers and crew of FS Poseidon for assistance and support during cruise 172. Especially we would like to thank A. Eisele for carefully preparing the figures and T. Elbr/ichter who performed many of the CFM analyses. The work was supported by the Deutsche Forschungsgemeinschaft (SFB 133 "Warmwassersph/ire des Atlantiks").

R E F E R E N C E S

AMBAR 1. (1983) A shallow core of Mediterranean water off western Portugal. Deep-Sea Research, 30,677-680.

AMSAR l. and M. R. Howe (1979) Observation of the Mediterranean outflow, I. Mixing in the Mediterranean outflow. Deep-Sea Research, 26, 535-554.

ARMI L., D. HEBERT, N. OAKEY, J. PRICE, P. L. RICHARDSON, T. TOSSBY and B. RUDDICK (1989) Two years in the life of a Mediterranean salt lens. Journal of Physical Oceanography, 19, 354-370.

BECKMANN A. and R. H. K.~SE (1989) Numerical simulation of the movement of a Mediterranean water lens.

Geophysical Research Letters, 16, 65--68.

GR~NDLINGH M. L. (1981) On the observation of a solitary event in the Mediterranean outflow west of Gibraltar.

"Meteor" Forsch.-Ergebn., set A, 23, 15-46.

HILLER W. and R. H. KASE (1983) Objective analysis of hydrographic data sets from mesoscale surveys. Bet. Inst.

Meereskd. Univ. Kiel, 116, 78 pp.

HINRICHSEN H.-H. and M. TOMCZAK (1993) Optimum multiparameter analysis of the water mass structure in the western North Atlantic Ocean. Journal of Geophysical Research, 98, 10155-10169.

HINRICHSEN H.-H., M. RHEIN, R. H. K.~SE and W. ZENK (1993) The Mediterranean water tonguc and its chlorofluoromethanes signal in the Iberian basin in early summer 1989. Journal of Geophysical Research, 98, 8405-8412.

Howe M. R. and R. I. TAIT (1972) The role of temperature inversions in the mixing processes of the deep ocean.

Deep-Sea Research, 19, 781-791.

K~.SE R. H., A. BECKMANN and H.-H. HINRICrlSEN (1989) Observational evidence of salt lens formation in the Iberian Basin. Journal of Geophysical Research, 94, 4905-4912.

RHE1N M. and H.-H. HINRICHSEN (1993) Modification of Mediterranean Water in the Gulf of Cadiz, studies with hydrographic, nutrient and chlorofluoromethane data. Deep-Sea Research, 40, 267-291.

SCHAUER U. (1989) A deep saline cyclonic eddy in the West European Basin. Deep-Sea Research, 36, 1549-1565.

SIEDLER G. and W. ZENK (1973) Variability of the thermocline staircase. Nature, 244(131), 11-12.

TOMCZAK M. and D. G, B. LARGE (1989) Optimum multiparameter analysis of mixing in the thermocline of the Eastern Indian Ocean. Journal of Geophysical Research, 94, 16141-16149.

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ZENK W. (1970) On temperature and salinity structure of the Mediterranean water in the N.E. Atlantic. Deep- Sea Research, 17,627-632.

ZENK W. (1975) On the origin of the intermediate double-maxima in T/S profiles in the North Atlantic. "'Meteor"

Forsch.-Ergebn., ser. A , 16, 35-43.